Abstract
Uniaxial and reversed torsion fatigue properties of spring steels SUP7, SUP9A and SUP12 are discussed from viewpoints of effects of inclusions on relationship between fatigue strength and hardness, comparing with the results obtained previously for rotating bending fatigue properties.
The conclusions obtained are as follows:
(1) As for uniaxial fatigue under stress ratios of -1 and 0, an inclusion was observed at the crack initiation site and the fatigue strength were smaller than that estimated from hardness.
(2) Uniaxial fatigue strength under a stress ratio of -1 was smaller than rotating bending fatigue strength, as the frequency of existence of nonmetallic inclusions is high under a condition of uniform stress distribution.
(3) As for reversed torsion fatigue, nevertheless no inclusion was observed at the crack initiation site, the fatigue strength were smaller than that estimated from hardness.
(4) Fine correlation between weighted value of inclusions and fatigue strength was not observed for uniaxial and reversed torsion fatigue strength normalized by Vickers hardness.
(5) The estimation curves of the relationship between fatigue strength and Vickers hardness and fatigue limit diagrams taking initial crack size as a parameter are proposed by using fracture mechanics approach.
