Analysis of transverse composite cross-ply laminates

Abstract: Advanced polymer composites are commonly based on thin prepreg plies stacked at different angles to form a laminate. When the laminate is loaded, intralaminar transverse cracking in off-axis plies is one of the first damage modes. In the presented thesis, cross-ply laminates are used to study the transverse cracking phenomenon. Micromechanical modeling was performed in order to predict thermoelastic properties of the laminates as a function of crack density. The stress state between two existing cracks was calculated from two models developed to different degree of accuracy, both based on the principle of minimum complementary energy. As compared with the established model by Hashin, the so called 2-dim 0-model included a non-uniform x-axis stress distribution across the 0-layer thickness, the 2-dim 0/90-model also has a non-uniform x-axis stress distribution across the 90-layer thickness. The predictions of the 2-dim 0/90 model show the best agreement with test data for brittle matrix composites. If the 90-layer is thin as compared with the 0-layer, the 2-dim 0-model shows better agreement as compared with Hashin's model. Models for prediction of the first transverse cracking strain and crack density versus applied load were also developed. Fracture criteria based on linear elastic fracture mechanics were used. A so-called through-the-thickness flaw model showed good agreement with test data in laminates with thin 90-layers. For laminates with thick 90-layers, the failure strains were underestimated. A model with flaw growth in two directions (thickness and width) was therefore developed. The agreement between first transverse failure strain predictions and experimental data was very good for all 90-layer thicknesses. Finally, transverse cracking in [90n/0m]s and [0m/90n]s laminates was compared and the differences in local delamination behaviour were explained based on an analysis of the stress state.