The influence of relative slip range and contact material on the fretting fatigue properties of 3.5NiCrMoV rotor steel

The influence of relative slip range and contact material on the fretting fatigue properties of 3.5NiCrMoV rotor steel has been investigated. Two generator rotor slot wedge materials, 1CrMo steel and 2014A aluminium alloy, were employed as contact materials. The results indicate that the observed relationships between both relative slip range and contact material and the fretting fatigue strength of 3.5NiCrMoV steel can be fully rationalized in terms of surface fretting contact stresses. Relative slip range and contact material are themselves of secondary importance. Fretting fatigue tests have been interrupted and specimens broken open, demonstrating that small cracks can initiate by fretting at stresses considerably below the fretting fatigue limit. The size of these nonpropagating cracks is related to the frictional and normal forces at the fretting contact and is independent of externally applied stresses and relative slip range, at least over the range of conditions investigated. The maximum non-propagating crack size is approximately 0.25 mm. Fretting cracks initially grow at a shallow angle to the specimen surface, probably by a shear mode of crack extension, and are highly extended along the specimen surface. The results of the investigation indicate that fretting fatigue strength may best be assessed by a method which uses fracture mechanics to model the continued propagation of these small cracks; models which relate fretting fatigue strength simply to relative slip range (such as the Engineering Sciences Data Unit route) are inadequate.