The response of a woven graphite fiber/polyimide composite to aging in nitrogen

The mechanical response of an eight harness satin (8HS) woven T650-35/PMR-15 composite to aging in nitrogen at 315 °C for up to 1000 h has been investigated both experimentally and numerically. The aging stresses in the composite were numerically predicted on a meso-scale using the concept of a unit-cell. Viscoelastic, age stiffening, and volumetric shrinkage effects were considered in the simulations to describe the mechanical behavior of the neat PMR-15 polyimide at 315 °C. It was found from the computations that the internal stresses in the composite increased due to aging and that the rate of the increase was not significant due to stress relaxation. The model has also shown that after 1000 h the aging stresses in the composite are much smaller than the thermal residual stresses caused by manufacturing. To verify the numerical predictions, the strength and stiffness of the composite were measured as a function of aging time using the ±45° tensile test. The numerical predictions agreed with the experimental results, which demonstrated no meaningful influence of aging in nitrogen both on the strength and stiffness of the composite.