The Fatigue Crack Propagation Mechanism of the Annealed and the Surface-Rolled Low Carbon Steels

Abstract

In order to clarify the fatigue crack propagation micromechanisms of the annealed and the surface-rolled low carbon steels, the microscopic observation near fatigue crack tip in both the materials have been performed by the use of the optical microscope, the X-ray microbeam technique and the electron microscopic replica method, and the fatigue propagation micromechanisms have also been discussed.The results are summarized as follows:(1) The fatigue crack propagation rate in the surface-rolled specimen is slower than in the annealed specimen.(2) It is found out from the optical microscopic observation near the fatigue cracks that the crack is fairly sharp in the annealed specimen, but broad in the surface-rolled specimen. This fact will show the formation of ramification of microcracks at the tip of the main fatigue crack in the surface-rolled specimen.(3) From the results of the X-ray microbeam diffration pattern and of the electron microscopic replica at the crack tip in the annealed and the surface-rolled specimens, the microstructure at the crack tip in crack both the specimens is similar and this microstructure shows distinct substructure, and the fatigue crack is propagated by jointing the microcracks which have been formed by jointing the pores in the plastic region in front of the crack tip.(4) Thus the fatigue crack propagation mechanism is the same in its basic nature in common with both the specimens, and the fatigue propagation will be considered as continuation of the crack initiated and accelerated by the stress concentrated at the crack tip.(5) The propagation of fatigue crack takes place more slowly in the surface-rolled specimen than in the annealed specimen. What is to account for the difference? The slower propagation of fatigue crack in the surface-rolled specimen than in the annealed specimen is to be traced ultimately to the difference in their initial microstructure hence the slower formation of substructure at the crack tip region of the surface-rolled specimen than of the annealed specimen. Then there occurs crack ramification in the surface-rolled specimen which goes to relax stress, hence the tendency to retard the propagation of crack, in other words, to make its propagation more slowly than it is in the annealed specimen.