Abstract
The purpose of this paper is to investigate the effect of such defects as minute thermal cracks, ghost cracks and non-metallic inclusions situated with machine parts on its tatigue strength. Steel specimens were made with artificially produced cracks by forging. The test results under rotary bending are follows. 1. Fatigue Strength of the specimen with axial crack decreases slowly as the length of crack increases and is about three times as strong as the round crack specimen. 2. Fatigue strength of the specimen with partial circumferential crack decreases rapidly in comparison with axial crack and approches to that of round crack specimen as the length of crack increases. 3. Fatigue strength of circumferential round crack specimen decresses no more rapidly as the crack deepens more than 0.5 mm and it is appreciated that the round crack specimen has characteristic fatigue strength for the material. It was found that artificial cracks are less detrimental under repeated torsion than under rotary bending with out distinction of its direction, axial or circumferential.
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