Estimation of the Fatigue Life under Program Load Including the Stresses Lower than Endurance Limit

Abstract

A method was proposed for the quantitative evaluation of the progress of fatigue damage caused by stresses below the endurance limit. The fundamental assumptions are that the material has a finite life at the stress σ₂ below the endurance limit just as it has a finite life at the stress σ₁ above the endurance limit, when σ₂ is combined with σ₁, and that the accumulation of the fatigue damage is proportional to the number of stress cycles. New S-N curves representing the relation between the stress and the life in the range below the endurance limit were obtained by conducting two-stress single-block and two-stress repeated-block fatigue tests. Program load tests were also conducted, and the lives obtained were compared with the estimated lives based on the above assumptions and the new S-N curves. It was ascertained that the evaluations based on the new S-N curves lead to reasonable estimations of fatigue lives for complex load histories within the range of the experiment.