Fatigue Crack Initiation Under Cyclic Torsion

Lin’s ratchet mechanism developed by Lin and his associates for fatigue crack initiation under tension and compression loading is here applied to fatigue under torsional loadings. Under the former loading, the most favorable slip system has a slip plane and a slip direction, both making 45 deg with the free surface; while under the latter the most favorable slip system has a slip direction parallel to the free surface. High cycle fatigue loading is considered. A slip direction parallel to the free surface does not produce extrusion or intrusion. A surface crystal with a slip plane normal to the shaft axis and with a slip direction making an angle β with the free surface was considered. Displacement component normal to the free surface is considered to be a measure of the amount of intrusion, which, in turn, is taken as a measure of crack initiation. It was found that the angle β, giving a maximum displacement component normal to the free surface, varies with the torsional stress. Under the same maximum range of shear stress (zero mean stress), the growth of the displacement component normal to the free surface is much larger in the tension and compression specimen than in torsional ones.