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Séminaires à venir

Friction, noise, rough surfaces and statistics

Alain Le Bot Laboratoire de tribologie et dynamique des systèmes, UMR CNRS 5513, Ecole centrale de Lyon, France.

Jeudi 16 mars 2017 - 14h00 - Amphi Joliot


The friction noise emitted when rubbing hands or small objects is a wide band noise generated by impacts between antagonist asperities of rough surfaces. Since microscopic shocks are random events, this is a problem of statistical mechanics. In this study, we propose to analyze the problem in terms of (vibrational) energy balance. A direct numerical simulation which consists in solving the elasto-dynamics equations with contact delivers a crucial information on the rate and strength of impacts. While experiments gives some empirical laws such as acoustical power versus sliding speed and roughness. We present an experiment which investigates the dependence of sound with the nominal contact area. It is found that in some cases, the acoustical power is proportional to the contact area while in some others, the sound is constant. This paradoxical result is explained by introducing a dissipation law of vibration at the interface of solids. In the regime where this dissipation process dominates, the sound is found to be constant.

Morphing soft structures with instabilities

Joël Marthelot Elasticity, Geometry and Statistics Laboratory, MIT, Cambridge

Jeudi 6 avril 2017 - 14h00 - Amphi Boreau


Fracture and buckling of slender structures are typically regarded as a first step towards failure. Instead, we envision mechanical instabilities in structures as opportunities for scalable, reversible, and robust mechanisms that are first to be predictively understood, and then harvested for function. I will first show how delamination and fracture cooperate in thin films leading to the propagation of robust fracture patterns that offer opportunities to use cracks as a tool to design surfaces at small scales. I will then focus on thin elastic shells to revisit the canonical mechanics problem of sensitivity of shell-buckling to geometric imperfections. Finally, I will move on to the post-buckling regime of shells where periodic dimpled patterns are observed when the shell is constrained from within by a rigid mandrel. We find that the geometry of the system is central in setting the surface morphology. This prominence of geometry suggests a scalable, and tunable mechanism for reversible shape-morphing of spherical shells.