Kinking and compressive failure in uniaxially aligned carbon fibre composite tested under superposed hydrostatic pressure

Abstract

Initiation and propagation of failure in uniaxially aligned 60% volume-fraction Type III carbon fibre-epoxide compressive specimens, strained parallel to the fibre axis, was studied at atmospheric and superposed hydrostatic pressures, H, extending to 300 MN m−2. The atmospheric axial compressive strength was approximately 1.5 GN m−2 and equal to the tensile strength, but mechanisms involving shear-operated failure of the fibres must be discounted since the failure process was very pressure sensitive above H∼ 150 MN m−2. The results also could not be satisfactorily interpreted by theories involving micro-buckling of individual fibres or laminae when the matrix shear modulus controls the compressive strength. For atmospheric tests and for H<150 MN m−2 the initiation of failure was associated with transverse cracking (longitudinal splitting) which was followed by kinking. Ahead of the propagating kink band, groups of fractured fibres were observed, which is consistent with failure of these groups by buckling; this process causes composite catastrophic failure. At higher pressures splitting was suppressed, as was interlaminar cracking in doubly-notched (in-plane shear) specimens, but kinking, which became increasingly more difficult to initiate, was the precursor of the failure process. An attempt was made to analyse failure using the fracture mechanics model of Chaplin with some success for the notched specimens.