1. Failure Mechanisms in Twill-weave Laminates:
FEM Predictions vs. Experiments by
Gianni Nicoletto and Enrica Riva
Dipartimento di Ingegneria Industriale
Università di Parma
Parma, Italy
COMPTEST 2003
Chalons en Champagne, France
Jan. 28, 2003

2. Outline Introduction e motivation Related works
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Outline
Introduction e motivation
Related works
Twill-weave laminate chacterization
Finite element modeling
Experimental observations and computational results
Conclusions

3. Motivation Cooperation with Dallara Automobili F3 racing car
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Motivation
Cooperation with
Dallara Automobili
F3 racing car
IRL racing car
Infinity Pro
CFRP Chassis

4. Woven Composites Definitions Advantages
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Woven Composites
Definitions
Yarns: bundle of thousands fibers
Warp yarns: parallel to load direction
Fill yarns: perpendicular to load direction
Texture: plain weave, twill weave, etc.
Crimp ratio: degree of yarn curvature
Advantages
With respect to unidirectional laminates:
Easier handling and shaping
Improved impact resistance
Superior out-of-plane stiffness
Balanced in-plane mechanical properties
Cost competitive
yarn

5. COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
Objectives of the work
Develop a finite element-based modeling approach to the mechanics of woven laminate composites.
Compare modeling results and experimental observations.
Analyze the role of texture on mechanical performance.
Develop tools for monitoring damage development in woven laminates.

6. Related Modeling Work Analytical approach T.W. Chu et al (1983 - )
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Related Modeling Work
Analytical approach
T.W. Chu et al ( )
N.K. Naik et al ( )
Models, such as mosaic, crimp and bridging models subjected to iso-strain or iso- stress conditions, predict adequately the stiffness of woven laminates.
These models are less satisfactory for strength prediction and micromechanical stress determination.
Convenient approach for texture design.
The plain-weave texture has been mainly considered.

7. Finite element approach V. Carvelli and C. Poggi (2001)
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Finite element approach
J. Whitcomb et al. ( )
V. Carvelli and C. Poggi (2001)
D. Blackketter et al. (1993)
Computational prediction of the mechanics of woven laminate composites.
The finite element method is used to geometrically model an elementary cell of the woven laminate.
Boundary conditions enforcing stress and strain periodicity are imposed to the representative volume (RV) .
Stress-based damage and stiffness discount technique to model damage progression.
Most studies deal with the plain-weave texture.

8. Electrical resistance method applied to unidirectional composites
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Related Experimental Work
K. Schulte et al ( )
J.C. Abry et al. (1998)
Electrical resistance method applied to unidirectional composites

9. Material and Experiments
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Material and Experiments
0° direction
Fiber: Toray T-300 carbon fibers
Fiber diameter: 7 μm
Fiber volume fraction Vf: 42%
Density ρ: 1.76 g/cm2
Strength su= 3200 MPa
Elastic modulus E: 228 GPa
Matrix: Epoxy Hexcel 1990S
Laminate
Lay-up: 8-ply
Texture: Twill-weave
Yarns: 3k fibers
Warp and fill yarns: Identical
Laminate thickness: 2.4 mm
Tensile tests according to: ASTM D3039
Servo-hydraulic testing machine: MTS 810
Resistance strain gages & Extensometer
Electric resistance measuring apparatus

10. Geometrical Characterization of Twill-weave Texture
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Geometrical Characterization of Twill-weave Texture
Yarn shape: circular arcs
Ply stacking: random a b g RT RL
2.04
0.17
1.13
6.11
6.15
All dimensions in mm

11. Tensile Tests and Evolution of Electrical Resistance
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Tensile Tests and Evolution of Electrical Resistance
Strain
Norm. elect. resistance vs. strain
Stress (MPa)
(R-R0)/R0
Stress vs. strain
Twill-weave laminates

12. Damage Observations Fill yarn Tow yarn Epoxy Fiber fracture
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Damage Observations
Fill yarn
Tow yarn
Epoxy
Fiber fracture
Fracture in yarn

13. Inter-ply delamination Delamination between orthogonal yarns
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Damage Mechanisms: a Summary
Inter-ply delamination
Crack
in fill yarn
Delamination between orthogonal yarns
Final longitudinal fiber fracture is preceeded by a number of mechanisms.
Matrix cracks develop in fill yarns.
Delamination occurs between orthogonal yarns.
Inter-ply delamination is observed

14. Homogeneization Method for Composite Materials
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Homogeneization Method for Composite Materials
Assumption of periodic microstructures which can be represented by unit cells
Asymptotic expansion of all variables and the average technique to determine the homogeneized (macroscopic) material properties and constitutive relations of composite materials
Prediction of microscopic fields of deformation inside the unit cell through the localization process

15. Texture and Representative Volume RV
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Texture and Representative Volume RV
Material models:
Yarn: transverse isotropic, linear elastic
Matrix: linear elastic
Twill-weave RV

16. Finite Element Modeling of RV
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Finite Element Modeling of RV
Parametric geometrical model (I-DEAS)
Finite element code (ABAQUS)
Convergence study
Optimized model: > elements
Geometric nonlinearity included
Progressive damage evolution routine in FORTRAN

17. Boundary conditions on RV
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Boundary conditions on RV
Free surface
Carvelli & Poggi (2001)
where
u(x) is the displacement field in the RV
u0 is a rigid displacement of the RV
W is a small rigid rotation of RV
E is the average strain (macroscopic)
of RV
ũ(x) is a periodic displacement associated to microscopic strain field within RV
Post, Han and Ifju (1994)

18. Damage Modes for Fiber Yarn
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Damage Modes for Fiber Yarn
M. Zako et al (2003)

19. Modeling Damage Development
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Modeling Damage Development
Discount method
Blackketter et al (1993)
Iterative procedure.
Evaluation at each integration point.
Normal stress criterion for failure.
Elastic modulus is reduced to 1/10 of its initial value.
Role of time step and mesh size.

20. Effect of Texture on Longitudinal Stiffness
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Effect of Texture on Longitudinal Stiffness
Plain weave
Twill weave
Lamina
Thick laminate
Strong influence of crimp ratio on stiffness.
Good correlation with experimental results.
A thick laminate is stiffer than a single lamina.
At high crimp ratios the twill-weave is stiffer than the plain- weave.

21. Effect of Crimp Ratio on Stress-Strain Curve
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Effect of Crimp Ratio on Stress-Strain Curve
Strong influence of crimp ratio on stress-strain curve.
Good correlation with experimental results.
Low crimp ratio shows a trend linear to failure.
Influence of computational parameters.

22. Stresses and Damage Step a a d c b
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Stresses and Damage a b c d
Step a
The critical stress is perpendicular to the fill yarn surface.
The wedge elements of the straight portion of the fill yarns fail first.
Initial damage occurs near the fill yarns.
The critical stress is representative of damage initiation in the matrix.

23. Stresses and Damage Step b b d c a
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Stresses and Damage a b c d
Step b
The critical stress direction does not change. It is perpendicular to the fill yarn surface.
Damage continues in the fill yarns.
Damage now involves the brick elements next to the wedge elements.
The damage spreads into the matrix.

24. Stresses and Damage Step c c d b a
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Stresses and Damage a b c d
Step c
The wedge elements, where the two perpendicular yarns are close to each other, fail.
Fiber failure occurs in the fill yarn.
Failure occurs where the yarn is curved to the maximum.

25. Stresses and Damage Step d d c b a
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Stresses and Damage a b c d
Step d
Failure extends to the neighboring brick elements up to final catastrophic collapse.
In this final stage different failure modes are activated such as transverse and longitudinal shear, and transverse direct stress.

26. Inter-ply delamination Delamination between orthogonal yarns
COMPTEST Chalons en Champagne G. Nicoletto & E. Riva
Qualitative Correlation
Experimental
Inter-ply delamination
Crack
in fill yarn
Delamination between orthogonal yarns
Computational

27. COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
Conclusions
Optical inspection of a twill-weave laminate during tensile testing showed different damage mechanisms.
Finite element modelling of an appropriate RV provided the macroscopic stress-strain relation of a woven laminate that were compared to experimental results.
The finite element model of the RV provided the microscopic stresses and strains within matrix and reinforcements.
An iterative procedure based on a damage routine has been developed to simulate damage evolution.
A first correlation between experimental observations and computed damage evolution in a twill-weave laminate is encouraging.