Neutron-Diffraction Determination of Residual Stresses in Advanced Composites

Lightweight, high temperature, advanced engineering composites consist of a metal or ceramic matrix that is reinforced with metal or ceramic fibers, whiskers, or particles. The mechanical properties of these composites make them potentially attractive for components in aircraft, space systems, and automobiles, as well as for use in cutting tools and bearings. However, because of differences in the coefficients of thermal expansion of the composite constituents, residual stresses are generated post-fabrication during cooling. Analytical methods for predicting the response of composites to mechanical and thermal loads during use are necessary for optimizing component design and applications. Knowledge of residual stresses is necessary to develop these models. Validation of computer models that predict residual stress distributions by finite-element calculations and that account for creep mechanisms, differential thermal expansion between constitutents, and strains due to transformation is also necessary. X-ray diffraction can be used experimentally to determine surface stresses, but these stresses do not necessarily represent properties of the bulk. Neutron diffraction, on the other hand, can be used to measure bulk elastic strains of the crystalline constituents of composites and does not suffer from the uncertainty associated with varying surface conditions. However, neutron diffraction can measure residual stress only to amore » resolution of about 1 mm; thus, to determine interfacial stresses, analytical models have to be developed to use the average bulk values. The experimental results from neutron diffraction can and have been used to validate theoretical models. In this paper, neutron diffraction technique for measuring strain, analytical models for predicting stresses and strains, and results of neutron diffraction experiments with metal- and ceramic-matrix composites are reviewed.« less