論文ID: 19-00131
Aiming at an enhanced fatigue strength of various metal materials, surface treatment named cutting and rubbing process was developed. The surface of processed specimen of a quenched and tempered SCM435 had an extremely large value of compressive residual stresses (-2100 to -2600 MPa). Although the compressive residual stresses significantly decreased during an early stage of stressing, it tended to saturate to -1800 MPa before the repetitions of N ≈ 4×104 independent of stress values, even after a longer fatigue regime in excess of N = 107. Fatigue limit stress of the processed specimens was enhanced to approximately 1.5 times of a smooth specimen finished with #1000 emery paper, and most of the fracture surface origin exhibited fisheye pattern. Fatigue tests on the processed specimens with reduced residual stresses due to the heat treatment showed that the enhanced fatigue strength by the processing was attributed to the effect of compressive residual stresses rather than that of the processing- induced hardened-layer. The actual fatigue limit was much higher than the evaluated value of 500 MPa calculated from the fatigue limit prediction equation by Murakami, suggesting a greater contribution of compressive residual stresses near the surface layer on non-propagating internal fatigue cracks. In order to investigate the effect of residual stress on non-propagation of internal fatigue cracks, the monitoring of internal cracks in the specimens fatigued by N = 5×107 cycles of the fatigue limit stress was conducted, showing no internal cracks in the specimen interior in a range of the present experiment. The internal cracks should be generated at the stresses above the expected fatigue limit of 500 MPa, suggesting the formation of non-propagating internal cracks under the actual fatigue strength of 710 MPa. It is unclear that a mechanism of higher fatigue limit (710 MPa) than an expected fatigue limit (500 MPa) calculated using parameters obtained from their crack origin. The extremely high compressive residual stresses over the surface may be a cause of non-propagation of internal cracks, whereas the present rough monitoring could not detect the internal cracks.
