composites module 4a. spring 2001 isat 430 dr. ken lewis2 an aside: stress – strain tension test...
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Composites
Module 4a
Spring 2001 ISAT 430 Dr. Ken Lewis 2Module 4a
An aside: Stress – Strain
Tension test Used to determine mechanical properties such as
Strength Ductility Toughness Elastic modulus Elongation to break stiffness
Relative mechanical Properties
Strength Hardness Toughness Stiffness Strength/Density
Glass Fibers Diamond Ductile Metals Diamond Reinforced Plas.
Graphite Fiber Cubic BN Reinforced Plas. Carbides Titanium
Kevlar Fiber Carbides Thermoplastics Tungsten Steel
Carbides Hardened steels Wood Steel Aluminum
Molybdenum Titanium Thermosets Copper Magnesium
Steels Cast Irons Ceramics Titanium Beryllium
Tantalum Copper Glass Aluminum Copper
Titanium Thermosets Ceramics Tantalum
Copper Magnesium Reinforced Plas.
Reinforced TS Thermoplastics Wood
Reinforced TP Tin Thermosets
Thermoplastics Lead Thermoplastics
Lead Lycra®
Spring 2001 ISAT 430 Dr. Ken Lewis
Spring 2001 ISAT 430 Dr. Ken Lewis 4Module 4a
Stress - Strain
There is a standard specimen for each type of material having a known initial gage length (l0) and diameter (cross sectional area (A0)
Metals l0 =~ 50 mm ( 2 in) A0 dia =~ 12.5 mm (0.5in)
Fibers l0 =~ 25 mm ( 10 in) A0 dia =~ the fiber
Spring 2001 ISAT 430 Dr. Ken Lewis 5Module 4a
Stress - Strain
Specimen is mounted in the jaws of a tensile testing machine.
Usually can be tested at various rates of extension and temperature.
Produces the important Stress – Strain curve.
0
, P
StressA
0
0
, l l
Strainl
Elastic Plastic
Fracture
YieldStress
Ultimate TensileStress
E
Spring 2001 ISAT 430 Dr. Ken Lewis 7Module 4a
Stress - Strain
In the elastic region, stress and strain are proportional
The ratio or slope of the line in the elastic region is called
Modulus of elasticity, E, Young’s modulus
The linear relationship is known as Hooke’s Law
0
, P
StressA
0
0
, l l
Strainl
Elastic Plastic
Fracture
YieldStress
Ultimate TensileStress
E
T. Young (1773 – 1829)
R. Hooke (1635 – 1703)
Spring 2001 ISAT 430 Dr. Ken Lewis 8Module 4a
Stress - Strain
The higher the E value The higher the load required to
stretch a specimen to the same extent
The stiffer the material
0
, P
StressA
0
0
, l l
Strainl
Elastic Plastic
Fracture
YieldStress
Ultimate TensileStress
E
Note Strain is dimensionlesstherefore
E has units of stress F/A.
Spring 2001 ISAT 430 Dr. Ken Lewis 9Module 4a
Stress - Strain
Brittle Materials Glass Most un-reinforced ceramics
Elastic to the end.
Spring 2001 ISAT 430 Dr. Ken Lewis 10Module 4a
Stress - Strain
Materials that strain harden Some steel alloys
Material is ductile until grain boundaries intersect
Movement stops More stress can then be borne.
Spring 2001 ISAT 430 Dr. Ken Lewis 11Module 4a
Stress - Strain
Elastomers Lycra® Rubbers
Small truly elastic region Large elongation with little
increase in stress At the end, crystallization occurs
and stress is borne until rupture.
Spring 2001 ISAT 430 Dr. Ken Lewis 12Module 4a
Stress – Strain - Plastics
Spring 2001 ISAT 430 Dr. Ken Lewis 13Module 4a
Stress - strain &Temperature
Cellulose Acetate Note
the large drop in strength The large increase in ductility
Spring 2001 ISAT 430 Dr. Ken Lewis 14Module 4a
Composites
A way of combining the benefits of different materials to influence the bulk properties
Make plastics competitive with steel Allow building a single piece boat hull Increase the elastic modulus and strength of light metals.
An important class of engineered materials.
Spring 2001 ISAT 430 Dr. Ken Lewis 15Module 4a
Composites
Definition Combination of two or more chemically distinct and insoluble
phases Properties and structural performance superior to those of the
constituents acting independently.
Common classes Reinforced Plastics Metal Matrix Composites (MMC) Ceramic Matrix Composites (CMC)
Spring 2001 ISAT 430 Dr. Ken Lewis 16Module 4a
Early examples
Brick reinforced with straw ( 3000 BCE) Concrete reinforced with iron rods (1800’s)
In truth,Concrete is itself a
Composite(sand, cement, gravel)
In both cases, the reinforcing material
(straw and iron)provide needed tensile
strength.
Spring 2001 ISAT 430 Dr. Ken Lewis 17Module 4a
Mechanical Properties
Material Ultimate Tensile Strength (Mpa)
Young’s Modulus
E (Gpa)
Elongation to break
(%)
Acetal 55 - 70 1.4 – 3.5 75-25
Acetal Reinforced 135 10 --
Epoxy 35-140 3.7-17 10-1
Epoxy Reinforced 70-1400 21-52 4-2
Polycarbonate 55-70 2.5-3 125-10
Polycarbonate reinforced 110 6 6-4
Polyester 55 2 300-5
Polyester reinforced 110 8.3-12 3-1
Nylon 55-83 1.4-2.8 200-60
Nylon reinforced 70-210 2-10 10-1
Spring 2001 ISAT 430 Dr. Ken Lewis 18Module 4a
Composite Classification
Matrix The material that surrounds the other component Provides the bulk form of the part Hold the reinforcing phase in place
Reinforcing phase The embedded material may be metal, ceramic, plastic Usually provides the strength
In terms of its effect, the shape of the reinforcing material is of particular importance.
Spring 2001 ISAT 430 Dr. Ken Lewis 19Module 4a
Matrix Function
To support the fibers in place Transfer stresses to them Let them carry the tensile load
To protect the fibers Physical damage Environment
Reduce crack propagation Greater matrix ductility (sort of, usually)
Spring 2001 ISAT 430 Dr. Ken Lewis 20Module 4a
Reinforcing phase
Fibers Continuous
Very long Discontinuous
L/D about 100 Whiskers
Hair like single crystals with diameters down to about 40 x 10-6 in. Very strong.
Spring 2001 ISAT 430 Dr. Ken Lewis 21Module 4a
Fibers of choice
Glass – cheapest and most widely used Glass fiber reinforced plastic (GFRP) Made by drawing molten glass through a platinum spinneret. Types
E – glass (calcium aluminosilicate glass) S – glass (magnesia aluminosilicate glass)
Spring 2001 ISAT 430 Dr. Ken Lewis 22Module 4a
Fibers of choice
Graphite – more expensive High strength, stiffness and low density Carbon fiber reinforced plastic (CFRP) Made by pyrolysis of organic precursors – usually polyacrylonitrile
(PAN) or pitch Two kinds of fibers
Carbon (80 – 95% carbon) Lower modulus and strength
Graphite (> 99% carbon) Crystalline and very high modulus and strength
Spring 2001 ISAT 430 Dr. Ken Lewis 23Module 4a
Fibers of choice
Aramids (among the toughest fiber) Kevlar® is the best example Have some elongation before rupture (~3%) so are very tough.
Mechanism used in the bullet proof vest! Absorb moisture which can degrade properties.
Spring 2001 ISAT 430 Dr. Ken Lewis 24Module 4a
Fibers of choice
Boron Formed by chemical vapor
deposition onto tungsten fibers. Very strong and stiff both in
tension and compression However, very heavy because of
the tungsten ( = 19.3 g/cm3)
Tungsten
Boron
Spring 2001 ISAT 430 Dr. Ken Lewis 25Module 4a
Particles and Flakes
Orientation in the matrix is usually quite random Properties therefore are isotropic Called fillers
Crack stoppers
Spring 2001 ISAT 430 Dr. Ken Lewis 26Module 4a
Effect of Fiber Type on Properties
Mechanical and physical properties depend on the reinforcing mediums
Kind Shape Orientation
Short fibers are less effective than long fibers
Spring 2001 ISAT 430 Dr. Ken Lewis 27Module 4a
Fiber Matrix Bond - Plastics
Strength of the fiber matrix bond is critical The load is transmitted through the fiber – matrix interface
Weak bonding can cause Fiber pullout delamination
Spring 2001 ISAT 430 Dr. Ken Lewis 28Module 4a
Strength as a function of fiber direction and content
In general… The highest stiffness and strength
is obtained when the fibers are aligned in the direction of the tension force.
Cool…. But, there is a caveat
This makes the composite very anisotropic.
Spring 2001 ISAT 430 Dr. Ken Lewis 29Module 4a
Strength as a function of fiber direction and content
Result of anisotropy… Other properties are anisotropic
Stiffness Creep Thermal & electrical conductivity Thermal expansion
Example – fiber reinforced packaging tape
Strong in the fiber direction Easily pulled apart in the width
direction
Reinforcing Fibers
Fiber Density g/cm3) Young’s Modulus (GPa)
Tensile Strength (GPa)
Steel 7.83 210 2.1
Tungsten 19.3 350 4.2
Beryllium 1.84 300 1.3
E-Glass 2.48 75 3.5
S-Glass 2.54 85 4.6
Alumina 3.15 320 2.1
SiC 3.0 400 2.8
Boron 2.6 420 4.0
High Modulus C 1.9 390 2.1
High Strength C 1.9 240 4.0
Kevlar® 29 1.44 83 2.8
Kevlar® 49 1.44 130 3.2
Spring 2001 ISAT 430 Dr. Ken Lewis 31Module 4a
The Rule of Mixturesc = composite
r = reinforcing phasem = matrix
The mass of the composite body is: c r mm m m
Equivalently: c c r r m mV V V
r r m mc
c
V V
V
If we let: the volume fraction of the reinforcing materialr
c
Vf
V
sincec r mV V V 1m
c
Vf
V
Spring 2001 ISAT 430 Dr. Ken Lewis 32Module 4a
The Rule of Mixtures
We get: (1 )c r mf f
This is the Rule of Mixtures
Each component contributes to the propertiesOf the composite in proportion
To its VOLUME fraction
Spring 2001 ISAT 430 Dr. Ken Lewis 33Module 4a
Fiber reinforcement
When the filler is in the form of thin fibers strongly bonded to the matrix
Properties depend on the fiber and the amount present There is a critical minimum volume fraction There is a critical length
Fiber should be continuous of If chopped long enough to accept stresses transferred from the
matrix
Spring 2001 ISAT 430 Dr. Ken Lewis 34Module 4a
Fiber reinforcement
If we use the criteria It takes more force to shear the matrix at the fiber boundary (pull out
the fiber) than to break the fiber
We get an approximate critical length lcr.
2cri
d Tl
D = fiber diameterT = fiber tensile strengthi = interfacial shear strength
Spring 2001 ISAT 430 Dr. Ken Lewis 35Module 4a
Tensile Strength
Unidirectional Composites Complex
Upon loading a single fiber breaks first The aim is to neutralize the effect of local failure.
Matrix should be ductile enough to not propagate a crack Matrix should be able to carry the shear stress on the fiber matrix
interface. The rule of mixtures is helpful but actual design is made using
actual statistical properties.
Spring 2001 ISAT 430 Dr. Ken Lewis 36Module 4a
Long Fiber Composites
Rule of mixtures works in the longitudinal direction.
c
or
E 1c r r m m
r r r m
E f E f E
f E f E
Perpendicular to the
longitudinal direction
' m rc
m r r m
E EE
f E f E
Spring 2001 ISAT 430 Dr. Ken Lewis 37Module 4a
Example
Suppose we are reinforcing an epoxy matrix whose elastic modulus is 2.7
GPa with 26% by volume of E-glass whose elastic modulus is 75 GPa What is the Elastic modulus of the composite in the longitudinal direction?
(1 )Lc f mE f E f E
0.26 75 0.74 2.7LcE GPa GPa
21.498LcE GPa
Spring 2001 ISAT 430 Dr. Ken Lewis 38Module 4a
Example
Suppose we are reinforcing an epoxy matrix whose elastic modulus is 2.7
GPa with 26% by volume of E-glass whose elastic modulus is 75 GPa What is the Elastic modulus of the composite in the transverse direction?
(1 )f m
Tcf m
E EE
f E f E
75 2.7
0.26 75 0.74 2.7Tc
GPa GPaE
GPa GPa
3.6TcE GPa
Recall
21.498LcE GPa
Spring 2001 ISAT 430 Dr. Ken Lewis 39Module 4a
Other Composite Types
A) conventional laminar structure
B) sandwich with a foam core C) sandwich structure using a
honeycomb
B and C gain stiffness using an increase in the moment of inertia
Spring 2001 ISAT 430 Dr. Ken Lewis 40Module 4a
Applications of Reinforced Plastics
1907 - First application was for an acid resistant tank made of a phenolic resin and asbestos.
1920’s – Formica, used for counter tops 1930’s – Advent of epoxy as a reinforcing material 1940’s – fiberglass/epoxy boats, some aircraft, sporting
goods 1970’s – beginning of “Advanced Composites” using hybrid
plastics and carbon fibers.
Spring 2001 ISAT 430 Dr. Ken Lewis 41Module 4a
Applications of Reinforced Plastics
Aircraft (DC-10, L-1011, 727, 757, 767, 777) The Boeing 777 is about 9% (weight) composites
Floor beams and panels Most of the vertical and horizontal tail
The Lear Fan 2100 passenger aircraft structure is almost all graphite/epoxy.
90% of the world circling Voyager is plastic composites The stealth bomber is made of carbon and glass fibers, epoxy resin
matrices, high temperature polyimides (and other neat stuff).
Spring 2001 ISAT 430 Dr. Ken Lewis 42Module 4a
Composite Sailboard
K. Easterling, Tomorrow’s Materials, p.133, Institute of Metals, 1990
Spring 2001 ISAT 430 Dr. Ken Lewis 43Module 4a
Metal Matrix Composites
The matrix is usually a low density metal, primarily Aluminum Aluminum - lithium Magnesium Copper titanium
The reinforcement is often SiC, Al2O3, or carbon
MMCsFiber Matrix Applications
Graphite Aluminum
Magnesium
Lead
Copper
Satellite, missile, and helicopter structures
Space and satellite structures
Storage- battery plates
Electrical contacts and bearings
Boron Aluminum
Magnesium
Titanium
Compressor blades and structural supports
Antenna structures
Jet-engine fan blades
Alumina Aluminum
Lead
Magnesium
Superconductor restraints in fission power reactors
Storage-battery plates
Helicopter transmission structures
Silicon Carbide Aluminum
Super alloy
High-temperature structures
High-temperature engine components
Molybdenum
Tungsten
Superalloy High-temperature engine components
Spring 2001 ISAT 430 Dr. Ken Lewis 45Module 4a
Ceramic Matrix Composites
In polymer matrix composites the reinforcement is always stronger and of much higher elastic modulus than the matrix
Thus a significant increase in strength can be had by transferring stresses from the matrix to the fiber through a strong interface.
This is also true of MMC (for low elastic moduli material such as Al, Mg)
Ceramic already have a high elastic modulus (except glasses) so the purpose of a CMC is to increase toughness.
Spring 2001 ISAT 430 Dr. Ken Lewis 46Module 4a
MonolithicCeramics
Fail completelyIn
BrittleCatastrophic
mode
Spring 2001 ISAT 430 Dr. Ken Lewis 47Module 4a
Ceramic Matrix Composites
Matrix materials Silicon carbide Silicon nitride Aluminum oxide Mullite (aluminum, silicon oxides)
Ceramics are strong and stiff, retained at high temperatures but are brittle.
Spring 2001 ISAT 430 Dr. Ken Lewis 48Module 4a
CMC Toughness
(A) Crack Deflection A crack meeting the
reinforcement is deflected along the interface where energy is used to effect separation
(B) Crack Propagation Barrier Reinforcement can force the
crack to bow out, increasing the stress necessary for propagation
Spring 2001 ISAT 430 Dr. Ken Lewis 49Module 4a
CMC Toughness
(C) Fiber Bridging Sometimes fibers bear the load
across the crack, putting the crack in compression
(D) Fiber Pull-out Most important, energy is used
in pulling the fiber out. The interfacial bond strength
must not be so high that the fiber breaks rather than pulling out.