K. Pereira

On the Convergence of Stresses in Fretting Fatigue

Fretting is a phenomenon that occurs at the contacts of surfaces that are subjected to oscillatory relative movement of small amplitudes. Depending on service conditions, fretting may significantly reduce the service life of a component due to fretting fatigue. In this regard, the analysis of stresses at contact is of great importance for predicting the lifetime of components. However, due to the complexity of the fretting phenomenon, analytical solutions are available for very selective situations and finite element (FE) analysis has become an attractive tool to evaluate stresses and to study fretting problems. Recent laboratory studies in fretting fatigue suggested the presence of stress singularities in the stick-slip zone. In this paper, we constructed finite element models, with different element sizes, in order to verify the existence of stress singularity under fretting conditions. Based on our results, we did not find any singularity for the considered loading conditions and coefficients of friction. Since no singularity was found, the present paper also provides some comments regarding the convergence rate. Our analyses showed that the convergence rate in stress components depends on coefficient of friction, implying that this rate also depends on the loading condition. It was also observed that errors can be relatively high for cases with a high coefficient of friction, suggesting the importance of mesh refinement in these situations. Although the accuracy of the FE analysis is very important for satisfactory predictions, most of the studies in the literature rarely provide information regarding the level of error in simulations. Thus, some recommendations of mesh sizes for those who wish to perform FE analysis of fretting problems are provided for different levels of accuracy.

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.

Effect of Short Crack Behavior on the Propagation Life Prediction for a Fretting Cylindrical Pad Configuration

The numerical prediction of fretting fatigue lives has been an important research topic over the past few decades. Generally, those predictions are based on the addition of an initiation life, e.g. computed by Continuum Damage Mechanics, and a propagation life, calculated from the analysis of the growth of an initial crack until final rupture of the material. The propagation life prediction often relies on an empirical description of the crack growth rate. Normally, regardless the size of grains in the microstructure, a Paris growth law for long cracks is used. In this paper, our goal is to incorporate the behavior of short cracks on the growth law and analyze its impact on the predictions of fretting lives. Our results show that short cracks have a considerable impact on the predictions of fretting fatigue at low stresses (higher lives), but for relatively high stresses (shorter lives), it has almost no influence and a simple Paris law provides good estimates.

Minimum shear stress range: a criterion for crack path determination

For problems under proportional mixed-mode conditions, various criteria are used to predict fatigue crack growth directions, most achieving reasonable accuracy. The crack propagation angle is often obtained by maximizing a quantity (for instance, energy or stresses) as function of the stress intensity factors KI and KII. This maximization is generally performed at the instant of maximum fatigue loading and a stress analysis at this instant is sufficient to predict the crack propagation angle and thus the fatigue crack growth direction. However, under non-proportional loading, the maximum values of KI and KII may occur at different instants of the fatigue cycle and so a simple analysis at the maximum loading instant is not appropriate; it is necessary to consider the entire loading cycle history. One possible criterion to treat problems under these circumstances is the minimum shear stress range criterion (MSSR). This paper presents a brief discussion of the most common criteria used for determination of crack propagation direction, focusing on an implementation of MSSR. Its performance is assessed in different conditions and the results are compared to literature data.