The stress applied to a specimen does not fully act to deform material. The effective stress which actually acts on dislocations can be related to the internal stress of material. The origin of internal stress and thus effective stress has been discussed here by means of a dislocation model, concluding that internal stress depends on the dislocation structure. Based on this conclusion, fatigue tests were carried out and the following results were obtained:
(1) The behavior of internal stress in the fatigue process showed a clear difference between the over-stressed and the under-stressed conditions.
(2) The fatigue failure occurred under many-fold multiple repeated stress in two stress levels below the fatigue limit. Such fracture was attributed to the instability of internal stress.
(3) The concept of internal stress and effective stress is useful to the comprehensive understanding of fatigue phenomenon.
