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