Subcritical interlaminar crack growth in fibre composites exhibiting a risingR-curve

Subcritical crack growth behaviour has been evaluated in composite laminates based on uniaxial carbon fibres in poly(ether-ether ketone) matrices. Double cantilever beam (DCB) specimens have been employed to give mode I loading and it is first shown that the materials exhibit a risingR-curve, i.e. the value of the interlaminar fracture energy,G _IC, increases as the crack propagates through the specimens. Secondly, when a DCB specimen is held at a constant displacement, subcritical crack growth is found to occur. The velocity of the subcritical crack growth,v, has been measured using a load-relaxation technique. Hence, values of the crack velocity,v, have been obtained as a function of the strain-energy release rate,G _I applied during subcritical crack growth. Owing to the presence of theR-curve, these data have been measured at various stages during the development of theR-curve. The relationships betweenv andG _I are modelled using power-law expressions. Finally, it is considered that theR-curve behaviour is most likely caused by the fibre bridging which develops behind the crack tip as the delamination propagates through the specimen. Fibre bridging allows stress to be transferred across the crack faces, behind the advancing crack tip, and so results in a shielding of the stress field at the crack tip from the applied stress. Therefore, the expression ascertained for the relationship between the velocity,v, of subcritical crack growth and the corresponding value ofG _I has been further refined and modelled to account for the presence of fibre bridging.