Fatigue Crack Initiation and Propagation Behaviors in Rotating Bending of SNCM439 Steel in Very High Cycle Regime

In order to ensure the long term durability of mechanical structures, the fatigue property of structural components should be clarified in the long life region such as the gigacycle regime. The rotating bending fatigue tests in very high cycle regime were carried out for a nickel chromium molybdenum steel for structural use of machines (JIS Material Code: SNCM439) in this study. Based on the initiation site of the fatigue crack, fracture modes were classified into the following typical three modes: (1) usual surface fracture, (2) surface defect-initiated fracture and (3) interior inclusion-initiated fracture, respectively. InS-Ndiagram, experimental data in the usual surface fracture mode appeared at higher stress levels with fewer loading cycles, whereas the data in the other two fracture modes appeared at lower stress levels with more loading cycles. Thus, the duplexS-Nproperty was confirmed for this steel in the very long life regime. In order to clarify the fatigue mechanism of the interior inclusion-initiated fracture, the quantitative evaluations were made by applying the stress intensity factor range. The fatigue crack initiation and propagation processes in the interior inclusion-initiated fracture were divided into four stages: formation of the fine granular area (FGA) due to initiation and coalescence of micro-debondings, formation of the fish-eye due to penny-shape crack propagation, crack propagation as surface crack and final catastrophic fracture.