Politecnico  Di  Milano  Facoltà  di  Ingegneria  dei  Processi  Industriali  

Degree  in  Materials  Engineering    

   

3D  Reinforcement  of  Composite  Materials  

   

   

Supervisors:   Valter  Carvelli  (Politecnico  di  Milano)         Giulio  Ventura  (Politecnico  di  Torino)         Carlo  Poggi  (Politecnico  di  Milano)          

Corinna  A  Conway  750155  

   

Academic  Year  2010  -­‐  2011  

 

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Dedicated  to  my  little  brother,  Michael  Conway      Who  taught  me  how  precious  life  truly  is  and  what  it  means  to  follow  

your  dreams.    

And  to  My  Parents,  Robert  and  Theresa  Conway  Who  made  this  all  possible  through  their  love  and    

   

 

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Table  of  Contents    

Abstract                     13  

Summary                   14  

Chapter  1  Introduction  to  Composites           16  

  1.1  Introduction                   16  

  1.2  Fiber  Reinforced  Composites  (FRC)           16  

    1.2.1  The  Matrix               17  

    1.2.2  The  Fibers               17  

  1.3  Reinforcement  Architectures             19  

    1.3.1  2D  Composites               20  

    1.3.2  3D  Composites               21  

  References                   22    

Chapter  2  Reinforcement  Fabric  Manufacturing       23  

  2.1  Introduction                 23  

  2.2  Weaving                   23  

    2.2.1  2D  Weaving               23  

    2.2.2  3D  Weaving               25  

2.2.3  3D  Orthogonal  Non-­‐Woven,  Multiaxial  Weaving    

and  Distance  Fabrics               26  

  2.3  Braiding                   27  

    2.3.1  2D  Braiding               27  

    2.3.2  Two  and  Four  Step  3D  Braiding         27  

    2.3.3  Multilayer  Interlock  Braiding           28  

  2.4  Knitting                   28  

  2.5  Stitching                   29  

  2.6  Z-­‐Pinning                   30  

 

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  References                   31  

 

Chapter  3  Composite  Manufacturing           32  

  3.1  Introduction                 32  

  3.2  Composite  Consolidation  techniques           32  

    3.2.1  Resin  Transfer  Molding           32  

    3.2.2  Resin  Film  Infusion             32  

    3.2.3  SCRIMP                 34  

  3.3  Consolidation  Equipment             34  

    3.3.1  Tooling  (mold)               35  

    3.3.2  Heating  and  Cooling             35  

    3.3.3  Injection  Equipment             36  

  3.4  Optimization                 36  

  References                   39  

 

Chapter  4  Textile  Fiber  Reinforcement  Properties       40  

  4.1  Introduction                 40  

  4.2  In-­‐Plane  Shear                 40  

  4.3  In-­‐Plane  Biaxial  Tension               42  

  References                   44  

 

Chapter  5  Composite  Modeling             45  

  5.1  Introduction                 45  

5.2  Fundamentals                 46  

5.3  Representative  Volume               48  

5.4  Rule  of  Mixtures                 50  

5.5  Basic  Models  for  2D  Woven  Composites         51  

  5.5.1  Mosaic  Model               51  

  5.5.2  Undulation  Model             53  

 

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5.6  Models  for  3D  Woven  Composites           56  

  5.6.1  Orientation  averaging             57  

  5.6.2  Iso-­‐Strain  and  Iso-­‐Stress  Model         57  

  5.6.3  Finite  Element  Model             60  

References                   61  

 

Chapter  6  3D  Woven  Composites             63  

  6.1  Introduction                 63  

  6.2  3D  Woven  Composites               63  

    6.2.1  Microstructure  Features  and  Crimp         63  

    6.2.2  Tensile  Properties             66  

    6.2.3  Compressive  Properties           67  

    6.2.4  Flexural  and  Interlaminar  Shear  Properties       68  

    6.2.5  Interlaminar  Fracture             68  

    6.2.6  Impact  Damage  Tolerance           69  

  References                   71  

 

Chapter  7  3D  Braided,  Knitted,  Stitched  and    Z-­‐Pinned  Composites                 72  

  7.1  Introduction                 72  

  7.2  3D  Braided  Composites               72  

    7.2.1  In-­‐Plane  Properties             72  

    7.2.2  3D  vs.  2D  Braided  Composites         73  

  7.3  3D  Knit  Composites               73  

    7.3.1  In-­‐Plane  Properties             73  

7.3.2  Interlaminar  Fracture  and  Impact  Toughness     75  

  7.4  Stitched  Composites               75  

    7.4.1  In-­‐Plane  Mechanical  Properties         76  

    7.4.2  Fracture  Toughness  and  Impact  Damage  Tolerance     76  

  7.5  Z-­‐Pinned  Composites               77  

 

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    7.5.1  Tensile  and  Compressive  Strength         77  

    7.5.2  Delamination  Resistance           78  

  References                   79  

Chapter  8  Concluding  Remarks             80  

 

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Figures    

Figure  1.1:    Depiction  of  fiber  type,  and  non-­‐woven  composite  architectures                   18  

Figure  1.2:  Layup  sequence                 18  

Figure  1.3:  Stress  vs.  Strain  graph  comparing  Carbon  (green),    Glass  (purple)  and  Aramid  (red)  fiber  properties         19  

Figure  1.4:  Braided,  woven  and  knit  fabric  structures         20  

Figure  1.5:  Comparison  of  in-­‐plane  and  through-­‐thickness    Properties                   21  

Figure  2.1:    Traditional  weaving  loom             24  

Figure  2.2:    Illustration  of  yarn  structure,  and  common  weave    Patterns                   25  

Figure  2.3:    3D  weave  geometries               26  

Figure  2.4:    Illustrating  the  ability  to  weave  slits  into  the  fabric    capable  of  creating  three-­‐dimensional  structures         26  

Figure  2.5:  Multilayer  interlock  braided  fabric           28  

Figure  2.6:  3D  knit  fabric                 29  

Figure  2.7:  Stitched  fabric                 30  

Figure  2.8:  Z-­‐pinning  process               30  

Figure  3.1:  RTM                   33  

Figure  3.2:  RFI                   34  

Figure  3.3:  Autoclave  for  composite  consolidation.    Image    provided  by  AAC  research               35  

Figure  3.4:    Viscosity  vs.  Time  –  temperature  dependence    of  thermoset  TGDDM  resin.  Image  taken  from  Understanding    of  Rheology  of  Thermosets               37  

Figure  4.1:    Biaxial  tension  test  (left)  and  Picture  frame  test  (right)     41  

Figure  4.3:  Illustrating  non-­‐linear  behavior  of  woven  fabric       42  

Figure  4.4:  Biaxial  testing  Machine               42  

Figure  4.5:  Biaxial  testing  sample               43  

Figure  4.6:    Clamps                   43  

Figure  5.1:    Illustrating  the  microscopic  heterogeneity  of  a  composite    structure.    Fibers  shown  in  grey  and  matrix  in  blue.       45  

Figure  5.2:    Illustrating  the  three  planes  of  symmetry  that  make    composites  orthotropic  materials.    Planes  are  shown  in  yellow.     46  

 

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Figure  5.3:  Examples  of  common  weave  geometries         51  

Figure  5.4:    CLT  modeling  of  a  layered  composite         52  

Figure  5.5:  Fiber  undulation  model             54  

Figure  5.6:    Types  of  3D  woven  fabrics             57  

Figure  5.7:    a)  Unit  cell  for  the  mixed  iso-­‐strain  and  iso-­‐stress    model.    b)  Division  of  the  unit  cell  into  4  blocks         58  

Figure  5.8:    Possible  assembly  directions  of  block  A  and  B       58  

Figure  5.9:  Example  of  a  3D  FE  model  of  a  unit  cell  of  a  3D    orthogonal  Woven  composite  material           60  

Figure  6.1:  Tensile  strength  at  different  stages  of  the  weaving    Process                   64  

Figure  6.2:  Illustration  of  the  crimping  in  2D  woven  fabrics       65  

Figure  6.3:  difference  between  Idealized  z-­‐binder  geometry  (a)    and  actual  (b)                   65  

Figure  6.4  Top  and  cross  sectional  view             66  

Figure  6.5:  Kinking  failure  in  compression           67  

Figure  6.6:  Mode  I  delamination  cracking             68  

Figure  6.7:    Effect  of  impact  velocity  on  delamination  damage  of    2D  and  3D  woven  composites             69  

Figure  6.8:  Effect  of  impact  energy  on  flexural  strength         70  

Figure  6.9:  Effect  of  impact  energy  on  the  compressive  strength       70  

Figure  7.1:    Warp  knit  (a)  Denbigh,  (b)  1x3  single  cord,  and  (c)  1x4    single  cord  architectures               74  

Figure  7.2:  Wale  and  course  directions  as  well  as  warp  and    weft  fabric  structure.                 75  

Figure  7.3:    Illustrating  mode  I  interlaminar  toughening    mechanism  of  stitched  composites             76  

Figure  7.4:  Depiction  of  z-­‐pinned  architecture  at  insertion  site       77  

Figure  7.5:    Depiction  of  weaving  and  deflection  caused  by  z-­‐pins     78      

Tables    Table  1.1:  Matrix  materials  costs,  application  temperature  and    

toughness.    (TP  –  thermoplastic  and  TS  –  thermoset)    14       17  

Table  1.2:  Glass  type  and  defining  characteristics         19  

Table  7.1:  Reported  results  from  Macander  et  al.  for  effects  of  braid    

 

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pattern  and  edge  conditions               73  

Table  7.2:    Tensile  properties  of  warp  knit  with  varying  knit  architectures                   74  

Table  7.3:    Tensile  properties  of  weft  knit  with  varying  knit        architectures                     74  

 

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Abstract    

Composite  materials  present  a  unique  opportunity  to  engineer  a  material   in  order   to optimize   its   physical,   thermal   and  mechanical   properties   for   specific  applications.    Offering  many  advantages  such  a  relatively  high  specific  strength,  stiffness,  fatigue  resistance  and  corrosion  resistance  with  respect  to  weight.    Due  to   their   exceptional   qualities,   composites   can   be   found   in   many   applications,  from   aircrafts,   helicopters   and   spacecrafts   to   submarines,   automobiles   and  sporting  goods.    However  their  wide  spread  use  has  been  inhibited  by  their  high  cost,   poor   delamination   toughness,   and  poor   impact   damage   resistance.    Many  prospects   have   been   investigated   as   methods   for   improving   these  characteristics,   however   composites   reinforced   with   3D   fabric   architectures  appear   to   be   the  most   promising   solution.     Here   an   investigation   of   3D   fabric  architectures   (3D   woven,   braided,   knit,   stitched   and   z-­‐pinned),   manufacturing  methods,   and   composite   properties   are   reviewed   in   order   to   have   a   better  understanding   of   the   pros   and   cons   of   such   a   material   as   well   as   potential  improvements  and  opportunities.    As  expected  3D  composites  solve  many  of  the  problems   faced   by   2D   composites,   however   these   improvements   are  accompanied  by  the  deterioration  of  in-­‐plane  properties.      Many  3D  composites  show   potential   for   applications   unsuited   for   2D   composites,   however  optimization  of  3D  fabric  manufacturing,  composite  production,  and  in-­‐  and  out-­‐of-­‐plane  properties  needs  further  investigation.      

 

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Summary    

I  materiali  compositi  presentano  una  opportunità  unica  per  progettare  un  materiale  in  modo  da  ottimizzare  le  sue  proprietà  fisiche,  termiche  e  meccaniche  per  applicazioni  specifiche.  Questi  materiali  offrono  molti  vantaggi  come:  una  relativamente  alta  resistenza  specifica,  rigidezza,  resistenza  a  fatica  e  resistenza  a  corrosione  se  confrontati  col  peso.  A  causa  delle  loro  qualità  eccezionali,  i  compositi  possono  essere  trovati  in  molte  applicazioni:  dagli  aeroplani,  elicotteri  e  veicoli  spaziali  ai  sottomarini,  automobili  e  merci  sportive.  Purtroppo  la  loro  larga  diffusione  è  limitata  dal  loro  alto  costo,  piccola  durezza  alla  laminazione  e  piccola  resistenza  all'impatto.  Molti  aspetti  sono  stati  investigati  come  metodi  per  migliorare  queste  caratteristiche,  comunque  i  compositi  rinforzati  con  architetture  di  tessuti  3D  appaiono  essere  la  soluzione  più  promettente.  Un  approfondimento  sui  metodi  di  produzione,  modellazione,  e  le  proprietà  dei  compositi  di  tessuti  3d,  3d  intreccaiti,  cuciti  e  z-­‐appuntati  architetture  di  rinforzo  è  stata  eseguita  in  modo  da  capire  meglio  i  pro  e  contro  di  questi  materiali  così  come  possibili  miglioramenti  e  opportunità.      

Molti  miglioramenti  sono  ancora  necessari  per  la  produzione  di  tessuti  di  rinforzo  3D.    I  tessuti  e  i  tessuti  intrecciati  possono  essere  prodotti  usando  speciali  macchine  o  modificando  i  tradizionali  macchinari  2D.  Comunque  le  architetture  e  i  costi  e  i  volumi  di  produzione  sono  correntemente  limitati  dalle  tecnologie  disponibili.  RTM  (Resin  transfer  molding)  and  RFI  (Resin  film  infusion)  or  SCRIMP  sono  i  metodi  più  efficienti  per  la  corrente  produzione  di  compositi  3D.  Ogni  metodo  ha  diversi  benefici  e  limitazioni.  Una  revisione  di  base  degli  attuali  metodi  di  prova  e  modellazione  per  i  compositi  3D  è  presentata  nei  capitoli  4  e  5.      

Come  ci  si  aspetta  i  compositi  3D  risolvono  molti  dei  problemi  che  hanno  i  compositi  2D,  comunque  questi  miglioramenti  sono  accompagnati  dalla  deteriorazione  delle  proprietà  nel  piano.  L'ottimizzazione  della  manifattura  dei  tessuti  3D,  la  produzione  di  compositi,  e  le  proprietà  nel  piano  e  fuori  dal  piano  hanno  bisogno  di  ulteriori  investigazioni,  comunque  molti  compositi  3D  mostrano  potenzialità  per  diverse  applicazioni  per  cui  non  possono  essere  usati  compositi  2D.  Per  esempio,  cuciture  e  z-­‐pinning  mostrano  eccezionali  potenzialità  per  il  rinforzo  dei  giunti,  mentre  I  tessuti  a  maglia  3D  mostrano  un  eccellemte  resistenza  all'impatto  e  sono  di  particolare  interesse  per  l'uso  nelle  protesi  ma  non  sono  utilizzabili  per  applicazioni  strutturali.  

 

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1  Introduction  to  Composites  

   1.1  Introduction  

 Over   the   past   years   composites   have   become   increasingly   popular.     Their  

popularity   is  due   to   their  ability   to  be  manipulated  and  engineered   in  order   to  optimize   physical,   thermal   and  mechanical   properties   for   specific   applications.    The  mechanical  properties  of  a  composite  depend  on  both  the  material  selection  as   well   as   the   orientation   of   the   reinforcements   within   the   component.     For  example,   fiber   direction   may   be   dictated   in   order   to   optimize   the   mechanical  properties  of   the  material   in  a  given  direction  and  materials  can  be  selected   to  optimize   performance   in   diverse   environments.     Composites   are   also  advantageous  from  a  weight  perspective,  as  they  display  a  relatively  high  specific  strength,   stiffness,   fatigue   resistance   and   corrosion   resistance   with   respect   to  weight.     Therefore   they   are   usually   chosen   for   applications   where   high  operational   properties   are   crucial   and   weight   management   is   critical.     Due   to  their  exceptional  qualities,  composites  can  be  found  in  all   types  of  applications,  from   aircrafts,   helicopters   and   spacecrafts   to   submarines,   automobiles   and  sporting  goods.1,2        1.2  Fiber  Reinforced  Composites  (FRC)    

The  definition  of  a  composite  material   is  that  it  must  be  made  up  of  at   least  two  distinguishable   constituents  demonstrating   significantly  different   chemical  or  physical  properties.    The  combination  of  these  constituents  into  a  composite  creates   a   new   material   that   displays   a   set   of   properties   different   from   the  individual   properties   of   each   of   the   constituent   materials.     There   are   many  different  composite  types,  however  for  the  purpose  of  this  thesis  we  will  focus  on  Fiber  Reinforced  (FR)  Composites.    FR  composites  consist  of  a  matrix,  usually  a  rigid   polymeric  material   embedded  with   fiber   reinforcements.     The   polymeric  material   of   the   matrix   is   made   from   either   a   thermoplastic   (e.g.   polyamide,  polypropylene,  etc)  or  a   thermoset  (e.g.  polyimides,  epoxy,  etc.)  material,  while  the  reinforcing  fibers  are  usually  made  from  glass,  carbon  or  aramid.1,2        

 

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 1.2.1  The  Matrix  

Selection  of   the   constituent  materials   depends  on   the  desired  properties   of  the  final  product.      The  two  most  common  matrix  materials  for  FR  composites  are  thermoplastics   and   thermosets.       Thermoplastics   are   characterized   by:   high  application  temperatures,  high  toughness  and  ease  of  repair.    On  the  down  side  thermoplastics   require   high   processing   temperatures,   and   can   be   difficult   to  handle  due  to  their  high  viscosity.    Thermoset  matrix  materials  are  characterized  by   their   low   viscosity   and   low   processing   temperature   with   drawbacks   in  application  temperature,  and  toughness  (see  Table  1.1).    Matrix  Material   Cost   Application  Temperature   Toughness  PAI  (TP)   >25  €/kg   >  300°C   Medium-­‐High  PEEK  (TP)   >25  €/kg   >  300°C   High  Polyimide  (TS)   10-­‐25  €/kg   200-­‐300°C   High  PES  (TP)   10-­‐25  €/kg   200-­‐300°C   Low-­‐Medium  Epoxy  (TS)   2.5  –  10  €/kg   120-­‐200°C   Low  Phenolic  (TS)   2.5  –  10  €/kg   120-­‐200°C   Low  PBT  (TP)   2.5  –  10  €/kg   120-­‐200°C   Low-­‐Medium  PA  (TP)   2.5  –  10  €/kg   120-­‐200°C   Medium-­‐High  Polyester  (TS)   <2.5  €/kg   <120°C   Low  PP  (TP)   <2.5  €/kg   <120°C    Table  1.1:  Matrix  materials  costs,  application  temperature  and  toughness.    (TP  –  thermoplastic  

and  TS  –  thermoset)  3      1.2.2  The  Fibers     The   fibers   of   the   FR   composite   can   be   varied   in   size,   shape,   length,  direction,  architecture,  and  material  in  order  to  engineer  a  composite  to  the  have  specific   properties.     The   length   of   the   reinforcing   fibers   can   be   whiskers  (short/staple)  or  continuous  (filament)  (Figure  1.1),  and  usually  have  an  ovular  or  circular  cross-­‐sectional  shape,  although  almost  any  shape  is  possible.  Whisker  reinforcement   fibers   are  used   to   create  non-­‐woven,  non-­‐structural   composites.    When   randomly   oriented   in   the   matrix   material   they   create   an   isotropic  composite,  while  orienting   the   fibers   can  give  more  strength   in   the  orientation  direction,  generating  an  anisotropic  composite.  

   

 

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 Figure  1.1:    Depiction  of  fiber  type,  and  non-­‐woven  composite  architectures  

 On   the   other   hand,   using   filament   fibers  makes   it   possible   to   engineer   the  

reinforcement  architecture.    This  can  be  achieved  through  the  prepreg  lay-­‐up,  or  by   using   woven,   braided,   stitched,   or   z-­‐pinned   fabrics.     A   prepreg   is   a  unidirectional  fiber  sheet  impregnated  with  uncured  matrix  resin.    The  layup  of  the  prepregs  determines  the  fiber  orientations  within  the  composite  (see  figure  1.2).     Fibers   may   be   oriented   in   one   direction   (e.g.   0°/0°/0°/0°)   giving  unidirectional   characteristics,   or   in   multiple   directions   (e.g.   0°/+45°/-­‐45°/90)  creating  quasi-­‐isotropic  properties.1  

 

 Figure  1.2:  Layup  sequence3  

   

 

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The  fiber  material  is  also  very  important.    Glass  fiber  reinforcements  are  the  most  common  due  to  their   low  cost  and  high  strength,  however   limitations  are  found  in  the  low  modulus,  high  density/weight,  low  fatigue  and  wear  resistance,  and   sensitivity   to   humid   environments.     Within   the   glass   fibers   there   are  different   types   that   can   be   selected   based   on   the   desired   properties   and  environmental  conditions  (see  Table  1.2)  

 Type   General  Characteristics  E   Low  cost,  General  purpose  S/R   High  stiffness  and  strength  D   Good  dielectric  properties  A/AR   Alkali  resistance  E-­‐CR   Acid  Resistance  C   Good  chemical  resistance  

Table  1.2:  Glass  type  and  defining  characteristics    Carbon   fibers  are  becoming  more  popular  and  are  of  high   interest  due   to   their  high   modulus,   high   strength,   and   low   density/weight,   however   they   are   still  extremely  expensive.        

 Figure  1.3:  Stress  vs.  Strain  graph  comparing  Carbon  (green),  Glass  (purple)  and  Aramid  (red)  

fiber  properties.  3    Aramid  fibers  have  advantages  in  its  high  toughness,  high  strength  and  low  cost,  but  suffer   from  low  UV  resistance,  and   low  compressive  strength  (although  the  low  compressive  strength  can  be  used  to  an  advantage   in  certain  applications).      These  are  the  three  most  common  fiber  reinforcement  materials,  whose  tensile  behavior   are   compared   in   Figure   1.3.     However   it   is   important   to   note,   that  boron,  basalt  and  ceramic  fibers  have  also  been  used  to  a  much  lesser  extent.1,2      1.3  Reinforcement  Architectures    

Using  more   complex   reinforcement  architectures  gives  another  engineering  possibility.   Woven,   knit,   braided,   stitched,   and   z-­‐pinned   architectures   provide  

 

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the   most   interesting   opportunities   at   the   moment   (figure   1.4).     Within   each  fabric   production   process   there   are   many   different   architectures   that   can   be  achieved.    For  example  with  3D  weaving  we  can  produce  angle  interlock  weaves,  orthogonal   weaves   or   through-­‐thickness   interlock   weaves.     The   properties   of  each  of  these  fabrics  differ  greatly,   therefore  the  fabric   itself  can  be  engineered  for  the  desired  properties.1,2  

 

   

   

Figure  1.4:  Braided,  woven  and  knit  fabric  structures      

1.3.1  2D  Composites  2D laminated composites are among the most common composites used in the

market today. In applications requiring high performance properties filament fibers are selected over whiskers and are oriented in the x-, y-directions of the composite. Some of the major disadvantages of 2D composites lie in their high cost, and low through-thickness mechanical properties due to the lack of z-directional fibers. Therefore the mechanical properties in the through-thickness direction are determined by the mechanical properties of the resin and the fiber-resin interface. A comparison of the in-plane and through-thickness strengths of 2D laminates, seen in figure 1.5 below, reveals that the through-thickness properties are usually less than 10% of the in-plane properties and therefore cannot be used in applications supporting high through-thickness or inter-laminar shear loads.2

 

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Figure 1.5: Comparison of in-plane and through-thickness properties1

Another issue with 2D composites is their poor impact damage resistance, delamination and the loss in mechanical properties caused by impact. A composite subjected to an impact in the through-thickness direction can suffer from degraded in-plane mechanical properties under tension, compression, bending and fatigue. Due to the threat of delamination, composite parts are often over-engineered by adding thickness, resulting in increased costs, weight and volume.2

Alternatives to improve through-thickness delamination resistance and post-impact mechanical properties include chemical and rubber toughening of resins, chemical and plasma treatment of fibers, and interlaying using though thermoplastic films. They have all shown improvements in low energy impacts but have other drawbacks, which have lead to the limitation of their use in large structures.2

   1.3.2  3D  Composites     3D  composites  were  introduced  as  a  solution  to  the  main  disadvantages  of  2D   composites:   high   fabrication   costs,   proneness   to   through-­‐thickness  delamination  and   low   impact  damage   tolerance.    Unlike   the  2D  composites,  3D  composites   have   fibers   in   the   x-­‐,   y-­‐,   and   z-­‐directions.     The   z-­‐directional   or   z-­‐binder   yarn   is   responsible   for   the   increase   of   these   out-­‐of-­‐plane   mechanical  properties.    3D  composites  can  be  made  from  3D  woven,  braided,  or  knit  fabrics  as  well   as   stitched   and   z-­‐pinned   fabrics.     The   rest   of   this   thesis  will   cover   3D  composites  and   their   reinforcements.    The   following  chapters  will   review   their  manufacturing,  composite  consolidation,  modeling  and  properties.2    

 

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References  1. M. Sc. Badawi, Said Sobhey A. M. Development  of  the  Weaving  Machine  

and  3D  Woven  Spacer  Fabric  Structures  for  Light  Weight  Composites  Materials.    Dresden Technical University. 2007  

2. Tong,  L.  Mouritz,  A.P.  and  Bannister,  M.K.    3D  Fibre  Reinforced  Polymer  Composites.    Elsevier  Science  Ltd.    Oxford,  UK.    2002.  

3. Poggi,  Carlo.    Composites  For  Structural  Application.    Politecnico  di  Milano.    Course,  2nd  Semester  2011    

   

 

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2  Reinforcement  Fabric  Manufacturing  

   2.1  Introduction  

    The  manufacturing  of  the  3D  fabric  reinforcement  plays  a  very  important  role   in   the   growth   of   the   3D   fiber   reinforced   composite   industry.     It   is   only  through   economical   production   of   the   3D   reinforcement   that  wide   spread   use  can  be  achieved.    There  are  many  different  ways   in  which  to  manufacture  a  3D  fabric  reinforcement,  however  the  most  common  methods,  and  those  that  will  be  discussed   here,   are:   weaving,   braiding,   knitting,   stitching,   noobing,   and   z-­‐pinning.  2,3      2.2  Weaving       Weaving   is   one   of   the   oldest   forms   of   fabric   production   and   is   already  used  extensively  within  the  composite  industry.    However,  the  fabrics  currently  being   used   are   mostly   2D   and   not   3D.     The   weaving   process,   at   the   moment,  allows   for   the  production  of   fabric  widths  between  1.8   –  2.5  meters   at   a   rapid  production   rate   making   this   type   of   reinforcement   good   for   components  requiring   large   surfaces   and   fast   production   rates.     An   appealing   aspect   of   the  current  weaving  process  is  that  the  2D  weaving  equipment  can  be  easily  altered,  at   little   cost,   to   attain   the   ability   of   producing   3D   fabrics,   however   yarn  directions  are  restricted  to  0  and  90  degree  directions.  1,3      2.2.1  2D  Weaving     Let  us  begin  with  a  description  of  the  traditional  2D  weaving  process,  as  the   3D   process   is   based   off   of   its   simpler   counterpart.     Weaving   is   the   act   of  interlacing  two  sets  of  yarns  to  produce  a  fabric2.    The  steps  of  weaving  are  in  the  order   of   shedding,   picking,   beating   up   and   taking   up3.     At   this   point   it   is  important  to  note  that  there  are  different  types  of  weaving  looms,  the  traditional  looms  (Figure  2.1)   that  can  produce   fabrics  of  plain  weave,   twill  and  satin,  and  those   called,   jacquard   looms,   which   can   produce   complicated   fabric   patterns.    Jacquard   looms   have   a   lifting   mechanism   controlled   by   a   computer   in   which  individual  warp  yarns  can  be  lifted  at  any  time  allowing  for  intricate  patterns  to  be   woven   into   the   fabric,   these   are   of   particular   interest   in   the   3D   weaving  process.  1,2,3  

 

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 Figure  2.1:    Traditional  weaving  loom3  

 The  weaving  process  starts  by  threading  or  feeding  the  warp  yarns,  those  

that   run   in   the  machine   direction   –   0   degrees,   into   the   loom   from   the   source  yarns.    The  source  yarns  are  run  through  a  series  of  rollers  in  order  to  maintain  and  control  the  tension.    These  yarns  are  then  fed  through  the  lifting  mechanism.    The  lifting  mechanism  lifts  the  warp  yarns  in  order  to  create  a  space,  or  shed,  for  the  weft  yarns  to  be  inserted.    The  weft  yarns  are  those  running  horizontal  to  the  machine,  or   in  the  90  degree  direction.    The  sequence   in  which  the  warp  yarns  are  lifted  and  the  weft  yarns  inserted  creates  the  pattern  of  the  fabric  (see  Figure  2.2).     It   is   important   to   note   that   the   fabric   architecture   greatly   influences   the  mechanical  properties  and  drapability  of   the   fabric  and   is  highly  dependent  on  the   weave   pattern,   fiber   or   tow   size,   weft   and   warp   yarn   concentration,   yarn  tension,   and   tightness   of   the   tows.1,2,3   Plain   weave   being   the   stiffest   (least  drapable)  and  weakest,  while  satin  is  the  strongest  and  most  drapable.3  

 

 

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 Figure  2.2:    Illustration  of  yarn  structure,  and  common  weave  patterns  

 Picking  is  the  process  of  inserting  the  weft  yarns  in  the  shed  created  by  the  lifting  mechanism.2     This   can   take   place   in   a   number   of   different   ways.     The   oldest  technique   for   insertion   of   the   weft   yarns   is   through   the   use   of   a   shuttle   to  transport   the   yarns   through   the   shed.       This   technique   is   slow,   but   creates   a  closed  edge  fabric.    Open  edged  fabrics  can  be  produced  at  much  quicker  rates,  using   a  mechanical   arm,   rapier,   or   high-­‐pressure   air   or  water   to   transport   the  weft  yarns  through  the  shed.3    The  next  step  is  the  beating  up  process,  in  which  the   inserted   weft   yarns   are   compacted   using   a   comb-­‐like   devise,   the   reed.    Finally,  in  order  to  have  a  continuous  process  the  fabric  is  advanced  forward  by  a  series   of   positively   driven   rollers,   this   is   called   take-­‐up.     This   process   is  continued   until   the   desired   length   of   fabric   is   created.     The   fabric   can   be  produced  continuously  and  cut   into  the   lengths  needed.    Also  different  types  of  yarns  can  be  used  for  warp  and  weft  to  help  created  a  fabric  better  suited  for  the  intended  use.  1,2,3      2.2.2  3D  Weaving     The  major   difference   between   2D   and   3D  woven   fabrics   is   the   need   of  multiple   layers   of   warp   yarns   in   the   3D   fabrics.     This   tends   to   be   a   major  disadvantage,  as  the  need  for  a  large  number  of  warp-­‐ends  and  the  time  required  to   prepare   the   loom   can   be   very   costly.     Therefore,   at   the   moment,   most   3D  woven   fabrics   are   used   in   the   production   of   narrow   products   reducing   the  number   of   warp   yarns   required.     As   stated   above,   the   traditional   weaving  equipment   can   be   easily   altered   to   create   a   3D   woven   fabric.     The   first  modification   is   to   use   a   lifting   mechanism   with   multiple   eyes,   allowing   for  layered  warp  yarns.    Jacquard  looms  are  normally  selected  for  the  production  of  3D  woven  fabrics,  given  the  distinct  advantage  of  improved  control  of  the  lifting  mechanism.     With   the   multiple   layers   of   warp   yarns,   comes   the   creation   of  multiple  sheds.    This  allows  for  multiple  insertions  of  the  weft  yarns  at  the  same  time,  and  is  the  second  modification  needed  in  order  to  have  3D  weaving.  1,2,3     In   the   formation  of  a  3D  woven   fabric,  pockets  are   formed  between  any  four  adjacent  warp  yarns.    These  pockets  can  be  filled  with  stuffer  yarns  that  do  not   interlace   with   the   weft   yarns.     The   pockets   can   be   filled   according   to  mechanical  needs  and  in  this  way  the  fabric  can  be  further  engineered  to  specific  applications.     In   order   to   maximize   performance,   majority   of   the   yarns   are  designed   to   lay   flat,   and   only   select   warp   yarns   are   used   to   bind   the   layers  together.     Examples   of   the   weaving   architectures   capable   of   being   produced  using   the   3D  weaving   procedure   are   given   in   the   figure   2.3.     Please   note   that  

 

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these   fabric   architectures   are   idealized   and   not   possible   in   reality.     Woven  materials  can  be  produced  in  types  of  solid,  shell,  tubular  and/or  combinations  of  these.  1,2,3  

 

 Figure  2.3:    3D  weave  geometries2  

 As  with  2D  weaving,   3D  weaving   is   limited   to   yarn  placement   in   the  0   and  90  degree   directions.     Therefore   its   use   is   limited   to   components   that   are   not  subjected  to  extensive  shear  and  torsion  stresses.    An  advantage  of  the  weaving  loom  is  its  capability  of  producing  fabrics  with  slits  that  can  then  be  opened  into  three-­‐dimensional   structures   (see   figure   2.4).     This   can   be   used   to   produce   I  beams  and  boxes  using  flat  fabric  and  have  already  been  used  in  civil  engineering  components.  3    

 Figure  2.4:    Illustrating  the  ability  to  weave  slits  into  the  fabric  capable  of  creating  three-­‐

dimensional  structures.3    

  Examples   of   3D   weaving   equipment   include,   3WEAVE   created   by   3tex.    This  machine  allows  for  the  use  of  multiple  filling  layers  at  a  time,  use  of  carbon,  aramid,   glass,   polyethylene,   steel   fibers,   etc.,   produce   a   fabric   thickness   up   to  25.4  mm,  and  a  fabric  width  of  1830mm.  2      2.2.3  3D  orthogonal  Non-­Woven,  Multiaxial  Weaving,  and  Distance  Fabrics     3D   orthogonal   non-­‐wovens   are   those   fabrics   produced   from   the   same  equipment  used  to  produce  3D  woven  fabrics,  but  that  do  not  contain  interlacing  

 

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yarns.     A   clear   advantage   of     the   3D   orthogonal   non-­‐wovens   is   that   they   are  easier  to  produce  close  to  the  ideal  architecture.  3     Multiaxial  weaving  allows  for   the  placement  of  yarns   in  directions  other  than   the   traditional  0   and  90  degrees.    However   it   is  not   suited   for   large   scale  production  and  the  equipment  tends  to  be  much  more  expensive.    Currently  this  type   of   technology   is   still   in   the   development   stages   and   much   research   is  needed  to  discover  to  true  potential.  3     Distance   fabrics   are   produced   using   a   similar   processed   used   in   the  production   of   velvet.     There   are   two   sets   of   warp   yarns,   spaced   at   specified  distance   apart,   that   are   woven   as   separate   fabrics   and   at   the   same   time  interlinked  by   transferring   specific  warp   yarns,   pile   yarns,   between   the   layers.    This   fabric   is   important   for   the   production   of   peel-­‐resistant   and   delamination  resistant  sandwich  composites.  3      2.3  Braiding         Braiding   is   also   commonly   found   in   the   production   of  many   composite  components:     golf   clubs,   yacht  masts   and   aircraft   propellers.     Unlike  weaving,  braiding  allows  for  a  much  larger  selection  of  shapes,  however  is  not  capable  of  producing   large   volumes   of   wide   fabrics,   therefore   is   better   suited   for   the  production  of  highly  specialized  parts.    The  disadvantages  of  braiding  fall  in  the  limited   size   of   performs   compared   to   the   size   of   the   equipment   as  well   as   the  limited  length  of  the  preform  before  the  yarns  need  to  be  refilled.  3      2.3.1  2D  Braiding     2D   braiding   is   usually   preformed   by   a   set   of   yarn   carriers   that   counter  rotate   around   a   circular   frame   to   form   the   braided   fabric.     Braided   fabric   is  characterized   by   the   high   level   of   yarn   interlinking   and   is   formed   as   either   a  tubular  or  flat  fabric.    A  large  benefit  of  the  braiding  process  is  that  braiding  can  be  preformed  over  a  mandrel  in  order  to  produce  intricate  perform  shapes.    The  shapes  achievable  can  have  varying  cross  sectional  shapes,  varying  dimensions  along  their  length,  and  attachment  points  or  holes  can  also  be  incorporated  into  the  preform.    By  incorporating  holes  and  attachment  points  it   is  possible  to  cut  costs   in   component   finishing   as   well   as   improve   mechanical   performance   by  allowing  for  unbroken  fibers  at  the  attachment  sites.    Another  large  benefit  of  the  braiding  process  is  the  ability  to  produce  fabric,  containing  yarns  at  angles  other  than  0/90  degree  directions.3      2.3.2    Two  and  Four-­Step  3D  Braiding     The  2D  braiding  equipment   is   insufficient   to  produce  3D  braided   fabric.    One  of  the  first  3D  braiding  processes,  was  developed  by  General  Electric  and  is  known  as  the  four-­‐step  or  row  and  column  braiding.    This  process  involves  a  flat  bed  containing  rows  and  columns  of  yarn  carriers  that  form  the  preform  shape.    The  name  comes   from   the   requirement  of   four   separate   sequences  of   row  and  column   motion   in   order   to   produce   the   braided   fabric   or   perform.     In   this  process,   the   yarns   are   mechanically   compacted   after   each   step,   similar   to   the  

 

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weaving   process.     The   braiding   process   can   be   controlled   in   order   to   produce  diverse  braid  patterns  and  allows  for  high  control  over  mechanical  properties  of  the  preform  in  the  three  principal  directions.3     Later   the   technology   was   developed   into   a   cylindrical   configuration  known  as,  Through-­‐the-­‐Thickness  braiding.    This  is  achieved  by  having  identical  rings   arranged   side  by   side   in   an  axial   arrangement.    The   rings   allow   the  yarn  carriers  to  move  from  ring  to  ring  in  the  axial  direction  while  the  rings  rotate  to  perform   braiding.     Cylindrical   braiding   equipment   is   advantageous   in   space  saving.3     Another   form  of  braiding  exists   in   the  two-­‐step  process.     In   this  process  majority  of  the  yarns  are  fixed  in  the  axial  direction,  and  a  small  number  of  yarns  are   used   to   braid.     The   shape   of   the   perform   can   be   controlled   by   the  arrangement  of   the  axial   carriers.    Here   the  braiding   carriers  move  completely  through   the   structure   between   the   axial   yarns.     It   is   advantageous   in   that   any  shape   can  be   achieved,   and   there   is  no  need   for  mechanical   compaction  of   the  yarns  reducing  the  risk  of  damage.3      2.3.3    Multilayer  Interlock  Braiding     This  method  of  3D  braiding  is  most  similar  to  the  traditional  2D  braiding  processes.     The   equipment   is   comprised   of   a   cylindrical   braiding   frame  containing   parallel   braiding   tracks   with   yarn   carriers   that   can   be   transferred  between   the   tracks,   allowing   for   the   interlocking   of   the   adjacent   layers.     This  version  of  3D  braiding  is  advantageous  in  that  the  interlocking  yarns  are  in  the  plane  of   the  structure  and  therefore  allow  the  preform  to  maintain  most  of   the  in-­‐plane  properties.    However,  to  achieve  the  same  number  of  yarn  carriers  the  multilayer   interlocking   braiding   equipment   needs   to   be   larger,   and   the  equipment   is   less  adaptable.    Figure  2.5   illustrates  multilayer   interlock  braided  fabric.3    

 

 Figure  2.5:  Multilayer  interlock  braided  fabric3  

   2.4  Knitting       At   the   moment   knitting   is   the   least   known   and   studied   of   the   fabric  production   techniques   for  use  as   composite   reinforcements.    However,   current  conventional   knitting   machines   are   already   capable   of   producing   3D  

 

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architectures   as  well   as   very   detailed   and   intricate   shapes   and   geometries.     A  downside   to   the   knitting   process   is   the   high   level   of   curvature   of   the   fibers,  which  is  one  of  the  main  causes  for  loss  in  mechanical  (see  figure  2.6).    However,  this   high   degree   of   curvature   can   be   beneficial   for   non-­‐structural   components  that   require   complex   shapes   or   require   the   preform   to   be   stretched   over   a  complex  geometry.    Also,   the  knitting  pattern  can  be  greatly  varied   in  order   to  engineer   the   fabric   properties,   it   is   even   possible   to   knit   fabrics   with   large  sections   of   straight   yarns   to   improve   in   plane  mechanical   properties.     Further  developments   in   the   electronic   control   of   the   needles   have   allowed   for   the  component   to   be   knit   in   a  way   that   allow   for   the   final   3D   shape   to   be   formed  automatically   without   further   alteration   after   the   knitting   process,   without  excessive  waste.  3  

 

 Figure  2.6:  3D  knit  fabric3  

 

  The  traditional  forms  of  knitting  are  either  warp  of  weft  knitting.    In  weft  knitting  there  is  only  a  single  yarn  fed  into  the  machine  at  a  90-­‐degree  direction  with  respect  to  the  fabric  production.    The  yarn  forms  a  line  of  interlocking  loops  to   form  the  knit   fabric.    While  with  warp  knitting,   there  are  a  number  of  yarns  feed   into   the   machine   at   the   0   degree   direction   with   respect   to   the   fabric  production.     With   warp   knitting,   multiple   types   of   yarns   can   easily   be   knit  together,  however  more  yarn  bundles  will  be  needed  and  therefore  can  be  more  costly.    The  interlocking  of  the  loops  is  achieved  through  a  needle  bed,  a  row  of  closely   spaced   needles   that   pull   the   yarns   through   the   previously   knit   loops.    Machines  with  two  or  more  needle  beds  are  capable  of  creating  3D  knit  fabrics.  3          2.5  Stitching  

 Stitching  is  the  simplest  and  cheapest  of  the  methods  for  producing  a  3D  

fabric  architecture.    The  process  involves  the  insertion  of  a  needle  carrying  a  z-­‐directional  yarn  through  layers  of  2D  fabric,  in  effect  stitching  the  layers  together  and   creating   a   3D   architecture   (see   figure   2.7).     The   z-­‐binding   yarns   are  most  commonly   aramid.     This   is   due   to   their   high   toughness   as   well   as   equipment  requirements.     Current   stitching   machinery   may   be   used   with   aramid   yarns  

 

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without  further  alterations.    However  attention  must  be  given  to  their  tendency  to  absorb  moisture  and  insufficient  binding  to  many  common  polymer  resins.3  

 

 Figure  2.7:  Stitched  fabric3  

 Creating   3D   architectures   through   stitching   provides   many   benefits.    

Among   those   is   the   possibility   to   use   the   process   with   traditional   2D   woven,  braided,   knit,   etc.   prepregs.     This   allows   for   a   great   degree   of   flexibility   in   the  fabric  lay-­‐up;  using  different  material  layers,  as  well  as  different  yarn  directions.    Also,  stitching  can  be  placed  only  in  the  areas  that  require  reinforcement  in  the  z-­‐direction,   as   well   as   complex   stitching   patterns   by   using   current   embroidery  machinery   and   software.     Another   great   advantage   is   the   ability   to   create  complex  3D  shapes  by  stitching  different  component  parts  together.3  

The  main  disadvantages  with   this  process   lay   in   the  reduction  of   the   in-­‐plane   properties.     This   is   due   to   local   fiber   damaged   caused   by   the   needle  insertion,   increased   crimp   induced   by   the   z-­‐directional   yarns,   and   resin-­‐rich  pockets  formed  by  the  bunching  of  fibers  contained  by  the  stitching  yarns.3      2.6  Z-­pinning       Z-­‐pinning  is  used  as  an  alternative  method  to  stitching.    The  process  uses  pre-­‐cured   reinforcement   fibers,   which   are   embedded   in   a   thermoplastic   foam  and  placed  on  top  of  the  prepreg  or  dry  fabric.    The  prepreg  and  foam  are    then  prepared  for  curing.    During  the  curing  process,  the  thermoplastic  foam  collapses  and   the   pressure   slowly   drives   the   reinforcing   fibers   into   the   component   (see  figure  2.8).    With  z-­‐pinning,   there   is   less   crimping   induced  by   the  z-­‐directional  reinforcing  fibers  as  well  as   less  damage  to  the  yarns  in  the  prepreg,  while  still  maintaining  the  high  level  of  control  over  reinforcement  placement.3    

 Figure  2.8:  Z-­‐pinning  process3  

 

 

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References  1. Long,  A.C.    Design  and  Manufacture  of  Textile  Composites.    Woodhead  

Publishing  Limited,  Cambridge  England.  2005.  2. M. Sc. Badawi, Said Sobhey A. M. Development  of  the  Weaving  Machine  

and  3D  Woven  Spacer  Fabric  Structures  for  Light  Weight  Composites  Materials.    Dresden Technical University. 2007  

3. Tong,  L.  Mouritz,  A.P.  and  Bannister,  M.K.    3D  Fibre  Reinforced  Polymer  Composites.    Elsevier  Science  Ltd.    Oxford,  UK.    2002.  

 

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3  Composite  Manufacturing  

   3.1  Introduction  

    There   are   many   different   ways   in   which   to   consolidate   the   preform   to  create   the   final   composite   component,   however   not   all   of   these   processes   are  suited   for   3D   preform   consolidation.     Methods   such   as   hand   impregnation,  pultrusion,  and  commingled  yarns  greatly  distort  the  fabric  architecture  during  composite   consolidation,   significantly   diminishing   the   final   mechanical  properties  of  the  component.    Therefore,  in  order  to  reap  the  benefits  of  the  3D  preform  production  technologies,   the  correct  consolidation  technology  must  be  chosen.     At   this   moment   the   only   manufacturing   process   that   is   successful   in  consolidating  3D  fiber  performs  is  Liquid  Molding  (LCM).    This  is  due  to  its  high  flexibility  regarding  component  shape.    For  preforms  of  complex  geometries  LCM  offers  opportunity  to  produce  a  high  quality  component  for  a  relatively  low  cost.  1,2  

LCM  consists  of  a  family  of  processes,  which  involves  the  impregnation  of  a   dry   reinforcement  with   a   liquid   thermosetting   resin.     The  most  widely   used  processes   of   the   LCM   family   are:     resin   transfer  molding   (RTM),   SCRIMP,   and  resin   film   infusion   (RFI).    Here  we  will   review   the   different   processes   and   the  opportunities   and   challenges   that   they   each   provide   with   respect   to   the  formation  of  3D  composites.  1,2      3.2  Composite  Consolidation  Techniques      3.2.1  Resin  Transfer  Molding     Resin  transfer  molding  is  the  most  commonly  used  of  the  liquid  molding  techniques.     It   consists  of   a   closed  mould   system,  which  produces   components  with  excellent  surface  finishes  and  fiber  volume  ranging  between  50-­‐60%.    It  is  perfect  for  production  of  high  quality  automotive  and  aerospace  components.    In  this   process   the   preform   is   place   between   a   closed   mould,   and   the   resin   is  pumped  into  the  mould  at  pressures  ranging  from  2-­‐20  bar  (see  figure  3.1).      The  resin   travels   in   the   in-­‐plane   direction   to   the   preform,   this   is   a   distinguishing  feature  of   this  process.    This  makes   impregnation   time  and  maximum  injection  length   (of   the   resin)   important   factors   and   considerations,   both   being  determined  by  pressure  gradient,   resin  viscosity,  permeability  of   the   fiber  bed,  

 

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and   resin   polymerization   rate.     Using   these   variants,   injection   time   and   length  can  be  determined  and  maximized  with  economical  considerations.        However,  the   tooling   used   in   RTM   is   often   expensive   due   to   the   high-­‐pressure  requirements,   and   component   size   is   limited  due   to  maximum   injection   length  (two  meters  is  generally  the  limit)  and  financial  considerations.  1,2    

 Figure  3.1:  RTM  

    Variations   to   RTM   include   Vacuum   assisted   RTM   (VARTM)   where   a  vacuum  is  applied  to  aid  in  consolidation,  air  removal  and  increase  the  velocity  of   resin   infiltration,   and   Structural   Reaction   Injection   Molding   (SRIM)   where  higher  injection  pressures  are  used  to  decrease  production  time.  1,2      3.2.2  Resin  Film  Infusion     Resin  Film  Infusion  (RFI)  is  an  alternative  to  the  RTM  method.    In  RFI  the  resin  is  present  in  the  form  of  a  film  instead  of  a  liquid.    The  resin  film  is  placed  on  the  tool  surface,  over  which  the  preform  is  placed.    On  top  of   the  preform  a  release   film   (to   allow   for   easy   component   removal)   and   breather  material   (in  order  to  form  a  vacuum)  are  added.    Everything  is  bagged,  vacuumed  and  placed  in  an  autoclave  to  be  heated  under  pressure  (see  figure  3.2).    The  resin  film  melts  and  is  sucked  up  into  the  preform  through  capillary  action,  thus  being  absorbed  in   the  thickness  direction.    The  pressure  can  be  varied   in  order   to  compact   the  component  to  the  desired  fiber  volume  fraction.  1,2    

 

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 Figure  3.2:  RFI2  

    RFI  has  many  advantages  and  disadvantages  of  RTM.    The  advantages  of  RFI   consist   of   the   relatively   low   tooling   costs   and   the   loss   of   the   maximum  injection  length  limitations.    However,  RFI  has  limitations  in  the  thickness  of  the  component.    Therefore  RFI  is  usually  used  with  thinner  larger  components  while  RTM   is   suited   for   smaller   thicker   components.    Another  disadvantage  of  RFI   is  the   relatively   high   costs   of   the   resin   film,  which   can   cost   up   to   two   times   the  price  of  the  pure  resin,  as  well  as  their  difficulty  to  handle.  1,2      3.2.3  SCRIMP     Seemann  Composite  Resin  Infusion  Process  (SCRIMP)  is  a  mixture  of  both  the  RTM  and  the  RFI  consolidation  processes.    SCRIMP  uses  a   liquid  resin  from  an  external  source,  like  with  RTM,  and  impregnates  the  preform  in  the  thickness  direction,  like  with  RFI.    To  achieve  resin  absorption  in  the  thickness  direction,  a  resin  distribution  medium  is  used.    This  medium  allows  the  resin  to  flow  quickly  over   the   preform   surface,   spreading   over   the   entire   surface   and   then   being  absorbed   in   a   similar   fashion   to   RFI   through   the   thickness   of   the   component.    The   preparation   is   similar   to   RFI,   with   the   layering   of   the   components   and  sealing  in  a  vacuum  bag.    The  prepared  setup  is  then  placed  under  vacuum  and  the   resin   is   sucked   into   the   freeform   through   a   resin   inlet   port.     The   vacuum  created   pressure   gradient   provides   the   driving   force   for   resin   infusion   and   no  other   injection   equipment   is   needed.     This   process   has   an   advantage   in   that  tooling   costs   are   cut   similar   to   RFI,   as   well   as   cost   reductions   in   the   raw  materials  as  in  RTM.    The  limitations  of  thickness  and  maximum  length  are  also  overcome  in  this  process.  1,2      3.3  Consolidation  Equipment       Just   as   for   the   selection   of   consolidation   techniques,   the   selection   of  equipment   to   optimize   composite   consolidation   is   based   off   of   many   variants  such  as  production  quantities  and  qualities,  material   selection,  process,   etc.     In  

 

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this   section   we   will   briefly   discuss   the   equipment   used   for   the   three   main  processes  described  above.          3.3.1  Tooling  (mold)  

For  the  RTM  process  a  closed  mold  is  used,  meaning  that  the  preform  is  completely   enclosed  by  a  mold,  while   for   the  RFI   and  SCRIMP  processes  a  one  sided   mold   or   open   mold   is   used.     The   most   important   consideration   is   the  material   used   to   produce   the   mold.     This   depends   on   cost   and   production  volume.     For   low   production   volume,   wood   and   plaster   are   generally   used   to  make  the  mold  due  to  the  ease  of  mold  production  and  low  costs.    However  for  large   production   volumes   (10,000s)   metals   such   as   steel   and   aluminum   are  chosen.    For  high  production  volumes,  it  is  more  cost  efficient  to  use  metals  due  to   their   high   durability   the   need   for   repair   or   replacement   is   greatly   reduced.      Also  metals  tend  to  produce  higher  quality  surface  finishes  and  allow  for  a  wider  range  of  processing  temperatures.  

   

3.3.2  Heating  and  Cooling  As   with   all   the   other   equipment,   the   heating   and   cooling   systems   are  

dependent   on   the   consolidation   process.     For   RFI   and   SCRIMP,   using   an   open  mold,   it   is   more   cost   effective   to   use   an   external   heating   sources,   such   as   a  convection   oven,   autoclave   (figure   3.3)   or   other   similar   heating   devices.     The  heating  system  selected  will  depend  on  component  size,  shape,  required  heating  rate  and  curing  temperature.    Cooling  is  generally  achieved  through  air-­‐cooling.  2,4  

 

 Figure  3.3:  Autoclave  for  composite  consolidation.    Image  provided  by  AAC  research  

    For  RTM  heating  using  external  sources  becomes  too  expensive.    Here  it  becomes  more  cost  effective  to  use  an  integrated  heating  system.    This  system  consists  of  a  series  of  internal  channels  that  allow  for  temperature  controlled  

 

  35  

water  or  oil  to  flow  through  the  mold.    The  heat  is  transferred  between  the  water  and  the  mold  to  control  the  temperature  of  the  mold.  2      3.3.3  Injection  Equipment     Injection  equipment  is  specific  to  the  RTM  process  as  it  is  the  only  process  requiring   the   pressurized   injection   of   resin   into   the   mold.     This   equipment  generally  consists  of  a  resin  storage  area,  resin  feed  apparatus  and  delivery  nose  and   is  highly  dependent  on  resin  choice  and  resin  handling  requirements.    The  first   option   to   consider   is   whether   to   have   the   resin   injection   controlled   by  constant  pressure  or  constant  flow  rate.    Constant  pressure  injection  is  beneficial  in   the   sense   that   pressures   can   be   controlled   and   therefore   will   not   exceed  equipment   capabilities,   however   with   constant   pressure   the   flow   rate   will  decrease  as  the  preform  becomes  impregnated  with  resin  and  if  resin  cure  rates  are  not  controlled  defects  may  form  in  the  final  component.    Constant  flow  rate  ensures   that   the   preform   is   impregnated   at   a   constant   rate   and   therefore   pre-­‐curing  is  no  longer  a  problem,  however  pressures  required  to  maintain  flow  rate  may  be  extremely  high,  requiring  expensive  equipment.  2     The   second   option   to   consider   is   whether   to   have   resin   and   hardener  injected  together  or  separately.    Premixed  and  simultaneously  injected  resin  and  hardener  have  better  mixing  and  curing  control.    The  equipment  is  simpler  and  cheaper   with   a   higher   flexibility   to   change   between   resins   and   lower  maintenance  costs.    However,  having  the  resin  and  hardener  premixed  runs  the  risk  of  curing  occurring  in  the  reservoir,  therefore  only  limited  amounts  can  be  stored  in  the  reservoir  at  a  time  and  often  leads  to  excess  waste.    Therefore  it  is  generally   better   suited   for   components   produced   in   low   volume   or   using  different  resin  systems.    On   the  other  hand,   separately   injected  resin/hardener  systems   reduce  waste   as   this   system  mixes   only   the   required   amounts   at   any  given  time.    However  it  is  difficult  to  switch  between  different  resins  and  due  to  the   increased   complexity   of   the   equipment,   maintenance   costs   are   increased.    Therefore  this  system  is  generally  used  in  production  lines  where  large  numbers  of  components  are  produced  and  flexibility  is  not  as  important.2      3.4  Optimization       Optimization  of   the   consolidation  process   is   very   important   to  maintain  product   quality,   reduce   waste,   and   reduce   costs.     Optimization   involves   the  correct   selection   of   materials,   equipment,   and   processing   requirements.     As  stated   above,   resin   selection   is   the   most   important   determinant   of   injection  equipment.    The  selection  of  a  resin  is  based  off  of  both  component  requirements  (mechanical,   environmental,   health,   and   costs)   and   the  manufacturing   process.    Here   we   will   focus   on   the   consideration   given   to   the   manufacturing   process  when  selecting  a  resin.    The  fist  most  important  consideration  is  the  viscosity  of  the  resin.    The  viscosity  must  allow  for  complete  infusion  of  the  preform  without  the  need  of  excessive  pressure.    Generally  the  pressure  range  must  fall  between  100kPa  –  700kPa,  and  obviously  pressure   is  determined  by  the  viscosity  of   the  resin   combined  with   the  permeability  of   the  preform  which   is   a   factor  of   fiber  

 

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volume  and   injection  distance,   however   it   is   a   general   rule   that   resin   viscosity  should  not  exceed  500cps  during  molding.2     Using  Darcy’s   law  we   can   relate   pressure,   flow   rate   and   resin   viscosity,  preform  permeability,  pressure  gradient,  and  injection  distance.1,2    Flow  Rate  =  [(Permeability  x  Cross  Section)/Viscosity]  x  (Pressure  drop/Length)    Re-­‐written  in  variables:   〈u〉  =  (-­‐K/η)⋅∇〈P〉f         (eq.  3.1)    

〈u〉  =  resin  velocity  vector  averaged  over  fluid  volume     K  =  permeability  tensor  of  the  textile  preform     η  =  resin  viscosity     ∇〈P〉f  =  pressure  gradient  averaged  over  the  fluid  volume    As   thermosets   are   the  most   commonly  used   resins   for  LCM  another   important  factor  to  consider  is  the  relationship  between  viscosity,  temperature  and  setting  time.     This   plays   an   important   role   in   the   selection   of   pressure   vs.   flow   rate  injection  equipment.1,2  

 Figure  3.4:    Viscosity  vs.  Time  –  temperature  dependence  of  thermoset  TGDDM  resin  

Image  taken  from  Understanding  of  Rheology  of  Thermosets3    

As   seen   in   the   illustration   (Figure   3.4),   initial   viscosity   of   thermoset   resins  decrease   with   increasing   temperatures,   however   curing   rate   increases   with  increasing  temperature,  causing  the  viscosity  to  increase  over  time.    Therefore  it  is  important  to  find  the  correct  balance  between  the  viscosity,  temperature  and  curing  time.2,3     The  architecture  of  the  textile  preform  forms  a  complex  network  of  channels  through  which  the  resin  flows.    Certain  architecture  types  can  create  preferential  flow  directions,  which  can  lead  to  entrapped  air.      Using  equation  3.1  above  it  is  possible  to  derive  an  equation  describing  the  mold  filling  process  by  taking  the  partial  derivative  of  flow  rate  with  respect  to  time:1,2  

 

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 The  resin  is  assumed  incompressible,  therefore:    

δ〈u〉/δt  =  0    

Viscosity  is  assumed  constant,  therefore:    

δ(K⋅∇〈P〉f)/δt  =  (δK/δt)⋅∇〈P〉f+  K⋅(δ∇〈P〉f/δt)  =  0    

Boundary  Conditions:       Mold  walls:     n  (K∇)  =  0     Flow  front:     P  =  0     Injection  gates:   P=Pi    

In  order  to  minimize  defects  in  the  resin  injection  process  it  is  common  to  us  Liquid  Molding  Simulation  (LIM).    LIMs  use  the  above  equations  with  boundary  conditions  to  preform  a  finite  element  analysis  simulating  resin  flow  in  the  mold  cavity.    The  variables  are  most  often:  Mold  geometry,  resin  and  preform  properties,  gate  location,  and  injection  conditions.    Using  LIMs  it  is  possible  to  optimize  resin,  gate  location,  and  determine  minimum  fill  time,  possible  disturbances  and  problems  in  the  filling  process  and  injection  conditions  to  cut  costs  and  reduce  defects.1  

 

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References    

1. Long,  A.C.    Design  and  Manufacture  of  Textile  Composites.    Woodhead  Publishing  Limited,  Cambridge  England.  2005.  

2. Tong,  L.  Mouritz,  A.P.  and  Bannister,  M.K.    3D  Fibre  Reinforced  Polymer  Composites.    Elsevier  Science  Ltd.    Oxford,  UK.    2002.  

3. Franck,  A.J.    Understanding  the  Rehology  of  Thermosets.    TA  Instruments  2004.  

4. http://www.aac-­‐research.at/products/products_AAC_CompositePolymer_de.html  

 

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4  Textile  Fiber  Reinforcement  Properties  

   4.1  Introduction  

    The  characterization  of  textile  reinforcement  properties,  especially  those  of   biaxial   tension   and   shear,   is   important   in   the   prediction   of   reinforcement  drapability   during   composite   forming   and   the   determination   of   the   final  composite  characteristics.5,  7    Some  of  the  composite  forming  process  are  similar  to   those   used   for   metal   forming,   however   the   discontinuous   nature   of   fabric  structures   cause   it   to   act   differently   than   continuous  materials   such   as   plastic  sheets   and   sheet  metal.1   The   drapability   of   the   textile   limits   the   3D   composite  geometries   that   can   be   produced.     Therefore   having   a   good   understanding   of  textile   reinforcement   drapability   is   essential  when   designing   and   choosing   the  components   of   the   composite.     In   this   section   we   will   discuss   the   different  methods   available   for   analyzing   the   biaxial   and   shear   behavior   of   3D   woven  textile  reinforcements.1,  5,  7          4.2  In-­Plane  Shear       In-­‐plane  shear   is  considered  the  primary  deformation  mechanism  of   the  textile  reinforcement  during  composite  forming.    This  is  of  great  importance,  as  it  will  be  the  main  determining  factor  in  the  limitations  to  the  final  3D  composite  shape.    The  main  objective  of   in-­‐plane  shear  testing  is  to  determine  the  limit  to  deformation,  which   is   characterized  by   the   locking   angle,   or  maximum   level   of  shear   deformation   before   wrinkling   occurs.     Among   the   testing   methods  available  we  will  discuss  bias  extension  and  picture  frame.7,  8,  10     Bias   extension   tests   have   been   popular   due   to   their   simple   procedures  and  useful  measure  of   the   lock  angle,  which  can  then  be  used  to  determine  the  deformation  limit.    In  this  test,  rectangular  samples  are  cut  in  the  bias  direction  and   placed   between   two   vertical   clamps   (see   figure   4.1).     The   sample   is   then  subjected  to  uniaxial  extension.    Some  of  the  drawbacks  of  this  test  are  that  the  deformation   field   within   the   sample   is   non-­‐uniform,   with   maximum   shearing  occurring   in   the  center  of   the  sample,   therefore  visual  monitoring   is  needed   to  determine   the   shear   angle   as   it   cannot   be   determined   by   crosshead  displacement.7,  8,  10  

 

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 Figure  4.1:    Biaxial  tension  test  (left)  and  Picture  frame  test  (right).8  

    The  picture  frame  test   is  usually  employed  to  test  high  shear  angles  and  produces  uniform  shear   throughout   the  sample.    The   fabric  sample   is  mounted  into  a  hinged  frame  paying  close  attention  to  ensure  the  fibers  are  parallel  to  the  frame,   as   a   slight   variation   in   fiber   direction   will   cause   drastically   different  results   (see   figure   4.1).     The   two   opposite   corners   are   pulled   in   opposite  directions.    In  order  to  achieve  pure  shear  the  yarns  in  the  frame  should  be  free  to  rotate,  however  this  is  extremely  hard  to  achieve  and  therefore  in  most  cases  the   fabric  edges  are  held  with  a   firm  grip.    Having  a   firm  grip  causes  the   fibers  near   the   frame   to   bend   leading   to   a   difference   between   the   shear   angle   in   the  fabric   and   in   the   frame.     In   order   to   reduce   the   effects   of   the   firm   grips,   it   is  common   to  use   a   cross-­‐shaped   sample   and   remove   the  parallel   yarns  near   the  frame.    Direct  measurement  of  axial  load  and  shear  angle  is  possible  through  the  following  relationships:7,  8,  10    

        (4.1)  

     

      (4.2)  

 Shear  force  (Fs)  is  determined  by  the  axial  force  (Fx),  the  slide  length  of  the  shear  frame  (l)  and  the  frame  angle  (Φ).    While  frame  angle  can  be  determined  directly  from  crosshead  displacement  (Dx).    Shear  angle  can  then  be  determined  from  the  frame  angle.    

        (4.3)  

      Photo   camera   and   video   cameras   are   employed   to  measure   yarn  width,  pitch  and  estimate  lock  angle.    As  mentioned  above,  lock  angle  characterizes  the  deformation   limit,   or   the   maximum   deformation   before   wrinkling   occurs.     In  

 

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order   to   determine   the   wrinkling   point,   samples   are   marked   with   horizontal  lines,  wrinkling  occurs  when  the  lines  buckle.7,  8,  10      4.3  In-­Plane  Biaxial  Tension    

Woven   fabrics   are   composed   of   perpendicular   interlacing   yarns,   which  are  in  turn  composed  of  fibers.    The  cross  sectional  area  of  the  fibers  is  so  small  that   it   is   assumed   they   are   only   subject   to   tensile   stress   in   the   fiber   direction.    The   interlacing   of   the   yarns   causes   them   to   become  wavy,   or   crimped.    When  placed  under  tension  the  yarns  begin  to  straighten.    If  tension  is  applied  in  only  one   direction   the   yarns   in   that   direction   will   straighten   completely   while   the  crimp  of  the  interlacing  yarns  will  increase  to  accommodate  the  straightening  of  the  other  yarns.    On  the  other  hand,  if  both  sets  of  yarns  are  placed  under  tension  a  crimp  equilibrium  will  be  reached.    This  is  a  biaxial  phenomenon.    Due  to  the  biaxial  behavior  and  yarn  undulations   the   tensile  behavior  of  a  woven   fabric   is  non-­‐linear  at  low  tensions  (see  figure  4.3).1,  3,  5,  7,  8  

 

 Figure  4.3:  Illustrating  non-­‐linear  behavior  of  woven  fabric8  

    Characterization   of   the   biaxial   tension   behavior   of   woven   fabrics   is  achieved   using   a   biaxial   testing  machine.     The  machine   is   equipped  with   four  independently   controlled   axes.     These   axes   are   computer  directed  allowing   for  independent  control  of  direction,  distance  and  speed  of  axes  displacement.    

 Figure  4.4:  Biaxial  testing  Machine10  

 

 

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 Figure  4.5:  Biaxial  testing  sample8  

 The  fabric  sample,  in  the  shape  of  a  cross,  is  attached  using  free  moving  clamping  rigs   (figure   4.4   and   4.5).     The   clamps   are   equipped   with   load   cells   to   record  forces   (see   figure  4.6).    Deformations  are  recorded  using  optical   techniques.    A  common  technique   is   to  use  a  monochromatic  CCD  video  camera   to  record   the  changes   in   fabric   geometry   and  use  digital   image   correlation   (DIC)   to   quantify  the   data.     DIC   requires   a   camera   to   be   positioned   perpendicular   to   the   fabric  surface   and   the   assumption   that   there   are   no   out   of   plan   deformations.     DIC  records  the  change  in  the  surface  of  the  sample  by  a  series  of  images  taken  of  the  sample  at  different  deformation  stages.    This  requires  points  of  reference,  with  textiles   this   can   be   naturally   created   from   weave   and   surface   textures,   or   a  speckled  paint  pattern  can  be  applied.  6  

   

    Figure  4.6:    Clamps10      In  the  end,  the  DIC  and  the  forces  recorded  on  by  the  clamps  gives  a  mapping  of  the  deformation  concentrations  for  the  induced  stresses.    This  data  is  important  in  knowing  how  the  fabric  reacts  under  biaxial  tension,  and  can  lead  to  a  better  understanding   of   how   the   fabric  will   behave   both   in   the   composite   as  well   as  during  forming.  

 

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References    

1. Buet-­‐Gautier,   K.   and   Boisse,   P.     Experimental   Analysis   and   Modeling   of  Biaxial   Mechanical   Behavior   of   Woven   Composite   Reinforcements.    Experimental  Mechanics.    Vol.  41,  No.  3,  September  2001.  

2. Bogdanovich,  A.E.,  and  Pastore,  C.M.    Mechanics  of  Textile  and  Laminated  Composites.  Chapman  &  Hall,  1996.  

3. Gasser,   A.,   Boisse,   P.,   Hanklar,   S.     Mechanical   Behavior   of   dry   Fabric  Reinforcements.     3D   Simulations   Versus   Biaxial   Tests.     Elsevier.    Computational  Material  Science  17  (2000)  7-­‐20.  

4. Ko,   Frank   K.,   and   Chou,   Tsu-­‐Wei.     Textile   Structural   Composites   North  Holland,  1989.  

5. Launay,   Jean,   Lahmar,   Fathia,   Boisse,   Philippe,   Vacher,   Pierre.     Strain  Measurement   in   tests   on   Fibre   Fabric   by   Image   Correlation   Method.    Advanced  Composites  Letters,  Vol.  11,  No.  1,  2001.  

6. Lomov,   S.V.,   Boisse,   Ph.,   Deluycker,   E.,   Morestin,   F.,   Vancloster,   K.,  Vandepitte,  D.,  Verpoest,   I.,  Willems,  A.    Full-­field  strain  measurements   in  textile  deformability  studies.        Composites:    Part  A  39  (2008)  1232-­‐1244.  

7. Lomov,   S.V.,   Willems,   A.,   Barburski,   M.,   Stoilova,   Tz.,   Verpoest,   I.    Experimental   Textile   Mechanics:     Characterization   of   Deformability   of  Reinforcements   for   Textile   Composites.    http://www.mtm.kuleuven.ac.be/research/c2/poly/index.htm  

8. Long,   A.C.     Design   and   Manufacture   of   Textile   Composites.    Woodhead  Publishing  Limited,  Cambridge  England.  2005.  

9. Luo,   Y.     and   Verpoest,   I.     Biaxial   tension   and   ultimate   deformation   of  knitted   fabric   reinforcements.     Department   of   Metallurgy   and   Materials  Engineering,  Katholieke  Universiteit  Leuven,  Belgium.      Composites:  Part  A  33  (2002)  197-­‐203.  

10. Quaglini,   Virginio,   Corazza,   Carola,   Poggi,   Carlo.     Experimental  Characterization   of   Orthotropic   Technical   Textiles   Under   Uniaxial   and  Biaxial  loading.      Composites:  Part  A  39  (2008)  1331-­‐1342.  

   

 

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5  Composite  Modeling  

   5.1  Introduction  

    Composites   are   composed   of   a   reinforcing   textile   or   fiber   and   matrix  materials.     Their   properties   depend   on   the   mechanical   properties   of   each  individual   component   as  well   as   the   interaction   between   them.     For   simplicity  sake   let   us   consider   a   composite   embedded  with   unidirectional   parallel   fibers.    On  a  microscopic  scale   the  composite  would  be  considered  heterogeneous,  due  to  the  alteration  between  sections  of  fiber  and  matrix  (see  figure  5.1).    However,  due  to  the  scale  in  which  composites  are  used  and  the  impracticality  of  modeling  a   heterogeneous   microstructure,   composites   are   usually   assumed   to   have   a  homogeneous  macrostructure.    Therefore,  when  modeling  composite  behavior,  it  is  extremely  important  to  select  a  representative  volume  that  accommodates  all  demonstrated  properties  of  the  composite  on  the  microscopic  level.12,  18,  23      

 Figure  5.1:    Illustrating  the  microscopic  heterogeneity  of  a  composite  structure.    Fibers  shown  in  

grey  and  matrix  in  blue.      It  is  also  important  to  distinguish  between  isotropic,  anisotropic  and  orthotropic  behaviors.     Composites   are  known   for   their   anisotropy  or  differing  mechanical  properties  depending  on  orientation,  and  fall  under  orthotropic  materials  as  they  generally  have  three  axes  of  symmetry  around  which  the  properties  remain  the  same  (see  figure  5.2).  12,  23      

 

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 Figure  5.2:    Illustrating  the  three  planes  of  symmetry  that  make  composites  orthotropic  

materials.    Planes  are  shown  in  yellow.    

Modeling  of  composite  materials   is  a  very  useful  tool   in  determining  the  mechanical  properties,  as  experimental  methods  tend  to  be  extremely  expensive  and   impractical.     In   this   chapter   we   will   review   the   fundamentals   on   which  composite  modeling  is  based  as  well  as  some  of  the  more  widely  used  methods  for  modeling  3D  woven  composites.  12,23      5.2  Fundamentals       To  begin  let  us  review  the  fundamentals  that  are  required  for  composite  modeling.    The   first  being   the  generalized  Hooke’s   law   -­‐   the   linear   constitutive  relationship  between  stress  and  strain.    It  is  represented  by  the  following:    

[σ]  =  [C][ε]           (5.1)  

 

 

   

 

              (5.2)    

 

 

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     [C]  is  the  elastic  stiffness  constant  and  [S]  is  the  inverse  matrix  of  [C].    Leading  to  the  following  relationships:    

            (5.3)  

 

      (5.4)  

 

        (5.5)  

   Where  ui  (i=1,2,3)  are  the  displacements  in  Cartesian  coordinates  and  xj  (j=1,2,3)  are  the  coordinates.  

 The   above   relationships   can   be   further   simplified   for   orthotropic  

materials  as  they  have  the  three  planes  of  symmetry.    Becoming:    

    (5.6)  

 

    (5.7)  

 

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 Where  E1,  E2,  E3  are  the  three  elastic  moduli,  G12,  G23,  G31  are  the  three  

shear  moduli,  and  ν12,  ν23,  ν31  are  the  three  independent  Poisson’s  ratios.    Remember  that:  

          (5.8)  

 Therefore  the  constitutive  relationship  for  elastic  behavior  of  orthotropic  materials  becomes:  

 ε1  =  1/E1    (σ1  -­‐  ν12σ2  -­‐  ν13σ3)  

 ε2  =  1/E2    (σ2  -­‐  ν21σ1  -­‐  ν23σ3)        

                    (5.9)  ε3  =  1/E3  (σ3  -­‐  ν31σ1  -­‐  ν32σ2)  

 γ12  =  τ12/G12   γ23  =  τ23/G23   γ13  =  τ13/G13  

   5.3  Representative  Volume       As   discussed   previously,   it   is   very   important   to   select   a   good  representative   volume.     The   representative   volume   must   be   large   enough   to  include   all   microstructural   features.     Think   of   it   as   a   type   of   monomer,   or  building  block   that  when  replicated   is  capable  of  reconstructing   the  composite.    By  determining  the  mechanical  properties  of  the  representative  volume,  we  can  use   continuum   mechanics   to   reproduce   the   properties   of   the   material   as   a  whole.12,  23     For   a   representative   volume   subject   to   a   homogeneous   macroscopic  stress  or  strain  and  no  body  forces,  the  average  stress  and  strains  are  defined  as  the   sum   of   the  micro-­‐stresses   and  micro-­‐strains   in   the   representative   volume,  divided  by  the  volume  and  can  be  represented  by  the  following  equations:    

 

            (5.10)  

 

   σij    and  εij      are   the  true  stresses  and  strains  (micro  stresses  and  strains)   in   the  representative  volume  V.  12,  23     The   boundary   conditions   for   iso-­‐strain   and   iso-­‐stress   on   the  representative  volume  are  expressed  by  the  following:         Iso-­‐strain:             (5.11)  

   

 

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    Iso-­‐stress:             (5.12)  

   When   there   is  perfect   interfacial  bonding  between   the   representative  volumes,  then     =       and   intrinsically     =   .     nj   is   the   unit     normal   vector   pointing  away  from  the  surface  of  the  representative  volume.  23     By   applying   the   homogeneous   boundary   conditions   to   the   stress-­‐strain  relationship  we  can  define  the  effective  properties  of  the  representative  volume  as  follows:  Iso-­‐strain:                       (5.13)  

 

 

 

 

            (5.14)  

           

 Iso-­‐stress:                     (5.15)    

            (5.16)  

    Solutions  for  true  stress  and  strain  can  be  obtained  with  either  analytical  or   numerical   approaches.     Analytical   approaches   are   cheaper   and   faster,   but  require   a   large   number   of   assumptions   and   therefore   may   exclude   certain  characteristics   and   their   input.     Finite   element  methods   (numerical)   allow   for  more  accurate  modeling,  however  can  be  quite  expensive.12,  23      

 

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     5.4  Rule  of  Mixtures       The   rule   of   mixtures   is   very   useful   in   many   analytical   approaches   for  determining   the   longitudinal   modulus   of   unidirectional   composites   and   the  major   Poisson’s   ratio   from   the   properties   of   the   individual   components.    However  for  determining  the  transverse  and  shear  moduli  modifications  must  be  made.    The  assumptions  made  with  the  rule  of  mixtures  are  that  the  composite  is  subject  to  uniform  or  iso-­‐stress  and  iso-­‐strain  conditions,  no  transverse  stresses,  and   that   the   load   carried   by   the   fiber   and  matrix   is   proportional   to   both   their  moduli  and  cross-­‐sectional  area.    For  determining  longitudinal  modulus  it  is  also  assumed  that  the  fiber  and  matrix  are  elastic  bodies  acting  in  parallel  resulting  in  the  following  relationships:                 (5.17)

                    (5.18)

   Vf  and  Vm  representing  the  volume  fractions  of  the  fiber  and  matrix  respectively,  and  Ef,  Em,  νf,  νm  represent  the  elastic  moduli  and  Poisson’s  ratios  of  the  fiber  and  matrix  components.13,  23  

To  approximate  the  transverse  elastic  properties  of  a  composite,  the  fiber  and  matrix  components  are  assumed  to  be  elastic  bodies  in  series,  however  this  is   an   inaccurate   method   for   approximation   because   in   reality   the   transverse  elastic  properties  lay  between  the  series  and  parallel  models.12,  23    

        (5.19)

 

 In  order   to  better  approximate  the  transverse  properties  of  a  composite  

from   the   individual   component   properties,   Halpin-­‐Tsai   equations   offer   a   good  approximation  by  taking  into  account  the  fact  that  the  properties  lay  somewhere  between  a  parallel  and  series  model.12,  23    

        (5.20)

 

 

                  (5.21)

 

 

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 Where:   M  represents  E2,  G12,  G23  of  the  composite       Mf/m  represents  Ef/m  or  Gf/m  of  the  fiber  or  matrix       Vf  is  the  fiber  volume  fraction  

ξ  is  a  constant  representing  the  way  the  load  is  shared  between  the  fiber  and  the  matrix.  (ξ=0    series,  ξ=∞    parallel)  

   5.5  Basic  Models  for  2D  Woven  Composites       The  first  step  to  applying  a  model  is  to  choose  the  representative  volume  of   a  woven   composite.     Fortunately  woven   textiles   are   formed   by   a   repetitive  geometric  pattern  represented  by  the  weave  pattern.    This  makes  selection  of  the  representative   volume,   or   unit   cell,   relatively   easy   as   it   is   the   smallest   volume  that  represents  the  weave  pattern.    This  pattern  is  represented  by  the  number  of  interlacing  weft  ( )  and  warp  ( )  yarns.  23  

 For  plain  weave   = =2  For  twill  weave   = =4  For  Satin  weave  (5-­‐harness)   = =5    This  pattern  or  weave  geometry  can  be  seen  in  figure  5.3  below,  demonstrating  the  most  common  weave  geometries.    

 Figure  5.3:  Examples  of  common  weave  geometries  

      Let   us   begin   by   discussing   several   basic   1D   methods   for   modeling   2D  woven  composites:  Mosaic  model  and  Fiber  Undulation  model.        These  methods  are   considered   1D   as   they   either   do   not   consider   fiber   undulation   or   consider  fiber  undulation  in  only  one  direction.  12,  23      5.5.1  Mosaic  Model  

The  mosaic  model  idealizes  the  composite  as  a  grouping  of  asymmetrical  cross-­‐ply  laminates  that  can  then  be  modeled  using  the  classical  laminate  theory  (CLT)  neglecting  shear  deformation  in  the  thickness  direction.  

 

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  CLT  assumes   that   the  composite   is   composed  of   thin  sheets  or   layers  of  composites  with  unidirectional  fiber  reinforcements.  Due  to  the  thinness  of  each  individual  layer  we  can  assume  for  each  layer  to  be  in  a  plane  stress  state  or  in  other  words   that  σ3=0.    With   these   assumptions  we   can   simplify   equation   5.6  into  the  following:    

      (5.22)

 

   

     

          (5.23)

 

 

 Figure  5.4:    CLT  modeling  of  a  layered  composite  

 Using   the   above   assumptions   the   constitutive   equations   are   given   by   the  following:    

          (5.24)  

 

 

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        (5.25)

 

 N  –  Stress  resultant  

      M  –  Bending  moment         ε  -­‐  In-­‐plane  strains  (mid-­‐plane)         k  –  Curvature  (mid-­‐plane)         Aij  –  In-­‐plan  stretching  stiffness         Bij  –  Bending/stretching  coupling         Dij  –  Bending  stiffness         Qij  –  Elastic  constants  of  a  lamina    Subscript  k  referring  to  the  kth  layer  and  z  referring  to  the  distance  between  the  mid-­‐plane  and  the  layers  boundary  as  seen  in  figure  5.4.     If   we   assume   an   iso-­‐strain   field   in   the   middle   plane,   and   that   our  composite  can  be  modeled  as  an  idealized  asymmetrical  cross-­‐ply  laminate,  then  we   can   express   the   stiffness   constants   by   the   following   equations:   (the  simplification   is   only   applicable   for   uniform  weave   geometries   and   not   hybrid  weaves)  2,  3,  11,  16,  18,  23    

     

      (5.26)        5.5.2  Undulation  Model     While   the   mosaic   model   is   good   for   predicting   the   upper   and   lower  bounds  for  effective  stiffness  and  compliance  constants  for  a  unit  cell  of  a  woven  composite,   it  does  not  take  into  consideration  non-­‐uniform  stresses  and  strains  that  tend  to  concentrate  in  the  fiber  interlacing  regions  or  fiber  undulation.    The  fiber  undulation  model  was  created  as  a  more  accurate  prediction  model.  2,  3,  23    

 

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 Figure  5.5:  Fiber  undulation  model  

    It   is   assumed   that   the   fiber   undulation   in   weft   and   warp   yarns   can   be  expressed  using  the  following  sinusoidal  equations:    

Weft:           (5.27)

 

 

Warp:     when  a0  <  x  <  a/2  

                  (5.28)  

      when  a/2  <  x  <  a2  

 au  representing  the   length  of   the  undulation  and  a0=(a-­‐  au)/2  and  a2=(a+  au)/2.    Undulation  in  the  warp  direction  is  neglected.    Refer  to  figure  5.5  for  definitions  of  other  variables.     As   can   be   seen   in   figure   5.5,   the   unit   cell   in   the   fiber   undulation  model  consists   of   two   straight   cross-­‐ply   regions   and   one   undulated   region.     The   two  straight   cross-­‐ply   regions   can   be   model   as   before,   while   the   elastic   stiffness  constants  of  the  undulated  weft  yarns  can  be  expressed  in  terms  of  weft  elastic  constants  (Qij)  and  the  undulation  angle  θ.    The  undulation  angle  is  represented  by  the  following  equation:    

 

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  (5.29)    And  the  elastic  stiffness  constants  of  the  undulated  weft  yarn  are  expressed  by:    

   

      (5.30)

 

 

      (5.31)

 

 Ishikawa  and  Chou6  obtained  the  following  formulas:          

 

                    (5.32)    where  superscripts  F,  W  and  M  represent  weft,  warp  and  matrix  respectively.  The  average  effective  properties  of  the  unit  cell  can  be  determined  by  the  sum  of  the  average  straight   cross-­‐ply   regions  and   the  average  undulated  region.    Here  we  have  the  average  compliance  equations,  inverted  matrices:    

 

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      (5.33)

 

 It   is   important   to   note   that   this   model   only   takes   into   consideration  

undulation  in  the  weft  yarn,  not  in  the  warp  direction.    To  take  into  consideration  yarn  undulation   in  both  directions  a  2D  model   is  needed.    Further,   to   take   into  consideration  yarn  undulation   in  both  directions  as  well  as  out-­‐of-­‐plane  elastic  constants   a  3D  model   is   needed.    Reviews  of   2D  models   can  be   found   in  work  completed  by  Naik  and  colleagues14,  15  and  for  3D  models  in  work  completed  by  Hahn  and  Pandey16  and  Vandeurzen  et  al.24,  25,  26  for  3D  models.      5.6  Models  for  3D  Woven  Composites       There   are   many   different   ways   to   model   3D   woven   composites,   many  depend  on   the  geometry  of   the  woven   fabric,  be   it  3D  orthogonal   interlock,  3D  through-­‐thickness  angle  interlock  or  layer-­‐to  layer  interlock  (see  figure  5.6).    The  three  main  models  we  will  be  discussing  here  will  be  the  orientation  averaging  model,  mixed  iso-­‐stress    and  iso-­‐strain,  and  finite  element  applications.2,  3,  11,  12,  16,  23  

 

 Figure  5.6:    Types  of  3D  woven  fabrics  

       

 

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5.6.1  Orientation  averaging     For   Orientation   averaging   models   the   composite   is   regarded   as   a  collection   of   small   volumes,   each   volume   being   modeled   as   a   unidirectional  composite  with  transversely   isotropic  properties.    This  model  has  been  applied  to  all  three  fabric  geometries  mentioned  above.    Each  composite  is  divided  into  suffer   (s),   filler   (f)   and   warp   weavers   (w1   and   w2).     The   effective   elastic  properties  are  modeled  using  the  simplified  3D  model  for  2D  woven  composites  for  non-­‐mixed  yarn  systems:    

        (5.36)

 

 As   ideal   geometry   differs   from   the   true   geometry,   it   is   necessary   to   take   into  consideration   the  effect  of   tow  waviness  on  the  elastic  properties.    Therefore  a  normal  distribution  is  calculated  for  the  out-­‐of-­‐plane  alignment  angle,  ξ:          

          (5.37)  

 With  a  density  function  of:    

                (5.38)

 

 Where   σξ   is   the   width   of   the   distributions.     In   order   to   reduce   the   Young’s  modulus  and  Poisson  ratio  caused  by  waviness,  a  waviness  knockdown  factor  is  introduced:    

        for  σξ  less  than  10°   (5.39)  

 This  method  provides  respectable  predictions  of  the  in-­‐plane  macroscopic  elastic  constants  and  a  reasonable  estimation  of  the  through-­‐thickness  elastic  constants.  2,  3,  22  

 5.6.2  Iso-­strain  and  Iso-­stress  model     For   prediction   of   mechanical   and   thermo-­‐elastic   properties   of   3D  orthogonal  and  angle-­‐interlock  composite  materials,  Tan  et  al.20,  21,  22  proposed  a  mixed   iso-­‐stress   and   iso-­‐strain   based   unit   cell   modeling   method.     For   a   3D  orthogonal   fabric   we   consider   the   structure   to   be   simplified   into   rectangular  cross-­‐sectional  shapes  in  three  mutually  orthogonal  directions.    An  example  of  a  unit  cell  is  shown  in  figure  5.7  (a).  

 

 

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 Figure  5.7:    a)  Unit  cell  for  the  mixed  iso-­‐strain  and  iso-­‐stress  model.    b)  Division  of  the  unit  cell  

into  4  blocks       The   Unit   cell   is   further   divided   into   smaller   blocks,   which   can   then   be  divided  again  to  create  individual  unidirectional  composite  blocks  (see  figure  5.7  (b)).     The   properties   of   each   unidirectional   block,   A,   B,   C,   D,   E,   F,   G   can   be  determined   using   methods   described   previously.     To   determine   the   overall  properties  of  the  unit  cell  it  is  possible  to  assemble  the  blocks  individually  in  the  x-­‐,  y-­‐  ,  and  z-­‐directions  (see  figure  5.8)    

 Figure  5.8:    Possible  assembly  directions  of  block  A  and  B  

 

 

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  For  an  x-­‐assemblage  of  NA    number  of  A  blocks  and  NB  number  of  B  blocks  in   the   strip,   the   overall   material   properties   are   represented   by   the   following  equations:    

         

 

         

               

         

 

              (5.40)

 

 Where  VA  and  VB  are  the  volume  fractions  of  block  A  and  block  B  in  the  strip,  and  subscripts  S,  A,  B  refer  to  strip,  block  A  and  block  B  respectively.     For  a  y-­‐assemblage  of  A  and  B  blocks:    

         

 

         

 

       

 

         

 

              (5.41)

 

    For  a  z-­‐assemblage  of  A  and  B  blocks:                

   

 

 

  59  

       

 

         

            (5.42)  

          In  figure  5.7  b)  it  is  seen  that  the  blocks  A  and  B,  D  and  E,  and  F  and  G  can  be   evaluated  using   the   x-­‐assembly   equations.     Strips  1   and  2,   and  3   and  4   can  then   be  modeled   using   the   y-­‐assembly   equations.     Finally   the   top   and   bottom  planes  can  be  evaluated  using   the  z-­‐assembly  equations,   estimating   the  overall  properties  of  the  complete  unit  cell.  2,  3,  12,  23    5.6.3  Finite  Element  Model     Finite  Element  Modeling  (FEM)  is  probably  the  most  accurate  method  for  predicting   elastic   and   failure   behavior   of   textile   composites.     It   allows   for  detailed  modeling  of  complex  geometries  and  varying  material  properties.    FEM  works   by   breaking   down   the   composite   structure   into   small   regions   whose  constitutive   properties   can   be   easily   evaluated.     Using   brick,   wedge   and  tetrahedral  elements  it  is  possible  to  generate  a  mesh  depicting  in  detail  the  true  geometry   of   the   unit   cell   as   seen   in   figure   5.9.     Determination   of   true   fabric  geometry   in   the   composite   is   of   course   is   limited   by   current   measurement  technologies  and  may  not  be  cost  effective.      

 Figure  5.9:  Example  of  a  3D  FE  model  of  a  unit  cell  of  a  3D  orthogonal  Woven  composite  

material.22    Yarns  are  modeled  as  orthotropic,  with  respect  to  the  principle  axes,  with  cross  sectional  shapes  of  rectangular,  circular,  elliptical  or  lenticular,  while  the  matrix  is  assumed  homogeneous  and  isotropic.    It  is  important  to  note  that  accuracy  of  FEM  depends  on  how  accurately  the  fabric  geometry  is  modeled.  2,  3,  11,  33  

 

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References  1.    Aboudi  J.,  1991,  Mechanics  of  Composite  Materials  -­‐  A  Unified  

Micromechanical  Approach,  Elsevier,  Amsterdam,  The  Netherlands.  2. Ansar,  mahmood,  Wang  Xinwei,  Zhou  Chouwei,  2011,  Modeling  strategies  

of  3D  woven  composites:  A  review.    Cmoposite  structures  93,  Elsevier.  3. Bogdanovich,  A.E.,  and  Pastore,  C.M.    Mechanics  of  Textile  and  Laminated  

Composites.  With  applications  to  structural  analysis.  Chapman  &  Hall,  London,  UK  1996.  

4. Chou  T.W.  and  F.K.  KO,1989,  Textile  Structural  Composites,  Volume  3  Composite  MaterialsSeries,ElsevierSciencePublishers,Amsterdam,NewYork,U.S.A.,  1989.  

5.      Hahn  H.T.  and  R.  Pandey,  1994,  A  micromechanics  model  for  thermoplastic  properties  of  plain  weave  fabric  composites,  J.  Eng.  Mat.  &  Tech.,  116:  517-­‐423  

6. Ishikawa  T.  and  T.W.  Chou,  1982a,  Elastic  behavior  of  woven  hybrid  composites,  J.  Comp.  Mat.  162-­‐19.  

7. Ishikawa  T.  and  T.W.  Chou,  1982b,  Stiffness  and  strength  behavior  of  woven  fabric  composites,  J.  Mat.  Sci.,  17:3211-­‐3220.  

8. Ishikawa  T.  and  T.W.  Chou,  1983a,  One-­‐dimensional  Micromechanical  analysis  of  Woven  Fabric  Composites,  AIAA  J.,  21:1714-­‐1721.  

9. Ishikawa  T.  and  T.W.  Chou,  1983b,  In-­‐plane  thermal  expansion  and  thermal  bending  coefficients  of  fabric  composites,  J.  Comp.  Mat.  17:92-­‐104.  

10. Ishikawa  T.  and  T.W.  Chou,  1983c,Nonlinear  behavior  of  woven  fabric  composites,  J.  Comp.  Mat.  17:399-­‐413.  

11. Ko,  Frank  K.,  and  Chou,  Tsu-­‐Wei.    Textile  Structural  Composites  12. Long,  A.C.    Design  and  Manufacture  of  Textile  Composites.    Woodhead  

Publishing  Limited,  Cambridge  England.  2005.  13. Mori  T.  and  Tanaka  K.,  1973,Average  stresses  in  matrix  and  average  

elastic  energy  of  materials  with  misfitting  inclusions,  Acta  Metalurgica,  21571-­‐574.  

14. Naik  N.K.  and  V.K.  Ganesh,  1992,  Prediction  of  on-­‐axes  elastic  properties  of  plain-­‐weave  fabric  composites,  Comp.  Sci.  &  Tech.,  45:135-­‐152.  

15. Naik  N.K.  and  P.S.  Shembekar,  1992a,Elastic  behavior  of  woven  fabric  composites:  I-­‐  lamina  analysis,  J.  Comp.  Mat.  26:2197-­‐2225.  

16. Onate,  Eugenio  and  Kroplin,  Bern.    Textile  Composites  and  Inflatable  Structures.  Springer,  The  Netherlands.    2005.  

17. Shembekar  P.S.  and  N.K.  Naik,  1992,Elastic  behaviourof  woven  fabric  composites:  II-­‐  Laminate  analysis,  J.  Comp.  Mat.,  26:2226-­‐2246.  

18. Stig,  Fredrik.    An  Introduction  to  the  Mechanics  of  3D-­Woven  Fiber  Reinforced  Composites.    KTH  School  of  Engineering  Sciences,  Stockholm,  Sweden.  April  2009.  

19. Tan  P.,  L.  Tong  and  G.P.  Steven,  1997a,  Modeling  for  predicting  the  mechanical  properties  of  textile  composites  -­‐  A  review,  Composites,  28A:903-­‐922.  

20. Tan  P.,  L.  Tong  and  G.P.  Steven,  1998,Modelling  approaches  for  3D  orthogonal  woven  composites,  J.  Rein.  Plastics  &  Comp.,  17545-­‐577.  

 

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21. Tan  P.,  L.  Tong  and  G.P.  Steven,  1999a,  Models  for  predicting  thermo  mechanical  properties  of  3D  orthogonal  woven  composites,  J.  Rein.  Plastics  &  Comp.,  18:151.  

22. Tan  P.,  L.  Tong  and  G.P.  Steven,  1999b,Micro-­‐mechanics  models  for  mechanical  and  thermo-­‐mechanical  properties  of  3D  angle  interlock  woven  composites,  Comp.,  30A:637-­‐648.  

23. Tong,  L.  Mouritz,  A.P.  and  Bannister,  M.K.    3D  Fiber  Reinforced  Polymer  Composites.    Elsevier  Science  Ltd.    Oxford,  UK.    2002.  

24. Vandeurzen  P.,  J.  Ivens  and  I.  Verpoest,  1996a,  A  three-­‐dimensional  micromechanical  analysis  of  woven  fabric  composites:  I.  Geometric  analysis,  Comp.  Sci.  &  Tech.,  56:  1303-­‐1315.  

25. Vandeurzen  P.,  J.  Ivens  and  I.  Verpoest,  1996b,  A  three-­‐dimensional  micromechanical  analysis  of  woven  fabric  composites:  11.  Elastic  analysis,  Comp.  Sci.  &  Tech.,  56:  1317-­‐1327.  

26. Vandeurzen  P.,  J.  Ivens  and  I.  Verpoest,  1998,  Micro-­‐stress  analysis  of  woven  fabric  composites  by  multilevel  decomposition, J. Comp. Mat., 32:623-651.  

 

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6  3D  Woven  Composites  

   

6.1  Introduction  

    3D   composites   have   been   developed   in   order   to   combat   the   problems  faced   by   2D   composites:   reduce   fabrication   costs,   increase   through-­‐thickness  mechanical  properties  and  improve  impact  damage  tolerance.    In  3D  composites,  fibers  are  aligned  not  only  in  the  x-­‐,  y-­‐direction,  but  also  in  the  through-­‐thickness  (z-­‐)   direction.     By   placing   fibers   in   the   z-­‐direction   mechanical   performance,  impact  damage  and  ease  of  processing  can  all  be  improved.14     3D   composites   consist   of   several   types,   including   woven,   knit,   braided,  stitched,  and  z-­‐pinned.    The  first  3D  composite  to  be  manufactured  was  a  braided  carbon-­‐carbon  composite   in  the  1960s  to  be  used  in  rocket  motor  components.      Soon   after   3D   woven   carbon-­‐carbon   composites   were   used   for   the   brake  components   of   jet   aircrafts.     3D   composites   were   meant   to   replace   high-­‐temperature  metal  alloy  parts  to  improve  durability,  heat  distortion  and  weight.    These  early  3D  composites  may  not  have  been  FR  composites,  however  the  idea  still  remains  valid.14      6.2  3D  Woven  Composites       At  the  moment  3D  woven  composites  are  used  in  only  a  few  niche  markets.    They   show   a   great   potential   for   applications   in   the   aerospace,   marine,  infrastructure,   military   and   medical   fields,   due   to   their   benefits:   potential  reduced   fabrication   costs,   increased   design   flexibility,   improved   impact  resistance  and  through-­‐thickness  mechanical  properties.    However  there  are  still  many  challenges  impeding  their  application,  such  as  current  production  cost  due  to  small  scale  production,  cost  of  certification  of  new  materials  for  load-­‐bearing  structures,  and  uncertainty  of  benefits.    Only  once   these  barriers  are  overcome  will   3D  woven   composites  make   a   name   for   itself   in   the  world   of   composites.    Here  in  this  section  we  will  discuss  microstructural  features  and  their  effect  on  mechanical   properties,   delamination   resistance,   impact   damage,   as  well   as   the  different  failure  mechanisms.14      6.2.1  Microstructure  Features  and  Crimp     The  microstructure   is   determined  mainly   by   the   fiber   architecture   of   the  

 

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preform   and   the  weaving   process.     Defects   in   composite  microstructure   come  mainly   from   the   weaving   of   the   fabric   reinforcement   and   include   abrasion,  breakage,  and  distortion  of  yarns.9    However  unequal  distribution  of  resin  during  the  composite  forming  stage  can  also  pose  a  problem.    Defects,  such  as  these,  can  significantly  degrade   the   composite  properties.    Abrasion  and  breakage  caused  by  weaving  cause  large  reductions  in  tensile  strength  of  the  yarns.14    

 Figure  6.1:  Tensile  strength  at  different  stages  of  the  weaving  process9  

 This  degradation  of   tensile  strength   is  seen   in   figure  6.1  above.    A  reduction   in  tensile  strength  of  about  30%  is  seen  at  the  later  stages  of  the  weaving  process.    The  extent  of  damage  and  reduction  of  properties  depends  on  the  weave,  loom,  as  well  as  yarn  material,  type  (twisted  or  untwisted),  and  diameter.    Glass  fibers  display  the  highest  loss  of  strength  in  comparison  with  carbon  or  Kevlar.14     Distortion   of   fibers   from   their   idealized   architecture   is   due   to   the  interlacing   of   the   yarns   during   weaving.     Misalignment   and   waviness   in   3D  performs  is  much  higher  than  that  of  the  2D  performs,  as  much  as  25%  higher,  and  can  cause  a   large  degradation   in  composite  properties.5    Crimping,  or  yarn  waviness,   is   possibly   the  most   important   factor   in  determining   the  mechanical  properties   of   the   final   composite.     The  more   crimped   the   yarns,   the   lower   the  strength  of  the  composite.    When  a  crimped  structure  is  subject  to  a  tensile  force  the  yarns  begin  to  straighten,  however  the  yarns  do  not  straighten  at   the  same  rate,   as   the   crimping   is   not   uniform   throughout   the   fabric.     Therefore   stress  concentrate   in  specific  regions  causing   those  regions   to   fail  earlier   than  others.    Crimping  also  introduces  shear  stresses  into  the  matrix  (see  figure  6.2).11,  14     Crimp  has  been  defined  in  several  ways  from  crimp  %,  crimp  angle  as  well  as   defining   the   geometry   of   crimp.     Describing   the   geometry   of   crimp   is  important  when  modeling   the   composite  properties   as  discussed  previously   in  chapter   5.     The   simplest   definition   of   crimp   is   the   crimp  %  or   the   ratio   of   the  difference  between  the  yarn  length  and  fabric  length  over  fabric  length:  

 

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 An  additional  concern   for  3D  woven  composites   is   the  crimp   induced  by   the  z-­‐binder  yarns.    They  cause  excess  bunching  of  the  surface  fibers  where  they  cross  over  the  in-­‐plane  yarns.    This  reads  to  regions  rich  in  fiber  content  and  creates  problems   for   the   resin   to   enter.     These   regions   lack   resin   and   have   increased  porosity.     At   the   same   time,   the   fiber   bunching   creates   spaces   fiber   poor   that  allow  for  excess  resin  to  enter.        The  result  is  fiber  rich  and  resin  rich  areas  on  the  composite  surface.14  

 

 Figure  6.2:  Illustration  of  the  crimping  in  2D  woven  fabrics5  

 

 Figure  6.3:  difference  between  Idealized  z-­‐binder  geometry  (a)  and  actual  (b)14  

 Misalignment  of   the  z-­‐binder  can  also  occur  due  to  high  tensile  stresses  during  weaving  as  well   as  excessive  pressure  during  consolidation.    As   can  be  seen   in  figures   6.3   and   6.4   the   actual   z-­‐binder   geometry   is   much   different   than   the  idealized.       This   is   important   to   note,   as   tensile   behavior   is   greatest  when   the  fiber   direction,   so   if   the   fiber   is   not   aligned   properly,   the  mechanical   behavior  will  be  greatly  reduced.14    

 

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 Figure  6.4:  Top  and  cross  sectional  view  12  

   

6.2.2  Tensile  Properties  The   tensile   properties   of   3D   woven   composites   are   only   recently  

beginning   to   become   understood,   and   are   often   compared   with   those   of   2D  woven  composites  with  similar  fiber  content.    The  Young’s  modulus  of  3D  woven  composites  is  commonly  lower  than  the  modulus  of  the  equivalent  2D  composite,  however  other  studies  show  the  modulus  being  higher.1,9    It  is  important  to  note  that  the  Young’s  modulus  is  not  significantly  influenced  by  the  z-­‐binder  content  or   fiber   structure,   and   is   most   probably   influenced   by   the   fiber   content  (increasing   with   increasing   fiber   content)   and   degree   of   fiber   waviness  (decreasing  with  increasing  waviness),  and  can  be  accurately  predicted  using  the  block  laminate  and  unit  cell  models.14     A  feature  displayed  by  3D  woven  composites,  but  not  by  2D,  is  an  onset  of  plastic   deformation   at   relatively   low   tensile   stresses,   displaying   a   reduction   of  stiffness  from  20-­‐50%.    This  softening  is  due  to  the  onset  of  plastic  deformation  in  the  most  heavily  distorted  load-­‐bearing  tows,  distortion  of  yarns  being  caused  by  the  z-­‐binders.    The  critical  tensile  stress  at  which  plastic  deformation  of  these  yarns  occurs  can  be  estimated  by  the  following  equation:14    

 

€

σa =fs τ13[ ]ξ[ ]

 

 

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Where   fs   is   the   volume   fraction   of   the   load-­‐bearing   tows,   τ13   is   the   axial   shear  strength  of  the  tow,  and  ξ  is  the  fiber  waviness  parameter,  defined  as  the  average  misalignment   angle   for   90%   of   all   load-­‐bearing   tows.     In   order   to   minimize  softening   it   is   necessary   to  minimize   the   in-­‐plane   fiber  waviness  or  use   a  high  yield  shear  strength  resin.14     As   tensile   stresses   increase   above   the   critical   value,   the   matrix   of   3D  woven   composites   begin   to   crack,   z-­‐binders   begin   to   de-­‐bond.   Failure   occurs  though   rupture   of   load-­‐bearing   tows.     3D  woven   composites   generally   have   a  lower   tensile   strength   compared  with   2D.     The   lower   tensile   strength   is  most  likely   due   to   fiber   damage   from   weaving,   fiber   waviness   and   pinching   of   the  surface   tows.     Prediction  of   tensile   failure   strength  of   3D  woven   composites   is  difficult   as   it   relies   heavily   on   fiber   damage   and   crimping   which   are   hard   to  accurately  measure.14      6.2.3  Compressive  properties  

In  most   cases   it   has   been   found   that   the   compression  modulus   of   3D   is  lower  than  that  of  2D  woven   laminate  with  similar   fiber  content,  due  to  higher  crimping   and   increased   waviness   of   the   3D   fibers.3     Studies   for   compressive  strength   determination   are   inconclusive   as   they   show   both   an   increase   and   a  decrease  in  strength.    The  cause  of  the  increased  compressive  strength  is  unclear.    However   the   decreased   compressive   strength   is   due   to   kinking   of   the   load-­‐bearing  tows  (see  figure  6.5).    Kinking  initiates  in  regions  with  low  resistance  to  permanent  shear  deformation  such  as  defects  or  misalignments,  which  are  much  more  prevalent  in  3D  woven  composites.  Kinking  arises  when  plastic  shear  flow  of   the   matrix   surrounds   the   axial   tow,   causing   a   rotation   until   break   of   the  individual  tows.14  

 Figure  6.5:  Kinking  failure  in  compression  

 Kinking  in  3D  woven  composites  is  usually  concentrated  in  the  surface  regions,  where   the   most   severely   distorted   tows   are   located.     Kink   bands   develop   as  discrete  geometric  flaws,  which  inhibit  catastrophic  failure  and  instead  cause  the  material  to  fail  gradually  under  increasing  strain,   leading  to  significantly  higher  

 

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strains  at  ultimate  failure.    3D  woven  composites  have  shown  to  have  significant  strength  even  after  being  exposed  to  compressive  strains  of  more  than  15%.14      6.2.4  Flexural  and  Interlaminar  Shear  Properties     In  most   cases,   the   flexural  properties  of  3D  woven  composites  have  been  found   to   be   lower   than   those   for   2D.     This   is   due   to   crimping   and   increased  misalignment  of  the  tows  by  the  z-­‐binder.4,14     On  the  other  hand,  the  interlanimar  shear  strength  of  3D  woven  composites  is  generally  the  same  or  slightly  higher  than  that  for  2D  composites.  3,4,13,14      6.2.5  Interlaminar  Fracture     The   advantage   of   3D   woven   composites   lies   in   their   high   resistance   to  delamination  cracking.    2D  laminates  are  prone  to  delamination  when  subject  to  out-­‐of-­‐plane   loads   or   impacts.     The   delamination   tendencies   of   some   2D  laminated  composites   significantly   inhibit   their  use   in  many  structures   such  as  aircrafts,  drawing  attention  to  3D  woven  composites.14     There  are  three  modes  in  which  a  composite  can  delaminate.    Mode  I  occurs  through   tensile   crack   opening   (see   figure   6.6),   mode   II   through   shear   crack  sliding  and  mode  III  through  tearing.    Mode  I  is  the  most  studied  and  well  know  of   the   three   delamination   modes,   and   it   has   been   found   that   delamination  resistance   of   3D   woven   composites   in   this   mode   is   superior   to   those   of   2D  laminates.10,13     This   improvements   in   delamination   resistance   can   be   achieved  with   small   amounts   of   z-­‐binder   reinforcements,   and   delamination   toughness  increases  with  increasing  volume  content,  elastic  modulus  ,  tensile  strength  and  pull-­‐out  resistance  of  the  z-­‐binders.    Guenon6  found  the  delamination  toughness  of  a  3D  woven  composite  with  z-­‐binder  content  of  1%  to  be  14  times  higher  than  for  2D  laminates.  

 

 Figure  6.6:  Mode  I  delamination  cracking14  

    This   dramatic   increase   in   delamination   toughness   is   caused   by   the  necessity  for  the  crack  tip  to  pass  through  the  z-­‐binders.    The  de-­‐bonding  of  the  z-­‐binders   as   the   crack   propagates   absorbs   a   small   amount   of   energy.     The  majority  of  the  toughening  is  caused  by  the  bridging  zone  (figure  6.6),  where  the  z-­‐binders   support   a   large   portion   of   the   applied   force   in   turn   reducing   the  stresses   acting  on   the   crack   tip   and   increasing  de-­‐bonding   toughness.     Further  toughening  occurs  from  crack  branching,  due  to  the  induced  toughness  of  the  z-­‐binders,   and   the   energy   absorbed   due   to   z-­‐binder   fracture   and   de-­‐bonding.    

 

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Delamination   toughness   decreases   with   decreasing   misalignment   of   the   z-­‐binders,  as  tow  strength  is  greatest  in  the  fiber/yarn  direction.14      6.2.6  Impact  Damage  Tolerance       The   impact   damage   tolerance   of   3D   woven   composites   is   of   particular  interest   to   the   aerospace   industry   as   aircrafts   are   subject   to   impact   loading  conditions,  such  as  hail,  bird  strikes  and  for  military  armored  planes,  bullet  fire.    Testing  under  lightweight,  low  speed  projectiles  was  carried  out  to  evaluate  their  potential   use   in   commercial   planes,   while   high-­‐velocity   bullets   were   used   to  evaluate  impact  damage  tolerance  for  military  aircraft  applications.  4,7,8,14       The  impact  damage  caused  to  3D  woven  composites  is  less  than  that  for  2D  laminates  with  the  same  fiber  volume  content  (see  figure  6.7).    This  resistance  to  impact   damage   is   due   to   the   high   delamination   toughness,  which   impedes   the  spread   of   the   delaminations   from   the   impact   site.     This   results   in   higher   post-­‐impact  mechanical  properties  (see  figures  6.8  &  6.9).1,3,14,15    

 Figure  6.7:    Effect  of  impact  velocity  on  delamination  damage  of  2D  and  3D  woven  composites2      

 

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 Figure  6.8:  Effect  of  impact  energy  on  flexural  strength15  

 Figure  6.9:  Effect  of  impact  energy  on  the  compressive  strength1  

 

 

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References  1. Arendts  F.J.,  K.  Drechler  and  J.  Brandt,  1993,  Advanced  textile  structural  

composites  –  status  and  outlook,  Proc.  Of  the  Int.  Conf.  on  Advanced  Composite  Materials,  ed.T.  Chandra  and  A.K.  Dhingra,  409-­‐416.  

2. Billaut  F.  and  0  .  Roussel,  1995,  Impact  resistance  of  3-­‐D  graphite/epoxy  composites,  Proc.  of  the  11”  Int.  Conf.  on  Comp.  Mat.,  Vol.  5,  Ed.  A.  Pourartip  and  K.  Street,  Whistler,  BC,  Canada,  14-­‐18  August  1995,  Woodhead  Publishing  Ltd.,  551-­‐558.  

3. Brandt  J.,  K.  Drechsler  and  Fa-­‐J.Arendts,  1996,  Mechanical  performance  of  composites  based  on  various  three-­‐dimensional  woven-­‐fibre  preforms,  Comp.  Sci.  &  Tech.,  56:  381-­‐386.  

4. Chou  S.,  H.-­‐C.  Chen  and  H.-­‐E.  Chen,  1992,  Effect  of  weave  structure  on  mechanical  fracture  behavior  of  three-­‐dimensional  carbon  fiber  fabric  reinforced  epoxy  resin  composites,  Comp.  Sci.  &  Tech.,  45:  23-­‐35.  

5. Cox  B.N.,  M.S.  Dadhkak,  W.L.  Morris  and  J.G.  Flintoff,  1994,  Failure  mechanisms  of  3D  woven  composites  in  tension,  compression  and  bending,  Acta  Metal.  et  Mat.,  42:3967-­‐3984.  

6. Guenon  V.A.,  T.-­‐W.  Chou  and  J.W.  Gillespie,  1989,  Toughness  properties  of  a  three-­‐dimensional  carbon-­‐epoxy  composite,  J.  Mat  Sci.,  24:4168-­‐4175  

7. James  B.  and  Howlett  S.,  1997,  Enhancement  of  post  impact  structural  integrity  of  GFRP  composite  by  through-­‐thickness  reinforcement,  Proc.  of  the  2ndEuropean  Fighting  Vehicle  Symposium,  27-­‐29  May,  Shrivenham,  UK.  

8. KO  F.K.  and  D.  Hartman,  1986,  Impact  behavior  of  2-­‐D  and  3-­‐D  glass/epoxy  composites,  SAMPE  J.,  July/August,  26-­‐30.  

9. Lee  B.,  K.H.  Leong  and  I.  Herszberg,  2001,  The  effect  of  weaving  on  the  tensileproperties  of  carbon  fibre  tows  and  woven  composites,  J.  Rein.  Plastics  &  Comp.,20:  652-­‐670.  

10. Mouritz  A.  P.,  C.  Baini  and  I.  Herszberg,  1999,  Mode  I  interlaminar  fracture  toughness  properties  of  advanced  textile  fibre  glass  composites,  Composites,30:859-­‐870.  

11. Stig,  Fredrik.    An  Introduction  to  the  Mechanics  of  3D-­Woven  Fiber  Reinforced  Composites.    KTH  School  of  Engineering  Sciences,  Stockholm,  Sweden.  April  2009.  

12. Tan  P.,  L.  Tong,  G.P.  Steven  and  T.  Ishikawa.  2000a.  Behavior  of  3D  orthogonal  woven  CFRP  composites.  I:  Experimental  investigation,  Composites,  31A:259-­‐271.  

13. Tanzawa  Y.,  N.  Watanabe  and  T.  Ishikawa,  1997,  Interlaminar  delamination  toughness  and  strength  of  3-­‐D  orthogonal  interlocked  fabric  composite,  Roc.  of  the  Eleventh  Int.  Conf.  on  Composite  Materials,  Ed.  M.L.  Scott,  14-­‐18  July,  Gold  Coast,  Australia,  V-­‐47  to  V-­‐57.  

14. Tong,  L.  Mouritz,  A.P.  and  Bannister,  M.K.    3D  Fibre  Reinforced  Polymer  Composites.    Elsevier  Science  Ltd.    Oxford,  UK.    2002.  

15. Voss  S.,  A.  Fahmy  and  H.  West,  1993,  Impact  tolerance  of  laminated  and  3-­‐  dimensionally  reinforced  graphite-­‐epoxy  panels,  Proc.  of  the  Int.  Conf.  on  Advanced  Composite  Materials,  Ed  T.  Chandra  and  A.K.  Dhingra,  591-­‐596.  

 

 

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7  3D  Braided,  Knitted,  Stitched,  and  Z-­pinned  

Composites    

 7.1  Introduction  

    In   this   section   we   will   briefly   review   the   mechanical   properties   of   3D  braided,  3D  knit,  stitched,  and  z-­‐pinned  composites.    This  is  meant  to  be  a  brief  review  and  therefore  data  is  not  presented  in  a  detailed  manner.    This  should  be  used   as   a   reference   of   the   general   knowledge   of   properties   concerning   these  composites  architectures.      7.2  3D  Braided  Composites       Braided  fabric  is  second  most  common  fabric  reinforcement  architecture,  after  woven.     2D  braided   carbon   and   glass   fabrics   have  been  used   in   products  such   as   golf   clubs,   yacht   masts   and   aircraft   propellers.     Just   like   3D   woven  fabrics,  3D  braided  fabrics  show  a  number  of  advantages  over  2D  fabrics,  such  as  higher   levels   of   conformability,   increased   drapability,   torsional   stability   and  structural   integrity.     Other   advantages   include   the   ability   to   form   intricately  shaped  performs,  including  changes  in  cross-­‐sectional  shape,  tapers,  holes,  bends  and  bifurcations.    Here  in  this  section  we  will  review  the  mechanical  properties  that   have   been   studied   in   regards   to   composites  made   from  3D  braided   fabric  reinforcement.15      7.2.1  In-­Plane  Properties     There   are   many   factors   affecting   the   in-­‐plane   properties   of   composite  materials.    For  3D  braided  materials  studies  have  been  performed  to  study   the  effect   of   braid   pattern   and   level   of   machining.     Macander   et.   al   examined   the  effect   of   braid   pattern   and   edge   condition   on   the   performance   of   3D   braided  composites.     The   results   are   found   in   table   7.1   below.     The   number,   e.g.   1x1,  denotes  the  braid  pattern.    The  first  number  represents  the  number  of  spaces  the  yarn  carrier  advances   in  the  x-­‐direction.    The  second  represents  the  number  of  spaces  the  yarn  carrier  advances  in  the  y-­‐direction.    The  final  number  indicates  the  number  of  carriers  fixed  in  the  axial  direction  (1/2F=50%).    As  can  be  seen  from  the  data   in  table  7.1,   there   is  a   large  difference  between  the  properties  of  cut  and  uncut  fabrics  of  the  same  braid  pattern.    This  shows  a  high  dependence  

 

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on  machining  damage,  as  cut  edged  fabrics  undergo  further  machining.    Another  large   difference   can   be   seen   between   weave   geometries.     The   1x1   weave  geometry   creates   a   much   tighter   weave   than   that   produced   for   the   3x1.     The  tighter  weave   results   in   a  higher   yarn   angle   or   crimping.       As  discussed   in   the  previous  chapter,  crimping   is  one  of   the  most   important   factors   in  determining  the  composite  properties.11,15    

 Table  7.1:  Reported  results  from  Macander  et  al.  for  effects  of  braid  pattern  and  edge  

conditions11      7.2.2  3D  vs.  2D  Braided  Composites     It  is  often  beneficial  to  compare  the  properties  of  new  materials  to  older  materials  in  order  to  have  a  better  idea  of  the  benefits  and  challenges  presented.    However,  this  often  difficult  as  it  is  can  be  unclear  as  to  what  variables  should  be  considered.    Gause   tested  a  1x1  and  1x1x1/2F  3D  braided  composite  against  a  24-­‐ply   laminate   of   AS1/3501   prepreg   designed   to   mimic   the   3D   architecture.    The  results  for  the  3D  composite  were  as  follows:  decreased  tensile  strength  in  all   directions,   increase   in   longitudinal   compressive   properties   and   tensile  modulus,  and   increased  ability   to  retain  tensile  properties   in   fabrics  containing  open   holes.     Further   studies   are   needed   to   further   characterize   3D   braided  composites.15      7.3  3D  Knit  Composites  

 Of  all  the  fabric  reinforcements  used  for  composite  construction,  knit  fabric  is  

the   least   understood.     Knitting   creates   a   fabric   by   looping   yarns   together,  creating  a  highly  curved  yarn  structure,  a  structure  with  relatively  low  structural  strength.   The   advantages   of   3D   knit   performs   lie   in   their   high   drapability   and  impact   damage   resistance.     They   are   ideally   suited   to   produce   non-­‐structural  parts  with  complex  geometries.    Due  to  its  exceptional  impact  damage  resistance,  it  is  being  considered  for  the  potential  use  in  medical  prosthesis,  bicycle  helmets,  and  automobile  doors.15  

   

7.3.1  In-­Plane  Properties     The   in-­‐plane  tensile  properties  of  3D  knit  composites  have  been  studied  the  most.    However,   little   information  was  given  on  knit  architecture  regarding  

 

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loop   length,   shapes   and   densities,   which   could   give   valuable   insight   into   the  mechanisms  behind  the  outcomes.    Tests  have  shown  that  the  tensile  strength  of  knitted  composites  is  similar  to  those  of  composites  constructed  with  non-­‐woven  randomly   oriented   fibers,   and   that   increases   in   fiber   volume   increased   tensile  strength.8,9,13     Wu   and   Leong   studied   the   effect   of   knit   architecture   on   the  mechanical  properties.10,16    They  found  that  it  was  possible,  to  change  the  tensile  properties   from   quasi-­‐isotropic   to   strongly   anisotropic   by   changing   the   knit  geometry.    The  tensile  properties  increased  with  increasing  fiber  orientation,  as  expected.     The   results   are   listed   in   tables   7.2   and   7.3   below,   and   figure   7.1  depicts  the  fiber  architectures  used  in  the  tests.  15  

 

 Table  7.2:    Tensile  properties  of  warp  knit  with  varying  knit  architectures16  

 

 Table  7.3:    Tensile  properties  of  weft  knit  with  varying  knit  architectures  10  

 Figure  7.1:    Warp  knit  (a)  Denbigh,  (b)  1x3  single  cord,  and  (c)  1x4  single  cord  architectures15  

    The  tensile  properties  can  also  be  controlled  by  changing  the  loop  length  or   stitch   density   (as   loop   length   increases   stitch   density   decreases).     Leong10  found   that   the   modulus   is   dependent   on   fiber   volume   fraction,   while   tensile  strength   and   failure   stain,   in   course   and   wale   directions,   decreases   with  decreasing  loop  length  for  both  weft  and  warp  knit  (see  figure  7.2).    On  the  other  hand  Wu16  found  the  exact  opposite;  in  the  course  direction  he  found  that  tensile  strength  increases  with  decreasing  loop  length  and  that  loop  length  had  no  effect  

 

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in   the   wale   direction.     This   shows   that   the   properties   of   the   knit   fabric   are  dependent  on  many  parameters  and  further  testing  is  needed.10,15,16  

 

 Figure  7.2:  Wale  and  course  directions  as  well  as  warp  and  weft  fabric  structure.  

    There  has  not  been  much  investigation  on  the  compressive  properties  of  knit  composites.    However,  with  what  little  has  been  done,  it  has  been  found  that  knit   composites   have   a   compressive   strength   better   than   the   tensile   strength,  while  compressive  modulus   is  about   the  same.    The  compressive  properties  do  not   depend   on   loop   length   and   are   much   more   isotropic   compared   to   tensile  properties.15      7.3.2  Interlaminar  Fracture  and  Impact  Toughness     Here  is  where  the  true  benefits  of  3D  knit  fabrics  lie.    Mouritz12    found  the  fracture  toughness  of  the  knit  fabrics  to  be  significantly  higher  than  that  for  2D  and  3D  woven,  2D  braided  and   stitched  composites.    This   is  due   to   the   looped  yarn   structure,   which   causes   crack   deflection   and   excessive   crack   branching,  consequently   increasing   the   toughness.     This   can   be   controlled   through   stitch  density,  as  density  increases  toughness  decreases,  due  to  the  tighter  structure  of  densely   stitched   knit   fabric.     Tight   structures   do   not   allow   for   intermingling,  which  is  responsible  for  crack  deflection.11,12,15     The  impact  performance  of  knit  composites  under  low  to  medium  energy  is   also   of   great   interest.     Chou3   found   that   knit   composites   are   capable   of  absorbing   up   to   2.4   times   more   impact   energy   than   woven   composites.     This  makes   knit   composites   attractive   candidates   for   components   requiring   high  impact  absorption  or  non-­‐structural  damage  prone  parts.3,15      7.4  Stitched  Composites       Stitching   involves   sewing   the   laminates   in   the   z-­‐direction   with   a   high  strength   thread.    The  high  strength   thread   is   inserted   through  a  stack  of   fabric  using  an  industrial  grade  sewing  machine.    The  through-­‐thickness  reinforcement  of   stitched   composites   is   between   1-­‐5%   and   is   comparable   to  woven,   braided  and   knitted   3D   composites.     The   act   of   stitching   the   fabric   prepregs   together  improves   the  delamination   resistance,   and   impact  damage   tolerance.     Stitching  can   also   be   used   to   connect   separate   composite   components   together,  

 

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eliminating  the  need  for  mechanical  fasteners,  reducing  costs  and  improving  the  joints  mechanical  properties.    Due  to  the  simple  nature  of  stitching,  it  is  possible  to   only   stitch   the   areas   that  would   benefit   from   the   z-­‐direction   reinforcement,  further   reducing   costs.   Up   to   this   point   stitching   has   proven   to   be   a   simple,  flexible   and   low-­‐cost  method   for   producing   3D   composites,   however   there   are  limitations  to  the  complexity  of  the  components.15      7.4.1  In-­plane  Mechanical  Properties     During  the  stitching  process  damage  can  occur.    This  damage  is  the  most  important   factor   to  consider  when  considering   the  mechanical  properties.    The  most   common   forms   of   damage   are   fiber   breakage,   fiber   misalignment,   and  crimping,   all   causing   sever   decreases   in  mechanical   properties.     Other   damage  can  occur  from  micro  cracking  at  stitch  insertion  sites  and  fiber  compaction.    Due  to  the  increase  in  fiber  damage  caused  by  stitching,  stitched  composites  tend  to  have  slightly  lower  tension,  compaction  and  flexural  properties  than  unstitched  composites,  although   there  are  many  contradictions  between  data.    As  stitched  composites   have   been   extensively   studied,   there   are   databases   containing  detailed   information   on   tension,   compression,   and   bending   modulus   and  strength  of  various  materials.15     The   interlaminar   shear  properties  of   stitched  composites  have  not  been  extensively   studied.     The   few   studies   that   have   been   carried   out   show  contradicting  results  and  are  therefore  inconclusive.15      7.4.2  Fracture  Toughness  and  Impact  Damage  Tolerance     Stitched   composites   show   a   large   improvement   in  mode   I   delamination  resistance.    The  mechanism  behind  the   fracture  toughness   is  similar  to  that   for  3D   woven   composites   (see   figure   7.3).     When   a   crack   forms   and   propagates  through  the  stitches  a  bridging  zone  is  formed,  where  the  stitches  exert  a  closing  force,   lowering   strain   in   the   crack   tip.     Stitched   composites   also   show   a   large  increase   in  mode   II   delamination   toughness,   however   not   as   great   as   in  mode  I.7,15    

 Figure  7.3:    Illustrating  mode  I  interlaminar  toughening  mechanism  of  stitched  composites7  

    Due   to   the   high   fracture   toughness   of   stitched   composites   it   is   not  surprising   that   they   have   good   impact   damage   tolerance.     Most   studies   for  

 

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impact  damage  tolerance  have  been  for  ballistic  projectiles  and  explosion  shock  wave  as  stitched  composites  have  been  of  particular  interest  to  the  military.    In  both  cases  an  increase  in  post-­‐impact  mechanical  properties  was  observed.15      7.5  Z-­Pinned  Composites       Z-­‐pinning   involves   the   insertion   of   short   metal   wire   or   pultruded  composite  pins  in  the  z-­‐direction.    The  pins  can  be  inserted  in  uncured  prepreg  taps  or  dry  fabrics  (see  figure  7.4).    For  further  details  on  the  z-­‐pinning  process  please  refer  to  Chapter  2.    As  this  is  a  relatively  new  technology  the  full  potential  and   application   possibilities   are   currently   being   investigated.     At   this   point   z-­‐pinning  has  demonstrated  the  ability  to  increase  joint  strength,  remove  the  need  for   fasteners   and   create   a  more   evenly  distributed   the   load  over   a   join   region.  However,   there   is   not   much   data   on   mechanical   properties   such   as   flexural  strength,   shear   strength,   fatigue,   etc.     The   data   that  will   be   reported   here  will  consist  of  tensile  and  compressive  strength,  and  delamination  properties.15    

 Figure  7.4:  Depiction  of  z-­‐pinned  architecture  at  insertion  site14  

   7.5.1  Tensile  and  Compressive  Strength     Steeves   and   Fleck   investigated   the   effect   of   z-­‐pinning   on   tensile   and  compressive   strength.14     Their   results   showed   an   average   decrease   in   tensile  strength   of   about   27%,   while   the   modulus   remained   same.     This   decrease   in  tensile   strength   is   believed   to   be   caused   by   the   stress   concentration   and   fiber  damage  at  the  pin  insertion  sites.    Freitas  showed  the  same  results  for  z-­‐pinned  materials  containing  more  than  1.5%  pinning.    Below  1.5%  the  tensile  properties  remained  almost  unchanged.4,5,6,14,15     The   effect   of   z-­‐pinning   on   compressive   properties   has   proven   to   be   very  similar  that  of  tensile,  with  a  30%  decrease  in  strength.14    However,   it  was  also  found  that  there  is  a  strong  correlation  between  fiber  misalignment  and  decrease  in  composite  strength.    This  correlation   is  caused  by  weaving  of   in-­‐plane  yarns  around   the   z-­‐pin   (see   figure   7.5),   as   weaving   increases,   compressive   strength  decreases.     Weaving   increases   as   the   insertion   angle   of   the   z-­‐pin   increases.    Insertion  angles  of  0°  showed  the  least  weaving,  while  weaving  increased  for  23°  and  45°  specimens.4,5,6,14,15    

 

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 Figure  7.5:    Depiction  of  weaving  and  deflection  caused  by  z-­‐pins14  

   7.5.2  Delamination  resistance     Z-­‐pinned  composites  show  a  high  resistance  to  interlaminar  cracking  and  through-­‐thickness   failure.     Cartei   and   Partridge   investigated   the   properties  under  mode   I   and   II   failure.     For  mode   I   failure   the   composite   showed  a   rapid  increase   in   delamination   resistance   with   increasing   z-­‐fiber   content   and  decreasing  pin  diameter.1,2,15     The  mechanisms  behind  the  increased  resistance  are  similar  to  that  of  3D  woven  and  stitched  composites.    The  presence  of  the  z-­‐directional  fibers  creates  the  bridging  zone,  which  relieves  strain  from  the  crack  tip.    For  more  details  on  the  mechanism  see  chapter  6.15    

 

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References    

1. Cartei  D.D.R.  and  I.K.  Partridge,  1999a,  Delamination  behavior  of  Z-­‐pinned  laminates,  Proc.  12*  Int.  Conf.  Comp.  Mat.,  5-­‐9  July,  Paris.  

2. Cartei  D.D.R.  and  I.K.  Partridge,  1999b,  Delamination  behavior  of  Z-­‐pinned  laminates,  Proc.  2"'  ESIS  TC4  Conf.,  Ed.  J.  G.  Williams,  13-­‐15  Sept.,  CH-­‐  Les  Diablerets,  Elsevier.  

3. Chou  S.  and  C-­‐J.  Wu,  1992,  A  study  of  the  physical-­‐properties  of  epoxy-­‐resin  composites  reinforced  with  knitted  glass-­‐fiber  fabrics,  J.  Reinforced  Plastics  &  Comp.,  11:1239-­‐1250.  

4. Freitas  G.,  T.  Frusco,  T.  Campbell,  J.  Harris  and  S.  Rosenberg,  1996,  Z-­‐Fiber  technology  and  products  for  enhancing  composite  design,  Proc.  of  the  83'   Meeting  of  the  AGARD  SMP  on  "BoltedKionded  Joints  in  Polymeric  Composites",  Sep.  2-­‐3,  Florence,  Italy,  pp.17-­‐1  -­‐  17-­‐8.  

5. Freitas  G.,  C.  Magee,  P.  Dardzinski  and  T.  Fusco,  1994,  Fiber  insertion  process  for  improved  damage  tolerance  in  aircraft  laminates,  J.  Advanced  Mat.,  25:36-­‐43.  

6. Freitas  G.,  C.  Magee,  J.  Boyce  and  R.  Bott,  1991,  Service  tough  composite  structures  usingz-­‐fiberprocess,Proc.9"  DoD/NASA/FAAConf.FibrousComp.,LakeTahoe,  Nevada,  Nov.  

7. He  M.  and  B.N.  Cox,  1998,  Crack  bridging  by  through-­‐thickness  reinforcement  in  delaminating  curved  structures,  Comp.  29:377.  

8. Hohfeld  J.,  M.  Drews  and  R.  Kaldenhoff,  1994,  Roc.  of  the  31d  Int.  Conf.  of  Flow  Processes  in  Composite  Materials,  July  7-­‐9,  120.  

9. Huang  Z.M.,  Y.  Zhang  and  S  .  Ramakrishna,  2001,  Modeling  of  the  progressive  failure  behavior  of  multilayer  knitted  fabric-­‐reinforced  composite  laminates,  Comp.  Sci.  &  Tech.,  61:2033-­‐2046.  

10. Leong  K.  H.,  P.  J.  Falzon,  M.  K.  Bannister  and  I.  Herszberg.  1998,  An  investigation  of  the  mechanical  performance  of  weft-­‐knit  Milano-­‐rib  glass/epoxy  composites,  Comp.  Sci.  &Tech.,  58:239-­‐251.  

11. Macander  A.B.,  R.M.  Crane  and  E.T  Camponeschi,  1986,  Fabrication  and  mechanical  properties  of  multidimensionally  (X-­‐D)  braided  composite  materials,  Composite  Materials:  Testing  and  Design  (7  Conf.),  ASTM  STP  893,  American  Society  for  Testing  and  Materials,  Philadelphia,  422-­‐443.  

12. Mouritz  A.  P.,  C.  Baini  and  I.  Herszberg,  1999,  Mode  I  interlaminar  fracture  toughness  properties  of  advanced  textile  fibre-­‐glass  composites,  Composites,  30:859-­‐870.  

13. Ramakrishna  S.,  N.  K.  Cuong  and  H.  Hamada,  1997,  Tensile  properties  of  plain  weft  knitted  glass  fiber  fabric  reinforced  epoxy  composites,  J.  Rein.  Plastics  &  Comp.,  16:946-­‐966.  

14. Steeves  A.,  Craig,  Norman  A.  Fleck,  2006,  In-­‐plane  properties  of  composite  laminates  with  through-­‐thickness  pin  reinforcement,  International  journal  of  solids  and  structures  43:  3197-­‐3212.  

15. Tong,  L.  Mouritz,  A.P.  and  Bannister,  M.K.    3D  Fibre  Reinforced  Polymer  Composites.    Elsevier  Science  Ltd.    Oxford,  UK.    2002.  

16. Wu  W-­‐L.,  M.  Kotaki,  A.  Fujita,  H.  Hamada,  M.  Inoda  and  Z-­‐I.  Maekawa,  1993,  Mechanical-­‐properties  of  warp-­‐knitted,  fabric-­‐reinforced  composites,  J.  Reinforced  Plastics  &  Comp.,  12:1096-­‐1110.  

 

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8  Concluding  Remarks  

        In   this   work   the   different   3D   reinforcement   architectures   have   been  discussed  in  some  detail.  As  a  concluding  remark,  it  is  helpful  to  summarize  the  pros  and  cons  associated  with  them.    In  General,  3D  reinforcement  architectures  are   characterized   by   reduced   in-­‐plane   mechanical   properties,   increased  delamination  toughness  and  increased  impact  damage  resistance.    The  reduction  of   the   in-­‐plane  mechanical   properties   is   due   to   increased   fiber   damage   during  processing  (especially  for  stitching)  and  increased  crimp.    The  improved  out-­‐of-­‐plane  mechanical  properties  are  attributable   to   the  z-­‐directional   fibers.    At   this  time,  the  manufacturing  of  3D  composites  is  still  being  explored,  however  there  are  high  hopes  for  the  future  that  this  type  of  reinforcement  architecture  will  be  able  to  improve  manufacturing  efficiency  and  ease,  eventually  reducing  costs.     Some  of  the  defining  characteristics  of  specific  3D  architectures  should  also  be  acknowledged.    Knit  fabrics  are  of  particular  interest  due  to  their  exceptional  impact   resistance,   however   are  unsuitable   for   structural   application.       Stitched  and   z-­‐pinned   are   among   the   simplest   and   most   inexpensive   methods   for  producing   3D   architectures   greatly   improving   mechanical   properties   in   joints  connections.    While  3D  braided  and  woven  allow  for  3D  shapes  to  be  produced  inherently  providing  a  means  for  enhanced  production  techniques.     3D   composites   and   their   reinforcement   architectures   need   to   be   further  studied   and   improvements   to   both   the   reinforcement   as   well   as   production  techniques   need   to   be   investigated   before   wide   spread   adoption   is   possible.    However,  3D  composites  are  a  very  promising  solution  to  many  of  the  problems  faced  by  2D  composites   and   can  be  expected   to  become  more  prevalent   in   the  future.