Ton Lubrecht

Damage model with delay effect: Analytical and numerical studies of the evolution of the characteristic damage length

This paper studies the delayed damage model in a one-dimensional transient analysis. It is well-known that kind of model prevents the mesh dependency when it is used in a finite element code. The model problem concerns a clamped uniaxial damage elastic bar submitted to a step load at its extremity. In order to guarantee the correct behaviour of the model and to be able to choose the appropriate mesh size before performing a finite element calculation, an analytical evaluation of the size of the damaged zone called characteristic length is given and compared to the converged numerical results. Finally, a two-dimensional example is treated with a damage with or without the delay effect.

Starved elastohydrodynamic lubrication theory: application to emulsions and greases

In classical fluid lubrication the film thickness is mainly determined by entrainment velocity and oil viscosity. At high pressure, elastic deformation occurs changing the local geometry: this is the elastohydrodynamic lubrication regime (EHL). When a limited amount of lubricant is available to the contact, a component failure due to lubricant starvation can result. A new starvation model is presented, using the amount of oil on the surfaces as the key parameter controlling starvation. It is successfully applied to describe the contacts lubricated with multi-phase fluids such as emulsions and greases, which combine starvation with a very complex rheology

A novel experimental analysis of the rheology of ZDDP tribofilms

In this study, the rheology and durability of tribofilm has been investigated. A ZDDP tribofilm (6×8 mm) has been partially removed by Ar+ ion etching (5keV), using an aluminum mask with a comb shape. A striped sample was obtained with a lateral alternation of etched zones and ZDDP tribofilm areas, respectively (about 1 mm width). On this striped sample, a friction test was performed using a reciprocating pin-on-flat friction machine. Tests were carried out in different lubricant environments at ambient temperature with the maximum contact presure of 0.5 GPa. The friction signal and the Electrical Contact Resistance (ECR) were plotted as a function of position and number of cycles. The results can be explained by a mechanical process in the tribofilm zone (its removal) and chemical reaction in the etched zone.

Shearing of adsorbed polymer layers in an elastohydrodynamic contact in pure sliding

The valve train wear of automotive diesel engines mainly depends on the kinematics of the contact, the metallurgy of the rubbing surfaces and the lubricant [1]. The presence of soot in the oil causes a dramatic increase in wear of the cam/tappet contact [2-3]. It has been shown, in this case, that the interactions between the soot particles and the rubbing solid surfaces are very important in the understanding of the wear and lubrication process. These interactions are partially governed by the dispersant polymer in solution in the automotive lubricant. Then, the tribological and the rheological behaviour of the boundary films anchored on to the surfaces must be considered. This paper focusses on the shearing of polymer films inside an elastohydrodynamic contact. The application of low speed alternative motions of the contact with successive periods of pure rolling (vanishing sliding speed) and pure sliding (vanishing entrainment speed) leads to the formation of film accumulations that are always entrained by the lubricant flow. The shearing and the geometry of these aggregates strongly depend on their adherence to the solid surfaces. The agglomeration process is stopped when the adsorbed layers of polymers have been consumed and totally peeled off from the solids. A numerical simulation including the peculiar kinematics of the contact in a pure sliding elastohydrodynamic lubrication regime gives the evolution of the oil film thickness versus time. The results of this model are compared to the experimental results. The consumption and the adherence of these layers to the substrates can be related to the wear process of the valve train.

The role of mechanical and chemical processes in anti-wear properties of ZDDP tribofilms

ZDDP additives have been used for their antiwear and antioxidant properties in lubricating systems for many years. In case of boundary lubrication, this additive generates a protective solid film on the surfaces called tribofilm. This work focuses on the rheological properties and durability of such films. In a previous paper, the feasibility of a particular experimental analysis of the rheology of ZDDP tribofilms has been demonstrated. This test procedure under realistic conditions (macrotribology) permits the visualization and quantification of the film rheology and its durability. It consists of different steps. First of all, a large ZDDP tribofilm (6×7 mm 2 ) is generated using a Cameron-Plint tribometer in cylinder-on-flat configuration under standard lubricated conditions (PAO and ZDDP). An aluminum mask with a comb shape is positioned on the tribofilm, which is then etched with an Ar + ion beam (5 keV), in order to remove the tribofilm where the mask does not cover the sample. Finally, a striped sample is obtained with a lateral alternation of etched zones and the original ZDDP tribofilm area (of approximately 1 mm width). On this striped sample, a Dual Zone Friction Test (DZFT) experiment is performed. It is in fact a pin-on-flat experiment run across a strip of tribofilm. The friction signal and the Electrical Contact Resistance (ECR) are plotted as a function of the position and the number of cycles. The pin-on-flat experiments are carried out dry (open air, room temperature) and under ultra high vacuum and the results were compared. Special attention is given to the evolution of the chemical composition inside the track during each experiment: this, in order to study the kinetics of the tribofilm disruption by coupling friction experiments with Auger Electron Spectroscopy analysis (AES) at intervals of the experiment. Both mechanical and chemical mechanisms seem to be involved in the removal processes of the ZDDP tribofilm.