Finite Element Analysis of Biaxial Stress Test Specimen for Graphite/Epoxy and Glass Fabric/Epoxy Composites

Finite element analyses of a diametrally loaded circular test specimen proposed for biaxial stress testing of composite materials are described. Two orthotropic material systems were investigated: unidirectional graphite/epoxy and glass/epoxy woven fabric. For the former, the stress distributions in the gage section were obtained for various loading angles. In general, the stresses did not agree with the uniform stress assumption based on force equilibrium except for the case corresponding to pure shear. Probable failure locations on the specimen boundary rather than in the gage section were identified for all loading angles by using a tensor polynomial failure criterion. Test results for the pure shear loading case confirmed the predicted failure location. Two geometry modifications were investigated as to their effect on this failure location. One of these, consisting of the removal of the small radius at the end of the gage section, warrants further investigation.

Examination of the E-glass/epoxy woven fabric material for the pure shear loading case showed a more uniform stress state in the gage section and the absence of a potential failure point on the boundary. Thus the test specimen shows promise for the determination of shear properties for such materials.