Structural Performance of Fiber-Placed, Variable-Stiffness Composite Conical and Cylindrical Shells

Fokker Aerostructures

Structural Performance of Fiber-Placed,V i bl Stiff C it C i lVariable-Stiffness Composite Conicaland Cylindrical Shells

Fokker Aerostructures B.V. is a company of the Stork Group

Agnes Blom, PhD – Boeing Seattle, January 2011

Fokker Aerostructures B.V. Agnes Blom, PhD - Boeing Seattle, January 2011

IntroductionIntroductionOutline• Introduction

• Variable-Stiffness Laminates

• Design Studies: g

• Axial Stiffness Variation on Conical Shells to Maximize Fundamental Frequencies

• Circumferential Stiffness Variation on a Cylinder to Maximize Load-Carrying Capability in BendingCapability in Bending

• Manufacturing using Advanced Fiber Placement (AFP)

• Experimental Validation:

• Modal Test

• Bending Test

• Conclusions

Structural Performance of Fiber-Placed, Variable-Stiffness Composite Conical and Cylindrical Shells

2


IntroductionIntroductionOverall research objectives

• Weight reduction and structural response improvements of tailored cylindrical and conical shells with fiber-placed laminateslaminates

• Understanding, predicting, and verifying structural g g y gresponse characteristics of variable-stiffness laminates


3


IntroductionIntroductionComposite tailoring

Fiber-placed, curved fibersConventional, straight fibers

Spatially varying stiffnessConstant stiffness

Courtesy of Ingersoll Machine Tools


4

Spatially varying stiffnessConstant stiffness


IntroductionIntroductionPrevious work: Gürdal, Tatting, Wu, Jegley

40Failure loadPfail

Experiments by Chauncey Wu, NASA LaRC

d, k

ips

30Panel with overlap

LAMINATE STACKING SEQUENCE

Pcr (kN)

Pfail (kN)

Baseline panel [±45] 17 120

Load

20

Critical Load, Pcr

Panel w/o overlap

[±45]9s0

Tailored Panel without Overlaps 41 125

Tailored Panel 60 183

10Test stopped at 8 kips

Baseline panel


5

Tailored Panel with Overlaps 60 183 0.05 0.200 0.10 0.15

Average end shortening, in.


IntroductionIntroductionImprovement due to load path tailoring

• Stiffer edges attract load• Center is softened, buckling is post-poned

W k th if th i t l h l !• Works the same if there is a central hole!


6



8


Case studiesCase studiesFiber angle variation in axial or circumferential directionConical and cylindrical shells with an

axially varying fiber angle,optimized for

Cylindrical shell with a circumferentially varying fiber angle, optimized for maximum bending load

maximum fundamental frequency carrying capability

ϕ(θ)ϕ(θ)

ϕ(x)


10


Finite Element PredictionsFinite Element Predictions

Fi i El A l i

AbaqusFinite Element Analysis:

• ABAQUS shell model

• Stiffness in terms of ABD matrixthrough user subroutine (more efficient than direct input)

• Local stacking sequence calculatedwith Fortran subroutine UGENS based ongeometry and laminate definitiongeometry and laminate definition

• Post-processing with python


11


Axial Stiffness VariationAxial Stiffness Variation

Variation in axial direction can improve the fundamental f b 20 t f l i ht!frequency by 20 percent for equal weight!

r0 (cm)

r1(cm)

A (cm)

α(deg)

fconstant(Hz)

f2-stage(Hz)

Increase (%)(cm) (cm) (cm) (deg) (Hz) (Hz) (%)

Satellite bus 30.0 30.0 72.5 0.0 334 393 17.7

Helicopter tail boom 30.0 35.0 100.0 2.9 227 273 20.4

Satellite end cap 12.5 80.0 80.4 40.0 187 211 12.8


12


Circumferential Stiffness VariationCircumferential Stiffness VariationCylinder Design

R = 305 mm (12 in), L = 813 mm (32 in), t = 24 plies Loading: Pure bending with clamped BC


13


Circumferential Stiffness VariationCircumferential Stiffness Variation

C hi k l i

Improvements in buckling load under bending• Constant thickness laminates:

• No constraints: 29 %• Realistic constraints: 17 %

• Strength constraints• Strength constraints• Manufacturability: curvature, fiber cutting• Equivalent 10 percent rule for robustness

• Overlap laminates (ply thickness scaled for equal mass):• No constraints: 357 %• Some constraints: 92 %

• Strength constraints• Manufacturability: curvature


14


Cylinder Design – Constant ThicknessCylinder Design Constant ThicknessConstant thickness laminates (AS4/8552), i l t th f t i d tiff t i t

Laminate Layup Buckling moment,

M

Material failure moment,

M

Comparison with baselineM /M 100%

incl strength, manufacturing and stiffness constraints

Mcr (kNm)

Mf(kNm)

Mcr/Mcr,b⋅ 100%

Baseline [±45,02, ±45,90,0,90, ±45,90]S 598 661 100

VS-1 [±45, ±ϕ1, ±ϕ2, ±ϕ3, ±ϕ4, ±ϕ5]S 687 689 115

VS-2 [±45, ±ϕ1,0,90, ±ϕ3,0,90, ±ϕ5]S 700 700 117

VS-3 [±45,0,90, ±ϕ2,0,90, ±ϕ4,0,90]S 678 678 114

Variation of in plane stiffness is dominant factor


17

Variation of in-plane stiffness is dominant factor


Cylinder Design – Constant ThicknessCylinder Design Constant ThicknessBest steered candidate (BMS8-276)(18% i i b kli l d)(18% increase in buckling load)

[±45 ±ϕ 0 90 ±ϕ 0 90 ±ϕ ][±45, ±ϕ1 , 0, 90 , ±ϕ3 , 0, 90 , ±ϕ5]s

T0 T1 T2 T3 T4

ϕ1 (θ) 10.0 10.0 10.0 10.0 24.7

ϕ3 (θ) 10.0 10.0 10.6 56.9 61.7

ϕ5 (θ) 10.0 12.0 10.0 34.2 68.9


18Tension side Compression side


Cylinder Design – Constant ThicknessCylinder Design Constant ThicknessFiber angle plots for: [±45, ±ϕ1, 0, 90, ± ϕ3 , 0, 90, ± ϕ5 ]s


19±ϕ1 ±ϕ5±ϕ3


Cylinder Design – Constant ThicknessCylinder Design Constant ThicknessStiffness variation around the circumference

Compressionside

Tensionside

Tensionside


20


Cylinder Design – Constant ThicknessCylinder Design Constant ThicknessAxial load around the circumference


21


Cylinder Design – OverlapCylinder Design Overlap Ply thickness is scaled, no constraintsLaminate Layup Buckling

moment,Mcr

(kNm)

Ply thickness

t (mm)

Spec. buckling moment

Mcr/m(kNm/kg)

Comparison with baselineMcr/Mcr,b⋅ 100%

Baseline [±45,02,±45,90,0,90,±45,90]S 627 0.183 58 100

VSo-1 [±45, ±ϕ1, ±ϕ2, ±ϕ3, ±ϕ4, ±ϕ5]S 1198 0.076 110 191

VSo-2 [±45, ±ϕ1,0,90, ±ϕ3,0,90, ±ϕ5]S 1070 0.101 99 171

VSo-3 [±45,0,90, ±ϕ2,0,90, ±ϕ4,0,90]S 927 0.117 85 148

VSo-4 [±45, ±ϕ1, ±ϕ2, ±ϕ1, ±ϕ2, ±ϕ1]S 1154 0.078 106 184

VSo-5 [±45, (±ϕ1)5]S 1210 0.074 112 193


23Out-of-plane stiffness is most dominant!


Cylinder Design – OverlapCylinder Design OverlapUnconstrained optimum overlap design [±45, (±ϕ1)5 ]S

Ply T0 T1 T2 T4 T5

±ϕ1 10 40 89 10 10


24


Cylinder Design - OverlapCylinder Design - Overlap

Number of plies, NEquivalent modulus, Ex

In-plane stiffnessE tExt


25


Cylinder Design - OverlapCylinder Design - OverlapUnconstrained optimum overlap design [±45, (±ϕ1)5 ]S

• Loads near θ=0˚ and θ=180˚ increase due to higher in-plane stiffness• Buckling load increases because bending stiffness increases more

than load


26Bending stiffnessIn-plane load distribution


Cylinder DesignCylinder DesignConstant thickness versus overlap• Constant-thickness laminates improve the performance by tailoring the

internal load distribution through in-plane stiffness variation• Overlap laminates improve the performance by increasing the laminate

thickness locally, thereby increasing the bending stiffness → this is opposite the constant-thickness principles

• Overlaps are function of angle variation → in-plane and out-of-plane stiffness are coupled

• Overlap laminates have potential, but more developments are needed to come up with realistic designs (limited amount of overlap, not too oriented, overlap decoupled from fiber angle variation)

• Constant thickness laminates require tow cutting/restarting, but can already be designed realistically


27


ManufacturingManufacturing


28


ManufacturingManufacturingSteered plies

Ply 4: T0 = -10.0˚T1 = -10.0˚T2 = -10.0˚2

T3 = -10.0˚T4 = -24.7˚

Ply 11: T0 = 10.0˚y 0 T1 = 12.0˚T2 = 10.0˚T3 = 34.2˚


37T4 = 68.9˚


Modal TestModal TestEigenmodes and eigenfrequencies – steered shell


41


Bending TestBending Test

• Measured:• Measured:• Force• Rotations• Strains

• Using:g• Load cell of machine• LVDT’s• Lasers• Lasers• Digital image

correlationSt i


42• Strain gauges


Bending Test ResultsBending Test ResultsGlobal response: baseline and variable-stiffness shell


50


Bending Test Results ztension

Bending Test ResultsDistribution of axial strains yxMy

compression


51


Bending Test ResultsBending Test ResultsAxial strains at 415 kNm bending moment

Tension

Compression


52baseline variable-stiffness


Bending Test ResultsBending Test ResultsPrediction of the buckling loads with different FEA

Baseline Variable-stiffness, preferred direction

Variable-stiffness, reversed direction

FE Model Mcr (kNm) Mcr (kNm) Mcr (kNm)Linear, clamped bc,

bifurcation 678 805(+19%)

477(-30%)

Nonlinear, clamped bc, bifurcation 647 763

(+18%)470

(-27%)

Nonlinear, flexible bc, 570 671 430, ,bifurcation 570

(+18%) (-25%)

Riks, flex bc, imperfections 488 589

(+21%)409

(-16%)


53

( ) ( )


Conclusions (1)Conclusions (1)Variable-stiffness composite design and verification• Developed laminate definitions for variable-stiffness, fiber-placed

conical and cylindrical composite shells• Implemented variable-stiffness definitions in a FEM• 20% improvement in fundamental frequency of conical/cylindrical

shells with axial stiffness variation (analytical)• > 18% improvement in bending load carrying capability of cylindrical

shells with circumferential stiffness variation (analytical + experimental)• Built 3 shells: 2 baselines and variable-stiffness shell• Modal test showed experimental eigenvalues are within 5% of p g 5%

analytical values• Bending test has good correlation with analysis: strains of VS shell

significantly lower & projected buckling load 20% higher


54

g y j g g


Conclusions (2)Conclusions (2)

D i

Many challenges remain!• Design:

• Other geometries• 2-D stiffness variation

• Optimization:• Optimization:• Combined/multiple load cases• Weight reduction through complete laminate optimization

• Strength predictions:Strength predictions:• Allowables for wide range of stacking sequences• Curved fibers• Tow drop areasp

• Certification:• Impact modeling• Progressive failure


55• Repair



56Thank you!

Documents

Structural Performance of Fiber-Placed, Variable-Stiffness Composite Conical and Cylindrical Shells