ON THE HIGH COMPRESSIVE STRAINS ACHIEVED IN BENDING TESTS ON UNIDIRECTIONAL CARBON-FIBER EPOXY

Specimens of 26 ply unidirectional XAS/913 were tested in a pinned-end buckling rig that applied a combination of bending and compression. Specimens of length 150 mm, 75 mm and 50 mm were used, with the longest ones failing in tension and the shortest by delamination from the ends. Overall failure initiating from the compression side was not found in any of the tests. Very high strains of about 2.5% were measured on the compression surfaces of the 75 mm specimens with no significant damage. It is suggested that the true ultimate compressive strain to failure of the material is in excess of 2.5%. Values of this magnitude are not normally attained in compression tests because of the presence of shear stresses and the tendency for overall buckling to occur as a result of the reduction in tangent modulus at high strain. In bending, the rest of the specimen provides support against overall buckling. Design of carbon fibre/epoxy structures under compression will inevitably be controlled by stability, but where support against buckling is available, it may be possible in some cases to use significantly higher values of allowable compressive strain than those currently accepted.