The Influence of Cool-Down Temperature Histories on the Residual Stresses in Fibrous Metal-Matrix Composites

Abstract

The vanishing fiber diameter model together with the thermoviscoplastic ity theory based on overstress including a recovery of state formulation is introduced. They are employed to analyze the effects of temperature rate and of annealing at constant temperature on the residual stresses at room temperature when unidirectional fibrous metal-matrix composites are cooled down from 1000°C during the manufacturing process. For the present analysis the fibers are assumed to be transversely isotropic thermoelastic and the matrix constitutive equation is isotropic thermoviscoplastic including recovery of state. All material functions and constants can depend on current temperature. Yield sur faces and loading/unloading conditions are not used in the theory in which the inelastic strain rate is solely a function of the overstress, the difference between stress and the equi librium stress, a state variable of the theory. Assumed but realistic material elastic and vis coplastic properties as a function of temperature which are close to W/9Cr-lMo steel com posite permit the computation of residual stresses. Due to the viscoplasticity of the matrix time-dependent effects such as creep and change of residual stresses with time are found. It is found that the residual stresses at room temperature change considerably with temper ature history. The matrix residual stress, upon reaching room temperature, is highest for the fastest cooling rate, but after 30 days rest the influence of cooling rate is hardly notice able since relaxation takes place. Annealing periods can reduce the residual stresses com pared to continuous cooling.