Yannick Desplanques

Fatigue of glass/epoxy composite in three-point-bending with predominant shearing

Unidirectional and multilayer composites are usually subjected to bending, and often fail in fatigue by interlaminar shearing. This work studies new experimental conditions in fatigue, using a three-point-bending test with predominant shearing, while tensile/compressive stresses are significant. The experimental device geometry is validated under monotonic loading by comparison between finite element results and experimental observations. The deflection amplitude vs. fatigue life curve is used as a pseudo-Wohler curve. An approximate drawing of maximum deformation vs. fatigue life curve is proposed using Fourniers model. Experimental results in fatigue are well accounted for by this model.

A tribometer for the study of materials under railway braking conditions

This paper presents a braking tribometer developed to study the behaviour of materials used in railway braking up to high speeds and energies. Its design is based on similitude rules which enable to reproduce at reduced scale, the same working conditions of materials tested at full scale. The validation of the testing machine is presented: it is based on the reproduction at the reduced scale of a performance test programme of the Thalys TGV friction materials, which has been previously performed on a test bench at full scale. A comparison of the obtained results with the ones available at full scale is achieved. A study of the friction physical mechanisms carried out from scanning electron microscopy shows that a stable third body, made of plane sheets, controls an S3M3-type velocity accommodation mechanism. These plane sheets are obtained by agglomeration and packing of particles resulting mainly from the Fe-CuSn sintered matrix of the brake pad. A specific study has enabled the genesis of the third body to be understood.

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.

Interactions between Third-Body Flows and Localisation Phenomena during Railway High-Energy Stop Braking

During railway braking, dissipation of high energy due to friction leads to transient and localised thermal phenomena such as hot bands and hot spots. These localisation phenomena interact with third body flows and friction mechanisms activated in the contact. To study these couplings, an experimental approach has been developed, based on an inertial tribometer able to reproduce high thermo-mechanical brake-disc loadings. This paper focuses on the coupling between flows of third-body and hot-band migration. Transient localised thermal phenomena are described for high-energy stop-braking. The monitoring of the disc track during friction in the visible and infrared wavelengths evidences the interactions between third-body flows and hot-band migration. The consequences relatively to load-bearing areas and third-body compaction in the contact are studied, allowing the proposal of a synthetic diagram of the coupling between friction mechanisms, third body flows, hot-band migration and first-body distortion.

Braking performance and influence of microstructure of advanced cast irons for heavy goods vehicle brake discs

It is well known that truck brakes dissipate several megajoules of energy every few seconds, which leads to high thermal stresses in the rubbing parts. Therefore, premature failure by cracking of truck brake discs is a matter of major concern. Improving the design and material of brake discs may enhance braking performance. This study focuses on the latter aspect and was carried out with the aim of developing new material solutions for increasing disc lifespan. To do so, braking experiments were conducted on a specially designed braking tribometer. The brake pads that were used were made from a commercial brake lining material. Two advanced cast irons with different graphite morphology were studied in comparison with the lamellar grey cast iron commonly used for brake disc. To verify the friction and thermal behaviour of the two cast irons, braking tests were carried out as a series of stop-brakings with increasing dissipated power and energy and as a series of slowdowns to achieve heat accumulation effects. Thermal phenomena were studied through bulk temperature measurements and infrared monitoring of the disc surface. Friction behaviour, braking performance and variations in thermal loading were analysed in relation to the level of energy dissipation. The two advanced cast irons and lamellar cast iron had equivalent braking performance and stored similar amounts of heat, according to their thermophysical properties. Observations of the rubbing surfaces indicated damage mechanisms affected by the graphite morphology. Less plastic deformation on the surface was observed with an interdendritic graphite.

Relationships between the heterogeneous microstructure, the mechanical properties and the braking behaviour of an organic brake lining material

Industrial brake lining materials are composite with complex formulations consisting of multiple constituents. Resulting from the fabrication process, the morphology and distribution of the constituents have significant influences on the future properties and braking performance. In this study, an in-depth analysis ranging from the microscale to the macroscale was performed to assess the relationships between the microstructure, the mechanical properties and the braking performance of an industrial brake lining material formulated for heavy vehicles. It was observed that the manufacturing process had different effects on the morphology and size of constituents and on their distribution in the phenolic binder. The morphologies of large organic particles such as rubber and graphite were affected by the mixing procedure, contrary to those of fibres and mineral particles. A transverse anisotropy consistent with fibre orientation due to cold preforming and hot moulding was observed. The microstructure displayed a strong local heterogeneity right up to the mesoscopic scale at which friction and wear mechanisms typically occur. The mechanical properties were analysed with regard to the heterogeneity of the microstructure to determine the scale at which these properties could be considered to be associated with a homogenised behaviour. The rubbing surface after braking showed that load-bearing localisation depends on the nature, morphology and orientation of constituents but that this heterogeneity can be of interest with regard to the braking ability.

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.

Characterisation of oxidation and wear oxidised surfaces of H13 steel/brass in dry sliding conditions

Although oxide scale has significant influence on surface quality of hot–worked products, deformation of the scale during hot working has not been understood sufficiently. The authors propose an experimental study to analyse the oxidation and the tribological behaviour of a ferrous alloy (X40CrMoV5–1, H13 steel) and non ferrous alloy (CuZn39Pb3, brass alloys) in dry sliding conditions. The tribological behaviour of the pair of materials is carried out on a pin–on disk wear tester after oxidation at 600°C for 70 h. It was found that: i) a loose oxide film wholly covered the surface of the steel; ii) Fe2O3 as the main oxide was identified except for a small amount of Fe3O4; iii) during friction, a compacted oxide film was established on the worn surface of the steel. The analysis of the worn surface showed two regions: a delamination region due to the delamination of tribo–oxide during wear and an undelamination region characterised by the dominance of Fe3O4 tribo–oxides on the worn surface.