| This dissertation addresses the problem of determining the thermal residual stresses in unidirectional and woven graphite/PMR-15 composites from the aluminum inclusions embedded between the first and second plies of 6-ply unidirectional and 4-ply woven laminates. The stresses in the inclusions have been measured using X-ray Diffraction (XRD) from 422 aluminum reflections. The experimentally obtained values of the stress field in the embedded inclusions have been subsequently used to calculate the average thermal residual stresses in the composites. The visco-elastic Eshelby method has been used to obtain the residual stresses in the polymer matrix from the stresses in the embedded inclusions. The magnitudes of the thermal residual stresses calculated from the XRD experiments and the Eshelby method have been compared with the results given by the visco-elastic CLPT (inter-laminar model) and the Eshelby method (intra-laminar model). In order to further verify the obtained results, the composite specimens have been subjected to 4-point bending conditions. The total stresses and strains in the Al particles caused by the residual thermal stresses in the composite and the applied stresses generated by a four point bending fixture have been measured by XRD for different bending moments. It has been shown that for small bending displacements the stresses in the embedded inclusions were increasing linearly with the applied bending moment, however for large bending displacements the stresses in the embedded inclusions stabilized, which suggested that the embedded inclusions were deforming plastically. The proposed methodology was also used to investigate the effect of the matrix degradation on the thermal residual stresses. The unidirectional and woven composite specimens have been aged in air and nitrogen for 1064 hours and 1170 hours at 315°C, respectively. The residual thermal stresses in the embedded aluminum particles have been monitored during this experiment. In the last part of this dissertation, the effects of inclusions shape, distribution, and formation of an aluminum oxide film on a surface of the inclusions, on the calculated thermal residual stresses in the composites was investigated. |
