Loading-frequency effects on fatigue crack growth behavior of a low carbon steel in hydrogen gas environment

In this study, loading frequency effects of a low carbon steel (JIS S10C) on the fatigue crack growth rate were investigated. In the case of many other metallic materials, the fatigue crack growth rate usually increases as loading frequency decreases. However, in this case fatigue crack growth rate shows rather complex behavior. There are both cases; acceleration and deceleration in spite of the low frequency. In this case, the decrease of fatigue crack growth rate at low strain rate could be explained by decrease of the area ratio of quasi-cleavage. To estimate the loading frequency effect of this material, it is necessary to consider the effects on the ductile fracture and on the brittle-like fracture. At ductile fracture area, crack is accelerated by low strain rate similar to many other materials. At the brittle-like area, it is considered that loading frequency affect forming the starter of brittle-like fracture and propagation of brittle striation. One of the possible effects of loading frequency is as follows. Based on the experimental results, brittle striation forming process starts opening crack tip by slip deformation and blunting. After that the crack tip starts propagating. Propagation process is similar to ductile tear fracture. Low strain rate makes slip deformation easier and helps blunting. Hydrogen also can help blunting. Low strain rate and load holding makes brittle striation becomes harder to start propagation. Therefore, quasi-cleavage fracture decreases. At brittle striation propagating process, slip deformation seem to occur. Therefore, it is possible that propagation process has load frequency effect. However, in this case, these did not appear clearly. The crack growth rate at low strain rate is determined by the mutual relationship between the brittle like fracture and ductile fracture. However, the study in the wide range of the condition should be conducted in the future.