抄録
In many cases of actual service loads, machine parts are being used for a longer time than 108 cycles. Most of the stress levels in these cases are lower than the endurance limit under a constant amplitude. However, in the case when the minor large-stress and the major small-stress are repeatedly loaded in a random manner, the small stress gives comparatively a large damage on the parts.
The present tests were conducted in the condition that the stress level distribution was similar to the actual one, that is, the minor over-stress and the major under-stress were repeated alternately more than 2×106 cycles. These tests were planned by the subcommittee of Fatigue Strength of JSME, and have been carried out by its members.
The results of these tests have been analysed with the fatigue function using a criterion that the fatigue process consists of both the damaging and strengthening processes. Furthermore, the new endurance limit which has been brought about by the damage under the over-stress repetitions is assumed to be smaller than that under the constant stress amplitude, and the strengthening factor of the damaged materials is assumed to vary largely with the stress amplitude. On the basis of the above processes and assumptions, a new fatigue curve is derived from the original fatigue curve. The fatigue lives of the respective tests are estimated from the new fatigue function which satisfies the new fatigue curve. These estimated values have a good agreement with the test results of the region of not only the finite lives but also the fatigue limits.
