The lubricants used to protect structures from rolling-contact fatigue (RCF) have been reported to reduce the mode I fatigue crack growth rate in steels to a different degree, depending on their composition and viscosity. This was attributed to a protection of the fresh metal surfaces at the crack tip from hydrogen-releasing reactions with moist air, or to oil-pressure-induced crack closure effects. Although in RCF, the cracks are loaded in mixed-mode, with a predominance of shear modes, experimental studies on the effect of lubricants on the growh kinetics of a single crack loaded in mode II are lacking. Existing works use tribological test benches, that reproduce service conditions, but do not allow an analysis of the crack growth kinetics, because 1) the 3D crack path is complex, 2) non-proportional mixed-mode loading prevails, and 3) the crack cannot be monitored directly. The present study, compares the effects of Aerogear 823 lubricant on mode I and mode II crack growth kinetics in 16NCD13 steel. Tests are run in air or in oil on precracked SENT or cruciform specimens loaded in mode II + biaxial compression.  An inverse numerical analysis of the measured crack face sliding displacement profiles allow the extraction of an “apparent friction coefficient” along the crack faces [1]. The evolutions of this parameter during crack growth in air (in relation with crack face oxidation or absence thereof), or in oil (due to friction-induced heating, change in oil viscosity and lubrication regime…), and the reasons why the lubricant reduces the mode I fatigue crack growth rate, but accelerates mode II crack growth are discussed, using also fractographic observations and chemical analyses, which suggest detrimental effects of oil degradation in mode II.

 [1] M. Zaid, V. Bonnand, V. Doquet, V. Chiaruttini, D. Pacou, P. Depouhon, Int. Journ. Fatigue, 2022, 163, 107074