Development of a micromechanics based failure criteria for transversely loaded composite materials

The present work has identified two competing failure initiation mechanisms occurring in a unidirectional model composite system when loaded transverse to the direction of the fibers. Matrix cavitation and fiber-matrix debonding are the failure modes that have manifested themselves as a function of fiber spacing in multi-fiber cruciform specimens. The model composite system used two transparent epoxy systems, a linear room temperature cured 828/D-230 system and a nonlinear high temperature cured 862/W system, with five 0.36 mm diameter stainless steel wires as fibers. The fibers were arranged such that a single fiber was placed at the intersection of the face diagonals of four fibers located at the corners of a square. Seven different fiber spacing groups were tested ranging in volume fraction from 64% to 4%. Failure initiation was optically detected in-situ via the reflected light method using multiple high resolution, high magnification microscope video cameras. Three dimensional (3-D) finite element models (FEM) for all fiber spacing groups tested were used to analyze the stress state in the cruciform specimen at failure initiation. Residual stresses of both epoxy systems were measured by photoelasticity methods for incorporation into the micromechanical FEM. Analytical results of the individual cruciform 3-D FEMs in conjunction with the experimental observations were used to evaluate fiber-matrix debond and matrix failure criteria. A linear interaction debond criterion expressed as the sum of the ratios of the interfacial normal stress to tensile strength and interfacial shear stress to shear strength best validated the observed debond limits at the fiber spacing exhibiting fiber-matrix debonding as failure initiation. For the matrix failure criterion, analytical results indicated that the Mohr-Coulomb criterion validated the fiber spacing exhibiting cavitation. This work has developed failure criteria that correctly identified the two competing failure initiation modes that occurred as a result of the varying internal stress state as a function of fiber spacing. The criteria accurately predicted the observed debonding limits and matrix cavitation of both matrix systems. The detailed understanding and findings of this work will assist the materials engineer by increasing the fidelity of composite solutions to meet the increasing aerospace structural demands.