Failure Mechanisms in Twill-weave Laminates: FEM Predictions vs. Experiments by Gianni Nicoletto and...
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Transcript of Failure Mechanisms in Twill-weave Laminates: FEM Predictions vs. Experiments by Gianni Nicoletto and...
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
Outline
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
• Introduction e motivation
• Related works
• Twill-weave laminate chacterization
• Finite element modeling
• Experimental observations and computational results
• Conclusions
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
CFRP Chassis
Cooperation with Dallara Automobili
F3 racing carIRL racing carInfinity Pro
Motivation
Woven Composites
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
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
Objectives of the work
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
• 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.
Related Modeling Work
• Analytical approach
T.W. Chu et al (1983 - )N.K. Naik et al (1991 - )
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
• 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.
• Finite element approach
J. Whitcomb et al. (1990-1998)V. Carvelli and C. Poggi (2001)
D. Blackketter et al. (1993)
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
• 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.
K. Schulte et al (1988 - )J.C. Abry et al. (1998)
Electrical resistance method applied to unidirectional composites
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
Related Experimental Work
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
Material and Experiments
Fiber: Toray T-300 carbon fibersFiber diameter: 7 μmFiber volume fraction Vf: 42%Density ρ: 1.76 g/cm2
Strength u3200 MPaElastic modulus E: 228 GPa
Matrix: Epoxy Hexcel 1990S
LaminateLay-up: 8-plyTexture: Twill-weaveYarns: 3k fibersWarp 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
0° direction
COMPTEST 2003 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 6.15All dimensions in mm
COMPTEST 2003 Chalons en Champaign G. Nicoletto & E. Riva
Tensile Tests and Evolution of Electrical Resistance
Strain
Norm. elect. resistance vs. strain
Stre
ss (
MP
a)
(R-R
0)/R
0
Stress vs. strain
Twill-weave laminates
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
Damage Observations
Fill yarn Tow yarn
Epoxy
Fiber fracture Fracture in yarn
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
Damage Mechanisms: a Summary
• 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
Inter-ply delamination
Crack in fill yarn
Delamination between orthogonal
yarns
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
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
Texture and Representative Volume RV
Material models:
• Yarn: transverse isotropic, linear elastic
• Matrix: linear elastic
RV
Twill-weave
COMPTEST 2003 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: > 30000 elements
• Geometric nonlinearity included
• Progressive damage evolution routine in FORTRAN
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
Boundary conditions on RV
Post, Han and Ifju (1994)
)x(u~xExu)x(u 0
Carvelli & Poggi (2001)
where
u(x) is the displacement field in the RVu0 is a rigid displacement of the RV is a small rigid rotation of RVE is the average strain (macroscopic)
of RVũ(x) is a periodic displacement associated
to microscopic strain field within RV
Free surface
Damage Modes for Fiber Yarn
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
M. Zako et al (2003)
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
Modeling Damage Development
Blackketter et al (1993)Discount method
• 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.
Effect of Texture on Longitudinal Stiffness
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
• 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.
Plain weave
Twill weave
Lamina
Thick laminate
COMPTEST 2003 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.
Stresses and Damage
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
0
200
400
600
800
0 0.4 0.8 1.2 1.6
Strain (%)
Str
ess
(MP
a)
ab
cd 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.
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
0
200
400
600
800
0 0.4 0.8 1.2 1.6
Strain (%)
Str
ess
(MP
a)
ab
cd 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.
Stresses and Damage
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
0
200
400
600
800
0 0.4 0.8 1.2 1.6
Strain (%)
Str
ess
(MP
a)
ab
cd 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.
Stresses and Damage
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
0
200
400
600
800
0 0.4 0.8 1.2 1.6
Strain (%)
Str
ess
(MP
a)
ab
cd 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.
Stresses and Damage
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
Inter-ply delamination
Crack in fill yarn
Delamination between orthogonal
yarns
Qualitative Correlation
Experimental
Computational
Conclusions
COMPTEST 2003 Chalons en Champagne G. Nicoletto & E. Riva
• 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.