A New Method of X-Ray Measurement of Residual Stress in Short-Fiber Reinforced Plastics
Keisuke TANAKA, Shohei TOKORO, Yuuki KOIKE, Noboru EGAMI and Yoshiaki AKINIWA
Abstract:The X-ray diffraction method is used to measure the residual stress in injection-molded plates of short-fiber reinforced plastics (SFRP) made of crystalline thermoplastics, polyphenylene sulphide (PPS), reinforced by carbon fibers with 30 mass%. Based on the orientation of carbon fibers, injection molded plates can be modeled as three-layered lamella where the core layer is sandwiched by two skin layers. The stress in the matrix in the skin layer was measured by Cr-K radiation by the sin2 method. Since the X-ray penetration depth is shallow, the state of stresses measured by X-rays in FRP can be assumed to be plane stress. The X-ray measurement of stress in carbon fibers was not possible because of high texture. A new method was proposed to evaluate the macrostress in SFRP from the measurement of the matrix stress. According to micromechanics analysis of SFRP, the matrix stresses in the fiber direction,1m and perpendicular to the fiber direction,2m , and shear stress 12m can be expressed as the functions of the applied (macro) stresses,1A,2A,12A as follows:1m=111A+122A, 2m=211A+222A,12m=6612A,where 11,12,21,22,66 are stress-partitioning coefficients. Using skin-layer strips cut parallel, perpendicular and 45 to the molding direction, the stress in the matrix was measured under the uniaxial applied stress and the stress-partitioning coefficients of the above equations were determined. Once these relations are established, the macrostress in SFRP can be determined from the measurements of the matrix stresses by X-rays. Microscopic phase stresses due to the mismatch of the thermal expansion coefficient between matrix and fiber was negligible in X-ray stress measurement of the skin layer. Key Words:Residual stress, X-ray stress measurement, Short-fiber reinforced plastics, Fiber orientation, Stress-partitioning, Coefficient, Micromechanics