Dennis De Pellegrin

Assessing the role of particle shape and scale in abrasion using ‘sharpness analysis’: Part II. Technique evaluation

The classical Euclidean paradigm of shape imposes false order and diverts our attention from the true complexity of nature. Fortunately, objects exhibiting solitary randomness coalesce en masse to produce outcomes with semblance of order. Particulate abrasion is one such example; each particle is unique, yet in large numbers, average behaviour emerges that is representative of the population of derivation. This work specifically investigates the role of particle shape and scale in abrasion. Existing characterisation techniques are appraised and the concepts of shape and scale are developed into definitions that are useful for the solution of the particle characterisation problem. The notion of sharpness is synthesised and then integrated into a new technique called sharpness analysis. The first of two parts, this paper provides a detailed description of sharpness analysis and its application to groups of particles. The resulting groove functions give a representation of particle shape that takes into consideration the penetration of the particles into a wearing surface.

A new technique for measuring particle angularity using cone fit analysis

Abstract The classical abrasion model, proposed by Rabinowicz, is founded on the premise that abrasive surfaces are composed of protrusions that are approximately conical. This premise is supported by Kruschov’s experiments which show that the wear resistance of pure metals in two body abrasion is linearly related to hardness. This leads to an important question — do most abrasive types exhibit conical behaviour or is it coincidental that the abrasives used by Kruschov conformed to the idealised model? To answer this question, a new particle characterisation technique has been developed. By fitting cones to projected particle profiles at different penetration depths, it is possible to judge how closely the protrusions resemble cones. This technique is also useful for assessing the directional properties of particles and characterising the angularity of particle samples in relation to wear rate. The technique has been called cone fit analysis (CFA) — its development and application are discussed in this paper, together with an experimental assessment of some common types of abrasive particles.

Abrasiveness of particles measured by Cone-Fit Analysis (CFA)

This paper presents a newly developed technique for the characterisation of abrasive particle shape. The method described is unique because it is synthesised from the classical abrasion model (proposed by Rabinowicz), where the asperities of abrasive surfaces are modelled by cones. This model is convenient because it predixts that wear rate is linearly related to load and inversely related to hardness. Kruschov provided evidence in favour of the cone paradigm by observing a highly linear relationship between the wear resistance and hardness of pure metals. Unfortunately, similar experiments with different materials do not provide equally convincing correlations. Therefore it is evident that the behaviour of most abrasive surfaces is more complex than predicted by the classical abrasion model. The initial objective of this work was to establish which types of particle, if any, possess geometrical attributes similar to cones. This was done by fitting cones to the projected boundaries of particles at different orientations and penetration depths (hence the acronym CFA for Cone-Fit Analysis ). CFA has since been used to characterise particle sharpness in order to understand its role in abrasion. Due to the large statistical variation of particle shape, CFA has been computer implemented to facilitate the analysis of large particle samples. Consequently, it has been possible to make objective assessments of shape-related abrasive potential of different particle populations. CFA shows that particle asperities do not behave like cones; this conclusion is also supported by pin-on-disk experiments. Application of CFA to different sizes of silicon carbide particles suggests that these are geometrically similar; this important attribute is credited for the lack of sensitivity that wear rate demonstrates in relation to particle size.

Experimental Depiction of the Hypothesis of Flake-Like Wear Debris Generation in Rolling Contact Fatigue of the Rail–Wheel Contact Interface

ABSTRACT Flake-like particles represent a common type of wear debris generated during the rolling contact fatigue wear test using a twin-disc test rig. It is argued that these flake-like particles are generated during the delamination process due to plastic shear strain accumulation at the wearing surfaces. This hypothesis has been developed in the last decades to explain the particle generation mechanism, yet it has not been proven conclusively. This research provides strong experimental evidence of the creation processes of wear debris propagation, aggregation, transfer, and compaction, therefore confirming the existing hypothesis and enhancing the understanding of wear mechanisms in the rolling contact interface.

Numerical Investigation of Hammer Erosive Wear due to Particle Impact

Hammers are the key machine element of high-speed hammer mills which lead to the coal pulverisation process. Progressive material loss from the hammer occurs due to the mechanical interactions between the coal particles and the hammer surface. Coal pulveriser industries implement extensive efforts to combat against premature material loss from the hammer surface due to coal particle impact which may result in premature failure. This work investigates the erosion wear mechanism through computational simulation. A numerical model is developed using Abaqus® to simulate the solid coal particle impacting onto the hammer (target).The Abaqus/Explicit® dynamic simulation solver is used for this analysis. The interactions between the solid coal particles and the target are modelled using the Abaqus/Explicit® element deletion method. The Johnson and Cook plasticity model is employed to analyse the flow stress behaviour of ductile materials during impact. The developed stress and plastic strain are analysed through simulation on the impact surface. This model is applied to different ductile alloys to determine the best erosion wear resistance hammer material for extending the operating life of hammers in the coal pulverisation process.

A simple model to estimate yield stress and variation of hardness in railheads

This technical note presents a macroscopic model capable of estimating the variation of hardness and yield stress at different railhead distances (depths) from the running surface. Published data, including results reported in previous works by the authors, have been utilised to calibrate and test the validity of the model. From this preliminary investigation, it was found that the model can accurately predict the measured hardness and yield stress values. It was also found that the model can represent the variation profile exhibited in the examined railhead material. This model, subject to further validation, has the potential to be used in practical applications.

Wear Particle Analysis and the Evolution of the Plastically Deformed Layer on Australian Rail Steel

Particle analysis methodology is presented, together with the morphology of the wear debris formed during rolling contact fatigue. Wear particles are characterised by their surface topography and in terms of wear mechanism. Rail-wheel materials are subjected to severe plastic deformation as the contact loading progresses, which contributes to a mechanism of major damage in head-hardened rail steel. Most of the current methodologies involve sectioning of the rail-wheel discs to trace material damage phenomena such as crack propagation and plastic strain accumulation. This paper proposes methodology to analyse the development of the plastically deformed layer by sectioning wear particles using the focussed ion beam (FIB) milling method. Moreover, it highlights the processes of oxidation and rail surface delamination during unlubricated rolling contact fatigue.

Abrasive Surfaces Measured by Digital Optical Stereopsis

The principle of stereopsis involves measuring an object’s geometry from a pair of images taken at slightly different viewing positions. This technique is frequently used for geographical mapping in satellite-based reconnaissance, however, the same practice has not been reliably applied at the other end of the scale spectrum: i.e. optical microscope imaging. The impediments have been identified and addressed in this work, concluding that optical stereopsis can be applied to microscopical surface examinations, and that the resulting digital elevation models can be of particular use in tribological investigations for performance and failure analysis.