Essais & Simulations n°116

Mesures et Methodes de

Mesures et Methodes de Mesure 2 - Experimental approach 2.1 Description of the TGV brake system The disc-brake system is composed of four discs on each wheels axle and sliding bodies that are constituted of two symmetric lining plates with cylindrical pads (18 pad for each side), as illustrated in Figures 1. The brakes are activated by the pneumatic system pressure and slow down rotation of the wheels by the friction caused by pressing brake pads against brake discs. Etudes expérimentales et simulation numérique du phénomène de crissement sur TGV Le crissement est un bruit strident fréquemment produit par les systèmes de freinage. Dans le milieu ferroviaire, des relevés de niveau acoustique ont montré que le crissement dû à l’arrivée en gare de certains trains pouvait atteindre 110 dB à un mètre du bord du quai. Ainsi, la problématique liée au crissement de freins à disque ferroviaires vise à traiter la gêne occasionnée par le crissement, principalement pour les passagers présents sur le quai lors de l’arrivée d’un train en gare, mais aussi pour les riverains et le personnel présent dans les gares. Cette étude vise donc à mieux comprendre les phénomènes vibratoires et mécanismes générés lors de l’apparition du crissement des freins à disque ferroviaires. Pour ce faire, des essais expérimentaux variés, ainsi que des confrontations avec des modèles éléments finis et simulations numériques complexes sont proposés. Cette étude s’insère plus globalement dans le projet de recherche AcouFren, subventionné par l’ADEME, dont l’objectif est de proposer de développer des outils d’aide à la spécification et à la conception de freins à disque ferroviaires optimisés vis-à-vis du crissement. Mots clés : crissement, dynamique non-linéaire, expérimentation, simulation numérique, frottement. Fig. 1. TGV brake system - (a) TGV bogie, (b) part of a brake pad 2.2 Experiments The evaluation of the squeal prediction and the dynamical behaviour of the TGV brake system under working conditions are performed with the help of dynamic tests on bench that is located at SNCF Agence d'Essai Ferroviaire. The TGV disc is brought up to speed, and then pressure is introduced to activate the brake. The test ends when the TGV disc stops. The spectrum of brake squeal and transient vibrations are obtained via the experimental measurement. For this, the TGV brake system is fully instrumented with accelerometers on the stationary part, as indicated in (Fig. 2). Vibration measurements for the rotating part (disc) are performed by using a vibrometer. Moreover, microphones are mounted near the disc (see (Fig. 2)). To have a more precise estimation of the range and variability of vibration instabilities at the origin of disc brake squeal, series of tests with the fully instrumented TGV brake system are performed for different operating conditions. Effects of the variation of the speed before braking system (25 km/h Fig. 2. Dynamic tests for TGV brake squeal Fig. 3. Continuous Wavelet Transform of experimental data (a) 15kN- 60km/h (b) 15kN- 25km/h (c) 8kN- 60km/h (d) 8kN- 25km/h and 60km/h), the rotational direction of the disc (positive and negative rotations defined by Rot+ and Rot-, respectively) and the compression force (8kN and 15kN) are more particularly undertaken. The continuous wavelet transform (CWT) based on the Morlet mother wavelet is used to study the time-history responses. Experimental results (vibrometer measurement) for four operating conditions (15kN-60km/h, 15kN-25km/h, 8kN-60km/h and 8kN- 25km/h with a positive rotation Rot+) are given in (Fig. 3). Moreover the repeatability of experiments is investigated by performing three identical tests for each deterministic operating condition. As shown in (Tab. 1), it appears that the response of the TGV brake system and the associated frequency content do not differ between three tests when a series of deterministic tests is performed in the same operating conditions. Tests allow identifying two main com- Essais & Simulations • MARS 2014 • PAGE 16

| Mesures et Methodes de Mesure plex nonlinear phenomena for TGV brake squeal due to the variation of the compression force. The first identified behavior is illustrated in (Fig. 3a) and (Fig.3b) (for 15kN- 60km/h and 15kN- 25km/h). The second one is given in (Fig. 3c) and (Fig.3d) (for 8kN-60km/h and 8kN-25km/h). Even if the nonlinear transient response and the associated CWT are not identical, the frequency content of TGV brake squeal appears to be globally the same for the four operating conditions, as indicated in (Tab. 1): TGV brake squeal appears at low/middle frequency in the 0–10000 Hz range (with a predominant frequency content in the 0–5000 Hz range). For the first identified behaviour of TGV squeal (15kN- 60km/h and 15kN-25km/h, see (Fig. 3a) and (Fig. 3b)), only one characteristic dynamic behavior is identified. At the beginning of transient vibrations, an evolution and increase of the squeal frequencies are observed (see (Fig.3a) and (Fig.3b) between t=[3 ; 4]s for 15kN-60km/h, and t=[2.5 ; 5] s for 15kN-25km/h). Moreover, it clearly appears that all the transient non-linear oscillations can become complex with the contribution of several frequencies. For the second identified behaviour of TGV squeal (8kN- 60km/h and 8kN-25km/h, see (Fig. 3c) and (Fig. 3d)), two dynamic behaviours are observed: firstly, a ‘‘simple” behaviour of the transient oscillations with only two main frequency resonances around 1000-2000Hz (see (Fig.3c) and (Fig.3d) between t=[2 ; 7]s for 8kN-60km/h, and t=[2 ; 11]s for 8kN-25km/h); secondly, a ‘‘complex” non-linear transient behaviour with the appearances of new contributions in the 2000-10000Hz range (see (Fig.3c) and (Fig.3d) between t=[7 ; 39]s for 8kN-60km/h, and t=[11 ; 17]s for 8kN-25km/h). As explained in [9], evolutions of the transient vibrations and the frequency content of the TGV brake squeal are governed by the modification of the sliding non-linear equilibrium point (i.e. initial static position due to the compression force) during self-excited vibration. This may lead to new instabilities in the TGV brake system and induces a transition from one to the other behaviour. Experimental analysis of TGV brake system with squeal frequencies identification 3 - Numerical simulation and comparison with experiments 3.1 TGV braking system and formulation of the problem The TGV brake model is composed of one disc, outer and inner pads (18 pins applied on either sides of the disc are taken into account) modelled using the finite element method, as illustrated in (Fig.4). Then, the backplate and support are considered by adding the flexibility of these pads’ supporting structures. Considering the description of the nonlinear interface, a Coulomb law with a constant friction coefficient µ is used. This formulation can be summarized as follow: where r is the contact reaction, u is the displacement field, T E C H N O L O G I E S ESSAIS MESURES TRAITEMENTS ANALYSES TRAITEMENTS COMPLEXES DONNEES / MESURES / IMAGES MODELISATIONS PROBABILISTE / STATISTIQUE MOYENS D'ESSAIS VALIDATION / ENDURANCE MODELISATION DYNAMIQUE, SIMULATION SIMULATEURS TEMPS REELS HIL / SIL INFORMATIQUE INDUSTRIELLE ELECTRONIQUE PRODUITS PARTENAIRE MULTI-TECHNOLOGIES DE VOS PROJETS DE R&D | Agrémenté C.I.R. PROCEDES MOYENS D'ESSAIS OUTILS LOGICIELS AXTRID 1 rue Georges Guynemer, Bât B8 78114 Magny-Les-Hameaux (SAINT QUENTIN EN YVELINES) www.axtrid.fr Info@axtrid.fr 01 61 37 45 46 Essais & Simulations • MARS 2014 • PAGE 17