Strain Distribution and Its Relationship to Crack Initiation and Propagation Behavior During Cyclic Bending Test on Commercial SUS301 Sheet

The fatigue crack that developed during the cyclic bending test in commercial SUS301 stainless steel with an inclusion was intermittently observed using the replica method, with the strain distribution around the fatigue crack visualized using the digital image correlation method for the replica films. The cyclic bending test was interrupted after 9000 cycles (N = 9000) when the fatigue crack penetrated the opposite side of the surface. The fatigue crack nucleated from the region where the inclusion was present beneath the specimen surface until N = 10. Cracks nucleated along the high-strain regions and coalesced to cause a large crack until N = 5000. Above N = 5000, i.e., the crack propagation stage, the crack grew independently of the strain distribution. The ease of crack nucleation depended on the crystallographic orientation, with cracks preferentially nucleating from grains in the <112> or <111>//loading direction (LD). The plane of the crack nucleated until N = 10 was ( \(\overline{1}\) $\overline{1}$ \(\overline{1}\) $\overline{1}$ 1), the slip plane in the austenite phase. The Schmid factor for the applied stress of the slip systems with the ( \(\overline{1}\) $\overline{1}$ \(\overline{1}\) $\overline{1}$ 1) slip plane in the grain was low, suggesting that the strain concentration around the inclusion induced the ( \(\overline{1}\) $\overline{1}$ \(\overline{1}\) $\overline{1}$ 1) slip system and the fatigue crack nucleation.