Effect of Plastic Strain and Preloading on Delayed Fracture Characteristics of High Strength Steel

Plastic strain takes place at the bottom of the thread, when fastening force is applied to the high strength bolts. The effect of plastic strain on the delayed fracture characteristics was studied using conventional strain rate test (CSRT). For this purpose two kinds of methods were used in the experiments: one is applying uniform plastic strain to the specimen before machining notch and the other is preloading of notched round bar (NRB) specimen. The hydrogen content increased as increasing plastic strain and in contrast, the relationship between fracture stress and hydrogen content using CSRT was independent of plastic strain, which indicates that plastic strain increases hydrogen content and results in decreasing fracture stress based on the above-mentioned relation. On the contrary, the preloading affected the fracture nominal stress obtained by CSRT of hydrogen prechargeci NRB specimen. Using finite element stress analysis, the maximum stress ahead of the notch tip for preloaded NRB specimen was obtained. The relationship between the fracture maximum stress and hydrogen concentration becomes unique irrespective preloading. It is concluded that the relation of local maximum stress the hydrogen concentration at the delayed fracture initiation site is the material constant and control delayed fracture.