INVESTIGATIONS ON THE FIBRE FRACTURE BEHAVIOUR OF CFRP LAMINATES UNDER BIAXIAL LOADING

The present failure criteria for the prediction of fibre fracture in composite laminates are simple maximum stress or maximum strain criteria. They do not consider any possible interaction of transverse stresses or strains. There are theoretical considerations by Hart-Smith and some rare experimental results that there might be a considerable impact of transverse strains on the longitudinal ultimate strain of CFRP. This is an important but presently unanswered question for the safe design of composite laminates. In this work, filament wound tubular specimens are tested under torsion to induce a biaxial stress state in a unidirectional CFRP ply. The specimens are accurately designed with consideration of nonlinear material behaviour by means of analytical methods for thick-walled tubes. The specimen laminate consists of a 45(degree) CFRP test layer and several +,-45(degree) GFRP support layers. A series of longitudinal tensile tests and longitudinal compression tests for one particular strain ratio is carried out. The experiments indicate that there seems to be an impact of transverse strains on the longitudinal strength, but the strength is not cut to half as postulated by Hart-Smith.