The structural strain method for fatigue evaluation of welded components: closed-form solutions

A consistent and unified analysis framework is proposed in this paper for extending the early traction-based structural stress method to low-cycle fatigue regimes in fatigue evaluation of welded structures through a novel structural strain method. Instead of local strain or notch strain, the structural strain here refers to a through-thickness strain distribution that satisfies plane-remains-plan conditions under elastic-plastic deformation conditions. This paper provides closed-form solutions to structural strain distributions corresponding to loading and unloading, structural strain range, an equivalent structural strain range, and the structural stress-strain relation by considering elastic-perfectly plastic material behavior. Both finite element method (FEA) simulations and in-house modeling tools confirm the accuracy of these closed-form analytical solutions. Although these solutions are derived based on elastic-perfectly plastic material assumption, the resulting structural strain range, as a unique fatigue damage parameter for welded components, is shown essentially the same as those obtained numerically by considering a modified Ramberg-Osgood material model under pulsating loading condition, i.e., stress ratio equals zero. Good data correlation between the equivalent structural strain range predicted results and the test results obtained for a welded steel structure under the same loading condition is achieved. Finally, the structural strain range-based parameter is proven effective in correlating a large number of both low-cycle and high-cycle fatigue test data of welded structures made of several structural materials into a narrow scatter band demonstrating the universality of the structural strain method.