Multiaxial Thermomechanical Deformation Utilizing a Non-Unified Plasticity Model 2000-01-0782

The ability to model inelastic deformation is of practical interest in many design applications, especially with the current emphasis on lighter weight structures and components operating at higher temperatures. Thermomechanical deformation poses several challenges in the sense that material properties change with temperature, and at higher temperatures time dependent phenomena such as creep and/or stress relaxation are active deformation mechanisms. Due to recent success modeling many time-independent plastic deformations, recent modifications of the Armstrong-Frederick type plasticity formulations are utilized as a basis for the current model. The choice to implement a non-unified model is based on the notion that distinct or independent mechanisms govern time dependent and independent plastic deformations. The literature also suggests a similar scenario with regard to the damage accumulation. Furthermore, it was deemed desirable to maintain the tensoral nature of both time-independent and “creep” stress-strain behavior especially for more complex multiaxial thermomechanical loadings such as those that may be encountered when analyzing residual stresses resulting from welding. A Sherby-Dorn creep stress power law relationship is utilized to model time-dependent deformation, along with a separate non-translating creep yield surface. Uniaxial isothermal experiments are employed to fit the modeling constants. Uniaxial, torsional and axial-torsional thermomechanical loadings are analyzed with three constraint conditions. Additional isothermal relaxation tests are examined to verify the basic concepts of the model. Omission of an adequate primary creep model is seen to inhibit the predictive capability at some temperature regimes, but the overall qualitative capabilities of the model are in agreement with the experimental results.