Thermodynamically Consistent Viscoelastic-Viscoplastic Models for Fatigue and Fracture

Materials operating in extreme environments, such as during space launch, re-entry, high speeds, or in the space environment experience quasi-conservative and dissipative stresses that cannot be modelled using elastic material behaviour. Instead, a thermodynamically consistent formulation is required that can account for the dynamic, mechanical, chemical and thermal loading experienced by the body. A continuum mechanics based approach is implemented using internal variables to account for the dissipative effects. The internal variables are solved through the application of thermodynamic orthogonality. A further issue for materials in extreme environments is the formation of cracks, deformed regions, and chemical changes. These aspects can be captured using multiple order parameter formulations of the continuum mechanics models. The internal variables and order parameters provide the necessary inputs to fully solve the resulting system of coupled partial differential equations. This approach is applied to a representative solid rocket propellant motor experiencing launch, and repetitive, on-orbit starts and stops.