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MEASUREMENT OF ACTUAL STRESSES DURING FATIGUE PROCESS

There are several investigations on the behavior of residual stress during fatigue process by means of x-rays in order to clarify the fatigue fracture mechanism and to predict the initiation of fracturelW4. However, it is difficult to detect the crack initiation from the change in residual stress because no significant change in residual stress occur during the fatigue process. Residual stress generally changes at the initial stage of whole fatigue life but thereafter it does not change until fracture. So it is impossible to detect the initiation of crack from the behavior of residual stress. However, if change in stress by x-rays at the maximum applied tensile load was monitored during fatigue process, we may expect the detection of crack initiation because such stress will be gradually released by the crack opening. In this study, actual stress is defined as stress measured by x-rays at the maximum applied stress position during fatigue process and the actual stress is considered as the sum of residual stress and the applied stress and it will change following the stress relaxation by crack opening. From this point of view, the changes in the actual stress were measured to make clear the relation between the actual stress and the crack initiation. For this purpose, we fabricated a cantilever type rotating bending fatigue testing machine mounted an x-ray stress analyzer based on the single exposure technique (SET). Then we measured both actual and residual stresses by x-rays.