Investigation of delamination and matrix cracking in quasi-isotropic GFRP laminates.

Intra-laminar and inter-laminar cracking in GFRP laminates has been studied. The epoxy matrix used gave a transparent composite and was compatible with polyurethane, enabling a modified system (containing 20% urethane) to be investigated in addition to the standard epoxy. Three stacking sequences of quasi-isotropic laminates, (+45/-45/0/90)s, (0/90/-45/+45)s and (+45/90/-45/0)s were tested. Data were obtained for the growth of damage and its effect on laminate stiffness under increasing quasi-static load and as a function of number of fatigue cycles at two different stress levels. Using the transparent systems enabled a more complete set of damage data to be obtained than by previous workers. The damage comprised 90-ply cracking followed by +45 and -45 cracking and then for (+45/-45/0/90)s [and to a limited extent in (+45/90/-45/0)s], delamination. The initiation and growth of damage was examined with regard to matrix type and stacking sequence. The onset of matrix cracking and delamination are both delayed in the urethane-containing laminates; fracture mechanics tests showed that the urethane system was significantly tourer. Moreover, at a given quasi-static stress or number of fatigue cycles the urethane-modified laminates retain a greater proportion of their initial modulus. The stacking sequence influences interlaminar stresses (thereby controlling delamination) and determines parameters such as ply thickness and neighbouring ply orientation which in turn influence intralaminar cracking. Fracture mechanics has been applied to model the initiation and growth of delamination under quasi-static and cyclic loading using a modified compliance technique. Shear-lag models have been used to determine the stiffness loss due to intralaminar cracking, enabling the stiffness reduction associated with delamination to be deduced empirically. This enables the energy release rates associated with delamination to be derived leading to more sensible results than those obtained using an unmodified technique