THE NONISOTHERMAL VISCOPLASTIC BEHAVIOR OF A TITANIUM-MATRIX COMPOSITE

The thermomechanical fatigue (TMF) response of a unidirectional SCS-6/Timetal 21S composite is determined using the unified inelastic-strain model of Bodner and Partom for the matrix response. The Bodner-Partom model captures the strain-rate sensitivity and time-dependent behavior of Timetal 21S for a wide range of temperatures (25-815-degrees-C) and strain rates (10(-3)-10(-7) s-1). For nonisothermal conditions, special terms are added to the inelastic-strain-rate expressions that account for changes in the temperature-dependent material parameters with changing temperatures. This viscoplastic model is implemented into the finite element package ADINA through user-defined subroutines. The unidirectional composite is represented by a concentric cylinder geometry formulated from axisymmetric elements. Numerical simulations predict an increase in residual stresses as the cooling rate from consolidation is increased. The composite response under in- and out-of-phase TMF loading compares well with experimental measurements. Compared to out-of-phase TMF, in-phase TMF shows considerably more ratchetting. For in-phase TMF, both the response and fatigue lives are more sensitive to variations in fiber-volume fraction than the out-of-phase case. The numerical model is consistent with the experimental observations in that the fibers control the in-phase TMF behavior, while the matrix controls the out-of-phase TMF behavior.