Initiation and Propagation of Fatigue Crack in High Strength Eutectoid Steel

Abstract

One of the authors (T. Yokobori) has made an investigation of the fatigue microcrack initiation and propagation leading to final fracture in plain low carbon steel and in low carbon tempered martensitic high strength steel using the plastic replication method and a scanning electron microscope. As a part of this research program, the present investigation was undertaken to make clear the nature of fatigue crack in high strength eutectoid steel.
It is found that almost all the fatigue microcracks are initiated from the aluminum oxide type inclusion. The initiation of these microcracks occurs at only 5–20% of the total life and they propagate in such a process that the microcrack initiated from a high stress concentrating inclusion joins another one originated from other inclusions, in which case the former grows predominantly to considerable size and finally leads to eventual fracture. The part through fatigue crack grows as a semicircular configuration with the inclusion as the center.
The fatigue crack propagation rate obeys the power relation, dc⁄dN=A(ΔK)^δ, where δ=1.8. Comparing this result with those of plain low carbon steel and low carbon tempered martensitic high strength steel, the higher the strength level, the lower the fatigue crack propagation rate. The value of δ decreases with increasing strength level. The trend is in good accord with the kinetic theory of fatigue crack propagation proposed by Yokobori.