Tribological adaption of a failure hypothesis for fretting fatigue based on the local parameters slip amplitude and contact pressure

Abstract

Fretting fatigue is a phenomenon that can lead to catastrophic damage. It can cause components designed in the classical way in relation to intrinsic material strengths to fail unexpectedly and prematurely. To solve this problem, a failure hypothesis is needed to assess fretting fatigue in a realistic way and thus predict the risk to the components. To this end, the specific fretting-fatigue strengths of the material pairing of 34CrNiMo6+QT vs. C45+N were determined for a permutation of the tribological parameters of slip amplitude and contact pressure. A double-actuated flat-pad test bench was used for this. This test bench can keep the aforementioned tribological parameters constant over the entire test period, regardless of the cyclical tensile load on the specimen. From these tests, an equation was developed that describes the fretting-fatigue strength as a function of slip amplitude and contact pressure. Additional experimental tests were then carried out on connecting-rod connections. The connecting rods failed due to fretting fatigue. The failure location and fatigue limits of the connecting rods were determined. By numerical analysis of the connecting rods, it was possible to determine the local value of mechanical stress, slip amplitude and contact pressure. This was followed by a comparison of equivalent stress and fretting-fatigue strength with the same parameter combination of slip amplitude and contact pressure for each point in the contact, whereby the equivalent stress was calculated using cutting-plane methods that can take the multiaxial stress state into account. The point with the highest degree of utilization is the calculated failure location. With this adapted failure hypothesis, the failure location could be determined with very good accuracy. However, the maximum error in determining the degree of utilization is about 25%.