BACKGROUND

• Fiber-reinforced composites can exhibit both high stiffness and strength
• Failure is a complex process accumulation of microstructural damage [1]

OBJECTIVES

• Create a comprehensive micromechanics-based 3-dimensional Finite element model of fiber-reinforced composite unit cells
• Use this finite element model to perform micromechanical analysis of unidirectional and woven composite unit cells

MODELING APPROACH

• Fiber-level modeling of woven composite unit cells is complex and time-consuming due to the 3D architecture of the inter-locking fibers in the unit cell.
• To overcome this complexity, automated model generation is accomplished through Python scripting in ABAQUS.
• Each Fiber is modeled as a separate part which enables modeling of hybrid composites
• Script can generate both woven as well as unidirectional composite unit cells
• Current models assume hexagonal packing
• Script-based modeling provides capability to model more realistic statistically equivalent [3] random fiber-packings

IMPREGNATED TOW WITH BROKEN FIBER

Objective : To study the effect of matrix properties on the stress-redistribution after a single fiber break in a glass fiber-ductile epoxy unidirectional composite
• 3D cylindrical unit cell with a single broken fiber
• Fiber-matrix interphase modeled using cohesive surface-to-surface contact with bi-linear mixed-mode traction-separation law
• Parameters of Mode II traction law are determined from finite element simulations of microdroplet experiment [4]
• Fiber breakage simulated through Boundary conditions, an approach that has been used in previously published works [5-7]

STRESS REDISTRIBUTION AFTER FIBER BREAKAGE

• When one fiber breaks at its weakest cross-section, the neighboring fibers will experience a stress concentration in the vicinity of the break
• If the stress concentrations are sufficiently high, it will lead to breakage of more fibers, ultimately causing complete rupturing of the composite
• Thus, reducing the stress concentrations around a broken fiber can play an important role in enhancing the strength of the composite

RESULTS AND DISCUSSION

Elastic Matrix
• Results are in good agreement with theoretical solution based on shear-lag analysis

Elastic-Plastic Matrix
• A stiffer matrix with lower yield strength leads to the least stress concentration [9]
• Stress concentrations are more localized in the case of matrix materials with higher yield strengths

DRY-FABRIC COMPRESSION

Objective : To verify the use of Kinematic constraints for maintaining periodicity at the boundaries
• Quarter-symmetric unit cell of plain weave fabric
• Model can be used to simulate compaction of dry fabrics
• Analysis illustrates that kinematic constraints can be used to efficiently enforce periodic boundary conditions in woven fabric unit cells

MODELING ROAD MAP

• Script-based parametric 3D Finite Element Model
• Capability to model plain-weave unit cells
• Mixed-mode Interface behavior can be captured
• Thermal residual stresses are accounted for
• Future work:
-Model Random fiber distribution
-Develop a methodology to evaluate Mode I properties of interface
-Add Statistical strength distribution in fibers

REFERENCES

[1] Chou, T.W., Microstructural Design of fiber composites, Cambridge University Press, 1992
[2] Hobbiebrunken et al., Composites Part A, 2006
[3] Vaughan and McCarthy , Composites Sci. Tech., 2010
[4] Sockalingam et al., Composites Part A, 2014
[5] Nedele and Wisnom, Composites Sci. Tech., 1994
[6] van den Heuvel et al., Composites Sci. Tech., 2004
[7] Swolfs et al., Composites Sci. Tech, 2014
[8] Cox, Brit. J. of Applied Physics, 1952
[9] Ganesh et al., Proc. of 29th ASC Tech. Conf., 2014, Submitted

ACKNOWLEDGEMENTS

This work is supported by ARL cooperative agreement number W911NF-06-2-0011.