Evaluation of Mode I interlaminar fracture toughness in asymmetric interlayer in CFRP laminates

Assuming the interlaminar failure in quasi-isotropic laminates, we evaluated the Mode I interlaminar fracture in asymmetric 0 degrees/45 degrees interlayer. To consider coupling effects caused by stacking sequence, we prepared two types of laminates with 0 degrees/45 degrees interlayer: one is that all coupling components are zero, and the other has non-zero D-16 and D-26 components. As for comparisons, we used unidirectional laminates and angle-ply laminates in which the midplane consisted of a +/- 22.5 degrees interlaminar. The laminates with the 0 degrees/45 degrees or +/- 22.5 degrees interlayers exhibited higher energy release rates than that of the 0 degrees/0 degrees laminates, owing to significant bridging fibers. It was experimentally confirmed that a failure in asymmetric interlayer, which has non-zero coupling components like typical quasi-isotropic laminates, did not propagate straight along a fiber direction on either side, but was distorted in fan-like shape by coupling effects.