Definition of a linear equivalent model for a non-linear system with impacts

International audienceModal characteristics of non-linear system are typically studied through response to harmonic excitation and using various definitions of non-linear modes. However, few results are available for systems under broadband excitation. The end objective sought here is to generate a linear system, in some sense equivalent to the non-linear system, whose modal characteristics evolve with a level of non-linearity. The considered application is the contact non-linearity found between the tubes of heat exchangers and their support plates. Such tubes, present in nuclear plants, participate to the nuclear safety and can be significantly excited by the fluid flow, so that their dynamic behavior is critical. The turbulent nature of the flow implies broadband excitation and the small gaps between the tubes and the support plate generate very significant non-linear behavior. The proposed equivalent linear system is based on a bilateral contact law whose stiffness and damping characteristics evolve with the amplitude of excitation. A non-linear model is first validated by correlation with experiments. It is then shown that three different indicators (bandwidth of main resonance, operational modal analysis of non-linear power spectral density and correlation of operational deflection shapes) lead to similar values of contact stiffness and damping in the equivalent linear model. This model is hus shown to be a very efficient tool to analyze the impact of the amplitude dependence of the non-linear behavior in the considered system

Fatigue analysis of catenary contact wires for high speed trains

The fatigue fracture is one of the most critical failures which may occur on the high speed network because it is undetectable and it has a huge impact on traffic disruption. The contact wire lifespan of a high speed line is estimated at more than 50 years and thus it is necessary to consider the risk of fatigue. The Railway Technical Research Institute in Japan studied this phenomenon for a long time and performed experimental tests. Using these results and by comparing with failures occurred in France, a preliminary analysis is carried out to identify parameters which significantly influence the fatigue phenomenon. This analysis consists in using the numerical software OSCAR© to evaluate the loads, perform a fatigue assessment of the contact wire. The procedure, using a one-dimensional and a three-dimensional model, is described in this article

OpenFEM: an open source finite element toolbox

International audienceOpenFEM is a finite element toolbox designed to be used within a matrix computing environment MATLAB and SCILAB. This toolbox provides general specifications for models, multiphysics elements, and self extending pre- and post-processing tools. This paper describes the software architecture and the existing tools.OpenFEM est une boîte à outils éléments finis développée dans les environnements de calculs matriciels MATLAB et SCILAB. Cette toolbox fournit des outils de générations de modèles, d'éléments multi-physiques, de pré- et post-traitement. Cet article décrit son architecture logicielle et ses fonctionnalités

Merging sensor data from multiple temperature scenarios for vibration monitoring of civil structures

The ambient temperature effect may result in limitations of vibration-based structural health monitoring (SHM) approaches for civil engineering structures. This paper addresses the issue of discriminating changes in modal parameters due to damages and changes in modal parameters due to temperature effects. A non parametric damage detection algorithm is proposed, which only assumes that several datasets are recorded on the safe structure at different and unknown temperatures, and smoothes out the temperature effect using an averaging operation

Integrating damping and non-linearities in a vibration design process

Classical vibration design uses modes and transfer functions generated with the superposition principle to allow the verification of design objectives. If redesign is needed, one optimizes mass and stiffness in order to modify the transfer until the specification is met. Integrating damping and non-linearities in the optimization of detailed industrial models is however still considered a major difficulty, even though the physical mechanisms are well known. Approaches to handle viscoelastic damping and time domain modal damping are thus discussed. Distributed non-linearities, such as contact and friction, are becoming accessible to transient simulation, but lead to responses where modes are no longer defined. It is however illustrated that operational deflection shapes, associated with a singular value decomposition of the response, give similar information. Finally, a fundamental aspect of non-linear vibration simulation is the volume of output and the associated numerical cost. Model reduction is a key ingredient of practical approaches and a perspective on related issues is given

Incorporating damping predictions in the vibroacoustic design process.

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Orthogonal Maximum Sequence Sensor Placements Algorithms for modal tests, expansion and visibility.

International audienceAfter discussing standard requirements on sensor configuration and addressing the issue of non triaxial measurements, the paper introduces a new class of sensor placement algorithms that locate sensors at the maximum response position of an orthogonal sequence of vectors. The methodology is first applied to target modes and provides similar results to standard placement algorithms at a fraction of the numerical cost. Fixed sensor modes are then defined and shown to provide an efficient mechanism to validate a given placement and place additional sensors. A placement strategy to detect modeshape changes is finally proposed to enhance the visibility of selected model parameters