Estimation of Fatigue Damage by Means of Internal Stress and Effective Stress

A new procedure is demonstrated to estimate fatigue damage from the viewpoint of internal stress and effective stress. Constant effective stress fatigue tests were carried out in order to get the ES-N (effective stress vs. number of cycles to failure) curve, which was similar to the S-N curve in its shape. Based on the ES-N curve, the cumulative damage or the cumulative cycle ratio was estimated using the fatigue test data under the constant stress amplitude and under the manifold multiple repeated stress in two stress levels. The main results are as follows: (1) The relationship between the effective stress and fatigue damage was independent of load history. (2) The values of cumulative cycle ratio for the broken specimens based on the ES-N curve were around 1. On the other hand, the values for the unbroken specimens were small, and those values were saturated in the early stage of their fatigue process. These estimations based on the ES-N curve were superior to those based on the Miner's rule (S-N curve). (3) It is important that the maximum stress in service is held below the critical effective stress (fatigue limit represented by effective stress) in designing a structural member to be used over a long period.