Very high frequency induced fatigue damage

Fatigue lifetime and fatigue endurance limit are enhanced by the increased strain rate for many materials. Consequently, ultrasonic fatigue testing yields an S-N curve well above the conventionally obtained ones for the strain rate sensitive materials. Thus, understanding of the fatigue damage mechanisms operating under very high frequencies and their relation to the conventional testing results is crucial for lifetime prediction in the very high cycle fatigue region. The increase in test frequency from 20 Hz to 20 kHz enhances the fatigue life and the fatigue endurance limit of the ferritic-pearlitic structure steel. On the other hand, the character of the created surface relief was similar for all the test conditions in terms of test frequency and stress amplitude applied. The increased testing frequency reduced the number of slip markings. Nevertheless, it seems that test frequency only slightly impacted the height/depth of the extrusions/intrusions. Focused ion beam tomography and 3D reconstruction of the volume below the slip markings demonstrated the cavity distribution within the material volume and along the slip plane. No slip markings were observed in the pearlite areas. Fatigue crack initiation occurred predominantly in the slip markings; however, subsurface cavity-assisted crack initiation was occasionally observed, too. Coalescence of cavities along the slip plane facilitated the fatigue crack initiation and early growth. No difference in damage mechanism was detected regardless of the test frequency or stress amplitude.