INTERLAMINAR FRACTURE-TOUGHNESS OF ARAMID EPOXY LAMINATES

The Mode I interlaminar fracture toughness of woven aramid-fibre/epoxy laminates has been investigated as a function of the fibre surface properties. Values of G(IC) were found to increase significantly with increasing crack length as a consequence of the presence of fibre bridging in the wake of the propagating crack. The plateau values of G(IC), determined at large crack lengths, were in the range of 0.5-2.5 kj/m2 depending upon the fabric fibre surface treatment. An analysis is presented which allows the size of the fibre bridging zone to be estimated by means of the reduced crack length approach. The fibre bridging zone sizes were calculated to be in the range of 4-25 mm, depending upon the nature of the fibre surface treatment. The zone size was observed to be, in general, proportionally related to the plateau values of G(IC). This analysis supports the contention that a fibre bridging zone, of constant size, progressively propagates itself in the wake of the true crack tip zone, and that a finite energy release rate was associated with the failure of this zone. In addition, the extent of the fibre bridging phenomenon may be influenced by the surface modification of the fabric.