Smooth Specimen Fatigue Lives and Microcrack Growth Modes in Torsion
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Published:1985
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Constant amplitude torsional fatigue tests have been performed on smooth specimens of quenched and tempered steel at two strength levels. Both unidirectional and fully reversed tests have been performed. It was found that there was no detectable effect of mean strain upon fatigue life of either strength level material, both at short and at long lives. Observations on microcrack behavior during the tests has demonstrated that initial crack growth always occurs on a shear plane, even if subsequent propagation is on a tensile Mode I plane at 45°.
As in the uniaxial situation, short lives are dominated by microcrack growth. However, in torsion crack growth dominates over an extended range of life, and, in addition, surface crack lengths are longer than uniaxial at similar fractions of life. It is concluded that whereas crack initiation on shear planes is relatively easy in torsion, the Stage I/Stage II transition, which occurs in tension at crack lengths of one or two grains, is extremely difficult in torsion.