The X-ray diffraction method is used to measure the residual stress in injection-molded SFRP plates made of crystalline thermoplastic matrix. The stress in the matrix was measured by sin^2 Ψ method using Cr-Kα radiation. According to micromechanics analysis, the matrix stresses in the fiber direction, σ^m_1 , perpendicular to the fiber direction, σ^m_2 , and shear stress τ^m_<12> can be expressed as the functions of the applied (macro) stresses, σ^A_1 , σ^A_2 ,τ^A_<12> as follows: σ^m_1 = α_<11> σA_1 + α_<12> σ^A_2 ,. σ^m_2 = α21 α^A_2 , τ^m_<12> = α_<66> τ^A_<12> . Using skin-layer strips cut parallel, 45° and perpendicular to the molding direction, the stress in the matrix was measured under the uniaxial applied stress. The proportional constants of the above equations were determined experimentally, and compared with the micromechanics prediction. Microscopic phase stresses due to the mismatch of the thermal expansion coefficients between matrix and fiber was negligible in X-ray measurements. A new method is proposed to determine the residual macrostress in SFRP from the X-ray measurements of measurement of matrix stresses.
