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

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.