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Plastic ratcheting plays a key role in wear and rolling contact fatigue crack formation at the wheel–rail interface. Tests to examine the wear and rolling contact fatigue behaviour of rail materials over a wide range of service conditions are expensive and can be impractical. A physical simulation of the deformation behaviour associated with ratcheting is an attractive replacement for such tests....
Ductile materials commonly exhibit plastic deformation at and near the contact surface and their flow behaviour at large strain has a clear effect on wear resistance. These materials almost always fail while under high levels of compression, but behaviour under these conditions cannot be investigated by standard materials tests at atmospheric pressure. In this work, the characteristics of the near-surface...
Wear rate in a rail–wheel system has been found to be a function of temperature rise between the rail and wheel. The flash temperature due to frictional heating causes the development of thermal stresses and reduction of the material strength at the rail undergoing wear. This thermal effect leads to a rapid increase in wear rate, known as wear transition. Rail wear transition behaviour as a result...
Surface roughness has a significant effect on how loads are transmitted at the contact interface between solid bodies. It causes high local pressures in the contacting roughness peaks, i.e., asperities. Even for a low friction coefficient the surface roughness will still play an important role in the early surface wear. A dynamic ratcheting model (Dynarat) for studying the wear rate of ductile materials...
Fluid penetration of cracks has been regarded as an important mechanism of crack growth for inclined surface breaking cracks under contact loading since the 1930s. However, there are only limited cases in which it is realistic to assume fluid is sealed in and pressurised by wheels crossing cracks of complex three-dimensional morphology in rolling contact fatigue affected rail. To investigate, modelling...
The quality of rail steel has improved greatly in recent years and the material is more resistant to wear, plastic deformation and crack initiation; but track forces have also increased, and cracking of rails is a major concern. Different steel microstructures have different wear and rolling contact fatigue (RCF) behaviours when subject to cyclic, rolling–sliding, compressive contact. In order to...
Railway rails accumulate severe plastic deformation at the surface as a result of loading by passing wheels. This process of plastic strain accumulation is plastic ratcheting. Ratcheting failure occurs when material accumulates a critical strain and failed material produces wear debris and rolling contact fatigue cracks. A computer model of strain accumulation, ratcheting failure and wear has been...
The tractive force transmitted through the contact of rail and wheel is often lower than the limiting friction force and such contacts are not fully slipping. The distribution of traction is no longer proportional to the distribution of normal pressure in the contact patch. The distribution of stress in the rail, and therefore also the shakedown limit and the ratchetting wear rate, is affected as...
Rolling contact fatigue (RCF) is currently one of the principal limitations of railway infrastructure productivity. Head checks in particular are prevalent in curves and switches where flange contact at the gauge corner results in increased slip and decreased wheel–rail contact area. These surface-initiated cracks can lead to complete failure of the rail and potentially derailment. The focus of the...
Material in railway rails is loaded repeatedly by the passage of the wheel. The maximum contact pressure which the material can carry elastically in the steady state is known as the 'shakedown limit'. With an operating contact pressure below the shakedown limit the rail would be expected to remain elastic with a very long life. However, examination of rail cross-sections shows severe plastic deformation...
A ductile material subjected to repeated rolling contact can accumulate very high levels of shear strain near the surface. At some point the material loses its integrity and fails, and this failure is manifested in the form of wear (the material detaching from the surface and producing debris) or rolling contact fatigue (initiation of micro-cracks which may subsequently propagate and branch). Models...
Surface textures of very small scale are able to carry pockets of lubricant into heavily loaded contacts, leading to reductions in friction and wear. Recently, such textures have been produced by a new method in which two materials of differing wear properties are combined to form a composite surface. In a typical application, a distribution of individual wear resistant islands of small dimension...
Metallic wear occurs by one of the following two mechanisms. Material may extrude out from the sides of the contact giving rise to thin slivers which, subsequently, separate to produce wear debris. Fracture plays no intrinsic role in the wear process, and occurs only in the separation of wear debris which would otherwise remain attached to the sides of the contact. This mechanism has been modelled...
Wear debris in the form of thin platelets is observed in wear of sliding, rolling and eroding components. The paper discusses two mechanisms in which such platelets can be generated: (i) extrusion of material from below the contact into thin slivers which subsequently break off to provide lamellar wear debris, and (ii) fracture of a thin layer leading to delamination wear debris. Both these mechanisms...
When a hard rough surface slides repeatedly on a softer component a system of protective residual stresses may be developed in the near-surface layers of the weaker material which enable loads sufficiently large to cause plastic deformation in the early cycles of loading to be accommodated purely elastically in the later stages of the loading history. This is the process known as shakedown and limits...
A mechanism of metallic wear is proposed in which laminar wear debris is generated by a process of plastic ratchetting brought about by repeated pummelling of the softer wearing surface by the asperities on a harder mating surface. Wear rate is found to be approximately proportional to (load) and an increasing function of a single non-dimensional parameter termed the plasticity index for repeated...
This study is aimed at the deterioration of rolling contact fatigue (RCF) life of pearlitic rail steel, under rolling-sliding conditions, where the wet phase of the test is preceded by different numbers of dry cycles. It is shown that initial dry cycles above a critical number causes sudden and significant deterioration in RCF life. This effect has been explained using the argument of plastic strain...
Most work on the mechanics of shakedown has been on half-spaces which have strength properties invariant with depth. In practice, however, materials for tribological applications are often surface engineered to impart a higher hardness near the surface. In some situations, such as wheels on railway tracks, considerable plastic flow is observed near the surface and, as a result, the initial hardness...
Many researchers have observed plate-like wear debris in tests on erosion of ductile metals at near normal impingement. We propose that this is due to 'plastic ratchetting' of a thin surface layer, which is progressively extruded out in the form of thin slivers which subsequently break off to provide wear debris. The process is driven by the repeated random impacts of the erodent. During the impact...
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