Mechanics of fretting fatigue crack formation

Friction is a very complicated phenomenon arising at the contact of surfaces. Experiments indicate a functional dependence upon a large variety of parameters, including sliding speed, acceleration, critical sliding distance, temperature, normal load, humidity, surface preparation, and, of course, material combination. In many engineering applications, the success of models in predicting experimental results remains strongly sensitive to the friction model. Furthermore, a broad cross section of engineering and science disciplines have developed interesting ways of representing friction, with models originating from the fundamental mechanics areas, the system dynamics and controls fields, as well as many others. A fundamental unresolved question in system simulation remains: what is the most appropriate way to include friction in an analytical or numerical model, and what are the implications of friction model choice? This review article draws upon the vast body of literature from many diverse engineering fields and critically examines the use of various friction models under different circumstances. Special focus is given to specific topics: lumped-parameter system models !usually of low order"—use of various types of parameter dependence of friction; continuum system models—continuous interface models and their discretization; self-excited system response—steady-sliding stability, stick/slip, and friction model requirements; and forced system response—stick/slip, partial slip, and friction model requirements. The conclusion from this broad survey is that the system model and friction model are fundamentally coupled, and they cannot be chosen independently. Furthermore, the usefulness of friction model and the success of the system dynamic model rely strongly on each other. Across disciplines, it is clear that multi-scale effects can dominate performance of friction contacts, and as a result more research is needed into computational tools and approaches capable of resolving the diverse length scales present in many practical problems. There are 196 references cited in this review-article. #DOI: 10.1115/1.1501080$

Friction modeling for dynamic system simulation

Many manufacturing processes can induce residual stresses in components. These residual stresses influence the mean stress during cyclic loading and so can influence the fatigue life. However, the initial residual stresses induced during manufacturing may not remain stable during the fatigue life. This paper provides a broad and extensive literature survey addressing the stability of surface and near-surface residual stress fields during fatigue, including redistribution and relaxation due to static mechanical load, repeated cyclic loads, thermal exposure and crack extension. The implications of the initial and evolving residual stress state for fatigue behaviour and life prediction are addressed, with special attention to fatigue crack growth. This survey is not a critical analysis; no detailed attempt is made to evaluate the relative merits of the different explanations and models proposed, to propose new explanations or models or to provide quantitative conclusions. Primary attention is given to the residual stresses resulting from four major classes of manufacturing operations: shot peening and related surface treatments, cold expansion of holes, welding and machining.

A literature survey on the stability and significance of residual stresses during fatigue

Relating Almen intensity to residual stresses induced by shot peening: a numerical approach

The effect of slip regime on fretting wear-induced stress evolution

— The crack initiation lives of peened specimens of aluminium alloys 7010 and 8090 are shorter than those of unpeened specimens. This is caused by the acceleration of crack initiation due to stress concentration in the rough peened surface, especially at fold-like defects. The crack growth rate in peened specimens is significantly reduced with increasing ΔK, i.e. with increasing crack length. At a crack length of approximately 0.3 mm this trend is reversed and the crack growth rate rapidly increases and attains the same level of crack growth rate as that in unpeened specimens. The point of smallest crack growth rate roughly corresponds to the point of maximum residual stress. The crack growth rate in a peened specimen has been modelled by assuming the effect of residual stress reduces to the equivalent stress ratio. The predicted results agree well with the experimental data.

The effect of residual stresses induced by shot‐peening on fatigue crack propagation in two high strength aluminium alloys

— —Recent work addressing the problems of fretting fatigue crack initiation and propagation under a carefully controlled axi-symmetric Hertzian contact is described. Both experimental work, enabling the fretting damage, sites of initiation, and crack trajectory to be viewed, and theoretical work, permitting a prediction of those processes are presented. Good correlation between the two strands of work is found. In particular, the initiation criterion proposed by Ruiz, Boddington and Chen for a very different geometry is found to work well, and would seem to indicate its potential as a design tool.

Initiation and growth of fretting fatigue cracks in the partial slip regime

It is shown that the residual stresses obtained during shot-peening are directly proportional to the treated material's hardness, and that the depth of material influenced depends on the velocity of approaching shot. The relationship between the shot size and depth hardened is developed, and experimental evidence is provided to verify the major points of the analysis. Useful curves, enabling the variation of residual stress with depth to be estimated, are included, and this enables the shot-peening treatment to be matched to alleviate any subsequent contact-pressure loading that the material may experience during service.

An analysis of shot peening

The effect of shot-peening on the fretting-fatigue strength of an age-hardened aluminium alloy (2014A) and an austenitic stainless steel (En 58A)

The ageing characteristics of a commercial Co-Cr-Mo-C alloy after solution treatment were investigated using optical and electron microscopy. An M23C6-type carbide was identified by X-ray and electron diffraction after ageing treatments between 650 and 1150° C. Nucleation and growth of this carbide took place on intrinsic stacking faults by Suzuki segregation in the cobalt matrix. High stacking-fault densities gave rise to intragranular striations which were visible after etching once precipitation had occurred. Ageing temperatures of 925° C and above increased the stability of the f c c cobalt matrix and led to precipitation on undissociated dislocations. Grain-boundary carbides were evident at all ageing temperatures.

R. B. Waterhouse

Papers

The effect of residual stresses induced by shot‐peening on fatigue crack propagation in two high strength aluminium alloys

Initiation and growth of fretting fatigue cracks in the partial slip regime

An analysis of shot peening

The effect of shot-peening on the fretting-fatigue strength of an age-hardened aluminium alloy (2014A) and an austenitic stainless steel (En 58A)

The metallography of a cobalt-based implant alloy after solution treatment and ageing