Difference in Low Cycle Fatigue Strength by Strain Controlled Axial Load and Deflection Controlled Bending Load

Abstract

The ratio of fatigue strength of deflection controlled bending load cycling to that of diametral strain controlled axial load cycling was investigated in the low cycle fatigue life range by testing four kinds of steel. Cyclic characteristics of surface strain at the test section of a bending fatigue specimen were measured by strain gauges, which provided good results about strain amplitude. Fatigue test results were plotted in the relation between total strain amplitude and visible crack initiation life, and a general tendency was observed that the bending fatigue strength was the same order of or higher than the axial fatigue strength. The fatigue strength ratio was increased with the increasing crack initiation life, showing that the maximum value of the ratio was 1.45 for a mild steel, 1.17 for a HW 50 steel, 1.26 for a HW 70 steel and 1.56 for a 9% Ni steel. By applying a concept of cyclic hysteresis energy to an effective volume from a surface of a bending specimen, an analysis of the fatigue strength ratio was made with an assumption that bending and axial load fatigue specimens should be cracked at an equal number of cycles when equal hysteresis energy is accumulated in both specimens.