Nondestructive evaluation of residual stress in short-fiber reinforced plastics by x-ray diffraction

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 skin layers. The stress in the matrix in the skin layer was measured using Cr-Kα radiation with the sin 2 Ψ 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, σ 1 m , and perpendicular to the fiber direction, σ 2 m , and shear stress τ 12 m can be expressed as the functions of the applied (macro-) stresses, σ 1 A , σ 2 A , τ 12 A as follows: σ 1 m = α 11 σ 1 A +α 12 σ 2 A , σ 2 m = α 21 σ 1 A + α 22 σ 2 A , τ 12 m = α 66 τ 12 A , where α 11 ,α 12 , α 21 , α 22 , α 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.