Ajay Kapoor

Deterioration of rolling contact fatigue life of pearlitic rail steel due to dry-wet rolling-sliding line contact

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 accumulation (ratchetting) in the surface layer during the dry phase when the coefficient of friction is above 0.25. A strong correlation was found between the degree of ratchetting and the deterioration in RCF life. An empirical relationship to estimate this deterioration was concluded.

Prediction of fatigue crack initiation for rolling contact fatigue

In finite element (FE) simulations of a twin disc test of a wheel/rail contact, fatigue crack initiation criteria for elastic shakedown, plastic shakedown and ratchetting material responses were evaluated for a pearlitic rail steel BS11 normal grade. The Chaboche material model for nonlinear isotropic and kinematic hardening was used in the FE simulations. The ratchetting material response results were compared with a constitutive ratchetting model, and there was good agreement with respect to the number of cycles to crack initiation and shear strain distribution below the contact surface. In addition, angles for critical planes for crack initiation were calculated for both plastic shakedown and ratchetting material responses. Results from simulations with the ratchetting model at constant contact pressures and varying friction coefficient showed asymptotic values of the friction coefficient at which crack initiation due to ratchetting will not occur.

Wear by plastic ratchetting

Abstract 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 are manifestations of plastic ratchetting of material in a thin sub-surface layer. The plastic deformation in one cycle can be small but over many cycles it accumulates to large values. It is driven by (i) stress concentration at the edges of the hard slider, (ii) roughness on the hard slider, which causes the high contact pressure at the taller asperities to transverse all over the surface, and (iii) erosion where instead of sliding contacts, impacting erodent subjects the sub-surface layer to high contact stresses. The paper describes these mechanisms and discusses the associated uncertainties in predicting accurately the wear rate.

Tribological layers and the wear of ductile materials

Abstract 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 recently. The second mechanism is the fracture of surface material, causing a piece of material to leave the surface as wear debris. This is the so-called ‘delamination’ wear suggested by NP Suh at MIT in the 1970s, and fracture is an intrinsic part of this wear mechanism. This paper presents a ratchetting failure based approach to modelling (delamination) wear. Ratchetting failure occurs when the accumulated deformation exceeds a critical value. Using mechanics it is possible to relate the wear rate to the applied load, the roughness of the contacting bodies, and the elastic and plastic properties of the material near the surface. The formation of a mechanically mixed layer (MML) or transfer layer, here denoted ‘Tribological Layer’ generically, leads to a significant difference in the wear response since the properties of the layer differ from those of the bulk. Wear rate is governed by the near-surface properties and so the Tribological Layer plays a critical role in wear prediction. In this work, the role of the Tribological Layer in altering the wear response has been studied by examining the effects of changes in the hardness, stress–strain behaviour and ductility of the layer.

Surface roughness and plastic flow in rail wheel contact

Abstract 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 in a sub-surface layer of a few tens of microns thickness; the contact patch size is in tens of millimetres. This raises two questions: firstly, why should plastic flow occur if the shakedown limit is not exceeded; and secondly, why should plastic flow be confined to a thin sub-surface layer? It is hypothesized that asperity contacts are responsible for the observed plastic flow. This hypothesis was investigated in experiments on a twin-disc machine and was found to be correct. Numerical analysis showed that roughness causes the contact pressure to deviate from the assumed Hertzian (smooth) to one which is spiky.

Computer simulation of wear and rolling contact fatigue

Abstract 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 of such contacts have been developed based on ductility exhaustion [Wear 245 (2000) 204; Int. J. Fatigue 22 (2000) 205]. For wear, the material is divided into layers and each layer accumulates shear strain dependent on the stress at that depth; once a layer has accumulated a critical shear strain it is deemed to have failed. In the work presented here, the models are improved by allowing variation with depth of material properties such as ductility and shear yield stress. This reflects the statistical variation of real materials arising from the microstructure. For rolling contact fatigue, ductility exhaustion has been taken to mean initiation of a micro-crack. However, this introduces ambiguity since ductility exhaustion is also the cause of wear of material from the surface. The suggestion, here, is that the dilemma can be resolved by considering a model which has a brick wall structure. Each material brick can lose integrity and thus fail. Dependent on whether the failed material is supported by adjacent bricks which are intact, the material may detach to produce wear debris. Bricks which fail but do not detach (either at the surface or below it) behave as micro-cracks; these can interact and grow, but can also be lost if the surface material is worn away later.

Product design engineering – a global education trend in multidisciplinary training for creative product design

Product design is the convergence point for engineering and design thinking and practices. Until recently, product design has been taught either as a component of mechanical engineering or as a subject within design schools but increasingly there is global recognition of the need for greater synergies between industrial design and engineering training. Product design engineering (PDE) is a new interdisciplinary programme combining the strengths of the industrial design and engineering. This paper examines the emergence of PDE in an environment of critique of conventional engineering education and exemplifies the current spread of programmes endorsing a hybrid programme of design and engineering skills. The paper exemplifies PDE with the analysis of the programme offered at Swinburne University of Technology (Australia), showing how the teaching of ‘designerly’ thinking to engineers produces a new graduate particularly suited to the current and future environment of produce design practice. The paper concludes with reflections on the significance of this innovative curriculum model for the field of product design and for engineering design in general.

Modelling Wear and Crack Initiation in Rails

Abstract Wear and crack initiation of steel rails is a problem of great significance to the railway industry. A high wear rate shortens the life of rails and the frequent rail replacement is expensive in terms not only of resources but also of track access time and delays affecting timetables. In addition, railhead profiles change gradually as the rails are worn and the greater the wear rate, the more often the rails need to be reground to maintain good train running performance. In contrast, a low wear rate means that cracks have time to develop in the plastically deformed rail steel and these may propagate deeper into the rails with potentially disastrous consequences. Finding the optimum combination of wear and grinding to maintain railhead profile and prevent cracks from growing is key to running a safe and cost-efficient railway. The large number of variables arising from track geometry, train dynamics, and wheel and rail profiles leads to wide variation in contact patch size and location. The two-dimensional model of ratcheting wear developed by Kapoor et al. has been developed to model the damage accumulation near the surface of the rail on the basis of a full three-dimensional contact stress distribution. Different rail steel microstructures can also be modelled and the effects of microstructure on wear and crack initiation are explored.

A review on hybrid nanolaminate materials synthesized by deposition techniques for energy storage applications

Nanostructured materials such as nanocomposites and nanolaminates are currently of intense interest in modern materials research. Nanolaminate materials are fully dense, ultra-fine grained solids that exhibit a high concentration of interface defects. They may be developed for engineering applications that take advantage of enhanced mechanical properties or for devices such as energy storage and memory storage capacitors. Nanolaminates can be grown using atom-by-atom deposition techniques that are designed with different stacking sequences and layer thicknesses. The properties of fabricated nanolaminates depend on their compositions and thicknesses. These can be demonstrated within the synthesis process by thickness control of each layer and interfacial chemical reaction between layers. In fact, dielectrics with the formed thin layer have efficient dielectric constant and high insulation characteristics. Dielectric materials with giant dielectric constants can be fabricated as modified single, binary and perovskite oxides. A review of the advantages offered by nanolaminate structures for high performance energy storage devices is presented. Developments of dielectric materials that are formed from a thin layer approach are evaluated. The influence of the interface layer on the dielectric constant of nanolaminate films is assessed from the perspective of conferring a giant dielectric constant and high insulation characteristics. The incorporation of dopants and site-engineering techniques, as well as layer-by-layer structures, which can both be suitable for improving dielectric properties of dielectric nanolaminates, is detailed. Finally, the current status and development of artificial dielectric materials for high performance energy storage devices formed by dielectric nanolaminates are presented.

Adaptive gain sliding mode observer for state of charge estimation based on combined battery equivalent circuit model

An adaptive gain sliding mode observer (AGSMO) for battery state of charge (SOC) estimation based on a combined battery equivalent circuit model (CBECM) is presented. The errors convergences of the AGSMO for the SOC estimation are proved by Lyapunov stability theory. The AGSMO has a capability of compensating modeling errors caused by the parameters variation of the CBECM and minimizing chattering level in SOC estimation. The lithium-polymer battery (LiPB) is used to conduct experiments for extracting the parameters of the CBECM and verifying the effectiveness of the proposed AGSMO for the SOC estimation.