THE MIXED-MODE DELAMINATION OF FIBER COMPOSITE-MATERIALS

The present paper is concerned with an investigation of the mixed-mode delamination of polymeric fibre composite materials. Various test geometries have been used to measure the interlaminar fracture energy, G(c), of both thermoplastic and thermosetting carbon fibre composites when subjected to various ratios of mode I to mode II loadings. In particular, the mixed-mode bending (MMB) delamination test has been studied in detail and the results from this test method compared to those obtained from the fixed-ratio mixed-mode (FRMM) test method. Further, for the FRMM results, two methods of partitioning the measured interlaminar fracture energy, G(c), have been employed: namely, by way of a local singular-field approach and by a global method based on a consideration of the applied energy release rates. It is shown that the latter approach is the more appropriate method. Finally, a general criterion for mixed-mode failure is proposed which assumes that a crack loaded with G(I) and G(II) will have an induced mode I component equal to the failure value, termed G0, such that: G0 = G(c)[(cos2(psi - psi0) + sin2 omega sin2(psi - psi0)] where G(c) is the measured fracture energy, psi is the phase angle of the applied loads, psi0 is the phase angle which arises from the elastic mismatch across a bimaterial interface (e. g. the fibre/matrix interface) and where omega can be regarded as the slope of the fracture surface roughness.