FATIGUE FAILURE AT STRESSES BELOW FATIGUE LIMIT

In the present study, the authors have focused attention on the internal stress in a material. A stress applied to a specimen does not fully act to deform the material. The effective stress which actually acts on dislocations depends on the internal stress in the material. The origin of the internal stress is attributed here to the dislocation structure. From this viewpoint, fatigue tests were carried out and the following results were obtained: (1) The origins of the internal stress and the effective stress were clarified by means of a dislocation model. (2) The behaviour of the internal stress during the fatigue process showed clear differences between over-stressed and under-stressed states. (3) The concept of internal and effective stresses assists in our understanding of the fatigue phenomenon. (4) The mechanism of fatigue failure in the under-stressed fatigue state was also studied, and the concept of an internal stress was applied to instances of practical fatigue situations.