Ivan Iordanoff

A Review of Recent Approaches for Modeling Solid Third Bodies

The paper considers the behavior of third bodies in dry contacts. A description of the mechanism operating in contacts is given and the influence of external parameters outlined. Both physicochemical and mechanical conditions greatly influence third body behavior. Depending on third-body composition, the external influence can be more or less dramatic. Due to difficulties with experimentation, numerical modeling is suggested as a complementary tool. Two approaches for such modeling are described, the continuum and the discrete approach. At present these models are at an early development stage (two-dimensional simulations), and great efforts should be made for their further development. While an experimental apparatus can study phenomena at only one scale; namely the nanometer with AFM, or the micron to millimeter with fretting machines, modeling is able to range from the microscopic properties (particle interactions like coefficient of restitution) to the macroscopic properties by integration or averaging (load capacity, friction coefficient).

A Granular Dynamic Model for the Degradation of Material

The work presented here is a model of the degradation of a material (by particle detachment), based on a two dimensional granular dynamic model designed to study the flows of third body particles inside a contact. As the detached particles (third body) cannot exit the contact, the detachments stop after a certain time and a stable layer of third body can be seen. It is shown that the thickness of this stable layer depends both on the conditions applied (normal pressure and sliding speed) and the physicochemical interactions between the detached particles. Such investigations provide better understanding of the mechanism leading to the degradation of material.

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.

An Original Definition of the Profile of Compliant Foil Journal Gas Bearings: Static and Dynamic Analysis

This article deals with a numerical analysis of the static and dynamic performance of a compliant journal gas bearing. The common approach found in foil bearing literature consists in calculating the carrying capacity for a given shaft position. In this study the external load is fixed (magnitude and direction) and the related shaft position is investigated. Nevertheless, a rigid profile, able to support high imposed loads, is no longer valid if one considers that the bearing becomes compliant. An original calculation method of the initial profile considering rigid surfaces is proposed to overcome this problem. The prediction of nonlinear dynamic behavior, i.e., stability and response to external excitation, is investigated. Finally, a viscous damping model is introduced into the dynamic model in order to obtain the amount of structural damping necessary to increase the stability of the compliant journal gas bearing.

A Continuum Description of Dense Granular Lubrication Flow

The present paper applies a recent continuum theory due to Aranson and Tsimring (2002, “Continuum Theory of Partially Fluidized Granular Flows ,” Phys. Rev. E, 65, p. 061303) for the dense granular flow of particles in sustained contact to lubrication flows. Such third body granular flow may apply to some solid lubrication mechanisms. The continuum theory is unique in that it addresses solidlike behavior and the transition to fully fluidized behavior. The continuum studies are complemented by a discrete particle dynamics model of Iordanoff (2005, “Numerical Study of a Thin Layer of Cohesive Particles Under Plane Shearing ,” Powder Technol., 159, pp. 46–54). Three problems are treated: (1) flow due to the gravity of a layer of granular material down an inclined plane, (2) simple shear flow of a layer confined between sliding parallel surfaces, and (3) lubrication flow of a layer confined between a curved surface and a sliding plane. The perspective of this paper is that a continuum model will be more useful than a discrete model in engineering design of solid lubrication systems for the foreseeable future. In the inclined plane problem, the discrete simulations are used to provide material property parameters to the continuum model. In the simple shear problem, for validation, predictions of the continuum model are compared to those of the discrete element computer simulations. Finally, the continuum theory is applied to a more complex lubrication flow.

Influence of muscle-tendon complex geometrical parameters on modeling passive stretch behavior with the Discrete Element Method.

The muscle-tendon complex (MTC) is a multi-scale, anisotropic, non-homogeneous structure. It is composed of fascicles, gathered together in a conjunctive aponeurosis. Fibers are oriented into the MTC with a pennation angle. Many MTC models use the Finite Element Method (FEM) to simulate the behavior of the MTC as a hyper-viscoelastic material. The Discrete Element Method (DEM) could be adapted to model fibrous materials, such as the MTC. DEM could capture the complex behavior of a material with a simple discretization scheme and help in understanding the influence of the orientation of fibers on the MTC׳s behavior. The aims of this study were to model the MTC in DEM at the macroscopic scale and to obtain the force/displacement curve during a non-destructive passive tensile test. Another aim was to highlight the influence of the geometrical parameters of the MTC on the global mechanical behavior. A geometrical construction of the MTC was done using discrete element linked by springs. Young׳s modulus values of the MTC׳s components were retrieved from the literature to model the microscopic stiffness of each spring. Alignment and re-orientation of all of the muscle׳s fibers with the tensile axis were observed numerically. The hyper-elastic behavior of the MTC was pointed out. The structure׳s effects, added to the geometrical parameters, highlight the MTC׳s mechanical behavior. It is also highlighted by the heterogeneity of the strain of the MTC׳s components. DEM seems to be a promising method to model the hyper-elastic macroscopic behavior of the MTC with simple elastic microscopic elements.

The GranOO workbench, a new tool for developing discrete element simulations, and its application to tribological problems

This work was supported by the Conseil Regional d’Aquitaine and was conducted under the auspices of the Etude et Formation en Surfacage Optique (EFESO 2) project.

First steps for a rheological model for the solid third body

Abstract Much progress has been made in our understanding of dry friction since the 1980s through the work of M.Godet et al. [1, 2, 3, 4], in particular with the introduction of the third body concept. As the third body approach is well adapted for phenomenological description, many theoretical studies have analysed artificial third bodies. These studies attempt to predict contact behaviour. The problem is so complex, however, that these studies are used to complement experimental observation. This paper first describes the main granular flow models that have been developed during the past 10 years : the kinetic model [5, 6, 7, 8], the granular model [10, 11, 12] and the quasi-hydrodynamic model [13-14]. The granular model is a very good tool for understanding and visualising the transition between microscopic and macroscopic phenomena. The quasi-hydrodynamic approach seems to give good qualitative and quantitative results. This model is based on a macroscopic rheological law where other studies are based on microscopic contact laws between grains. The rheological law can be found by experimental studies whereas this is not the case for contact laws. As the quasi hydrodynamic approach can not be applied to all real contacts, the macroscopic thought process is retained extended to real conditions: first, an experimental study was carried out with an artificial third body: TiO 2 Rutile powder. A visco elastic-visco plastic behaviour was found. It was also shown that in many cases, the greatest part of velocity accommodation occurs at the interface between first and third body. Because of this, the influence of the interface was studied. It is then obvious that the interface has to be taken into account in the rheological model of the third body : the slipping phenomenon is not a specific property of the powder but depends on the interface : this leads to a new definition of the limiting shear stress τ 1 presented in previous rheological models [14].

Discrete-Continuum Coupling Method to Simulate Highly Dynamic Multi-Scale Problems: Simulation of Laser-Induced Damage in Silica Glass

Complex behavior models (plasticity, crack, visco-elascticity) are facing several theoretical difficulties in determining the behavior law at the continuous (macroscopic) scale. When homogenization fails to give the right behavior law, a solution is to simulate the material at a mesoscale using the discrete element model (DEM) in order to directly simulate a set of discrete properties that are responsible for the macroscopic behavior. Originally, the discrete element model was developed for granular material.