Analysis of the Thermoviscoplastic Behavior of [0/90] SCS-6/TIMETAL®21S Composites

Micromechanical modeling is used to determine the stresses and strains due to both mechanical and thermal loads in [0/90] titanium matrix composites (TMCs) subjected to cooldown from the processing temperature and subsequent thermomechanical fatigue (TMF) loading conditions. The [0/90] composite is treated as a material system composed of three constituents: fiber and matrix in the [0] ply, and a [90] ply. The [0/90]_S layup is modeled by a uniaxial stress rule of mixtures model for the [0] ply and adding a parallel element to the [0] model and invoking strain compatibility and stress equilibrium in the loading direction. The fiber in the [0] ply is treated as elastic and the matrix in the [0] ply is treated as viscoplastic with temperature dependent mechanical properties. The [90] ply is characterized as a viscoplastic material including damage from fiber/matrix interface separation. Computations are made for isothermal fatigue as well as in-phase and out-of-phase TMF conditions for the crossply SCS-6/Timetal®21S composite. Effects of frequency and maximum temperature on the composite and constituent stress-strain behavior are evaluated. Fiber/matrix separation and strain ratchetting are found to be important factors in describing the response. Fiber stresses are shown to be dominant in isothermal fatigue at low frequencies as well as under in-phase TMF conditions. Matrix stresses dominate the behavior under high frequency isothermal fatigue and out-of-phase TMF. The use of tenth cycle constituent stresses is shown to be a good compromise between capturing the fully relaxed behavior and computational efficiency.