Sakae Takeuchi1 and Tsuneo Homma1
1The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai
In order to study the mechanism of fatigue at elevated temperatures, the process of fatigue failure under reversed bending and torsional stresses was observed microscopically on a single crystal and polycrystal specimens of pure metals, 99.99%Al and Pb, over the testing temperatures from 0.5 to 0.85 Tm°K. The results are as follows:
(1) The surface markings consisting of a series of sharp valleys and peaks were observed under the reversed bending stress in the temperature range of about 0.5 Tm°K. The shape of the valleys and hills varied with testing temperatures. The most sharp markings were found in the vicinity of 0.5 Tm°K and these markings became indistinct at temperatures lower and higher than 0.5 Tm°K.
(2) At the critical temperature of 0.5 Tm°K, it was found that the fatigue cracks of the tensile type due to the stress concentration were initiated at the bottom of the sharp valleys, and also that the cracks of the shear type due to the sliding of subgrains as described in a previous report(3) occurred along the boundaries between the markings and the matrices.
(3) It was ascertained that these markings were consisted of unique deformation bands which were developed remarkably by the climbing and alignment of dislocations during repetition of the reversed deformation.
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