Braking is mainly controlled by the engine. Friction between a pair of pads and a rotating disc converts the kinetic energy of the vehicle into heat. High temperatures can be reached in the system which can be detrimental for both, components and passenger safety. Numerical techniques help simulate load cases and compute the temperatures field in brake disc and brake pads. The present work implements a Finite Element (FE) toolbox in Matlab/Simulink able to simulate different braking manoeuvres used for brake dimensioning mainly in the early phase of car development process. The brake pad/disc geometry is considered as an axisymmetric body assuming negligible temperature gradient along the circumference of the disc. Calibration using three control factors namely: heat coefficient during braking , acceleration  and emissivity  for the implemented thermal model is performed using experimental investigation at Volvo Car Corporation (VCC) for three specific severe load cases. The thermal model is extended to measure brake fluid temperatures to ensure no vaporisation occurs. Simulation results of the brake disc and brake pad show good correlation with the experimental tests. A sensitivity analysis with the control factors showed convective coefficient during acceleration  the most sensitive, with temperature change of around 16%.

L’objectif de cette thèse est de proposer de nouvelles solutions matériaux moins sensibles à la fissuration par fatigue thermique induite en frottement, première cause de remplacement des disques de frein de véhicule « poids lourd ». Les performances en freinage doivent être équivalentes à celles obtenues avec le couple de matériaux actuels : disque de frein en fonte à graphite lamellaire- garniture en matériau de friction composite à matrice semi-métallique. L’étude repose sur une approche multidisciplinaire des phénomènes et des couplages induits en freinage. Elle comporte quatre parties : la première partie expose les principaux enjeux industriels et scientifiques du freinage des véhicules « poids lourds » ; la seconde développe une approche modèle-expérience originale pour caractériser les sollicitations de freinage conduisant à la dégradation de disques de frein échelle 1 en fonte à graphite lamellaire ; enfin les nouvelles solutions matériaux considérées sont évaluées suivant une étude tribologique (troisième partie) et en cyclage thermique (quatrième partie), développée en laboratoire / This PhD thesis was carried out with the aim of developing new materials for truck brake discs, so as to increase disc lifespans. Premature failure is the major problem encountered in the operation. Braking performances of new materials should be equivalent to those obtained with the current material couple: lamellar grey cast iron brake disc and commercial semi-metallic brake lining material. To do this, the study is divided in four parts: Firstly, major industrial and scientific aspects in braking are identified in literature and with investigations on truck brake discs used on road. Next, a coupled numerical-experimental approach enable us to characterize thermomechanical loadings induced by braking and leading to damage mechanisms of the lamellar grey cast iron disc. Finally, two aspects of new materials solutions are investigated in laboratory: tribological behaviour and thermal cycling behaviour

Cette thèse vise à développer une démarche de dimensionnement à la fatigue thermomécanique de disques de freins automobiles en fonte à graphite lamellaire. La première étape est la proposition d'une loi de comportement cyclique permettant de représenter les phénomènes non-linéaires ainsi que la forte dissymétrie de comportement entre traction et compression observés sur la fonte GL. Le modèle s'appuie sur l'introduction d'un tenseur d'endommagement d'ordre 2 induit par le chargement. A partir de l'analyse des mécanismes d'endommagement, on propose ensuite un critère de fatigue fondé sur la densité d'énergie dissipée par cycle, permettant de prédire l'amorçage de fissures dans les zones critiques. On s'intéresse enfin à l'étude de la formation d'un réseau de faïençage observé sur les pistes des disques de frein. / This thesis aims at developing an approach for thermomechanical fatigue design of automotive brake discs made of flake graphite (grey) cast-iron. The first step of this work consists of modeling the nonlinear cyclic behavior and tension/compression strong dissymmetry of grey cast iron. The proposed model is based on the introduction of a second-order induced damage tensor. From the analysis of the damage mechanisms, a fatigue criterion based on the dissipated energy per cycle, allowing crack initiation in the critical areas to be defined, is proposed. Finally, the formation of crack networks on braking discs is studied.

Os principais mecanismos de desgaste, presentes em discos de freio ventilados automotivos, foram determinados utilizando ensaios de bancada em dinamometro inercial. Foram medidas a temperatura, rugosidade, taxa de desgaste, tensões residuais, coeficiente de atrito e desempenho de frenagem. Foi analisado o tribocontato através de técnicas de microscopia eletrônica e ótica. / The main mechanims of wear, present in ventilated brake disc had been determined, when submited to the braking test in inertial dynamometric simulator. Were measured wear taxes, temperature, residual stresses, friction coeficient, topographic of relief and analised their tribographics.

Os principais mecanismos de desgaste, presentes em discos de freio ventilados automotivos, foram determinados utilizando ensaios de bancada em dinamometro inercial. Foram medidas a temperatura, rugosidade, taxa de desgaste, tensões residuais, coeficiente de atrito e desempenho de frenagem. Foi analisado o tribocontato através de técnicas de microscopia eletrônica e ótica. / The main mechanims of wear, present in ventilated brake disc had been determined, when submited to the braking test in inertial dynamometric simulator. Were measured wear taxes, temperature, residual stresses, friction coeficient, topographic of relief and analised their tribographics.

The aim of this thesis is the analysis of three types commonly used automotive brake discs in terms of their thermal and mechanical resistance on a defined braking maneuver. Another objective is to make an evaluate of the different structural solutions and compare them by each other.

No / Hot spots are high temperature thermal gradients and localisations that are circumferentially distributed on a disc surface which can occur during heavy duty braking. Vibrations and noise can be triggered by hot spotting as well as damage to the disc surface. The experimental investigations suggest that the trigger condition and distribution of hot spots are related to the disc geometry, especially for ventilated discs. To investigate the effects of geometry and structure of a ventilated disc on hot spotting, a 3D finite element model was established. A fast simulation method of hot spotting in 3D was implemented in the model to enable a parametric analysis to be performed more efficiently. The results were validated using experimental data from a laboratory dynamometer.

no / The migration of hot banding is the phenomenon whereby hot bands or hot spots on the brake disc surface periodically migrate radially inward and outward. These migrations can cause the undesired brake torque variation (BTV) and further induce vibration problems such as brake judder. To investigate the forming and migration of hot banding problem, transient thermal mechanical finite element models of repetitive braking considering the effects of wear have been performed. The displacement, temperature, stress, and contact pressure distribution against time were obtained in this model. The thermal buckling, thermo-elastic instability (TEI) and hot band migration phenomena have been captured and investigated. The results suggest a cause-effect chain of radial hot band migration. Its determinants include mechanical loading, disc thermal buckling, and most importantly the transient interactions between TEI and wear.

Yes / Hot spotting is a thermal localisation phenomenon in which multiple hot regions form on a brake disc surface during high energy and/or high speed braking events. As an undesired problem, hot spots can result in high order brake judder, audible drone and thermal cracking. This paper presents a finite element model for hot spot modelling which introduces the classical axisymmetric assumptions to the brake pad in 3D by scaling the material properties combined with a subroutine to simulate the heat generation instead of modelling the rotation of the brake pad. The results from the initial feasibility models showed significant improvement in computing efficiency with acceptable accuracy when compared to a traditional FE model without such simplifications. This method was then applied to the 3D simulation of hot spotting on a realistic ventilated brake disc/pad pair and the results showed good correlation with experiments. In order to improve the understanding of the hot spotting mechanism, parametric studies were performed including the effects of solid and ventilated disc geometry, rotational speed and energy, pins, disc run-out, and brake pad length. Based on the analysis of the results, it was identified that the vents and pins affected the hot spot distribution. Speed was shown to be more important on the hot spot generation time and distribution than either the pressure or total energy input. Brake disc run-out was shown to affect the magnitude of both hot spot temperature and height due to the non-linear relationship between local deformation, contact pressure and heat generation. Finally, increasing the brake pad length generated fewer hot spots but the temperature of each hot spot increased.

Carbon fibre reinforced ceramic composites (Cf/C-SiC) have been explored as lightweight and durable disc in a friction brake. This composite was manufactured through infiltration of liquid silicon into a Cf/C perform. It has heterogeneous microstructure, composed of three key phases, silicon carbide, Cf/C, and un-reacted residual silicon. The development of the transfer layer on the friction surface of Cf/C-SiC was studied through microstructural image registration of the surface after a range of braking stops on a laboratory-scale dynamometer test rig. When an organic pad was used as the counter face brake pad, it was found that a steady transfer layer was developed in silicon regions right after initial stops; in carbon-fibre/carbon (Cf/C) regions and most of the silicon carbide region, the friction surfaces were unsteady and any possible friction transfer layers were hardly built up. Large voids and cracks/crevices likely became pools to quickly and efficiently collect the transferred materials generated by the friction, but the compacts formed inside the pools were susceptible to be stripped off by further braking operation. Three types of friction surfaces were generalized: type I, the friction transfer layer had a steady relationship with the matrix and respectable longevity; type II, the transfer layer had an unstable relationship with the matrix and poor durability; type III, the friction transfer layer had a steady relationship with the matrix but short lifetime. After testing against organic pads under the laboratory scale dynamometer testing condition, the friction surface of each key phase in Cf/C-SiC composites disc was studied by transmission electron microscopy (TEM). It was found that the transfer layer developed on Si consists of fine particles of metal silicides, silicates and minerals. The substrate damage of Si was not observed, while the precipitates having high oxygen content were found in the substrate. Formation of an interfacial bonding between transfer layer and silicon substrate is believed to be the key factor for the formation of a stable transfer layer on Si. However, the interfacial bonding between transferred materials and SiC was not detected. Kinks are common features developed on the friction surface of SiC. The interface between carbon fibre and carbon matrix was experienced mechanical damage, in form of microcracks, and the transferred material was developed in the interface. Instead of transfer layer, a thin amorphous film, produced by friction induced amorphisation of carbon fibre, was developed on top of carbon fibre.