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Estimating Life Scatter in Fretting Fatigue Crack Initiation

October 1, 2013

KEY WORDS

Fretting Fatigue; Life Prediction; Finite Element Analysis

ABSTRACT

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.

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INTRODUCTION

Fretting occurs when mechanical machine components in contact experience small oscillatory motion. There are two steady-state fretting regimes that are differentiated by the relative motion occurring between the two components. In the partial slip regime, a portion of the contact sticks while the remainder of the contact slips. In the gross slip condition, the entire contact undergoes a smallscale relative motion. The two main failure mechanisms associated with fretting contacts are wear and cracking. Wear is primarily associated with the gross slip condition, whereas cracking is mainly encountered in partial slip contacts. When bulk stresses are also present in one of the components, cracking due to fretting can lead to a significant reduction in fatigue life.

Many different approaches have been proposed and developed to analyze the crack initiation process in fretting contacts. Due to the multiaxial nature of the stresses developed in fretting contacts, critical plane-based approaches have received significant attention. The criteria that have been used most often include the Smith-Watson-Topper (SWT; Szolwinski and Farris1; Lykins, et al.2; Fouvry, et al.3; Araujo, et al.4), Fatemi- Socie (FS; Lykins, et al.2; Fouvry, et al.3; Araujo, et al.4), and Dang Van (Fouvry, et al.3; Araujo, et al.4) criteria. Recently, continuum damage mechanics has also been employed to analyze the fretting crack nucleation process (Quraishi, et al.5; Aghdam, et al.6). The majority of the focus has been directed toward assessing the risk of crack initiation (Fouvry, et al.3; Araujo and Nowell7), predicting the initiation life (Szolwinski and Farrisl; Lykins, et al.2; Quraishi, et al.5; Aghdam, et al.6; Araujo and Nowell8), and explaining an apparent size effect associated with the large stress gradients in the contact region (Araujo and Nowell7,8). But for a few notable exceptions (Golden, et al.9; Kumari and Farris10), little effort has been expended toward explaining or quantifying scatter occurring in fretting fatigue life.

The focus of the present article is on developing an approach to estimate the crack initiation life of a fretting contact while accounting for the variability in the fatigue response. This was accomplished by developing a modified application of the SWT fatigue criterion. A combined finitediscrete 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 domains. The scatter in the generated data was quantified using Weibull statistics. The life predictions were compared against those of a previous application of the SWT equation that assumes quasi-uniaxial conditions at the trailing edge of the contact.

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