Nicolas Peyret

MICRO-SLIP INDUCED DAMPING IN THE CONTACT OF NOMINALLY FLAT SURFACES

It is well known that the friction between interfaces at bolted joints plays a major role in the damping of assembly structures. Friction can be either induced by macro-slipping or micro-slipping. The aim of this paper is to model and quantify the dissipated energy by micro-sliding down to the scale of roughness between two flat surfaces in order to compute the damping ratio. It was assumed that the coefficient of friction between two materials is constant and that friction is the only source of energy dissipation. An experimental study was conducted to measure static normal load and dynamic tangential load without any coupling between these two main directions. A rheological contact model based on the Extended Greenwood Model with micro-contacts and statistical distributions was developed and studied. Experimental results and simulations are compared in order to assess and discuss the model.

Vibration reduction of an assembly by control of the tightening load

Due to their lightness, the aerospace structures are vulnerable to vibrations. Their amplitudes need to be mitigated through damping devices. The aim of this paper is to control the tightening forc...

Identification of Nonlinear Viscoelastic Parameters Based on an Enhanced Oberst Beam Method

This work deals with payload vibration insulation in aeronautic applications. The main objective is to design stabilization devices for optical devices. To achieve this goal, polymer materials have been used because they provide damping and flexibility in order to isolate the optical devices from vibrations and shocks. This kind of material exhibit a mechanical behavior that strongly dependent on the temperature, the strain amplitude and the frequency. The purpose of this paper is to give a new identification method of the viscoelastic parameters based on the Oberst beam test. The aim is to carry out the dependence of the elastomer mechanical properties on the strain amplitude. By coupling this test with Dynamic Mechanical Analysis, it is possible to obtain the mechanical behavior of viscoelastic material according to the strain amplitude and the frequency. To achieve this goal, the experiment derived from the Oberst beam set-up and ASTM E756-05. The time response signal is post-processed using nonlinear unconstraint optimization method in order to identify the instantaneous frequency and damping ratio of the first eigenmode. Then, it is possible to recover the storage modulus and the loss factor of the polymer according to the strain amplitude using a finite element model of the setup. Finally, the identified frequency and amplitude dependent models are taken into account to carry out numerical simulations on the whole mechanical device.

The Cut Beam Benchmark System: Developing Measurement Techniques for Nonlinear Damping and Stiffness in Frictional Interfaces

This chapter deals with the damping caused by friction in joints. A new test bench is presented and justified by comparisons made with devices described in the literature. The purpose of this academic bench is to measure the damping induced by partial slip and friction in a planar joint. A new method for the so-called stopped-sine excitation was developed. It allows more precise monitoring of the evolution of the vibration frequency and damping of nonlinear modes, and it allows uncoupling normal static and dynamic tangential forces. This method is associated with piezoelectric exciters for greater efficiency when stopping excitation. A large number of experimental results are presented and discussed, which are used to characterize the damping induced by micro-sliding in the bonds.

Microslip Induced Damping in the Contact of Nominally Flat Surfaces with Geometric Defects

It is well known that the friction between interfaces at bolted joints plays a major role in the characterization of damping. Friction can be either induced by macroslip or microslip. The aim of this chapter is to model and quantify the dissipated energy by microslip in the joints in order to compute the damping ratio. It is assumed that the coefficient of friction between the nominally flat surfaces is constant and that friction is the only source of energy dissipation. An experimental study is reported to measure static normal load and dynamic tangential load without any coupling between these two main directions. A rheological contact model, Extended Greenwood Model (EGM), based on microcontacts and statistical distributions is developed and studied. Experimental results and simulations are compared in order to assess and discuss the model.

Dynamic Behavior of Very-High Speed Rotors at Non-stationary Conditions

Speed reducers with input shafts spinning at very high speeds (up to 42 000 rpm) are generally associated to electric motors, which are more and more used especially in the automotive field, in order to bring the rotational speed to the most efficient window. Accurate modeling of those rotating machinary behavior is crucial to improve product reliability and to prolong machinery life. Many studies are conducted with an imposed angular speed, which is in most of the cases considered as constant or, in best cases, which follows a given variation law. In this paper, the study is performed with no assumption on the rotational speed. A variable driving torque is induced to the input shaft and the instantaneous angular speed (IAS) is deduced from the dynamic problem coupled to an angular approach. As a result, the IAS takes into account not only the induced torque perturbations but also the periodic geometry of the whole structure (e.g.: bearings and gears). The aim of this work is to extend the existing model based on the Finite Element Method by introducing an enhancement of the gyroscopic effect matrices without any assumption on the spinning speed. This model will lead to the introduction of coupling between the flexural and torsional degrees of freedom as well as to a non-linearity in the modeling of the studied system. The aim is to improve the accuracy of simulations for the rotor dynamics in non-stationary conditions especially when getting through critical speeds.

Compact Model Synthesis for Partially Observed Operational Systems

This work proposes a Compact Model Synthesis (CMS) for Partially Observed Operational Systems (POOS) without using the complete knowledge of models. Series of “grey boxes” fed with partial observations are built in order to synthesize target variables with compact models. The recursive process for real time computation is based on Kalman Filters (KF). This stochastic approach allows to converge in line toward deterministic models with estimated uncertainties and without intrusion on the complete model process. Mathematical context is described first and illustrated secondly with two examples.Copyright

Dynamics of Assembled Structures: Taking Into Account the Surface Defects in Interfaces

In structural dynamics, the prediction of damping remains the biggest challenge. This paper deals with the energy losses caused by micro-slip in a nominally planar interface of a structure. This paper proposes an analytical and experimental study of flexural vibrations of a clamped-clamped beam with innovative position of the interfaces.The objective of this test bench is to characterize the global rheology of the interface. The proposed model aims to characterize this rheology based on local settings of the interface.First, the test bench is described and the choice of the position of the interface is justified. The experimental bench and the dynamic behavior of this structure are presented. We propose to illustrate the mechanism of energy losses by micro-slip by making a comparison between the behavior of a “monolithic” beam and a sectioned beam.Secondly, a modeling of the interface taking into account the surface defect is presented. The energy dissipated by friction in the interface is calculated during a loading cycle. This leads to a computation of the dissipated energy and thus to a nonlinear loss factor.Finally, we confront the loss factor calculated analytically and the measured one.Copyright