Mode-I interlaminar fracture of carbon/epoxy cross-ply composites

Mode-I double-cantilever beam (DCB) tests were performed on carbon/epoxy [0°/90°] 12 specimens. The starter crack was created at mid-thickness, between the 0 and 90° mid-layers. During the tests, however, the crack also propagated along the neighbouring 0°/90° interface and within the 90° mid-layer. Nevertheless, the test results were apparently consistent with the assumptions of the corrected beam theory (CBT) that was used to obtain the interlaminar critical strain energy release rate, G Ic. The measured values were higher than those of unidirectional [0°] 24 specimens, especially the final propagation values. A finite-element analysis confirmed the applicability of the CBT for interlaminar propagation along the two 0°/90° interfaces. The results also indicated that the intralaminar G Ic is significantly smaller than the interlaminar G Ic. This will prevent pure interlaminar propagation in multi-directional specimens with high interlaminar fracture toughness.