Crack Growth and Threshold Characteristics of Surface Fatigue Crack in Plate Specimen of S 45% Carbon Steel

概要

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Crack growth, threshold and closure characteristics of surface fatigue cracks were examined, using surface notched plate specimens of a 0.45 % Carbon steel with different grain sizes in air and at room temperature. The results obtained are summarized as follows : (1) In the case of constant load range tests, the aspect ratio of a surface crack decreases with flattening of the crack shape as the crack grows, because the bending stress component increases with a decreasing ligament of the specimen. (2) The grain size influences the crack growth rate and the aspect ratio. (3) In the case of decreasing load range tests, the values of threshold ΔK_6lt;th> in the depth direction are about 8 and 4% higher than those at the surface, in the cases of fine and coarse grain size, respectively. This is caused by flattening of the crack shape. The grain size influences strongly the threshold characteristic. In both directions, ΔK_<th> of a specimen having coarse grain size is about 40% higher than that of a specimen having fine grain size.
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