Pressure sensitivity and strength-differential effect of fiber-reinforced polymer matrix composites

Based on an energy approach, a simple nonlinear theory for the fiber-reinforced polymer matrix composite is developed. The yield stress and elastic moduli of the polymer matrix are taken to be pressure-dependent, and the ensuring pressure sensitivity and the strength-differential effect of the overall nonlinear response are then investigated. The stress-strain curves of the transversely isotropic composite are calculated at several levels of superimposed hydrostatic pressure, and it is found that the flow stress, except for a pure tension or compression along the fiber direction, can increase markedly under a high pressure. The strength-differential effect, which characterizes the different responses of the material between tension and compression, also exhibits a strong dependence on the loading mode and the applied pressure. The simple theory is shown to be accurate enough to compare favorably with an exact solution; it also yields results which are in close agreement with the experimental data of a grap hite/epoxy system at several selected pressures.