微小疲労き裂の成長に対する線形破壊力学の適用限界とその組織依存性

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The characteristics of small fatigue crack growth were inverstigated under rotating bending on two specimens of a low alloy steel having prior austenite grain sizes of d=15 μm (fire grain) and d=91μm (coarse grain). The influences of grain boudary on the crack growth rate and the aspect ratio of small fatigue cracks were clarified, and the critical crack length above which linear elastic fracture mechanics (LEFM) was applicable, was evaluated for growing small fatigue cracks. When the surface crack length was shorter than 3d, the crack growth rate decreased near grain boundary, and the aspect ratio varied widely by the effect of microstructure. Cracks longer than 3d had no influence of the microstructure, but they grew faster than would be expected based on LEFM untill the lengthj reached to 3d+150μm. This behavior may be attributed to the difference in crack closure between small cracks and large ones. If the contribution of the crack closure to the small fatigue crack growth were to be established experimentally or analytically, the critical crack length would be 3d. However, it is now difficult to evaluate the cracl closure and therefore 3d+150μm is regarded as the critical crack length for engineering application at present.
社団法人日本材料学会の論文
1985-10-15