A New Method of X-Ray Measurement of Residual Stress in Short-Fiber Reinforced Plastics

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-Kα radiation by the sinψ 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, 1 m σ and perpendicular to the fiber direction, 2 m σ , and shear stress 12 m τ can be expressed as the functions of the applied (macro) stresses, 1 A σ , 2 A σ , 12 A τ as follows: 1 11 1 12 2 m A A , σ α σ α σ = + 2 21 1 22 2 m A A σ α σ α σ = + , 12 66 12 m A τ α τ = , where 11 12 21 22 66 , , , , α α α α α 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.