The behavior of high-strength unidirectional composites under tension with superposed hydrostatic pressure

Three types of high-strength unidirectional composite materials were studied under longitudinal tension with superposed high hydrostatic pressure. Reinforcing fibers were T1000G carbon, S2 glass and Zylon PBO fibers; the Ciba 5052 epoxy resin was used as matrix. The composites were tested under external pressure varying from atmospheric pressure up to 500 MPa. It was found that the longitudinal moduli of elasticity of tested composites were practically pressure independent (within the pressure interval examined). By contrast, the longitudinal tensile strength showed clear dependence upon superposed hydrostatic pressure. The pressure dependence of the tensile strength appeared to be of a non-monotonic character: The segments of increasing and decreasing strengths were present in the plots. It was found that the magnitude of the superposed hydrostatic pressure significantly affected the failure mode of unidirectional composites. The tests showed that there existed a tendency for the localization of the failure zone with increasing hydrostatic pressure for all three composites, i.e. one could observe the change from dispersed failure modes uniformly distributed within the whole volume of the test specimen to failure modes of localized types.