Measurement of Residual Stresses in Metal Matrix Composites

Metal matrix composites (MMC) are expected to develop internal residual stresses on cooling from fabrication due to the large thermal expansion mismatch between reinforcing fibers and the matrix. This work was undertaken to measure experimentally these residual stresses and compare them with analytical calculations in order to establish clearly their levels and dependence on material parameters. Two techniques for residual stress measurement were investigated: (1) X-ray diffraction (sin²-psi method) and (2) neutron diffraction. Both techniques gave results in good agreement with analytical predictions for several systems (SCS-6/Ti-24Al-11Nb, W/NiAl, and Al₂O₃NiAl). The results obtained showed a dependence of residual stresses on thermal expansion coefficients, elastic moduli, volume fraction fibers, and matrix yield strengths. The fibers showed compressive stress states, and the matrix, tension. Average stresses were higher in the fiber direction than transverse to fibers.