Internal Stress and Effective Stress during Fatigue of Steel

A new procedure is demonstrated to comprehend fatigue phenomena. The internal stress and the effective stress in a material are defined to analyze the fatigue process. The internal stress is attributed to the dislocation structure developed during fatigue. From this viewpoint, fatigue tests were carried out and some characteristics such as internal stress, effective stress and hysteresis loop were measured. The internal and effective stresses were measured by means of the strain dip test. The main results are as follows: (1) The internal stress increased with the progress of fatigue. (2) The effective stress obtained during fatigue under a maximum stress larger than the fatigue limit (over-stress) was larger than that under or below the fatigue limit (under-stress). (3) During fatigue under manifold multiple-repeated stress in two stress levels (over-stress and under-stress), both the effective stress and the plastic strain range measured at the under-stress increased by the over-stress loading. (4) The effective stress depended on plastic strain rate. From those results, the concept of internal stress is anticipated to contribute not only to the understanding of fatigue mechanism but also to the establishment of a cumulative damage rule.