This research was focused on the development of physics-based models to capture the response of metals at high strain rates. We used the Unified Mechanics Theory (UMT) to model different dynamic events. UMT is the unification of the second law of thermodynamics and the second law of Newton. In the UMT, the degradation in the material is represented by the Thermodynamic State Index (TSI), as a function of entropy change.
In the first phase of this research, a UMT-based dynamic equilibrium equation was derived and applied on a mass-spring single degree of freedom system subjected to Coulomb friction. The proposed model replaced the damping coefficient ‘C’ in the Newtonian equilibrium equation with a thermodynamic state function.
Secondly, the UMT was applied to model the low cycle fatigue in metals. Continuum-level analytical and computational constitutive models were developed and applied to predict monotonic tensile and compressive responses and low cycle fatigue life of Ti-6Al-4V alloy.
Finally, micromechanics-based constitutive models for the strain rate-dependent pre-yield and post-yield stress-strain responses of metals were developed. The pre-yield model was extended to include temperature dependence. The predictions using the derived post-yield model are in agreement with the test data for a wide range of strain rates and the model gives insight into the various aspects of micro-mechanisms.