THE INFLUENCE OF COOL-DOWN TEMPERATURE HISTORIES ON THE RESIDUAL-STRESSES IN FIBROUS METAL-MATRIX COMPOSITES

The vanishing fiber diameter model together with the thermoviscoplasti city theory based on overstress including a recovery of state formulat ion is introduced. They are employed to analyze the effects of tempera ture rate and of annealing at constant temperature on the residual str esses at room temperature when unidirectional fibrous metal-matrix com posites are cooled down from 1000-degrees-C during the manufacturing p rocess. For the present analysis the fibers are assumed to be transver sely isotropic thermoelastic and the matrix constitutive equation is i sotropic thermoviscoplastic including recovery of state. All material functions and constants can depend on current temperature. Yield surfa ces and loading/unloading conditions are not used in the theory in whi ch the inelastic strain rate is solely a function of the overstress, t he difference between stress and the equilibrium stress, a state varia ble of the theory. Assumed but realistic material elastic and viscopla stic properties as a function of temperature which are close to W/9Cr- 1Mo steel composite permit the computation of residual stresses. Due t o the viscoplasticity of the matrix time-dependent effects such as cre ep and change of residual stresses with time are found. It is found th at the residual stresses at room temperature change considerably with temperature history. The matrix residual stress, upon reaching room te mperature, is highest for the fastest cooling rate, but after 30 days rest the influence of cooling rate is hardly noticeable since relaxati on takes place. Annealing periods can reduce the residual stresses com pared to continuous cooling.