FATIGUE STRENGTH EVALUATION FROM SURFACE YIELDING DATA

Surface yielding was detected by X-ray diffraction. The surface yield strength _sy was significantly less than the bulk yield strength, and depends on the physical state and roughness of the surface. However _sy does not change with changes in surface residual stress. Determination of surface yielding, measured under static loading, indicates that a remarkable number of grains have been plastically deformed at the _sy stress level, which will result in damage accumulation under cyclic loading. Crack initiation predominates at the level of the fatigue endurance life for smooth parts and experiments indicate that _sy equals the smooth specimen fatigue limit, particularly that associated with a high survival probability.