A NEW GENERALIZED NONLINEAR SUPERPOSITION THEORY AND ITS APPLICATIONS IN MODELING CORROSION-FATIGUE

Corrosion-fatigue and stress corrosion cracking have long been recognized as the principal degradation and failure mechanisms of materials under combined corrosive environment and sustained/cyclic loading conditions. These phenomena are strongly material and environment dependent, and cycle-dependent fatigue and time-dependent matter diffusion/chemical reaction at the crack tip can be operational simultaneously. How to include these cycle-dependent and time-dependent phenomena in a single model and how to study the failure mechanisms interaction are big challenges posed to material scientists and engineers. In this paper the current linear superposition theories for modeling cycle-dependent and time-dependent corrosion-fatigue and stress corrosion cracking phenomena are reviewed first. Subsequently, a generalized nonlinear superposition theory is proposed to incorporate possible nonlinear interaction or synergistic effect among the underlying mechanisms. The unified model derived from the new theory, depending on the specific materials and loading condition and environment, can be reduced to pure corrosion, pure fatigue, stress corrosion cracking and corrosion-fatigue. Finally, for the first time, a new breakthrough parameter is defined in this paper to quantitatively describe the interaction or synergistic effect between two different operating mechanisms, such as time- and cycle-dependent mechanisms.