Influence of Hard Secondary Phase Area on Fatigue Behavior of Carbon Steel Thin Films under Cyclic In-Plane Shear Loading

Fatigue behaviors in thin films and bulk materials exhibit differences, even when they are composed of the same material. Studies indicate that the proportion of crystals exhibiting slip reached saturation at 0.1 under cyclic shear loading. This research aims to elucidate the slip behavior and crack initiation in carbon steel thin films under cyclic shear loading. We examined the influence of carbon content, film thickness, and hardened region on slip and crack formation. Pure iron, S10C, S25C, and S45C were chosen as test specimens to investigate the impact of carbon content. To study the effect of film thickness, we conducted tests with a film thickness to grain size ratio (t/D) of less than 0.80 and greater than 1.00. A Shimadzu TB-10 torsion bending tester was utilized for these tests. The results revealed that in-plane slips and cracks are more likely to occur with a larger volume fraction of the hard phase area, while out-plane slips are more likely with a smaller volume fraction. An increase in the t/D ratio led to an increase in out-plane slips, but did not affect in-plane slips and cracks. This study led to three key observations: in-plane slips occur more frequently when the volume fraction of pearlite or martensite is large, which is thought to be due to strain concentration; out-plane slips occur more frequently when t/D exceeds 1.00, suggesting that they are caused by the deformation of crystal grains in the film thickness direction; and the larger the amount of in-plane slip, the more cracks are observed, indicating that the cracks are a result of in-plane slip.