X-ray diffraction experiments on aged graphite fiber/polyimide composites with embedded aluminum inclusions

Unidirectional and woven graphite fiber (T650-35)/polyimide (PMR-15) composites with embedded aluminum inclusions were investigated for their aging behavior either in nitrogen or air at 315 °C for up to 1170 h. Residual strains and stresses in the inclusions were determined as a function of aging by performing X-ray diffraction (XRD) measurements. Subsequently, residual strains and stresses in the interlaminar regions of the composites were numerically estimated using the visco-elastic Eshelby/Mori-Tanaka method and classical lamination plate theory (CLPT). It was shown in this work that the residual strains and stresses in the inclusions as determined by XRD were noticeably affected by the aging conditions. For the composite aged in nitrogen good agreement was found between XRD and numerical determinations of residual stresses in the inclusions and in the interlaminar matrix. However, for the composites aged in air significant differences were observed between the experiment and the model. Large amounts of damage to the composites caused by oxidation and the volumetric shrinkage of the PMR-15 resin in air, which were not incorporated into the models, were responsible for the significant differences between the numerical predictions of the residual stresses and those determined from the XRD experiments.