MODE-I INTERLAMINAR FRACTURE OF SYMMETRICAL CROSS-PLY COMPOSITES

The composite material structures commonly used in the aeronautical industry are designed with lay-ups of the type [+/- theta]. By contrast with what happens in a unidirectional composite, a delamination defect can exist and propagate between the differently-oriented laminae in a +/- theta composite. Consequently, the strain and stress fields, and also the fracture mechanisms, become more complex. An experimental method and analysis has been used on double-cantilever beam (DCB) specimens of a carbon-epoxy laminate, using strain gauges and acoustic emission to detect the appearance of microcracking at the crack tip and the onset of delamination growth. Experimentally, the strain energy release rates for the initiation cracks and the delamination growth, G(IC) and G(IP), respectively, are very different according to whether the defect is situated between the [0/0], [0/90] or [90/90] layers. Microfractographic analysis shows and explains why the [0/90] and [90/90] layers have higher strain energy release rates than that of the unidirectional [0/0] layers.