Another mechanism for fatigue strength improvement of metallic parts by shot peening

Fatigue crack source in shot-peened specimens may be located either at the surface or in the interior, beneath the hardened layer. In this paper, the mechanism for fatigue strength improvement of shot-peened specimens with internal fatigue crack source was studied. Un-peened and shot-peened specimens made of quenched and low-temperature tempered 40CrNi2Si2Mo2V steel were used. The fatigue crack source in shot-peened specimen is found in the interior beneath the hardened layer. X-ray diffraction analyses of both kinds of specimen fatigue tested at stress equal to their apparent fatigue limit show that obvious changes have taken place in the surface layer for un-peened specimens, while for shot-peened specimens, such changes are observed in the sub-surface layer beneath the hardened layer. The calculated actual critical stress at the fatigue source position (the internal fatigue limit) for shot-peened specimen is about 138% of the (surface) fatigue limit of un-peened specimen. According to an analysis about the micro-meso-processes of fatigue crack initiation in metals, a concept of internal and surface fatigue limits of metal has been proposed. It is believed that the fatigue crack source transfers into the interior, Also, the internal fatigue limit of metal is higher than its surface fatigue limit, and is another mechanism for the improvement of apparent fatigue limit of shot-peened specimen.