Gaël Chevallier

A new identification method of viscoelastic behavior: Application to the generalized Maxwell model

This paper focuses on the generalized Maxwell model (GMM) identification. The formulation of the transfer function of the GMM is defined, as well as its asymptotes. To compare identification methods of the parameters of the GMM, a test transfer function and two quality indicators are defined. Then, three graphical methods are described, the enclosing curve method, the CRONE method and an original one. But the results of graphical methods are not good enough. Thus, two optimization recursive processes are described to improve the results of graphical methods. The first one is based on an unconstrained non-linear optimization algorithm and the second one is original and allows constraining identified parameters. This new process uses the asymptotes of the modulus and the phase of the transfer function of the GMM. The result of the graphical method optimized with the new process is very accurate and fast.

Viscoelasticity Measurement and Identification of Viscoelastic Parametric Models

Generally speaking, the behaviour of viscoelastic material is more complicated than the behaviour proposed by classical models as Voigt, Maxwell or Zener. The stiffness of such materials is a frequency dependent complex function. Above 1000Hz, classical measurements techniques are unable to achieve accurate measurements of the stiffness. In this paper, a new Dynamical Mechanical Analysis (DMA) tester is presented. It allows the characterization of the shear stiffness of preloaded viscoelastic materials between 200 and 3500Hz and without using frequency-temperature equivalences. Then the Generalized Maxwell model is used to describe behaviours measured with the DMA tester. A new iterative identification method of the parameter of the Generalized Maxwell model is described. This identification method is based on the asymptotes of the model.Copyright

A Numerical Quasi-Static Method for the Identification of Frictional Dissipation in Bolted Joints

The hereby paper investigates a way to compute the micro-sliding dissipations that occurs in built-up structure using modal coordinates. This numerical method extends the former quasi-static approach to modal displacements through the use of finite element analysis. Considered structures are supposed to behave linearly except for a lumped bolted joint. It is firstly assumed that mode shapes of such structures are few affected by contact non-linearities in joint interfaces. This assumption allows to apply the normal eigenmodes of the linearized structure as boundary conditions on a model reduced to the bolted joint. The method relies on a corrected quasi-static analysis associated with the Masing rules. Those assumptions enables to avoid the considerable numerical expense due to non-linear dynamics. In order to improve the simulation, a mode shapes correction is also implemented. The formulation of the method is detailed and investigated on the classical lap-joint benchmark.Copyright

Viscoelastic Damping Effect on Brake Squeal Noise

Brake squeal remains a widespread cause for discomfort in automobiles. Manufacturers overcome this problem by adding damping materials in their systems. The purpose of this work is to take into account the damping in the modeling. As the materials exhibit a viscoelastic behavior, the authors chose to model the damping with the Generalized Maxwell model. Moreover, the authors have tested their method on a detailed Finite Element-model of a brake system. To compute the complex poles of the model, the authors have established a state-space formulation of the viscoelastic model with a new assumption that allows one to reduce the number of states. Making the computation on the whole model is rather difficult due to the number of Degrees Of Freedom, the model is thus reduced on a basis constituted with the eigenvectors of the undamped model. Several results are also presented and discussed as the observed phenomena are rather different from the results obtained with undamped systems.Copyright

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.

Design of thermally adaptive composite structures for damping and stiffness control

Viscoelastic materials are widely used to control vibrations. However, their mechanical properties are known to be frequency and temperature-dependent. Thus, in a narrow frequency bandwidth, there is an optimal temperature that corresponds to a maximum loss factor and it is tricky to get a high damping level over a wide frequency range. Furthermore, an optimal temperature for a maximum structural damping leads to a poor static stiffness because the peak of the loss factor is obtained during the glass transition when the storage modulus is decreasing. Additionally, in industrial applications, the requirements might change according to the system life-cycle. For instance, the stabilization functions that are used for optronics applications require high stiffness for positioning steps, and high damping for filtering functions. To achieve this goal, engineers usually use several viscoelastic materials with functionally graded damping properties. This allows obtaining a high loss factor over a wide frequency range. This solution is however not adaptive. In order to be able to adjust the properties in real time, we suggest in this paper to use a single material which properties are functionally graded thanks to a non-homogeneous temperature field over the structure. A composite structure has been numerically designed integrating a viscoelastic core and a heat control device. The optimal temperature field has been obtained based on the static and dynamic elastic strain energy densities that reflects the compromise between structural damping over a wide frequency band and high static rigidity.

Tracking and Removing Modulated Harmonic Components with Spectral Kurtosis and Kalman Filters

This work describes an automatic method for removing modulated sinusoidal components in signals. The method consists in using the Optimized Spectral Kurtosis for initializing Series of Extended Kalman Filters. The first section is an introduction to vibration applications with Kalman Filters and modulated sinusoids. The detection process with OSK is described in the second section. The third section concerns the tracking algorithm with SEKF for amplitude and frequency modulated sinusoidal components. The last section deals with the complete process illustrated with an experimental application on a rotating machine.

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