The effect of hydrostatic pressure on the mechanical properties of glass fibre/epoxy unidirectional composites

Abstract

An investigation has been carried out to show the effect of hydrostatic pressure up to 850 MPa on the mechanical properties of pure epoxy and unidirectional (UD) E glass/epoxy lamina. Longitudinal tension, transverse compression and 10° off-axis tension tests were carried out on the composites and compression tests were carried out on the resin. Strain gauges were used to measure the strains during tests at pressures up to 500 MPa. From the longitudinal tensile tests on UD lamina, the longitudinal tensile Young's modulus was found to increase slightly with increasing pressure, while the longitudinal tensile strength was found to fall, with a change in failure mode from axial splitting to a transverse break at elevated pressures. From the transverse compression tests on UD lamina, the transverse compressive Young's modulus and transverse compressive strength were both found to increase markedly with pressure. A knee in behaviour was seen in the transverse compressive strength at a pressure of about 300 MPa: below this pressure the failure appeared flaw dominated, while above this pressure the failure appeared yield dominated. The in-plane UD lamina shear strength, determined from the 10° off-axis tension tests, showed a similar behaviour to the transverse compression tests, with a transition in the failure behaviour at a pressure of around 300 MPa. Compression tests on pure epoxy samples showed that the compressive Young's modulus increased significantly with increasing pressure whilst the compressive strength showed a modest increase with increasing pressure. By using a micromechanical model and the measured pressure dependence of the epoxy resin, it was found that the pressure dependent increase in the transverse modulus for a UD lamina could be predicted accurately.