Mathieu Renouf

Thermal Study of the Dry Sliding Contact With Third Body Presence

The objective of this paper is to highlight the influence of the rheology of a third body in dry sliding contact conditions. It has been shown that the local cohesion of the third body can create an asymmetric dissipative field through its thickness. The present study puts forward the consequences from a thermal point of view, overcoming the inherent experimental difficulties at this microscopic scale.Copyright

Friction Coefficient as a Macroscopic View of Local Dissipation

This paper presents an overview of a discrete element method approach to dry friction in the presence of a third body. Three dimensional computer simulations have been carried out to show the influence of the third body properties (and more specifically their adhesion) on friction coefficient and profiles of dissipated power. Simple interaction laws and a cohesive contact are set up to uncouple the key parameters governing the contact rheology. The model is validated through a global energy balance. As it is shown that dynamic friction coefficient can be explained only in terms of local energy dissipation, this work also emphasizes the fact that mechanism effects and third body rheology have important consequences on the energy generation and dissipation field. Therefore, asymmetries can arise and the surface temperature of first bodies can be significantly different even for the same global friction coefficient value. Such investigations highlight the fact that friction coefficient cannot be considered in the same way at the mechanism scale as at the contact scale where the third body plays a non-negligible role, although it has been neglected for years in thermal approaches to study of surfaces in contact.

Modeling Wear for Heterogeneous Bi-Phasic Materials Using Discrete Elements Approach

A proper understanding of the processes of friction and wear can only be reached through a detailed study of the contact interface. Empirical laws, such as Archards, are often used in numerical models. They give good results over a limited range of conditions when their coefficients are correctly set, but they cannot be predicted: any significant change of conditions requires a new set of experimental coefficients. In this paper, a new method, the use of Discrete Element Models (DEM), is proposed in order to tend to predictable models. As an example, a generic biphasic friction material is modelled, of the type used in aeronautical or automotive brake systems. Micro-scale models are built in order to study material damage and wear under tribological stress. The models show what could be achieved by these numerical methods in tribological studies, and how they can reproduce the behavior and mechanisms seen with real-life friction materials without any empirical law or parameter.

Self-lubricating composite bearings: Effect of fibre length on its tribological properties by DEM modelling

Self-lubricating polymer-based composites are used in space and in aircraft mechanisms as materials for solid lubricated systems. Such composites mostly consist of a polymeric matrix and fillers of two kinds: hard fillers (fibres made of glass, or of minerals) and solid lubricating particles (made of MoS 2). Their advantages are that they provide their own lubrication, and they can be used in both very high and very low temperatures (from −40 up to ~200 F). Precision ball bearings with these composites are manufactured since the 60s in these bearings the retainer material itself provides the lubrication. From the experimental analyses implemented (X-ray tomography, SEM observations, and experiences in a tribometer); it is possible to observe that the geometry of the fillers has a strong influence on the third body rheology. Nevertheless, the confined nature of the contact does not allow in-situ observation. To overcome this difficulty a combined numerical/experimental approach is carried out. To be able to reproduce the evolution of third-body particles within the contact, Discrete Element Methods (DEM) is used. Such an approach allows to represent wear: by the construction of an equivalent continuous medium resulting from the incorporation of interaction laws between the discrete particles. The motivation to this work is the understanding of the impact of filler geometry o tribological behaviour of these materials. More specifically, the goal is to study the influence of the fibre length in the tribological behaviour of self-lubricating composites by Discrete Element Methods (DEM).

Role of Third Body on Bolted Joints' Self-Loosening

Bolted joints are frequently subjected to self-loosening (gradual loss of clamping force) causing multiple failures, especially leaking and breaking of mechanical systems. Such physical phenomena would occur whatever the considered coating (Ag, MoS2, Zn–Ni and others). To enlighten this phenomenon, which remains rather misunderstood due to the confined nature of bolted joint contacts, a coupled experimental-numerical approach is adopted on a bolted joint with silver coating. Indeed, from tribological expert assessments of disassembled joints without loosening, a local view of nut/screw threads contacts is proposed, using discrete element method. This method becomes essential in tribology since it offers the ability to model the dynamic behavior of a contact interface. The model is based on a Non-Smooth Contacts Dynamics approach. The case of third body formed in contacts during tightening process, which has been ignored so far, is placed at the focus of self-loosening phenomenon.

Thermo-Mechanical Investigations of a Tribological Interface

Numerical methods are essential to understand tribological behaviors since it is difficult to measure directly a closed contact or write representative analytical equations. In this paper, a focus on the complexity of a contact is done with the modeling of thermo-mechanical phenomena in connection with tribological triplet (mechanism, first bodies, third body). Discrete element method is chosen to have a dynamic view of a contact and is interesting to represent both damage of first bodies and cohesion of third ones. Thermo-mechanical models are described for first and third-bodies and are adjusted as a function of continuity of the body. Results regarding damage, rheology and thermal effects are studied as a consequence of cohesion of third body and applied energy by the mechanism (pressure, velocity). Because mechanical and thermal behaviors have a narrow but unclear relationship, a balance between local energy (cohesion of third body) and global energy (applied forces by mechanism) is recommended.

Solveurs parallèles pour la simulation de systèmes multicontacts

Dans le cadre des systèmes multicontacts, et en particulier des milieux granulaires, la méthode de dynamique des contacts (Non Smooth Contact Dynamics) intègre un solveur de type Gauss Seidel non linéaire à chaque pas de temps. Nous proposons ici deux implémentations de cet algorithme aux comportements différents dans leur version parallèle. Un nouvel algorithme de type Gradient Conjugué, intrinsèquement parallèle, est également présenté.

First-Body Versus Third-Body: Dialogue Between an Experiment and a Combined Discrete and Finite Element Approach

The present paper proposes to analyze relations between the behavior of two bodies in contact (local stress, vibration modes) and the rheology of third-body particles. Experiments are performed on a system composed of a polycarbonate disk in contact with a steel cylinder, where birefregent property of polycarbonate allows to observe shear stress isovalues. Multi-scale numerical simulations involve the coupling between finite elements and discrete elements to model simultaneously non-homogeneous third-body flows within a confined contact and dynamical behavior of the bodies in contact. Comparisons between experiments and simulations are performed on the dynamic response of the system, the stress distribution as well as the evolution of third-body particles within the contact. Such comparisons exhibit not only qualitative results but also quantitative ones and suggest a new approach to study in deeper third-body rheology.