Fatigue Life of Circumferentially Notched Bars of Carbon Steel under Cyclic Torsion with and without Static Tension
Keisuke TANAKA, Tomoki ISHIKAWA, Junichi NARITA and Noboru EGAMI
Abstract:Torsional fatigue tests were conducted for circumferentially notched bars of carbon steel (JIS SGV410) under cyclic torsion without static tension (case A) and with static tension (case B). The direct electrical potential method was used to monitor the initiation and propagation of cracks from the notch root. For both cases A and B, the lives of crack initiation, Nc, and propagation, Np, decreased with increasing stress concentration. The ratio of Nc to the total life, Nf, was smaller for sharper notches under lower stress amplitudes, and was larger when the static tension was superposed. The fatigue fracture surfaces under high stress amplitudes for case A was flat with rubbing marks, and that under low stress amplitudes was factory-roof shaped. The electrical potential change in one cycle indicated the contacts of fracture surfaces, which will cause the retardation of crack propagation. For case B, the fatigue@fracture surfaces became flatter, and no evident retardation was observed in crack propagation. Contrary to the case of austenitic stainless steel (JIS SUS316L), the amount of crack retardation due to fracture surface contact in SGV410 was much smaller and the notch-strengthening did not take place. The difference in the crack propagation path at the notch root is responsible for the amount of crack surface contact, resulting in notch-weakening for SGV410 and notch-strengthening for SUS316L steel for case A. Key Words:Fatigue, Notch effect, Carbon steel, Torsional fatigue, Crack initiation, Crack propagation