D. Nowell

Mechanics of fretting fatigue tests

Abstract A popular fretting fatigue test, in which oscillating tension is applied in phase with the fretting force, is analysed. The configuration is a generalization of the well-known Mindlin contact problem, and it is shown that the addition of bulk tension has a substantial effect on the stick-slip geometry and the shear traction at the interface. The largest tension induced, which is thought to be responsible for the initiation of fatigue cracks, is also slightly increased.

Crack initiation criteria in fretting fatigue

Abstract This paper is a description of a series of fretting fatigue tests on an Al/Cu alloy in which the contact size was varied while the other relevant parameters were held constant. The fretting fatigue life was found to be infinite below a certain critical contact width. The configuration has been analysed using elastic stress analysis and stresses and displacements have been calculated. The variation of life with contact size can be explained by a variation in amplitude of micro-slip. Composite parameters proposed by Ruiz et al. are shown to characterize the severity of fretting damage and the probability of crack initiation, and hence explain the observed results. The physical basis of these parameters and their use in design calculations is discussed.

Contact of dissimilar elastic cylinders under normal and tangential loading

Abstract T he interfacial tractions induced by normal contact between dissimilar elastic cylinders are first re-evaluated, exploiting self-similarity and allowing for the influence of shear tractions on normal surface displacement. Subsequently, a tangential force less than that necessary to cause sliding is applied, and the modified traction distribution deduced. The effect of cycling the tangential force is also investigated, thus simulating the behaviour of the contact under fretting conditions. Results are compared with those predicted by the classical Mindlin-Cattaneo analysis for elastically similar bodies.

Contact problems incorporating elastic layers

Abstract In this paper, plane elastic contact between a thin strip and symmetric rollers is considered. Various loading regimes, including factional sliding, frictionless and frictional indentation, and the effect of applying a tangential force less than that necessary to cause sliding are treated. For each case, the surface tractions are found, and, for the last two problems, a detailed analysis of the stick and slip zones is presented.

Stress gradient effects in fretting fatigue

This paper highlights the importance of high stress gradients in fretting fatigue. These pose a serious difficulty in attempting to employ plain fatigue data to predict fretting fatigue performance. Stress gradient effects may also be responsible for a size effect which is found to occur in many fretting situations. Here we review a number of possible approaches for dealing with high stress gradients. We then explore a notch analogy which might be used to generate equivalent stress gradients in plain fatigue. Some sample comparisons are given for typical dovetail geometries.

Applications of the boundary element and dislocation density methods in plane crack problems

Abstract Crack modelling using the dislocation density method has become an increasingly popular technique for plane crack problems and allows the fast, efficient determination of stress intensity factors. The range of geometries which may be solved by the basic technique is, however, somewhat limited and this paper outlines a hybrid formulation combining the dislocation density technique with constant displacement discontinuity boundary elements to satisfy a range of far boundary conditions. The method described can analyse configurations with an arbitrary number of buried or surface breaking cracks and can be implemented on a personal computer to provide an efficient tool for crack analysis.

Designing against fretting fatigue in aeroengines

Abstract Gas turbines used for aircraft propulsion incorporate a large number of mechanical joints, where fretting fatigue is an important design consideration. Chief amongst them are the dovetail and firtree joints between blades and disks, particularly in the fan assembly and in the low pressure stages of the compressor where the operating temperature is sufficiently low to exclude the possibility of creep. This paper reviews some early designs and, after a brief description of the mechanics of fretting fatigue, presents current research of direct application to design.

Plane cracks near interfaces

Abstract This paper deals with the determination of stress intensity factors for plane cracks which are at or near interfaces. The interface may be straight or curved and be a free surface or between two elastically dissimilar materials. Any bulk stress field may be catered for as may partially closed cracks. The solution technique used is to distribute line singularities or ‘dislocations’; details of the formulation of the relevant integral equations and their solution are discussed.

Analysis of crack propagation

Fatigue cracks may propagate either from an existing flaw, such as an inclusion or a defect in a weld, or they may grow from damage nucleated as part of the early stages of fatigue itself. It is into this second category that fretting fatigue falls. The fretting process must first initiate an embryo crack and then propagate it in order to produce a failure. The life of a component suffering fretting fatigue may therefore be conveniently divided into initiation and propagation phases. In contrast to plain fatigue the initiation phase of fretting fatigue life is often, although not always, quite short. In reality the term fretting fatigue encompasses a range of conditions from mild contact tractions accompanying bulk stress amplitudes sufficient to cause failure in plain fatigue, to severe fretting in the presence of relatively low bulk stresses. The spectrum of loading conditions can be conveniently summarised in a diagram such as that shown in fig. 8.1. It is possible for fretting fatigue to affect significantly both crack initiation and crack propagation. Thus, a full understanding of fretting fatigue requires a consideration of the effect of fretting on both initiation and propagation phases of crack life. Initiation occurs at a microscopic scale and a detailed understanding can be achieved only by a micromechanics analysis, although some progress can be made by considering bulk properties of the contact. In contrast, once a crack has developed and is larger than several material grain sizes, it should be possible to explain its propagation by employing the same techniques of fracture mechanics as are used for other types of fatigue. Indeed, crack growth depends entirely on conditions at the crack tip and it is impossible for a crack to ‘distinguish’ whether the propelling stresses arise from a contact loading or from some other far field. Thus, the analysis of crack propagation would appear to be far more tractable than that of initiation and it will be addressed first. Crack initiation will be discussed further in Chapter 9.

Fretting Fatigue Tests

So far, we have presented several of the elements of calculation necessary to analyse a range of idealised fretting contacts. They merely provide tools for determining the state of stress, strain and displacement, but do not, in themselves, permit production of initiation or crack propagation criteria, without extensive experimental data. The object of fretting fatigue tests must be to permit monitoring of fretting fatigue crack propagation and, more importantly, initiation, under laboratory conditions, so that the influence of different load histories, surface treatments, surface finishes and indeed underlying materials may be found. There are three general categories of tests which might be envisaged, viz.