A unified expression of elastic-plastic notch stress-strain calculation in bodies subjected to multiaxial cyclic loading

In this paper, a simplified thermodynamics analysis of cyclic plastic deformation is performed in order to establish an energy transition relation for describing the elastic-plastic stress and strain behavior of the notch-tip material element in bodies subjected to multiaxial cyclic loads. Based on the thermodynamics analysis, it is deduced that in the case of elastic-plastic deformation, Neuber's rule inevitably overestimates the actual stress and strain at the notch tip. while the equivalent strain energy density (ESED) method tends to underestimate the actual notch-tip stress and strain. According to the actual energy conversion occurring in the notch-tip material element during cyclic plastic deformation, a unified expression for estimating the elastic-plastic notch stress-strain responses in bodies subjected to multiaxial cyclic loads is developed, of which Neuber's rule and the ESED method become two particular cases, i.e. upper and lower bound limits of the notch stress and strain estimations. This expression is verified experimentally under both proportional and non-proportional multiaxial cyclic loads and a good agreement between the calculated and the measured notch strains has been achieved. It is also shown that in the case of multiaxial cyclic loading, the unified expression distinctly improves the accuracy of the notch-tip stress-strain estimations in comparison with Neuber's rule and the ESED method. The unified expression of the notch stress-strain calculation developed in this paper can thus provide a more logical approximate approach for estimating the elastic-plastic notch-tip stress and strain responses of components subjected to lengthy multiaxial cyclic loading histories for local strain approach-based fatigue-crack-initiation life prediction. (C) 2008 Elsevier Ltd. All rights reserved.