On the Fatigue Crack Propagation in Structural Steels (1st Report)

Abstract

In this paper, it is tried to clarify the behaviour of the fatigue crack propagation in the, central slitted specimens with finite width of structural steels, mild steel, 60 kg/mm² high strength steel (HT 60) and 80 kg/mm² high strength steel (HT 80), under pulsating tension. As a result of the fatigue test, the following facts were repealed:
1) As shown in Fig. 2, S-N curves of mild steel and high strength steels (both of HT 60 and HT 80 are almost same) cross each other at the stress level of about 12 kg/mm², and high strength steels are more advantageous than mild steel in the range of higher stress level but are not at lower stress level.
2) The difference of fatigue strength of the central slitted specimen with finite width is dependent on the fatigue crack propagation process. In mild steel, the incubation phenomenon is very remarkable, as shown in Fig. 7, and it has effect on its fatigue life.
3) "Fourth power law" proposed by Paris et al. can be applied to the behaviour of the fatigue crack propagation in the central slitted specimen with finite width of high strength steels (Figs. 8 & 9). However, it can not be applied to that of mild steel because of a remarkable incubation phenomenon (Fig. 10).
4) The plastic behaviour near the crack tip is related to the fatigue crack propagation. The plastic zone size and the deformation of the crack tip are small in high strength steels as shown in Photo. 2 and the fracture mode would be subjected to maximum tensile component in stress field at crack tip. However, they are very large in mild steel (Photo. 3), and the mode would be related not only to maximum principal stress at the crack tip but also to other stress components, slip deformation, work hardening, etc.