E.M. Kopalinsky

An investigation of the different regimes of deformation which can occur when a hard wedge slides over a soft surface: The influence of wedge angle, lubrication and prior plastic working of the surface

Abstract Experiments are described in which a tool steel wedge was indented vertically into the horizontal surface of an aluminium specimen with subsequent sliding of the wedge along the surface in a direction normal to its edge. The conditions of the tests were approximately plane strain. The experiments covered a wide range of wedge angles and various lubrication conditions. Tests were made on specimens in the as-received condition and on specimens in which the test surface had been plastically worked prior to a test. Horizontal and vertical forces were measured and a record obtained of the surface deformation of specimens at the end of a test. The experimental results obtained are analysed and discussed in terms of rigid-perfectly plastic slip-line field models which have been proposed for the different regimes of deformation. It is shown in explaining certain aspects of the results that the perfectly plastic assumption is inadequate and that account must be taken of the strain-hardening properties of the aluminium material used for the specimens in the experiments. The relevance of the investigation to the processes of sliding metallic friction and wear is considered.

Explaining the mechanics of metallic sliding friction and wear in terms of slipline field models of asperity deformation

It is shown that the force which opposes the sliding of a hard relatively smooth surface over a softer surface can be explained as the force needed to push waves of plastically deformed material along the soft surface ahead of asperities on the hard surface. For rougher surfaces and/or poorer lubrication it is shown how the wave can be torn off or material removed by a chip formation process and wear particles formed. Coefficients of friction predicted from the corresponding asperity deformation models are shown to give good agreement with experimental results. For smooth well lubricated surfaces the wear of the softer surface is shown to occur as a result of the progressive damage to this surface brought about by the repeated passage of waves across it. Equations for predicting wear are derived from the asperity deformation models and a comparison made between predicted and experimental wear results. The paper ends by considering possible future trends in research into the mechanics of friction and wear.

An investigation of the role of low cycle fatigue in producing surface damage in sliding metallic friction

Abstract Experiments are described in which a hard wedge was indented into a relatively soft surface and then slid along it under well-lubricated conditions. Results are given of a metallographic investigation of the damage in the deformed layer caused by this process. Particular attention is paid to the influence of repeated uni-directional sliding of the wedge over the same length of surface. The relevance of these results to the prediction of wear in sliding metallic friction is considered.

Analysis and experimental investigation of a simplified burnishing process

A slipline field model of a simplified burnishing process is used to derive equations for estimating the burnishing force, the depth of the burnished layer and the plastic strains in this layer. A detailed comparison is made between predicted and experimental results. An observation of considerable practical interest is the high quality surface finish (low surface roughness) which can be obtained by the burnishing process described.

Sliding metallic friction with boundary lubrication: An investigation of a simplified friction theory and of the nature of boundary lubrication

Abstract By simply resolving the forces acting at the interface between contacting asperities it is shown that the coefficient of friction is given by μ = tan(α + arctanc) where α is the asperity angle and c is the constant in the linear shear strength-normal pressure relation τ = cn, which is assumed to be applicable to the lubricant film acting at the interface. An experimental investigation of the theory is described. Consideration is given to the nature of the lubrication at asperity contacts using the experimental results as a basis. Finally the simplified friction theory is looked at in terms of the inclined plane model of early friction theories.

Sliding metallic wear test with in-process wear measurement: a new approach to collecting and applying wear data

Abstract A wear test is described in which the edge of a hard wedge is loaded against the periphery of a rotating disc of softer specimen material. The applied normal load is kept approximately constant during a test. As the test progresses and the disc diameter is reduced by the wear taking place the wedge moves radially inward. By measuring this inward movement during a test it is shown how the wear can be continually monitored. Results are given and it is shown how these might be applied in practice, taking into account the influence of both surface roughness and lubrication.

Metallic sliding friction under boundary lubricated conditions: Investigation of the influence of lubricant at the start of sliding

Experiments are described in which a hard half-wedge, representing a scaled up model asperity, was indented into the horizontal surface of a relatively soft specimen with the specimen then moved in a direction parallel to its surface and normal to the wedge edge. Forces measured at the wedge-specimen interface are used to investigate the state of lubrication at the interface at all stages of the experiment, i.e. from initial indentation to final steady-state wave formation. The results are used to offer possible explanations for the nature of boundary lubrication and of stick-slip motion.

An investigation of the mechanics of sliding metallic wear under lubricated conditions

Tests are described in which a hard wedge is loaded against the periphery of a rotating disc of softer specimen material. The inward movement of the wedge, which occurs as the disc wears, is measured continuously during a test, as are the tangential (friction) and normal forces acting on the wedge. Five liquid lubricants are used in the tests. The wear results are explained in terms of a low cycle fatigue type equation. The results indicate that wear is occurring as a result of the combined action of ratchetting and low cycle fatigue.

An investigation of the interaction of model asperities of similar hardness

Abstract Experiments are described in which records were obtained of the forces and deformation during the life cycle of the interaction of model wedge-shaped asperities of similar hardness and geometry. The results show that the asperities moved past each other by a combination of plastic deformation of the asperities and sliding at their common interface. It appears that the fracture process which in previous work has been assumed to end the cycle did not occur. Slipline field models are presented to describe the deformation and good agreement is shown between stresses and velocities calculated from the slipline fields and those measured experimentally.

The wave model of metallic sliding friction: an investigation of the transition from initial indentation to steady-state wave formation in a model asperity experiment

Experiments are described in which a hard half wedge was indented vertically into the horizontal surface of a relatively soft specimen with subsequent movement of the specimen in a direction parallel to its surface and normal to the edge of the wedge. A liquid lubricant was used in the experiments. Forces and displacements were measured during transition from initial indentation to the point where the edge of the wedge reached the surface with a wave of plastically deformed specimen material pushed ahead of it. Slip-line fields were constructed using the experimental results as a basis to represent deformation during the transition. The results obtained are used in considering mechanisms of boundary lubrication and of stick-slip motion.