Failure investigation of graphite/polyimide fabric composites at room and elevated temperatures using the biaxial Iosipescu test

The biaxial and modified biaxial Iosipescu shear test methods were applied to determine the shear dominated, biaxial mechanical response of graphite/P MR-15 and graphite/Avimid-R woven fabric composites at room and elevated te mperatures. Three different composite architectures were examined: T650-35 warp-aligned, 8-harness satin (8HS) fabric in a PMR-15 matrix, T650-35 warp -aligned, 8HS fabric in an Avimid-R matrix and T650-35 0 degrees/90 degrees , 8HS fabric in an Avimid-R matrix. Several biaxial Iosipescu tests were pe rformed at room temperature under shear, shear-tension, and shear-compressi on loading conditions to characterize damage and obtain biaxial, shear domi nated failure properties. Shear tests were also conducted at elevated tempe ratures approaching 316 degrees C to determine the effects of temperature o n the shear strengths of the composites investigated. A nonlinear finite el ement analysis is briefly introduced to evaluate the effects of specimen sl iding and geometric nonlinearities on the stress and strain distributions i n the biaxial Iosipescu specimens. In addition, the effects of different lo ading block geometries on the stress distribution in the Iosipescu specimen s subjected to biaxial loads were also investigated. Within this investigat ion, it was found that graphite/Avimid-R was more resistant to biaxial, she ar dominated failure at room temperature in comparison to graphite/PMR-15. However, the graphite/PMR-15 composite system exhibited better shear streng th properties at elevated temperatures above 232 degrees C. It was also fou nd that the effect of compression along the notch root axis generated by th e loading blocks did not affect the loads at failure.