Laurent Baillet

Brake squeal as dynamic instability: An experimental investigation

This paper presents an experimental analysis performed on a simplified brake apparatus. In past years a common approach for squeal prediction was the complex eigenvalues analysis. The squeal phenomenon is treated like a dynamic instability: when two modes of the brake system couple at the same frequency, one of them becomes unstable, leading to increasing vibration. The presented experimental analysis is focused on correlating squeal characteristics with the dynamic behavior of the system. The experimental modal identification of the setup is performed and different squeal conditions and frequencies are reproduced and analyzed. Squeal events are correlated with the modal behavior of the system as a function of the main parameters. A clear distinction between squeal events involving the dynamics of the pad and squeal events involving the dynamics of the caliper is performed. The effect of the adding of damping is also investigated on the squeal phenomenon. Two opposite roles of the modal damping are described: a large modal damping can either prevent the rise of squeal instabilities or enlarge the squeal propensity of the brake apparatus. The robustness of the obtained squeal events permits a further analysis on the triggering mechanism of the squeal instability during braking.

Finite element simulation of dynamic instabilities in frictional sliding contact.

This paper describes tools for numerical modelisation which enable the understanding of the appearance of the vibration of structure generated by the frictional contact between two bodies (the excitation source being friction). The dynamic finite element code PLASTD is used to reproduce transitory phenomena generated at the contact interface. This code includes contact and friction algorithms based upon the formulation of Lagrange multipliers. A numerical study of the dynamic response of a 2D mechanical model composed of a deformable plane in relative translation and unilateral contact with Coulomb friction with a rigid surface is presented. The steady sliding solution is generically unstable and leads to a dynamic response which leads to the generation of instabilities characterized by the appearance of sliding-sticking or sliding-sticking-separation waves. It is important to notice that those instabilities appear even with a constant friction coefficient. These simulations have permitted to obtain the local contact conditions (kinematics, tribological state, contact stresses, etc). The kinematics shows the existence of local impacts and sliding at high frequencies. Furthermore, local normal pressure is much higher than that expected for a smooth surface. Finally, a 3D simulation of braking is carried out. We focused on the vibration of the disc and the brake pad which caused noises due to the generation of interface instabilities.© 2003 ASME

Structural-change localization and monitoring through a perturbation-based inverse problem

Structural-change detection and characterization, or structural-health monitoring, is generally based on modal analysis, for detection, localization, and quantification of changes in structure. Classical methods combine both variations in frequencies and mode shapes, which require accurate and spatially distributed measurements. In this study, the detection and localization of a local perturbation are assessed by analysis of frequency changes (in the fundamental mode and overtones) that are combined with a perturbation-based linear inverse method and a deconvolution process. This perturbation method is applied first to a bending beam with the change considered as a local perturbation of the Youngs modulus, using a one-dimensional finite-element model for modal analysis. Localization is successful, even for extended and multiple changes. In a second step, the method is numerically tested under ambient-noise vibration from the beam support with local changes that are shifted step by step along the beam. The frequency values are revealed using the random decrement technique that is applied to the time-evolving vibrations recorded by one sensor at the free extremity of the beam. Finally, the inversion method is experimentally demonstrated at the laboratory scale with data recorded at the free end of a Plexiglas beam attached to a metallic support.

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

This 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.

From railway dynamics to wheel/rail contact mechanics, an approach for the modelling of the wheel/rail contact: elasto-plastic response of the railhead

Abstract This wheel/rail contact study presents a combination of two complementary mechanical fields: railway dynamics and tribology. The first one gives an estimation of the contact forces between wheels and rails because of the movements of the bodies. The multi-body representation is combined with a contact model describing the pressure field at the interface, including the tangential stresses and the sliding hypothesis on discretized patches between wheel and rail. These values are then used as the inputs to finite-element models developed to investigate the rail elasto-plastic response to the action of a rolling wheel. Predicted plastic distortions are compared with those observed on sites.

Characterization of the 3‐D fracture setting of an unstable rock mass: From surface and seismic investigations to numerical modeling

The characterization of the fracturing state of a potentially unstable rock cliff is a crucial requirement for stability assessments and mitigation purposes. Classical measurements of fracture location and orientation can however be limited by inaccessible rock exposures. The steep topography and high-rise morphology of these cliffs, together with the widespread presence of fractures, can additionally condition the success of geophysical prospecting on these sites. In order to mitigate these limitations, an innovative approach combining non-contact geomechanical measurements, active and passive seismic surveys and 3-D numerical modeling is proposed in this work to characterize the 3-D fracture setting of an unstable rock mass, located in NW Italian Alps (Madonna del Sasso, VB). The 3-D fracture geometry was achieved through a combination of field observations and non-contact geomechanical measurements on oriented pictures of the cliff, resulting from a previous laser-scanning and photogrammetric survey. The estimation of fracture persistence within the rock mass was obtained from surface active seismic surveys. Ambient seismic noise and earthquakes recordings were used to assess the fracture control on the site response. Processing of both datasets highlighted the resonance properties of the unstable rock volume decoupling from the stable massif. A finite-element 3-D model of the site, including all the retrieved fracture information, enabled both validation and interpretation of the field measurements. The integration of these different methodologies, applied for the first time to a complex 3-D prone-to-fall mass, provided consistent information on the internal fracturing conditions, supplying key parameters for future monitoring purposes and mitigation strategies.

Non linear analysis of vibrations generated by a contact with friction

The aim of this paper is to study vibrations generated at contact with friction for two different applications. The first one is an investigation of friction-induced vibrations of a beam-on-beam system in contact with friction. For this study the complementary use of linear and nonlinear analyses drives to the understanding of physical phenomenon induced in these vibrations. The second parts consists in investigating numerically dynamic rupture of a biomaterial interface. The numerical Finite Element model is composed of two homogeneous and isotropic elastic solids which are brought in contact with friction by remote normal compression and shear traction. The rupture is nucleated by decreasing instantaneously the friction coefficient to zero at nucleation area. The properties of the obtained ruptures (velocity, generated waves, interface state…) are analyzed.

Finite Element Analysis of a Contact With Friction Between an Elastic Body and a Thin Soft Layer

When studying a mechanical system involving contact between two bodies such as a disc and brake pad system, finite element simulations are often used to predict the phenomena involved. However, due to model size and calculation time problems, when modeling this type of mechanical system on a scale of about 100 mm, it is difficult to model as well a layer (for example a third body layer) on a scale of approximately 10 pm. In quasi-static problems it is possible to simulate the contact between an elastic body and a thin elastic layer bonded to a rigid surface, by considering the contact between this elastic body and a rigid surface with a specific contact law. This paper shows that it is possible to implement this specific contact law in a dynamic finite element code to simulate thin layers undergoing quasi-static and dynamic problems without physical contact instabilities. This specific contact law saves a large amount of calculation time. Once the specific contact law has been validated, the influence of the layer thickness is studied.

Interplay Between Local Frictional Contact Dynamics and Global Dynamics of a Mechanical System

Friction affects almost the entirety of the mechanical systems in relative motion. In spite of intense and long-time research activities many aspects of this phenomenon still lack of a meaningful interpretation. Some of them could be explained by not focusing only on the interface properties. In fact recent literature confirms the picture of a macroscopic frictional behaviour of a mechanical system as the outcome of a complex interaction between the local dynamics at the frictional interface (wave and rupture nucleation and propagation) and the global dynamics of the system. This paper presents the results of a 2D non-linear finite element analysis under large transformations of the onset and evolution of sliding between two isotropic elastic bodies separated by a frictional interface. The aim is to investigate the trigger of the dynamic rupture at the interface, which preludes and goes with the sliding and its interaction with the global dynamics to determine the observed macroscopic frictional behaviour (stick-slip, continuous sliding). The analysis is focused on the observed phenomena during the onset of the sliding (micro-slips, precursors, macro-slips), accounting for the frictional properties and the inertial and elastic properties of the system.

Monitoring Local Changes in Granite Rock Under Biaxial Test: A Spatiotemporal Imaging Application With Diffuse Waves

Diffuse acoustic or seismic waves are highly sensitive to detect changes of mechanical properties in heterogeneous geological materials. In particular, thanks to acousto-elasticity, we can quantify stress changes by tracking acoustic or seismic relative velocity changes in the material at test. In this paper, we report on a small-scale laboratory application of an innovative time-lapse tomography technique named Locadiff to image spatio-temporal mechanical changes on a granite sample under biaxial loading, using diffuse waves at ultrasonic frequencies ( 300 kHz to 900 kHz). We demonstrate the ability of the method to image reversible stress evolution and deformation process, together with the development of reversible and irreversible localized micro-damage in the specimen at an early stage. Using full-field infrared thermography, we visualize stress induced temperature changes and validate stress images obtained from diffuse ultrasound. We demonstrate that the inversion with a good resolution can be achieved with only a limited number of receivers distributed around a single source, all located at the free surface of the specimen. This small-scale experiment is a proof of concept for frictional earthquake-like failure (e.g. stick slip) research at laboratory scale as well as large scale seismic applications, potentially including active fault monitoring.