Jean-Pierre Laine

Pantograph-Catenary Dynamics Simulation

This paper presents research activities carried out in the field of pantograph–catenary dynamics simulation. Lump mass models are used to represent the pantograph and two levels of modelling are used to represent the catenary. The first level consists of a mono-dimensional analytical model which uses displacement decomposition in sine series. This method being continuous (as sine series are), it is very useful to validate results from finite-element simulations and to test some fundamental mechanical hypotheses. The second level consists of a three-dimensional finite-element model which will be used by overhead line designers. Defects and singularities can be included in the models to compare the obtained results with measurement data, with the goal of locating and identifying defects in the catenary using an instrumented pantograph. Before working on complex overhead line geometries, the relevance of some hypotheses was assessed on a simply supported tensioned beam, and the correlation of both models was verified.

Wave Finite Element Method Based on Reduced Model for One-Dimensional Periodic Structures

In this paper, an efficient numerical approach is proposed to study free and forced vibration of complex one-dimensional (1D) periodic structures. The proposed method combines the advantages of component mode synthesis (CMS) and wave finite element method. It exploits the periodicity of the structure since only one unit cell is modelled. The model reduction based on CMS improves the computational efficiency of unit cell dynamics, avoiding ill-conditioning issues. The selection of reduced modal basis can reveal the influence of local dynamics on global behavior. The effectiveness of the proposed approach is illustrated via numerical examples.

Cracked Blade Detection From Bladed Disk Forced Response

The primary task of this study is to offer reliable and accurate model of a bladed disk containing cracked blade. This model allows simulation of bladed disk dynamic behavior for various crack positions and lengths. Due to absence of cyclic symmetry caused by crack presence in the disk, a reduction procedure was implemented to simulate full bladed disk. It is proposed to use crack location as an interface between two substructures for subsequent fixed-interface method application. Harmonic balance method was applied to take into account crack nonlinear behavior under periodically varying loads. The method implementation considers contact interaction between crack sides at the crack being closed. The contact force is calculated using penalty method of contact force calculation. Relative vertical displacements between nodes in contact were used as nonlinear degrees of freedom (DOFs). Developed bladed disk model is able to take into account external excitation forces phase lag caused by difference between number of rotor and stator blades. Also presence of mistuning was considered. It was shown that certain level of mistuning can directly affect cracked blade detectability. Cracked blade dynamic behavior localization plays here very important role. Absence of cracked blade localization results in impossibility to separate cracked blade response at any mistuning level. Validity of zig-zag diagram for structures with disrupted symmetry is shown using developed bladed disk model with presence of certain level of mistuning.Copyright

Piezoelectrically induced electronic confinement obtained by three-dimensional elastic relaxation in III–V semiconducting overhanging beams

In this work, we demonstrate theoretically that the piezoelectric effect can be used to achieve confinement over quantum distances in systems grown on [001] GaAs substrates. Such an effect can be achieved by making use of elastic relaxation of micromachined strained structures. At the free corners of the overhanging beams, shear deformations appear which induce a three-dimensional V-shape potential. Calculations show the creation of quantum dots near the corners of the overhanging beams.

Modal Tests and Analysis of a Radial Impeller at Rest: Influence of Surrounding Air on Damping

HCF risk assessment for turbomachinery blades requires the prediction of vibratory levels, which in turn requires fine damping quantification. This issue is especially sensitive for structures with low structural damping such as monobloc centrifugal compressor disks (blisks). The material composing blisks and aero-dynamic flow both contribute to damping phenomena. A strategy for non-aerodynamic damping characterization is to perform experiments in vacuum.This paper focuses on the use of modal tests in vacuum to estimate material damping under non-rotating conditions. Experiments are performed on an isolated impeller manufactured from a single piece in a vacuum chamber at different air pressure levels ranging from 10 mbar to 1 bar. Strong dependency of damping ratios on pressure can be found on the first flexural mode, leading to two types of application.Firstly, measurements enable assessing the validity of extrapolations of non-aerodynamic damping from measurements sometimes performed under less thorough vacuum conditions. Basic fluid-structure interaction models are used to interpret and quantify the evolution of modal quantities when air is progressively removed. Secondly, vacuum measurements can give frequency response functions (FRFs) with much greater separation between resonance peaks. In this study, the damping ratio found in vacuum condition are 3% of these at ambient pressure corresponding to a magnitude 30dB higher at resonance peaks. This contrasts with in-air measurements on cyclic symmetry structures, like blisks, with high modal density that make the direct interpretation of FRFs and their modal analysis more difficult.Copyright

Simulation of Tip-Timing Measurements of a Cracked Bladed Disk Forced Response

The study intends to simulate the process of the blade tip amplitude calculation by the tip-timing method. An attention is focused on tip-timing measurements for detection of a cracked blade from the bladed disk forced response. The cracked blade is considered within frameworks of the bladed disk dynamic model that takes into account mistuning presence. Nonlinear formulation of a crack behavior is done with the harmonic balance method in its combination with the contact analysis that allows simulation of crack breathing. In order to make the cracked blade detection process evident, the crack length and location are set in such a way as to produce the cracked blade frequency localization. Reconstruction of the blade tip amplitudes is attained with the arriving time of measured probes of the blade tips. The results are compared with the blade forced response obtained by the bladed disk dynamic model. A possibility is also considered how to reconstruct time-history of the bladed disk forced response with tip-timing data.Copyright

Multi-Scale Homogenization of Transversal Waves in Periodic Composite Beams

This paper deals with the deduction of new homogenized models for the flexural wave in bi-periodic beams. According to the homogenization theory, the long-wave assumption is used and the valid frequency range of homogenized models is limited to the first Bragg band gap. However, the classical homogenization method, whose idea is taking the component’s mean values as effective material properties, has limitations in mimicking the dispersive behavior and the real valid frequency range is far less than the limit. Thus, enriched homogenized models, derived by the multi-scale asymptotic homogenization method, are proposed to provide more accurate homogenization models with larger real valid frequency range. The new homogenized models are validated by investigating the dispersion relation in the infinite case and the frequency response function in the finite case. Wave finite element method (WFEM) are used to provide associated references. A parametric study is carried out in the infinite case while two different boundary conditions are considered in the finite case.

Modélisation du comportement viscoélastique des élastomères autour d'une précharge

We introduce a model for the viscoelastic behaviour of elastomers around a large static load. We give an explicit form for the dynamic modulus as a function of the preload and of viscoelastic parameters. Several experiments are made on simple bushings for different levels of preload and frequencies. A comparison between the linearized model and experiments is presented.

Modelling the behaviour of a PWR core by a homogenization technique

Abstract Nuclear safety requires proper dimensional design of the sensitive components. In particular, it is indispensable to determine the dynamic behaviour of a pressurized water reactor core during seismic excitation (major risk). But full core modelling calls for a very large number of degrees of freedom. This is why it is indispensable to perform homogenization of the fluid and structures: in other words, a homogeneous model dynamically equivalent to the core must be determined. Once the equations governing the homogenized medium have been formulated, to solve them is necessary. By building suitable finite elements, it is possible to recreate the dynamics of the initial system. It is then possible to perform full core simulations.

DYNAMIC NON-LINEAR ANALYSIS OF A CRACKED BLADE

Abstract A cracked blade non‐linear dynamic analysis was performed, taking into account contact interaction between crack sides. The contact‐induced non‐linear problem was solved by using the harmonic balance method. Accuracy and computational efficiency are demonstrated by comparing the results with the time integration of the systems motion equation Problem size reduction was performed using system fixed‐interface method. They suppose that a crack forms an interface between two sub‐structures and use a relative degree of freedom to describe the motion of crack sides. The influence of centrifugal forces was investigated in order to understand the necessity of problem non‐linear formulation depending on crack location and frequency of rotation.