The Fatigue Crack Propagation Behavior of the Annealed and the Surface-Rolled Low Carbon Steel

Abstract

In order to clarify the behaviors of low carbon steels, the annealed and the surface-rolled, in their propagation of fatigue crack, the microfractographic observation of the specimens both of these descriptions of materials, has been performed by the electron microscopic replica method, with particular respect to their fatigue fractured surfaces. The results of the observation have been fully discussed, and the macroscopic investigation has been made of the specimens concerning their behaviors in propagation of fatigue crack.The results are summarized as follows:(1) The fatigue crack propagation rate in the surface-rolled specimen is undoubtedly slower than in the annealed specimen.(2) The spacing in striation on the fatigue fractured surface in both the sorts of specimens is larger than what was first estimated from the fatigue crack propagation rate, dl/dN. This fact shows that there is the latent period in the fatigue crack propogation process in both the sorts of specimens.(3) The various patterns observed on the fatigue fractured surfaces are similar in both the sorts of specimens, therefore the fatigue fracture mechanism in both the sorts of specimens is considered to be essentially the same.(4) The platy on the fatigue fractured surface in the surface-rolled specimen is smaller in size than that in the annealed sort, and the striation on the platy in the surface-rolled specimen runs in locally different direction from the main crack propagation line. It can be considered from these facts that in the surface-rolled specimen there have been formed, in consequence of micro internal stress in the work hardened surface layer, branched microcracks at the tip of the main fatigue crack.(5) The slower fatigue crack propagation rate in the surface-rolled specimen is due to the longer latent period that has come, so it is considerd, from the structural difference between the annealed specimen and the surface-rolled specimen, and also from the stress relaxation at the branched crack tip in the surface-rolled specimen.