Tests are conducted using stitched laminates in order to calculate the unnotched failure stress and the compression after impact (CAI) strength of the stitched laminates. Unnotched laminates, laminates with a hole, and laminates with impact damage are used in the tests. All the specimens are made of two different types of layup sequence of laminates with three different stitching directions - 0, 45, and 90. In this paper, an FEM model is developed using the MSC PATRAN software to predict the CAI strength of stitched laminates. Two failure criteria are applied to the model to predict the CAI strength, the point stress criterion (PS) and the fiber breakage in damage zone (FD) criterion. The influence of the moduli retention ratio (Mr) is investigated. The predicted CAI strengths show a good agreement when compared with the test results.

Tests were conducted with stitched laminates in order to calculate CAI (compression after impact) strength and TAI (tension after impact) strength for stitched laminates. Two levels of low velocity impact energy were used. All the specimens were made of two different types of lay-up sequence laminates with three different stitching directions of 08, 458, and 908. FEM model was developed using MSC/PATRAN software to predict the CAI and TAI strength of stitched laminates. PS criterion (point stress) and FD criterion (fiber breakage in damage zone) were applied to predict CAI strength. It showed that the predicted residual strengths were in agreement with the test results. Stitching improves the residual strength of the laminates.