STRESS-CONCENTRATION PHENOMENON IN GRAPHITE EPOXY COMPOSITES - TENSION/COMPRESSION EFFECTS/

In composite materials, fiber/fiber interaction and the resulting stre ss concentration phenomenon due to a fiber failure are crucial in dete rmining the composite fracture behavior. A number of mathematical mode ls have been developed for calculating the stress concentration factor (SCF) and predicting its dependence on the properties of the composit e constituents. None of the existing models separately considers the c ase of tension and compression. This could be due to the force-balance approach used, which assumes the phenomenon to be the same in both ca ses. In this study, micro-Raman spectroscopy (MRS) was used to investi gate the stress concentration phenomenon and measure experimentally th e stress concentration factor in graphite/epoxy composites subjected t o tensile and compressive stress. The results showed that the stress c oncentration in tension is different from that in compression. The rad ius of the zone of influence is much higher when the composite is unde r compressive stresses. The maximum SCF in both cases, however, was 1. 5 +/- 0.05, limited by the ability of the interface to transfer shear stresses. The difference in stress concentration is attributed primari ly to the difference in the interfacial behavior of the composite unde r tension and compression. The results are discussed and explained fro m both fracture mechanics and energy viewpoints. (C) 1997 Elsevier Sci ence Limited.