On the effect of out-of-phase cyclic normal load and axial stress ratios on fretting fatigue crack initiation and propagation

In this paper, we present a novel investigation into the influence of out-of-phase cyclic normal load and axial stress ratios on fretting fatigue crack initiation and propagation behaviours. Unlike previous studies that primarily focus on constant normal load, our research employs advanced numerical models to provide for the first time a detailed analysis of the effect of out-of-phase cyclic normal load on fretting fatigue crack initiation and propagation behaviours. Due to the non-proportional loading conditions, the frequency of loading, normal load ratio, and cyclic stress ratio play important roles in both crack initiation and propagation phases, which are not yet fully understood. Some experimental results reflect the effect of normal load ratio and cyclic stress ratio on the total fretting fatigue lifetime and wear scar, but there are no direct conclusions. This paper uses the Critical Plane Method and the Theory of Critical Distance to estimate crack initiation behaviour and lifetime. At the same time, the Extended Maximum Tangential Stress criterion from Linear Elastic Fracture Mechanics theory is applied to analyse the impact of load stress ratio on crack propagation path and lifetime, saving experimental time through numerical modelling. The enhancement ratios of crack propagation lifetime are less significant than those of crack initiation lifetime with varying axial stress ratios. Moreover, the impact of the stress ratio of both loads shows discrepancies under high and low cycle fatigue regimes.