Residual stresses in unidirectional graphite fiber/polyimide composites as a function of aging

In this research, the magnitude and distribution of residual aging stresses in unidirectional graphite fiber/polyimide composites were investigated. First, a viscoelastic aging material model was developed by characterizing the isothermal aging strain and stress relaxation of neat PMR-15 polyimide. Then, the aging material model was applied to numerically evaluate the effect of matrix aging and cooling on the residual stress state of a unidirectional graphite fiber (T-650)/polyimide (PMR-15) composite. It has been shown in this work that residual stresses in the composite are significantly affected by high temperature aging. Most importantly, the magnitude of the residual stress caused by aging in N2 at 316 °C for one month was found to be comparable to the cooling stress in the composite.