Delayed Yielding and Hysteresis Phenomenon of Low Carbon Steel under Tensile Fatigue Load

Abstract

Tensile fatigue tests were carried out on plain specimens of annealed S20C steel with two stress conditions of constant minimum and constant mean stresses, and observations were made on the variations of stress-strain hysteresis and permanent fatigue strain with special attention to the occurrence of delayed fatigue yielding. Marked delayed yieldings were observed when the maximum stress σ_max was in the range between the upper and lower yield points of the material, with delayed period ranging from 10³ to about 5×10⁶. This yielding was characterized by a sudden increase of permanent strain ε_f and simultaneous initiation of hysteresis loop. After yieldings, ε_f and Δε_p, the half width of hysteresis loop, showed characteristic variations according to the magnitude of σ_max. Discussions were made on these points in some detail, and also on the relation between the total hysteresis energy and the fatigue life.