Nadeem Ali Bhatti

A Comparison Between Critical-Plane and Stress-Invariant Approaches for the Prediction of Fretting Fatigue Crack Nucleation

The phenomenon of fretting fatigue can cause crack nucleation at the contact interface due to multiaxial loading conditions. The nucleation phase may take a significant proportion of life under fretting fatigue, which leads to micro crack initiation. There are several approaches in vogue to estimate crack initiation life. The current study, however, aims to compare two approaches, namely, Critical Plane (CP) approach and Stress Invariant (SI) approach. Smith Watson Topper (SWT), McDiarmid (MD) parameters, which represent critical plane approach, whereas, Crossland parameter (CL), which represents SI approach, are adopted for this purpose. These parameters are applied to cylinder on a flat configuration and predicted numerical results are also compared with experimental results from literature. It is observed that, under fretting fatigue conditions, life prediction capability of critical plane approach is better than stress invariant approach especially at large number of cycles to failure.

Numerical investigation of contact stresses for fretting fatigue damage initiation

Fretting fatigue phenomena occurs due to interaction between contacting bodies under application of cyclic and normal loads. In addition to environmental conditions and material properties, the response at the contact interface highly depends on the combination of applied loads. High stress concentration is present at the contact interface, which can start the damage nucleation process. At the culmination of nucleation process several micro cracks are initiated, ultimately leading to the structural failure. In this study, effect of ratio of tangential to normal load on contact stresses, slip amplitude and damage initiation is studied using finite element analysis. The results are evaluated for Ruiz parameter as it involves the slip amplitude which in an important factor in fretting fatigue conditions. It is observed that tangential to normal load ratio influences the stick zone size and damage initiation life. Furthermore, it is observed that tensile stress is the most important factor that drives the damage initiation to failure for the cases where failure occurs predominantly in mode I manner.