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
m1 in the distribution (that of the shallower cracks under 40∼50μm) and
m2 (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.
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