On 'multi-stage' fatigue life diagrams and the relevant life-controlling mechanisms in ultrahigh-cycle fatigue

In recent years, several fatigue studies on different metallic materials have indicated that fatigue failures can occur even at amplitudes below the conventional high-cycle fatigue (HCF) fatigue limit in the gigacycle or ultrahigh-cycle fatigue (UHCF) range (number of cycles to failure in excess of ca. 10(7)-10(8)). In the latter case, fatigue failures were observed to originate from internal defects (non-metallic inclusions). The S-N curves displayed a multi-stage shape, sometimes with a second lower fatigue limit in the UHCF range. The present paper specifies the conditions under which internal and/or surface fatigue failure can occur at low load levels below the conventional HCF fatigue limit. The relevant fatigue thresholds are discussed for two classes of materials, namely pure single-phase metallic materials (type I) without internal defects and materials such as steels or cast materials (type II) with internal defects (inclusions or pores). In the case of type II materials, the probability of surface versus internal fatigue is discussed in terms of the volume density, size and location of the inclusions and the relevant cracking mechanisms. It is argued that, in both type I and type II materials, multi-stage S-N curves or Manson-Coffin plots can be expected under certain conditions.