低炭素鋼の疲労微視き裂の観察 : 発生と初期成長の分布特性について

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Rotating bending fatigue tests in the high cycle region were performed on unnotched specimens of low-carbon steel. The initiation and the early propagation stage of micro-cracks were observed and the statistical property of crack depth distribution was investigated. (1) At the fracture surface of stage I cracks, parallel straight lines as the shear mode pattern were observed and the distribution of their initiated angle showed the maximum at the angle division of 40°〜50°. The average aspect ratio b/2a of stage I cracks was about 0.30 and this value was independent of the stress amplitude σ_a and the number of cycles N. (2) From the observation of the longitudinal section of specimens, the places of initiated region of micro-cracks were mostly in ferrite grains, and the ratio of cycles at the initiation was about 20%. The average depth of the initiation cracks (which included the stage I crack and the transient region) was 40〜50μm, being about equal to two grains. (3) The distribution of crack depth showed the composite Weibull distribution which was approximated to two straight lines separated by a separation parameter δ. Shape parameters m_1 in the distribution (that of the shallower cracks under 40〜50μm) and m_2 (that of the deeper ones) were about 0.5 and 1.0, respectively, and their values were independent of σ_a and N. The value of δ was about 50μm as similar to that of the initiation crack depth, and it increased with decreasing σ_a and with increasing average grain size.
社団法人日本材料学会の論文
1982-06-15

低炭素鋼の疲労微視き裂の観察 : 発生と初期成長の分布特性について

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