Macroscopic frictional contact scenarios and local contact dynamics: at the origins of “macroscopic stick-slip”, mode coupling instabilities and stable continuous sliding

Local contact behavior and its interaction with the global dynamics of the system are at the origin of innumerable contact issues concerning several different disciplines like tribology, geophysics, vibration mechanics or fracture mechanics. When two elastic media are in relative motion with a frictional interface, friction induced vibrations arise into the system. By a macroscopic point of view, the “macroscopic stick-slip” scenario occurring during relative motion is characterized by sudden friction force drops (sliding state) along the time, separated by periods of elastic energy accumulation (stick state). Instead, the mode dynamic instability occurs when a vibration mode of the mechanical system becomes unstable, due to frictional contact forces. This kind of instabilities, generated by frictional forces, have been mainly object of papers dealing with specific issues in different domains such as brake squeal, hip endoprosthesis squeaking, wheel-rail vibrations, earthquakes, etc. In this context, experimental and numerical analyses have been focused here on understanding how the local interface behavior affects the macroscopic frictional response of the system, and, conversely, during instability scenarios. The macroscopic frictional scenarios (macroscopic stick-slip instability, mode coupling instability, stable continuous sliding) arising between two simple elastic media in relative motion have been investigated numerically and experimentally. A newer experimental setup (TRIBOWAVE) has been developed and it allowed to reproduce and to investigate the different scenarios under well-controlled boundary conditions. The same frictional scenarios have been reproduced by transient numerical simulations. A dedicated friction law as a function of adherence (sticking) time has been recovered by means of experimental tests. The obtained friction law has been implemented in the numerical model, leading to a quantitative validation of the simulated scenarios by the experiments. Nonlinear transient simulations, complex eigenvalue analyses and experimental tests allowed for drawing instability maps as a function of system key parameters. The numerical model, validated by the comparison with the experimental global measurements (forces, accelerations/velocity), allowed for investigating the coupling between the local contact behavior (contact status distribution, wave and rupture propagation, precursors) and the system dynamic response during macroscopic stick-slip instability, mode coupling instability and stable continuous sliding. The understanding of the coupling between contact and system dynamics will bring to further improvements on the control of contact instabilities and related wear issues

Frictional response of reinforced polymers under quasistatic and fast-transient dry contact conditions

Reinforced polymers have recently gained interest because of their high stiffness associated with the classical features and cost-effectiveness of polymers. A further characteristic, suitable for several applications, is the possibility to provide high frictional and wear resistance. The frictional response of commercially available reinforced materials was here investigated in a wide range of contact boundary conditions. Experimental tests were performed on different test benches, to investigate the material frictional response under either quasistatic or fast-dynamic contact solicitations. While carbon-fiber-reinforced material exhibits a stable but low friction coefficient, the glass-fiber-reinforced material leads to the suitable combination of high friction and low wear. The PPS material, 40% (wt) glass-reinforced polymer, sliding against the Ti6Al4V titanium alloy, provided high static friction coefficients (>0.4). The same material pair was then tested in endurance under fast-dynamic contact solicitations, highlighting their resistance to wear

Scénarios macroscopiques de frottement de contact et contacts dynamiques locaux : A l'origine de "macroscopique stick-slip", mode d'instabilités de couplage et glissement stable continu

Local contact behavior and its interaction with the global dynamics of the system are at the origin of innumerable contact issues concerning several different disciplines like tribology, geophysics, vibration mechanics or fracture mechanics. When two elastic media are in relative motion with a frictional interface, friction induced vibrations arise into the system. By a macroscopic point of view, the “macroscopic stick-slip” scenario occurring during relative motion is characterized by sudden friction force drops (sliding state) along the time, separated by periods of elastic energy accumulation (stick state). Instead, the mode dynamic instability occurs when a vibration mode of the mechanical system becomes unstable, due to frictional contact forces. This kind of instabilities, generated by frictional forces, have been mainly object of papers dealing with specific issues in different domains. In this context, experimental and numerical analyses have been focused here on understanding how the local interface behavior affects the macroscopic frictional response of the system, and, conversely, during instability scenarios. The macroscopic frictional scenarios (macroscopic stick-slip instability, mode coupling instability, stable continuous sliding) arising between two simple elastic media in relative motion have been investigated numerically and experimentally. A newer experimental setup (TRIBOWAVE) has been developed and it allowed to reproduce and to investigate the different scenarios under well-controlled boundary conditions. The same frictional scenarios have been reproduced by transient numerical simulations. A dedicated friction law as a function of adherence (sticking) time has been recovered by means of experimental tests. The obtained friction law has been implemented in the numerical model, leading to a quantitative validation of the simulated scenarios by the experiments. Nonlinear transient simulations, complex eigenvalue analyses and experimental tests allowed for drawing instability maps as a function of system key parameters. The numerical model, validated by the comparison with the experimental global measurements (forces, accelerations/velocity), allowed for investigating the coupling between the local contact behavior (contact status distribution, wave and rupture propagation, precursors) and the system dynamic response during macroscopic stick-slip instability, mode coupling instability and stable continuous sliding. The understanding of the coupling between contact and system dynamics will bring to further improvements on the control of contact instabilities and related wear issues.Le comportement local au contact et son interaction avec la dynamique globale du système sont à l'origine d’innombrables problèmes de contact concernant plusieurs disciplines telles que la tribologie, la géophysique, la mécanique de vibration ou la mécanique de la rupture. Lorsque deux corps élastiques sont en mouvement relatif avec une interface de frottement, des vibrations induites se produisent dans le système. Dans un point de vue macroscopique, le scénario macroscopique de stick-slip survenant pendant le mouvement relatif est caractérisé par la chute soudaine de la force de frottement (état de glissement), séparées par des périodes d'accumulation d'énergie élastique (état d’adhérence). Autrement, une instabilité dynamique se produit quand un mode de vibration du système mécanique devient instable en raison des forces de frottement. Ces types d'instabilités, générées par des forces de frottement, ont été principalement objet de publies traitant de problèmes spécifiques dans différents domaines. Dans ce contexte, des analyses expérimentales et numériques ont été ici mis en place pour comprendre comme le comportement de l'interface locale affecte la réponse macroscopique du système et vice-versa, au cours de scénarios d'instabilité. Les scénarios macroscopiques (instabilité de « stick-slip macroscopique », instabilité modale, glissement continu stable), survenant entre deux milieux élastiques simples en mouvement relatif, ont été étudiés numériquement et expérimentalement. Un dispositif expérimental dédié (TRIBOWAVE) a été développé et a permis de reproduire et examiner les différents scénarios de frottement dans des conditions aux limites bien contrôlées. Les mêmes scénarios de frottement ont été reproduits par des simulations numériques transitoires. Une loi de frottement en fonction du temps d’adhérence (stick) a été définie à partir des essais expérimentaux. La loi de frottement obtenue a été mise en œuvre dans le modèle numérique, conduisant à une validation quantitative des scénarios de frottement par les expériences. Les simulations transitoires non linéaires, l’analyse aux valeurs propres complexes et les tests expérimentaux ont permis de dessiner des cartes de scénarios d'instabilité en fonction des paramètres clés du système. Validé par la comparaison avec les mesures des signaux expérimentaux globaux (forces, accélérations / vitesse), le modèle numérique a permis d'étudier le couplage entre le comportement du contact local (distribution de l'état du contact, propagation des ondes et des ruptures, précurseurs) et la réponse dynamique du système au cours du « stick-slip macroscopique », de l’instabilité due au couplage modale et du glissement continu stable. La compréhension du couplage entre le contact et la dynamique des systèmes apportera de nouvelles améliorations sur le contrôle des instabilités de contact et les problèmes d'usure connexes

Estimation of contact stiffness for frictional composite material

International audienc

Response surface model of a brake system to optimize structural modifications for squeal noise suppression

In spite of decades of investigation, brake squeal is still an unresolved problem. Many attempts have been made by industry and researchers to establish a general approach aimed at preventing squeal in brake design. Nowadays, the lock-in theory is one of the most accepted approaches for squeal generation and particular attention is given to the dynamics of brake systems. Moreover, one of the main difficulties encountered in studying squeal is the complexity of a real brake system. Thus many researchers approached the problem by conducting experimental and numerical analysis on simplified brake systems, and then trying to correlate the results with theoretical models. In this paper, an approach to identify appropriate changes of the physical properties of a brake system is developed, in order to suppress the squeal occurrence. First a sensitivity approach is developed to discard the less effective physical parameters. These selected parameters are those that can be modified. Subsequently, a simple mathematical model (Response Surface Model) that represents how the selected parameters affect system eigenvalues, can be obtained using Design of Experiments

Numerical and Experimental Analysis of Nonlinear Vibrational Response due to Pressure-Dependent Interface Stiffness

Modelling interface interaction with wave propagation in a medium is a fundamental requirement for several types of application, such as structural diagnostic and quality control. In order to study the influence of a pressure-dependent interface stiffness on the nonlinear response of contact interfaces, two nonlinear contact laws are investigated. The study consists of a complementary numerical and experimental analysis of nonlinear vibrational responses due to the contact interface. The laws investigated here are based on an interface stiffness model, where the stiffness property is described as a nonlinear function of the nominal contact pressure. The results obtained by the proposed laws are compared with experimental results. The nonlinearity introduced by the interface is highlighted by analysing the second harmonic contribution and the vibrational time response. The analysis emphasizes the dependence of the system response, i.e., fundamental and second harmonic amplitudes and frequencies, on the contact parameters and in particular on contact stiffness. The study shows that the stiffness–pressure trend at lower pressures has a major effect on the nonlinear response of systems with contact interfaces

Interaction between contact behaviour and vibrational response for dry contact system

This work wants to provide insights on the coupling between contact behaviour (local scale) and vibrational response (global scale) which brings to different contact scenarios arising in dry frictional systems. A newer setup, named TriboWave, has been developed in order to reproduce and investigate the system response to frictional contact, under well-controlled boundary conditions. The experimental results highlighted how a simple frictional system can switch from stable friction-induced vibrations to unstable vibrations, i.e. either macroscopic stick–slip instabilities or mode coupling instabilities. The effect of the contact surface roughness on the reproduced frictional scenario has been investigated too

Role of damping on contact instability scenarios

In the last years, many studies have been dedicated to investigate sliding contact issues between deformable bodies. Local frictional behavior and its interaction with the global dynamics of the system has been the subject of many works in several disciplines as tribology, geophysics, vibration mechanics and fracture mechanics. Experimental and numerical papers have focused the attention on the understanding how local interface dynamics (wave and rupture propagation) affects the macroscopic frictional behavior of the system during instability regime (stick-slip instability, mode coupling instability) and conversely. The stick-slip regime is characterized by sudden friction force drops (sliding state) along the time, separated by period of elastic energy accumulation (stick state). Instead, the modal dynamic instability occurs when a vibration mode of the mechanical system becomes unstable, due to frictional contact forces. This kind of instabilities, generated by frictional forces, has been mainly object of papers [5] dealing with a specific issue named brake squeal. However general and common mechanical system can generate harmonic acoustic emission comparable to brake squeal noise during relative motion with frictional contact. In such context, the role of material damping on the frictional dynamics has been investigated by a simple frictional elastic model. Comparison between nonlinear transient simulations and complex eigenvalues analysis allowed for investigating different instability scenarios for general systems in frictional contact and drawing a qualitative map in function of damping parameters. Moreover, the results show the importance on defining a good estimation of damping to modeling system with frictional contact, in order to have reliable results. Finally a brief comparison between numerical and experimental results has been carried out

Investigation of the role of contact-induced vibrations in tactile discrimination of textures

Tactile perception of texture is meaningful for numerous industrial and social applications. While the sense of touch is provided by several contributing factors, each related to different sensory cutaneous units, textures with non-low spatial gradients can be appreciated only by the relative motion between the finger and the object. This work focuses on investigating the role that the vibrations induced during sliding contact play in the origin of tactile perception and surface texture discrimination. We shall focus in particular on the effect of the different spectral contents of the induced vibrations. To this end, first the vibration signals produced by touching different sample surfaces are recovered and analysed under controlled boundary conditions. Then, a collection of data from different subjects is recorded and reproduced by a dynamic exciter. Finally, the reproduced signals are randomly submitted to a panel of subjects in order to test their ability to distinguish the different surfaces only through their perception of the reproduced vibrations. The results, while highlighting the key role played by the spectral distribution of friction-induced vibrations, simultaneously reveal the limits of taking only vibrational signals into account if we are to achieve satisfactory recognition of the whole panel of tested textures