The stress in polyphenylene sulfide (PPS) resin reinforced by glass fibers of 26 mass% was measured by the diffraction method using synchrotron with energy of 18 keV. The stress in the matrix was determined by the sin2ψ method with iso-inclination optics of transmitted X-ray diffractions. Tensile coupon specimens were cut from the skin-layer of injection molded plates in three directions, parallel, perpendicular and 45°, to the molding direction. The matrix stresses in the loading and transverse directions of specimens were measured under several uniaxial applied stresses. They changed proportionally to the applied stress. The proportional constants of the matrix stresses to the applied stress were determined, and named the stress-partitioning coefficients. The elastic constant and the matrix stress of the composite were calculated theoretically using micromechanics for two cases of fiber orientation: unidirectional (UD) and varied orientation distribution (VD). Fiber orientation tensor was determined from microscopic measurement of the cross-sectional shape distribution of fibers. Compared with the calculation based on UD fiber orientation, the analytical values based on micromechanics considering the variation in fiber orientation are closer to experimental values of elastic constants and stress-partitioning coefficients.
