A Direct,Unified Approach Toward Modeling Multi-axial Fatigue Failure Of Metals And Alloys

In this dissertation,new thermo-coupled finite strain elastoplasticity models with nonlinear combined hardeing are proposed for the purpose of simulating multi-axial thermo-coupled fatigue failure of metals and alloys,and a direct,unified approach toward evaluating the fatigue lives under various cyclic and non-cyclic thermomechanical processes is established based on these new models.As contrasted with usual elastoplasticity models,novelties in the following respects are incorporated in the new models:(i)the new models are of much simpler structure and totally free,in the sense that both the yield condition and the loading-unloading conditions need not be introduced as extrinsic coercive conditions but are automatically incorporated as inherent constitutive features into the models;(ii)the new models ensure a smooth transition from the elastic to the plastic state,thus bypassing the strong discontinuity issue concerning the tangent moduli;(iii)the new models can directly characterize thermo-coupled fatigue failure behavior of metals and alloys,in the sense without involving any usual damage-like variables as well as any ad hoc additional criteria for failure;(iv)both the high cycle and low cycle fatigue of metal materials can be automatically simulated in a unified manner;(iv)the thermodynamically consistency stipulated by the second law may be identically fulfilled,in a further sense that both the specific entropy function and the Helmholtz free energy function may be presented in explicit forms.Novel results in three respects are available directly from model predictions,namely,(i)complex features of thermo-coupled fatigue failure may be automatically represented by simple asymptotic properties of the hardening functions introduced;(ii)critical failure states may be derived with a criterion in unified form,and,in particular;(iii)direct procedures may be established for determining fatigue lives under cyclic and non-cyclic thermomechanical processesNumerical examples for model validation are presented for a number of typical cases of the metal fatigue.Results show that model predictions are in good agreement with relevant experimental data.