RESIDUAL THERMAL STRESS PREDICTION AND MEASUREMENT IN A METAL MATRIX COMPOSITE--GRAPHITE/ALUMINUM (X-RAY DIFFRACTION)

Prediction of residual thermal stresses in fiber-reinforced metal matrix composites generally assumes isotropy of higher thermal expansion coefficients of both fiber and matrix. In the present study, a thick-walled cylinder model for fiber-reinforced composites is extended to show the effect of anisotropy of fiber thermal expansion on thermal stresses. For graphite/aluminum composites, the present model predicts transverse thermal stresses which are 75% lower than stresses calculated for isotropic fiber expansion. When these stresses are examined for incipient plasticity, using the maximum shear stress theory, it is found that anisotropy of fiber thermal expansion shows far significantly larger temperature changes without plastic deformation of the matrix. In fact, with a consideration of viscoelastic effects, anisotropy of fiber thermal expansion can result in the absence of plastic deformation after cooling from processing temperatures. In view of the importance of anisotropy, the interface is included as a third entity to consider its effect on thermal stress. When the fiber has anisotropy of thermal expansion, the interface contributes significantly to thermal stress. Further, interfacial characterization by Transmission Electron Microscopy is done, and local residual stresses are measured by high-resolution X-ray diffraction.