Takeo Yokobori1, Yozo Sawaki1, Shiro Shono2 and Akiyoshi Kumagai2
1Department of Mechanical Engineering II, Faculty of Engineering, Tohoku University, Sendai
One of the authors has made an investigation of the fatigue microcrack initiation and propagation leading to final fracture in plain carbon steel and low carbon tempered martensitic high strength steel using the plastic replication method and a scanning electron microscope. As a part of this study, 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 arise from the aluminum oxide-type inclusions. The initiation of these microcracks occurs at only 5 ∼ 20% of the total life and its behavior of propagation shows that the microcrack generated from severe stress concentrated inclusion joins the other originating from an other inclusion, but the former is preferentially grown to the degree of considerable size and finally leads to eventual fracture. The fatigue crack propagation towards the inside of the specimen is revealed to occur in the semicircular form with the inclusion as the center.
The fatigue crack propagation rate obeys the power relation, dc/dN=A(Δ K)δ where δ=1.8. Comparison between this result and that of plain low carbon steel and low carbon tempered martensitic high strength steel reveals that the strength level becomes higher the lower the fatigue crack propagation rate. As for the value of δ , it decreases with increasing strength level. This trend is in accord with the kinetic theory of fatigue crack propagation proposed by Yokobori.
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