The present paper is concerned with an investigation of the mixed-mode
delamination of polymeric fibre composite materials. Various test geo
metries have been used to measure the interlaminar fracture energy, G(
c), of both thermoplastic and thermosetting carbon fibre composites wh
en subjected to various ratios of mode I to mode II loadings. In parti
cular, the mixed-mode bending (MMB) delamination test has been studied
in detail and the results from this test method compared to those obt
ained from the fixed-ratio mixed-mode (FRMM) test method. Further, for
the FRMM results, two methods of partitioning the measured interlamin
ar fracture energy, G(c), have been employed: namely, by way of a loca
l singular-field approach and by a global method based on a considerat
ion of the applied energy release rates. It is shown that the latter a
pproach is the more appropriate method. Finally, a general criterion f
or mixed-mode failure is proposed which assumes that a crack loaded wi
th G(I) and G(II) will have an induced mode I component equal to the f
ailure value, termed G0, such that: G0 = G(c)[(cos2(psi - psi0) + sin2
omega sin2(psi - psi0)] where G(c) is the measured fracture energy, p
si is the phase angle of the applied loads, psi0 is the phase angle wh
ich 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.