Design Guidelines Laminates should be SymmetricTo avoid warping from coupling between in-plane strains and out-of-plane rotations (curvature & twists),unless warping is desired. [B]=0 for symmetric laminates.

Laminates should be BalancedTo reduce coupling between in-plane normal strains and in-plane shear strains (A16=A26=0 for balancedlaminates) to eliminate or reduce coupling between curvatures and twists (D16=D26=0 for unsymmetricbalanced laminates i.e., D16=D26→0 for symmetric & balanced laminates with many plies)

Laminates are usually designed with 3 major fiber orientations: 0°,45°,90°To reduce the complexity of the design for which there are an infinite numbers of orientations possible.

Use 0° plies to carry axial loads, 45° to carry Shear loads and 90° plies for transverse loads.

Laminates carrying predominantly Shear loads but some normal loads (certain webs and skin panels)should be 10% 0°’s, 80% 45°’s,10% 90°’s.

Laminates carrying Axial loads (certain stiffeners, beam chords etc) should be 60% 0°’s, 30% 45°’s,10% 90°’s or even a higher percentage of 0’s.

Laminates should have a minimum of 8% fibers in each major direction (0°,45°,90°) to account forunexpected loads.

0°& 90° plies should be separated by a + or -45° ply, or preferably a +45° and a -45° ply toavoid delamination between the 0° ply and the 90° ply caused by Poisson’s mismatch.

Avoid stacking more than 4(and usually 3) plies in any one orientation together to help preventdelamination between the similarly oriented ply stack and the adjacent plies.

Avoid placing a stack of similarly oriented plies at the outer surface of the laminate since theyare more susceptible to damage. For example, 0, 90 (or fabric) is more damage tolerant than0,0.

45,90,-45 sequences are often used at outer surfaces since these sequences are more damageresistant and more damage tolerant than stacks of similarly oriented plies. This also contains theload carrying similarly oriented stacks within the laminate and helps prevent delaminationunder load.

Use fabric for outer ply to provide abrasion resistance, since fabric is more abrasion resistantthan tape (also Kevlar is more abrasion resistant than graphite or glass). Fabric also reducesfiber breakout when drilling holes for fasteners.

Load carrying plies should be located towards center for protection. For example, avoidplacing the 0° plies of a beam chord or stiffener at the outer surface to decrease chance ofdamage.

Avoid free edges to discourage delamination. Delamination from repeated loading begins at thefree edges where interlaminar (out-of-plane) stresses are the highest.

CROSS SECTIONS Solid Laminate Cross sections (I’s, C’s, Z’s, Angles) are typically used for compositestructural members such as skin panel stiffeners, Web stiffeners, Wing Spars, Floor beams,Wing Ribs and Fuselage frames. Sections such as I’s, C’s and angles, to this date, have beenused most often primarily because of their efficiency and ease of fabrication and NDI.

Sections formed by two or more laminates should be Symmetric (see figure 6.5.1) toavoid warping from coupling between in-plane strains and out-of-plane rotations(curvatures and twists).

Differences in Poisson’s ratios of discreet elements should be minimized (see figure 6.5.2)to prevent debonds or delamination under loads.

Observe thickness limitations with regard to Skin Stiffener flanges (see figure 6.5.3). ifthe stiffener flange is too thin, it will not adequately contain the skin (or plank); if it is toothick it may delaminate from the skin.

All inside radii should be 0.125 inches minimum (see figure 6.5.4) smaller radii are difficult to fabricate and cause high stress concentrations.

Observe flange length minimums (see figure 6.5.4) small flange lengths may cause stiffenercap delamination and stringer pull-off under load because of the small amount of bonded orco-cured area.

Locate flange-to-web ply drop offs after outer bend radius tangent point (see figure 6.5.5)to help prevent delamination.

Avoid cutting out any portion of flange and number of holes should be kept to a minimumbecause these continuities reduce the structural integrity of the section (true for anystructure)

Laminate guidelines should be followed in the design of laminates for complex sections.

PLY DROP-OFF Make ply drop-offs as gradual as possible to avoid stress concentration. Specify no more

than 6 plies every 0.2 inches transverse to the load and use a 20 to 1 slope in the loaddirection.

Drop-off reinforcement should be in pairs rather than singles to make the plies lay up lesslabor intensive.

Do not machine tapers into solid laminates to prevent exposing fiber ends, which increasesmoisture absorption and ply peeling.

Allow at least 1 inch from end of ply drop-off to edge of the part. Parts which require lessthan 1 inch from part edge should be fabricated larger than required, then cut to therequired size.

Locate flange-to-web ply drop-offs after outer bend radius tangent point to help preventdelamination (see figure 6.5.5)

See Figure 6.6.1 for preferred ply drop-off design.

10 TIPS FOR DESIGN AND ANALYSISTip 1 - Use unidirectional ply orientations to your advantageUnidirectional composites have the unique properties of having a higher stiffness and strength in the direction parallel to the fibers. Use this to your advantage:

0 deg. layers - provide axial strength and stiffness, ideal for beams and columns that have the design purpose of resisting axial loads.

+/- 45 deg. layers - provide shear/torsional strength and stiffness, ideal for torsion shafts and shear webs (I-beam webs).

90 deg. layers - provide transverse strength and stiffness, used primarily as a consolidating layer (keep everything together) and to provide most of the structural resistance in pressure vessels.

Tip 2 - Use quasi-isotropic layups when unknown or multiple direction loads are appliedA quasi-isotropic layup [0/45/90/-45] is able to support axial, transverse, and shear loading. Do not use a quasi-isotropic layup by default as a substitute for metal, use composites to their advantage:

A quasi-isotropic layup is only 40% as stiff as a 0 deg. layup in the axial direction! A quasi-isotropic layup is only 60% as stiff as a +/- 45 deg. layup in shear!

Tip 3 - Space ply angles as much as possible

[0/45/90/-45](3) instead of [0(3)/45(3)/90(3)/-45(3)]. This will help prevent delamination andwill increase damage tolerance by resisting crack propagation.

Tip 4 - Minimize the angle change between plies

[0/45/90/-45] instead of [0/90/45/-45]. This will help prevent delamination due to through-thickness shear stresses induced by high angle changes in adjacent plies. Plies orientedorthogonally to each other will try to Poisson in different directions under and applied load,upping the chance of delamination.

Tip 5 - When using carbon fiber unidirectional plies, place a glass fabric ply on theoutside to reduce the likelihood of damaging the stronger and stiffer carbon plies.

Tip 6 - Keep at least 10% of the plies in a laminate in 1 of the 4 quasi-isotropic directions

Even for an axially loaded laminate, keep at least 10% of the plies oriented in the 90, 45, and -45 deg. directions. A 50/40/10 (50% 0's, 40% +/- 45's, 10% 90's) is a typically used laminateto resist primarily axial loads.

Tip 7 - To get a quasi-isotropic layup for filament would parts use a [30/-30/90] layupA [0/45/90/-45] may not work for filament wound parts because of the 0 deg. layer and thedifficulties winding this angle. A [30/-30/90] will work for filament wound parts.

Tip 8 - Estimate G23 as E33/3.0 for carbon/epoxy plies and E33/2.82 for glass/epoxy plies

A unidirectional ply can be assumed to be transversely isotropic with 5 independent materialproperties: E11, E22, G12, v12, and G23. The first 4 are typically provided by material datasheets but G23 is often not. For transversely isotropic materials, G23 = E33/[2*(1+v23)].v23 can be estimated as 0.5 for carbon/epoxy plies and as 0.41 for glass epoxy plies.

Tip 9 - Utilize that online material dataAGATE is a great source for common aerospace composite material data. Other sourcesincludeProspector:Composites, MIL-HDBK-17, MatWeb, and manufacturer's data sheets.

Tip 10 - Become (or stay) a lifelong learner

Get on the internet, there is so much available. To get started, check out Tech Tidbitswritten by Dr. Scott Beckwith (SAMPE International Technical Director).

Subscribe to this blog! There is plenty more coming in the years to follow.

DESIGN DATA: 0-45-90 LAMINATE PROPERTIES

The material used in the examples here is Gr-Ep, 350 oF cured, types II, III and IV, classes 1and 2, as defined below:-

Type I: A Bleed system material with a non-self-adhesive resin content of 42% ± 2% byweight in its purchased state. The resin will be bled during fabrication.

Type II: A non-bleed material with a non-self-adhesive resin content of 35% ± 2% byweight in its purchased state. No bleeding is required during fabrication.

Type III: A non-bleed material with a non-self-adhesive resin content of 37% ± 2% byweight in its purchased state. No bleeding is required during fabrication.

Type IV: A non-bleed material with a non-self-adhesive resin content of 40% ± 2% byweight in its purchased state. No bleeding is required during fabrication.

Class 1: A unidirectional resin impregnated tape.

Class 1 tape material is further described by Grades

Class 1 Grades specify the Graphite Only areal weight in gm/m2.

Example: grade 145

145 gm/m2 (Approx)

Class 2: A woven, resin impregnated fabric.

Class 2 woven material is further described by Styles.

Class 2 Styles specify the number of filaments per fiber, the manufacturers’ approximate thickness and the weave pattern.

Example 3K-70-PWPlain weave-worked

7 Mil (approx)3000 filaments

Nominal cured ply thickness, inchType I Type II Type III0.0073 0.0075 0.0083

Table II-1 Ply Thickness

Class GradeNominal cured thickness, inch

Type - II Type - III Type - IV

Tape

1 95 0.0037 0.0039 -

1 145 0.0056 0.0059 -

1 190 0.0074 0.0078 -

Fabric

2 3K-70-P 0.0075 - -

2 3K-70-PW 0.0075 - 0.0083

2 3K-70-CSW 0.0073 0.0075 -

2 3K-135-8H 0.0144 0.015 -

2 1K-50-5H 0.0048 0.005 -

Table II-2 Prepreg Weight

ClassMaterial per BMS8-212

Type - II Type - III Type - IV

(35% resin content)

(37% resin content)

(40% resin content)

1

Grade 95 0.03 0.031 -

Grade 145 0.046 0.047 -

Grade 190 0.06 0.062 -

2

Style 3K-70-P 0.061 - -

Style 3K-70-PW 0.061 - 0.066

Style 3K-70-CSW 0.059 0.061 -

Style 3K-135-8H 0.115 0. 118 -

Style 1K-50-5H 0.039 0.04 -

Gr-Ep Tape & Fabric 350°F Cure Allowable Strain

Tension (єt) 0.0035

Compression (єc) 0.0027

Shear (єs) 0.0053

Table II-3 Allowable Strains