SUBCRITICAL INTERLAMINAR CRACK-GROWTH IN FIBER COMPOSITES EXHIBITING A RISING R-CURVE

Subcritical crack growth behaviour has been evaluated in composite lam inates based on uniaxial carbon fibres in poly(ether-ether ketone) mat rices. Double cantilever beam (DCB) specimens have been employed to gi ve model loading and it is first shown that the materials exhibit a ri sing R-curve, i.e. the value of the interlaminar fracture energy, G(1C ), increases as the crack propagates through the specimens. Secondly, when a DCB specimen is held at a constant displacement, subcritical cr ack growth is found to occur. The velocity of the subcritical crack gr owth, v, has been measured using a load-relaxation technique. Hence, v alues of the crack velocity, v, have been obtained as a function of th e strain-energy release rate, G(1), applied during subcritical crack g rowth. Owing to the presence of the R-curve, these data have been meas ured at various stages during the development of the R-curve. The rela tionships between v and G(1) are modelled using power-law expressions. Finally, it is considered that the R-curve behaviour is most likely c aused by the fibre bridging which develops behind the crack tip as the delamination propagates through the specimen. Fibre bridging allows s tress to be transferred across the crack faces, behind the advancing c rack tip, and so results in a ''shielding'' of the stress field at the crack tip from the applied stress. Therefore, the expression ascertai ned for the relationship between the velocity, v, of subcritical crack growth and the corresponding value of G(1) has been further refined a nd modelled to account for the presence of fibre bridging.