Measurement of Residual Stresses in Fiber Reinforced Composites Based on X-Ray Diffraction

1. ABSTRACT X-ray diffraction (XRD) m easurements have been made to determ ine residual stresses in embedded Al particles placed between the first and second ply of both unidirectional and fabric graphite/PMR-15 composite specimens. In order to further verify the results obtained, the effect of externally applied four-point bending on the measured strains and stresses in the Al inclusions was investigated. In the modeling part of this research, the state of stress in these inclusions was calculated using visco-elastic laminated plate theory and the Eshelby method. The numerical solutions for different concentrations of inclusions were subsequently compared with the experimental results. It has been shown that the thermal residual stresses in graphite/polyimide composites can be determined with reasonable accuracy by using X-ray diffraction from embedded inclusions in conjunction with the application of the Eshelby method. 2. INTRODUCTION The therm al residual stresses in composite materials are caused by the mi smatch between coefficients of thermal expansion (CTE) of the matrix and the reinforcements. The magnitude of these residual stresses depends on the difference of CTE between the matrix and the reinforcements multiplied by the temperature change ∆T. Because of the fact that the CTE for the polymer is higher than the CTE for the graphite fibers, the generated thermal residual stresses are tensile in the matrix and compressive in the fibers. The proper evaluation of these tensile stresses in the matrix becomes particularly important when high temperature composites are considered since ∆T for these types of composites is high enough to generate residual stresses on the order of the tensile strength of the polyimide resin. Residual stresses inside embedded crystalline inclusions in polymer m atrix composites were first measured by Predecki and Barrett [1] by performing XRD measurements. It was found that residual and applied strains transferred to various crystalline inclusions were high enough to be detected by X-ray diffraction. A new methodology for the evaluation of residual stresses in the polymer matrix of unidirectional polymer matrix composites based on XRD measurements of residual stresses in embedded crystalline inclusions has been recently developed by Benedikt et al. [2-3]. First, the residual thermal stresses in a unidirectional graphite/PMR-15 polyimide composite were determined by performing visco-elastic computations assuming interlaminar and intralaminar composite architectures. Second, the residual stresses in the Al inclusions were obtained through XRD measurements and from the application of the visco-elastic Eshelby method for multiple ellipsoidal inclusions. The residual stresses in the polymer matrix were extracted from the X-ray strains in the inclusions and subsequently compared with the strains obtained from the interlaminar and intralaminar analyses. The newly developed methodology has also been applied for the determination of the residual thermal stresses in 8 HS fabric graphite/PMR-15 composites, however, in the case of fabric composites, only interlaminar