Evaluation of Failure Criteria for Transversely Loaded Unidirectional Model Composites

A leading reason for the limited use of laminated composite materials in primary structural applications is that failure initiates in the ply oriented transverse to the direction of the applied load at a much lower strain than that which would cause the ultimate failure of the laminate. Previous studies indicate that transverse failure is manifested as either cavitation induced brittle failure of the matrix system or fiber-matrix debonding. The mechanism initiating the failure event is not conclusively known and depends on the local stress field of the constrained matrix that is a function of fiber spacing. In this work a model composite system using a transparent matrix is employed in a cruciform shaped specimen to evaluate several transverse failure theories. The cruciform shaped specimen utilizes a brittle 828/D230 RT cured epoxy and stainless steel wires arranged such that a fiber is placed at the intersection of the face diagonals of the four remaining fibers located at the corners of a square. The transverse failure mechanism is observed in situ via the reflected light method and recorded utilizing high resolution, high magnification microscope cameras. Three dimensional finite element models are used to analyze the stress state in the specimen at failure initiation. The results of the 3-D FE models in conjunction with the experimental observations is used to evaluate the transverse failure theories suggested in the literature. In addition this data will be used to develop a comprehensive failure criterion for transversely loaded multi-fiber composites that encompasses the dependence on fiber spacing.