Philippe Dufrenoy

Third Body Influence on Thermal Friction Contact Problems: Application to Braking

The aim of this study is to evaluate the temperature and the heat distribution into the two components of a disc brake system by combining macroscopic and microscopic effects. The major difficulty of the thermal problem is to determine how the heat is generated and how it is distributed in the two components in contact during transient state. Contrary to classical approaches assuming equal temperature at the contact surfaces, a contact interface is introduced in the model as a thin layer of third body with uniform volumic heat generation. This micro-macro model gives original indications on the temperatures near the contact surfaces, on the thermal gradients between the two components and on the heat partition between the two bodies during the braking time. Comparison with classical thermal models is discussed.

PROGRESSIVE WAVINESS DISTORTION: A NEW APPROACH OF HOT SPOTTING IN DISC BRAKES

ABSTRACT This paper deals with the mechanisms of the appearance of hot spots in railway brake disks. From infrared measurements, a classification of hot spots is presented. The most damaging thermal gradients are identified as macroscopic hot spots uniformly distributed on the friction surface. An explanation of the origin of these macroscopic hot spots is presented based on a scenario of progressive distortion of the disk. Three major effects have to be considered: the structural response of the disk and the pads; realistic boundary conditions, both geometric and loading; and the thermoelastoplastic behavior of the material. Finite element simulations based on the proposed model agree with experimental observations.

Copper Substitution and Noise Reduction in Brake Pads: Graphite Type Selection

Graphite is commonly used in brake pads. The use of graphite powder has the main goal of solid state lubrication and friction coefficient stabilization. In this article results on resin bonded brake pads with focus on noise performance and heat dissipation are presented. Experimental tests are based on model friction materials with a known formulation and a reduced number of components for a better identification of the role of the graphite type. Results clearly indicate that both noise performance and thermal conductivity are strongly affected by the type of graphite. Guidelines for the selection of graphite types for optimized friction materials are given.

Prediction of the Initial Residual Stresses in Railway Wheels Induced by Manufacturing

This article presents a prediction of the residual stresses that are introduced by the heat treatment in the manufacture of railway wheels. It is well known that this treatment induces the formation of a layer of residual circumferential (hoop) compressive stresses on the tread surface. This stress field may be reversed during service loading, especially tread braking. Tensile stresses at the wheel tread that arise from repeated braking may lead to the initiation of surface cracks. The work presented here defines a methodology for determining the as-manufactured residual stress field prior to service loading. A thermal and mechanical finite element model was used to study the residual stress evolution during the heat treatment. The heat transfer coefficient during quenching was determined by means of a dedicated experimental setup. The results show that the numerically computed residual stresses correlate strongly with the experimental residual stresses determined by ultrasound when the model material includes time-dependent effects such as creep or viscoplasticity.

Failure Mechanisms of TGV Brake Discs

The CRISFIS project (supported by ADEME agency) consists in jointly studying the squealing and cracking aspects of the high power disc brakes for TGV very high speed trains. This paper deals with the progress concerning the cracking part. An experimental and modelling strategy is adopted in order to better understand and predict brake disc cracking. Braking tests conducted on an industrial scale-one test bench are presented. In a first step, the influence of the pad-type on the thermal loading applied to the disc is studied by means of an infrared camera and thermocouples embedded in the pads and in the disc. In a second step, the thermal maps extracted from thermographic monitoring are used as input data for thermal-mechanical calculations. Finally, the results of modelling and tests are compared to the damage observed on the brake discs.

Infrared characterization of thermal gradients on disc brakes

The heat generated in frictional organs like brakes and clutches induces thermal distortions which may lead to localized contact areas and hot spots developments. Hot spots are high thermal gradients on the rubbing surface. They count among the most dangerous phenomena in frictional organs leading to damage, early failure and unacceptable braking performances such as brake fade or undesirable low frequency vibrations called hot judder. In this paper, an experimental study of hot spots occurrence in railway disc brakes is reported on. The aim of this study was to better classify and to explain the thermal gradients appearance on the surface of the disc. Thermograph measurements with an infrared camera have been carried out on the rubbing surface of brake discs on a full-scale test bench. The infrared system was set to take temperature readings in snap shot mode precisely synchronized with the rotation of the disc. Very short integration time allows reducing drastically haziness of thermal images. Based on thermographs, a classification of hot-spots observed in disc brakes is proposed. A detailed investigation of the most damaging thermal gradients, called macroscopic hot spots (MHS) is given. From these experimental researches, a scenario of hot spots occurrence is suggested step by step. Thanks to infrared measurements at high frequency with high resolution, observations give new highlights on the conditions of hot spots appearance. Comparison of the experimental observations with the theoretical approaches is finally discussed.

Relationships Between Surface Thermal Gradients and Disc Distortion During Stop-Braking with High Energy Dissipation

This paper focuses on the study of the relationships between brake disc surface temperatures and disc distortion for various high-energy stop-braking conditions. An original thermal metrology method combining an infrared camera and a fibre-optic two-colour pyrometer was used to record the spatial and temporal variation in disc surface temperature during braking. Disc distortion was investigated in situ by means of a high-frequency displacement sensor. In addition, an optical trigger kept track of disc revolutions and enabled the synchronization of the IR camera, two-colour pyrometer and displacement sensor measurements. This experimental set-up was successfully used to determine the surface temperature and investigate thermal localization and waviness distortion during braking. The results were correlated with each other in relation to the level of energy dissipation. It was shown that the highest temperature was reached in the hot spot regions at an early stage of stop-braking. By contrast, the greatest disc distortion appeared much later, during the last stage of stop-braking.

Monitoring of temperature and emissivity during successive disc revolutions in braking

The main difficulties of brake disc infrared temperature measurements are the unknown of disc surface emissivity, non-uniform and time-varying, and the high rotation speed and deceleration of the disc, which makes it difficult to follow the special distribution of the temperature. To provide information on the emissivity variation during braking, brake disc temperature and emissivity were investigated by an original optic-fiber two-color pyrometer combined with an infrared camera, which allows monitoring the true friction areas. In addition, an optical top-tour was used to identify disc revolutions. The evolutions of the surface temperature and emissivity on successive disc revolutions were successfully determined. It was showed that circumferential thermal gradients form on the surface and maintain their angular position, they can, however, win or lose in intensity with braking progress.

Monitoring of Transient Phenomena in Sliding Contact Application to Friction Brakes

This work focuses on the study of transient phenomena, in particular the non-uniformity and space–time variation of friction forces and surface temperature of a brake disc during stop-braking. Friction tests were conducted on a braking tribometer. The friction forces in the contact were measured using a 3D piezoelectric sensor, while the disc surface temperature was investigated by means of a high frequency fibre-optic two-colour pyrometer. An optical lap-top device was used to keep track of disc revolutions, and an original programme was written to plot the space–time variations of the measured parameters. This new original approach helps better understand the coupling between thermal and tribological phenomena occurring during braking.

An improved lagrangian thermography procedure for the quantification of the temperature fields within polycrystals

Polycrystalline metallic materials are made of an aggregate of grains more or less well oriented with respect to the loading axis. During mechanical loading, the diversity of grain orientations leads to a heterogeneous deformation at the local scale. It is well known that most of the plastic work generated during the deformation process reappears in the form of heat, whereas a certain proportion remains latent in the material and is associated with microstructural changes. To access the local stored energy during deformation processes, experimental energy balances are needed at a suitable scale. Thus, simultaneous measurements of thermal and kinematic fields were made in-house at the microstructural scale of a 316L stainless steel submitted to a macroscopic monotonic tensile test. The aim of the present study is to propose a complete calibration strategy allowing us to estimate the thermal variations of each material point along its local and complex deformation path. This calibration strategy is a key element for achieving experimental granular energy balances and has to overcome two major experimental problems: the dynamics of each infrared focal plane array sensor that leads to undesired spatial and temporal noise and the complexity of the local loading path that must be captured by simultaneous complementary measurement. The improvement of such a multifield strategy is crucial for performing properly the experimental and local energy balances required to build new energetically based damage criteria.