Experimental observation and modeling of resin pocket cracking in unidirectional laminates with ply discontinuity

Abstract Experimental observations revealed that a resin pocket in a unidirectional laminate with a discontinuous ply may form several cracks prior to initiation of delamination cracks. The observed cracks typically occur within the resin pocket, rather than at the interfaces with the discontinuous ply, and are curved towards the center of the pocket. To study this unintuitive cracking behavior, a simple variational stress analysis based on the principle of minimum complementary energy is developed. It allowed to estimate the stress field in the laminate with a discontinuous ply, with or without cracks in the resin region. The observed cracking pattern is explained by assuming a higher level of residual stresses due to chemical shrinkage of the resin during curing in the confined conditions of the pocket. It is shown that the crack locations are best determined using the principle of maximum energy release rate, while the curved crack path can be governed by the distribution of the maximum principle stress. The predicted cracking sequence and the crack curvatures agree well with experimental observations.