An engineering model for low cycle fatigue life based on a partition of energy and micro-crack growth

This paper gives some experimental results of low cycle crack growth from artificial through notch in tubular cylindrical specimens of a ferritic stainless steel. Tests were carried out in symmetric tension compression at 300 °C. Tomkins model often used for LCF tests under significant plasticity could not explain the results for the variation in crack length nor the variation in loading parameters. An engineering model based on a partition of energy density into plastic distortion energy density and elastic opening – positive dilation – energy density is proposed for predominant mode I cracking under low cycle fatigue. This energy is computed using a constitutive model with non-linear kinematic hardening. This partition is expected to reflect knowledge of cracking mechanisms on a microscopic level. Functional dependence is assumed to be the same for crack length and each energy contribution. A good description of crack growth tests can be obtained for both rate and crack length variation with the number of cycles. Integration from a grain size can give an estimate of the life of smooth specimens. The influence of number of computation of cycle for a full model with non-linear isotropic hardening is shown to illustrate the robustness of the model.