VACUUM FATIGUE TESTS OF HIGH STRENGTH STEEL TO SIMULATE SUB-SURFACE FRACTURES

摘要

To investigate the reason for the lower fatigue strength of sub-surface fractures of high strength steel in VHCF regime, this study focused on vacuum-like environment around sub-surface crack and carried out rotating bending fatigue tests in high vacuum. The material used was low-temperature-tempered Cr-Mo steel. Two kinds of small defects (√area = 30μm and √area = 100μm) were processed on specimens to simulate crack initiation site such as an inclusion and to investigate the effect of defect size on fatigue properties. Regardless of defect sizes, vacuum prolonged fatigue lives compared with air. However, the effects of vacuum on fatigue strengths were different between different defect sizes. Fatigue strength of 100um-defect was higher in vacuum than in air whereas that of 30um-defect was lower in vacuum than in air. Namely, fatigue properties of 30um-defect in vacuum resembled those of sub-surface fractures. Based on the evaluation of ΔK_(th) for both defect sizes, it was clarified that the dependency of ΔK_(th) on defect size, which is well known properties in short cracks, was much stronger in vacuum than in air. Conseguently, the lower fatigue strength of sub-surface fractures can be explained by the reduction of ΔK_(th) due to the synergistic effect between vacuum environment and a very small crack initiation site such as an inclusion.