Identification of dynamic contact instabilities generated by braking materials

The occurrence of unstable friction-induced vibrations is a major issue for braking manufacturers, as they lead to annoying noise, structure vibrations and brake surface degradation. Understanding the underlying causes of frictional instabilities, arising during the sliding between two bodies, is necessary for developing solutions and countermeasures. For this purpose, in this work, an experimental and numerical investigation of contact instabilities has been performed. Mode coupling and negative friction-velocity slope instabilities have been numerically investigated by both lumped-parameter and finite element models. As well, an experimental campaign has been carried out for recovering the frictional and vibrational response of braking materials under different boundary conditions. The comparison between numerical and experimental results allows validating a new methodology, based on the study of the phase shift between the tangential and normal vibrational responses, in order to distinguish the different types of contact instabilities

Dynamic Finite Element Simulations for Understanding Wheel-Rail Contact Oscillatory States Occurring Under Sliding Conditions

International audienceThis paper presents a temporal study using dynamic finite element methods of the dynamic response of a 2D mechanical model composed of a deformable rotating disk (wheel) in contact with a deformable translating body (rail) with constant Coulomb friction. Under global sliding conditions, oscillatory states at specific frequencies occur in the contact patch even in the case of a constant friction coefficient. A parallel is drawn between the frequencies of these states and the modal analysis of the entire mechanical model. The influence on local contact conditions of parameters such as normal load, global sliding ratio, friction coefficient, and the transient value for applying sliding conditions is then evaluated. Finally, the consequences of these states on local rail plastic deformation are presented and correlated with rail corrugation occurring on straight tracks under acceleration and deceleration conditions

Consequence of contact local kinematics of sliding bodies on the surface temperatures generated

International audienceWhen studying contact with friction between two bodies, it is not possible to obtain data on real contact conditions on the basis of steady-state situations. Indeed, contacts with friction usually lead to dynamic instabilities generated at the contact interface. It is therefore necessary to take into account contact dynamics in order to better understand the phenomena involved during sliding with friction. The explicit dynamic finite element code PlastD in 2D is used to simulate the contact between two bodies. A constant Coulomb friction coefficient is imposed at the interface. The simulations carried out permitted identifying local contact conditions (kinematics, tribological state, stresses, etc.). They revealed that different instability regimes can be generated (stick–slip, slip–separation, stick–slip–separation, etc.). Local contact stresses and the sliding velocity oscillate through time when instabilities are generated and their maximum values can be much higher than those expected for steady-state conditions. The aim of this paper is to analyse the frictional instabilities and their consequences on the heat generated in the contact. First, the influence of the different instability regimes is studied on a simple contact. Then, an industrial mechanism is studied (wheel–rail contact) to investigate the influence of local contact conditions on the temperature of the rail surface

Contact instability identification by phase shift on C/C friction materials

Carbon-carbon (C/C) composite material is currently among the most promising engineering materials for friction applications, where excellent tribological properties, lightweight and good thermal stability are needed. As a result, the industrial demand for C/C composite leads to the need to characterize in detail the frictional and vibrational response of such material, when adopted for high performance braking applications. In this context, the present work shows an experimental and numerical characterization of unstable friction-induced vibrations caused by frictional contact between C/C specimens. The results provide information on the C/C material behavior at high-temperature conditions as well as additional tools to distinguish the occurrence of different vibrational phenomena. The phase shift between vibrational signals has been correlated to different kind of contact instabilities (either mode coupling or negative friction-velocity slope), that can arise and bring to high amplitude oscillations and noise emission. Such correlation has been observed experimentally and reproduced numerically

Competition between 3rd body flows and local contact dynamics

When contact occurs between two bodies in relative motion, instability state – i.e. stick-slip or stick-slip-separation up so several kHz – often occur in the contact . Such phenomenon has been numerically highlighted and studied by Baillet and Massi using dynamic finite element modelling (FEM) with constant Coulomb’s friction coefficient. Nevertheless, such model doesn’t take account for the 3rd body flow inside the contact and its interactions with contact dynamics. Recently, Renouf has coupled finite element method with discrete element method (DEM) to compensate for this lack. The purpose of the present work is so to validate experimentally the numerical results obtained from both contact dynamics and 3rd body flows . An experimental set-up, called “PhotoTrib”, has so been developed to reproduce the birth conditions of both instability state and 3rd body particles

Numerical tribology of a dry contact

Tribologists are confronted on a daily basis by the need to understand the causes and consequences of friction on the behaviour of bodies in contact. Understanding contact behaviour is not only a scientific curiosity but the key to solving numerous industrial issues. Numerical tools have been developed to overcome the problems encountered in experiments due to limitations in the local dynamic analysis of multi-scale systems (mechanisms, bodies in contact, interfaces). More than an exhibition of numerical results, the present paper proposes reviewing the literature on the numerical tribology of dry contacts by analysing the different scales involved. © 2011 Elsevier Ltd. All rights reserved

Simulation of Dynamic Instabilities Induced by Sliding Contacts

When dealing with complex mechanical systems that include sliding contact, it is necessary to account for the coupling between the dynamic behavior of the system and the local behavior at the contact. A particular consequence of interaction between system dynamics and contact behavior is the occurring of vibrational instabilities of the mechanical system, induced by the frictional contact. The dynamics of bodies in sliding contact can thus become unstable, due to the modal coupling caused by the normal and frictional components of the contact forces. Friction induced instabilities are at the origin of several everyday issues such as squeaking of door hinges or brake squealing. In literature, a large number of works deal with this kind of instabilities and are mainly focused on applied problems such as brake squeal noise. This paper shows a more general numerical analysis focused on a simple system constituted by a deformable cylinder that rotates around a rigid cylindrical surface with friction. The parametrical complex eigenvalue analysis and the transient numerical simulations show how the friction forces can origin dynamic instabilities due to the coupling between two system modes, even for such a simple system (one deformable body). The simplicity of the system allows for a deeper analysis of contact instabilities. Results from the experimental analysis allow for validating the model and confirm the occurring of the simulated dynamic contact instabilities

Modal dynamic instabilities generated by frictional contacts

Presence of contact interface between deformable bodies can have very different purposes in mechanical systems. In some application the aim is to maximize the energy dissipation (e.g. brake systems); on the contrary in other cases the aim is to allow for a relative displacement between the two bodies, maximizing the efficiency (joints). In both cases frictional forces at the contact interfaces allow for coupling contact and system dynamics, and they can give origin to dynamic instabilities. From a numerical point of view the contact forces introduce an asymmetry on the system stiffness matrix, acting as a cross coupling factor between tangential and normal deformation at the contact interface [1]. Typical examples of this phenomenon, largely investigated in literature [2], are the squeal noise emission in automotive brakes or railway wheels. The coupling between system and contact results on unstable vibrations of the system. During vibration, the deformations due to the dynamic response of the system modify substantially the contact stress distribution. This variation bring to the onset of contact nonlinearities, such as the local transition between the different contact status (sliding, sticking, detachment) modifying the boundary conditions of the system. In case of dynamic instabilities, "Friction induced vibration" can result in high amplitude vibrations characterized by a harmonic spectrum at an eigenfrequency of the mechanical system and generally associated to noise emission. Between the mechanisms retained at the origin of such vibrations, the unstable coupling between two modes of the system is one of the most commonly adopted. The presence of friction forces governed by Coulomb Low couples the tangential and normal modal deformation at the contact of two coalescing modes. In order to generalize the mode coupling phenomenon an analysis developed on a simplified system, able to reproduce in-plain modal instabilities a single deformable body, is here proposed