Modelling of residual stress relaxation around cold expanded holes in carbon steel

In cases of cold expanded holes, taking residual compressive stresses into account in strength estimation, without considering their relaxation under cyclic load, leads to inaccurate mathematical models for the prediction of fatigue life of structural components. This article presents the outcomes from experimental and numerical investigations of residual stress relaxation around cold expanded holes in medium-carbon steel under cyclic tension. An approach to experimental modelling of the relaxation is proposed. This approach reduces to minimum the volume of the experiment and at the same time provides a relatively simplified mathematical model applicable to engineering practice. A generalized mathematical model of residual stress relaxation has been obtained and it predicts the relaxation as a function of the number of cycles, parameters of the cycle and hole expansion rate. A metallographic analysis has been made for clarifying the physical nature of the residual stress relaxation. On this basis two FE models have been developed for quantitative estimation of the errors from the basic formulations in the study as well as for estimation of the residual stress relaxation. The experimental and the numerical outcomes are in a good agreement.