Failure of laminated composites at thickness discontinuities under complexloading and elevated temperatures

Failure initiation of laminated composites with discontinuous thickness has been studied in terms of typical structural load description (tension, she ar force and bending moment) rather than in terms of micromechanics conside rations. Four types of specimens of different stacking sequence were examin ed to determine failure initiation, analyzed subsequently via a finite elem ent analysis (ABAQUS)and divided into two groups that evoke cross-ply failu re, on the one hand, and delamination type failure on the other. For uni-di rectional fiber orientation in the tension direction and across the interfa ce, failure occurs through cracking and delamination. While the initiation strength for this failure mode is significantly higher than for cross-ply c onfigurations, the residual strength after initiation increases only margin ally (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 t han the corresponding failure initiation loads. The data are analyzed in te rms 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 treatm ent of the step "discontinuity". Assuming that time dependent aspects of th e failure process are not dominant, elevated temperatures did not change th e general results of how bending and tension loads interact, provided one a ccounts for stresses induced thermally in the tests; however the magnitude at which the failures occur depends on the temperature, with increasing tem perature leading to decreasing load tolerance. (C) 2000 Elsevier Science Lt d. All rights reserved.