Fatigue Deformation Preceding Fracture Under Combined Cyclic and Steady Loads

Abstract

In the problems of materials fatigue, the most researches have been made on fatigue brittle fracture. But when both the steady and cyclic loads work on the specimen, the fatigue deformation (creep) occurs and it often becomes larger than that under the static load of the same maximum value. So the behavior of the fatigue deformation must be investigated to clarify the mechanism of the fatigue fracture. In this paper the investigations were made to obtain and consider the fatigue deformation at the room temperature under push-pull loads with various stress amplitudes and various mean stresses on ferrous and non-ferrous metals. Opposed to the ordinary concepts, the permanent plastic tensile strain is observed to be generated even under completely reversed push-pull loads. The stress generating no fatigue deformation is in the range with the compressive mean stress. The fatigue yield point is generally different from the static yield point, and also the value of fatigue deformation under the tensile mean stress is different from the same absolute value of that under the compressive mean stress. It seems to be materials nature. The new relations between stress conditions and fatigue deformations were discovered and represented by a criterion.