Strain-rate effects on the plastic indentation and abrasion of elastohydrodynamic contacts by debris particles

Abstract

A numerical model of plastic indentation and abrasion of elastohydrodynamic contacts by debris particles previously developed by the author is extended to include the dependence of material flow stress on strain rate. Using the JohnsonCook viscoplasticity model, the flow stress of all materials involved in the indentation process is expressed as a function of plastic strain, strain rate and temperature. This complements other elements of the model, including strain-gradient plasticity, work-hardening, frictional heating from particle extrusion, thermal softening, melting and material loss due to adhesion. Following a laborious programme of experimental validation and numerical comparisons, the predictions of the model are shown to be in excellent agreement with the experimental results on soft and hard particles in rolling and rolling-sliding elastohydrodynamic contacts. The incorporation of strain-rate effects further improved the agreement between theoretical and experimental results previously established with simpler versions of the model that ignored the strain-rate factor. Strain rate is also shown to affect several parameters in the process of surface damage, including the magnitude of contact stresses and flash temperatures, as well as the behaviour of a particle in a concentrated contact. It is also shown that for an optimum contact velocity linked to strain-rate effects and fluid film thickness in lubricated contacts, surface damage is minimised, particularly for large and hard particles.

Particle extrusion in elastohydrodynamic line contacts: Dynamic forces and energy consumption

“…He 51 also added thermal effects from particle frictional heating, including conductive and convective heat losses. Nikas 61 added viscoplasticity effects, i.e. thermodynamically varying material flow stresses, to incorporate the effect of the rate of plastic strains during indentation and particle deformation.…”

“…For the sake of completeness and clarity, only an educative summary is presented in the next pages. The thermal part of the analysis is not detailed here and readers are referred to studies by Nikas, 51,61,62 particularly article 62 that contains a recapitulation of the whole model barring the pile-up functionality. On the other side, everything that is directly related to the force analysis is detailed next, yet as briefly as possible.…”

“…where p is the block contact pressure, G p is the shear modulus of the particle at its average temperature p , " is a representative temperature of the solids adjacent to the particle disc to calculate temperature-affected material properties defined by Nikas, 51,62 and k is the kinetic friction coefficient between a block and an EHD contact surface. For non-sticking blocks, the slip velocity vector on a contact surface is 51,[60][61][62]…”

“…51 Given the complexity of the contact mechanics to analyse the passage of even a single particle through a contact, it comes as no surprise that numerical models are rare and variable in their features and limitations. According to the author's own research, [51][52][53][54][55][56][57][58][59][60][61][62][63] the model of the passage of a single particle through a concentrated contact with clearance smaller than the particle's minimum dimension includes the following elements: (a) particle entrainment via the lubricating medium prior to reaching the contact; 53 (b) particle entrapment based on force-equilibrium criteria once the particle is pinched; [57][58][59] (c) stress, strain and damage analysis during particle compression, including thermoelastic effects, [54][55][56] elastoplastic effects, 60 thermoelastoplastic effects 51 or thermoviscoplastic effects, 61 depending on the operating conditions, as well as geometrical effects in case of surface pile-up around dents 63 and tribochemical effects in cases of high frictional heating. 62 With a numerical model as the one just described, calculating contact forces on a squashing particle is part of the stress analysis.…”

“…In the author's latest ductile debris model, 51,60-62 recapitulated in Nikas 62 and with sub-models on particle entrapment in non-conformal line contacts 57 and both conformal and nonconformal elliptical contacts, 58,59 the previously stated shortcomings and simplifying assumptions are all eliminated. The model was gradually constructed with rigorous experimental validation in each of five successive development stages, adding micro-hardness modelling, 60 thermoelastoplasticity, 51 thermoviscoplasticity, 61 tribochemistry 62 and geometrical (surface pile-up) effects, 63 although the last two are not of concern in the present study. It is thus used here to calculate the normal and frictional forces on ductile, originally spherical particles passing through EHD, rolling-sliding, line contacts where the conditions favour surface sinking-in (no pile-up) as explained later.…”