Surface and subsurface rolling contact fatigue characteristic depths and proposal of stress indexes

Abstract

The rolling contact fatigue is distinguished into subsurface initiated (spalling and case crushing) and surface initiated (pitting and micropitting). A characteristic depth was identified for each of these mechanism. The characteristic depth of the case crushing is the hardening depth, while for the spalling it is the maximum cyclic shear stress depth. The pitting depth is the size of the crack for which the mode I stress intensity factor range, due to the fluid pressurization, is higher than the threshold. This depth can be similar to the micropitting depth, in the order of 10 µm, for heavily loaded small radius contacts. Rolling contact fatigue cyclic shear stress indexes are then defined on the basis of the characteristic depths, and they identify the load intensity of each rolling contact fatigue mechanism. The characteristic depths and the stress index approach can be used to relate specific tests to component design, without any size effect misinterpretation

Investigation of Sliding Wear Mechanism and Causes on a Diesel Application Roller Follower

“…Pitting is considered the least severe mode of RCF, and consists in microcracks formation related to the friction orientation. A proof of that is the opposite orientation of these microcracks on the adendum and on the dedendum driver gear (see Figure 9) [5]. In summary, the three forms of contact fatigue beginning that are interesting for this work are: 1.…”

“…In this case, the phenomenon can be understood as low or high cycle fatigue depending on presence or absence of plastic deformation. According to [5], spalling-starter cracks don't come from pittings coalescence. They come actually from material defects (e.g.…”

“…Mode III has a secondary role on the crack propagation. When spalling originates from subsurface, the main cause is frequently high Hertzian stress solely, and not poor lubrication [5]. Pitting is considered the least severe mode of RCF, and consists in microcracks formation related to the friction orientation.…”

“…The crack then propagates in an angle around 30° with the surface (Figure 7). This mode occurs generally when lubrication is not adequate and asperities contact occur [5]. 3.…”

“…Inclusion or material defect starts a crack due to high contact stress below surface ( Figure 5) and it propagates through mode II. This can occur without asperities contact, when lubrication regime is adequate: elasto-hydrodynamic (EHL) [5].…”