Trevor S. Slack

A Review of Rolling Contact Fatigue

Ball and rolling element bearings are perhaps the most widely used components in industrial machinery. They are used to support load and allow relative motion inherent in the mechanism to take place. Subsurface originated spalling has been recognized as one of the main modes of failure for rolling contact fatigue (RCF) of bearings. In the past few decades a significant number of investigators have attempted to determine the physical mechanisms involved in rolling contact fatigue of bearings and proposed models to predict their fatigue lives. In this paper, some of the most widely used RCF models are reviewed and discussed, and their limitations are addressed. The paper also presents the modeling approaches recently proposed by the authors to develop life models and better understanding of the RCF.

Fretting Wear Modeling of Coated and Uncoated Surfaces Using the Combined Finite-Discrete Element Method

A new approach for modeling fretting wear in a Hertzian line contact is presented. The combined finite-discrete element method (FDEM) in which multiple finite element bodies interact as distinct bodies is used to model a two-dimensional fretting contact with and without coatings. The normal force and sliding distance are used during each fretting cycle, and fretting wear is modeled by locally applying Archard’s wear equation to determine wear loss along the surface. The FDEM is validated by comparing the pressure and frictional shear stress results to the continuum mechanics solution for a Hertzian fretting contact. The dependence of the wear algorithm stability on the cycle increment of a fretting simulation is also investigated. The effects of friction coefficient, normal force, displacement amplitude, coating thickness, and coating modulus of elasticity on fretting wear are presented.

Estimating Life Scatter in Fretting Fatigue Crack Initiation

A new approach was developed to estimate the crack initiation life of a fretting contact while accounting for the variability in the fatigue response. This was accomplished using a modified application of the Smith-Watson-Topper (SWT) fatigue criterion. A combined finite–discrete element model of a cylinder-on-flat configuration incorporating material randomness and disorder via a Voronoi tessellation was developed. The SWT equation was used with this model to predict the crack initiation lives for multiple material domains. The predicted crack initiation lives displayed a large degree of scatter and were less conservative than previous deterministic implementations of the SWT parameter. The scatter in the predicted initiation lives was quantified using Weibull statistics.

A Phenomenological Discrete Brittle Damage-Mechanics Model for Fatigue of MEMS Devices With Application to LIGA Ni

Fatigue initiation and failure of various microelectromechanical systems (MEMS) is of significant importance as they gain widespread acceptance in sensors and electronics. This paper presents an approach for utilizing available experimental fatigue data to evaluate the fatigue lives of MEMS components. The approach is based on a phenomenological discrete material representation in which a domain is represented by a collection of rigid elements that interacts via springs along their boundaries. The principles of continuum damage mechanics are used to degrade the spring stiffnesses as brittle damage occurs when the domain is subjected to fatigue loading. The model utilizes experimental stress-life data for LIGA Ni to identify the material properties used in the model. The proposed model captures the statistical distribution of material properties and geometrical randomness of the microstructure commonly observed in a wide variety of MEMS. Consequently, simulations that account for the variability in fatigue life can be readily performed. The model is applied to a dog-bone-shaped specimen to evaluate the influence of material heterogeneity and material flaws on fatigue crack initiation life and scatter. The ability of the model to predict the fatigue life of different types of MEMS devices and loading conditions is also demonstrated by simulating the fatigue stress-life behavior of a MEMS resonator support beam.

EHL Modeling for Nonhomogeneous Materials: The Effect of Material Inclusions

Inclusions are common in bearing materials and are a primary site for subsurface fatigue crack initiation in rolling element bearings. This paper presents a new approach for computing the pressure, film thickness, and subsurface stresses in an elastohydrodynamic lubrication (EHL) contact when inclusions are present in the elastic half-space. The approach is based on using the discrete element method to determine the surface elastic deformation in the EHL film thickness equation. The model is validated through comparison with the smooth EHL line contact results generated using linear elasticity. Studies are then carried out to investigate the effects of size, location, orientation, and elastic properties of inclusions on the EHL pressure and film thickness profiles. Both inclusions that are stiffer than and/or softer than the base material are seen to have effects on the pressure distribution within the lubricant film and to give rise to stress concentrations. For inclusions that are stiffer than the base material (hard inclusions), the pressure distribution within the lubricant film behaves as though there is a bump on the surface, whereas for inclusions that are less stiff than the base material (soft inclusions), the pressure distribution behaves in a manner similar to that of a dented surface. Inclusions close to the surface cause significant changes in the contact stresses that are very significant considering the stress life relationship. For inclusions that are located deep within the surface, there is little change in the EHL pressure and film thickness.