Failure of laminated composites at thickness discontinuities under complex loading and elevated temperatures

Abstract

Failure initiation of laminated composites with discontinuous thickness has been studied in terms of typical structural load description (tension, shear force and bending moment) rather than in terms of micromechanics considerations. Four types of specimens of different stacking sequence were examined to determine failure initiation, analyzed subsequently via a finite element analysis (ABAQUS) and divided into two groups that evoke cross-ply failure, on the one hand, and delamination type failure on the other. For uni-directional fiber orientation in the tension direction and across the interface, failure occurs through cracking and delamination. While the initiation strength for this failure mode is significantly higher than for cross-ply configurations, the residual strength after initiation increases only marginally (10%) beyond the initiation point. For cases involving cross-plies on either side of the interface, failure initiation occurs by matrix cracking. In these cases the residual load bearing capability was 25 to 35% higher than the corresponding failure initiation loads. The data are analyzed in terms of the Tsai-Hill criterion and in terms of an energy release criterion that has been discretized in a manner consistent with a non-singular treatment of the step “discontinuity”. Assuming that time dependent aspects of the failure process are not dominant, elevated temperatures did not change the general results of how bending and tension loads interact, provided one accounts for stresses induced thermally in the tests; however the magnitude at which the failures occur depends on the temperature, with increasing temperature leading to decreasing load tolerance.