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, GIc. 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 GIc is significantly smaller than the interlaminar GIc. This will prevent pure interlaminar propagation in multi-directional specimens with high interlaminar fracture toughness.