Please fin below the list of past PhD theses directed by Etienne Balmes
| Author | PhD Thesis | Links |
|---|---|---|
 G. Malacrida Alves2025 | Experimental modal analysis of time varying non-linear systems. Application to brake squealAbstract[...]Brake squeal is a complex phenomenon resulting from a mode coupling instability. Despite being a subject of study for many years, no robust design method exists for its prevention and correcting measures are heavily based on prototyping and validation, which is time-consuming and expensive. Squeal testing can be separated into three main activities: classification of squeal occurrences based on acoustic levels, detailed spatial characterization of limit cycles, and modal characterization of components and assemblies. This work thus seeks to propose and analyze the usefulness of different methodological changes to testing methods. A specificity of squeal is that vibration depends on multiple operating parameters pressure, wheel speed, temperature, … which vary in time relatively slowly compared to the frequencies of squeal limit cycles. It is thus necessary to take this specificity into consideration to design testing methods and analyze their results. As a mean to gain further insight on the effects of slowly changing operating conditions, a new functional model for squeal is proposed based on the well-known Hoffmann model. The novelty lies in replacing the variable friction assumption by a nonlinear contact law, which introduces a sensitivity to the applied pressure (as the static-state is modified), and amplitude (through its effect on the mean pressure). Analyzing and comparing the response of this model in frequency and time domain provides a path to understanding the relations between squeal limit cycle vibration and parameters. The definition of a Harmonic Balance Vector (HBV) signal model and the use of demodulation for its estimation, then allows tracking of quasi-periodic squeal signals without resolution limitations of the Fourier transform. For parametric squeal tests, this method improved occurrence classification by extracting the evolutions of features such as instantaneous frequency, global vibration amplitude, generalized coordinates and shape. For the detailed shape analysis using 3D-SLDV measurements, the HBV signal estimates led to results improved over current methods based on short time Fourier transforms. Applications are illustrated in a contact test bench to obtain the evolution of shapes with pressure, and in a full-scale brake test to obtain the limit cycle shapes. The HBV signal estimation can also extract spatially detailed shapes for higher harmonics. Two indicators (harmonic modulation and harmonic perturbation) are then proposed, based on the notion of instant stiffness, as a mean of evaluating where in a period the system is stiffer or softer. Finally, for in operation parametric Experimental Modal Analysis (EMA), full scale tests showed that modes away from squeal can be consistently identified and tracked. Near squeal, however, the presence of a coherence loss indicates that other methods are required such as the feedforward phase resonance tracking proposed in the analysis of the simplified contact test bench. | Download PhD defense |
 R. Penas Ferreira2021 | Models of dissipative bushings in multibody dynamicsAbstract[...]Multibody simulations are used in the automotive industry to ensure that important design targets are met. A key point in the accuracy of such simulations is the proper modeling of rubber bushings and mounts. Indeed, they do require precise modeling of large deformation for high loading maneuvers, typically associated with active safety, and a correct modeling of dissipation, for most comfort applications. The current modeling process of these mounts and bushings considers tests to generate 0D models in each separate direction with linear viscous dissipation.The tests proposed and realized in this work were used to characterize the effects of large deformation, rate independent hysteresis and viscoelasticity separately. Models for each one of those behaviors are categorized into non-parametric models, extracted directly from data, selected order parametric models, whose accuracy depend on the number of internal states, and order independent models. Utilization of ratios of the nonlinear static branch for all the other branches is shown to be an effective way to model coupling of dissipation with large deformations. Transition between hysteresis and viscoelasticity is presented as a consequence of nonlinear viscoelasticity and provides good agreement with tests demonstrated by the comparison of the proposed instant modulus.The influence of loading in transverse directions on a given axis is very difficult to obtain from tests, yet necessary for accurate bushing models. Finite element computations are quite suitable for such studies, and as they require 3D models, the translation of the proposed 0D model into a 3D material model is detailed. Two different identification routines for both the 3D and 0D models were proposed: one with classic objective functions for the material model and a graphic one with order selection based on the non-parametric models. %Synthetic plots illustrating all behaviors are made for both material and part specimens, reuniting most of the responses.Despite the usefulness of FE routines to link geometry and material behavior, their typical computation times are orders of magnitude too large to be acceptable for the envisioned multibody applications. A combination of kinematic reduction and hyper-reduced integration of the model equations is thus detailed and shown to provide sufficient accuracy and the needed speedup in computation times. The mathematical frame for integrating the reduced 3D models into multibody routines is described.Finally, two multibody illustrations are detailed. The first case highlights the fact that hysteretic and viscous dissipations may lead to notably different transient responses, confirming the need for the propositions made for 0D models. The second one shows that replacing the current relatively coarse 0D models with those developed in this work should induce notable modifications of the response. | Download |
 F. Conejos2021 | Multiscale homogenization for a modal damping objective in the design of 3D woven composite bladesAbstract[...]The goal of this work is to control the damping of the first modes of the fan blade during the design phase.A numerical method for the viscoelastic homogenization of 3D woven composite materials is developed. Two homogenizations are made, at the yarn's scale and at the woven one. The complex modulus of the resin, the elastic properties of the yarns and the geometry of the microstructure allow a characterization of the damping in frequency and temperature. The matrix formulation of the homogenization, for a finite element implementation, allowed the use of model reduction techniques in parametric studies on the viscoelastic properties of the composite's constituents.A comparison of the numerical predictions with the modal damping obtained by modified Oberst test, for low frequencies and a range of temperature between -40°C and 120°C, provided coherent results in terms of absolute values and trends. These tests also show areas of stress gradient which are small compared to the weaving pattern size motivating the introduction of a verification methodology of the scale separation hypothesis at different wavelengths. The definition of the elastic and dissipated energy fractions by decomposition of the constitutive law into singular values allows an analysis of the main contributions to the damping at both 3D woven and fan blade scale.The insertion of a hybridized 3D woven composite with elastomer tows within the fan blade is proposed in order to increase the modal damping of the first flexion. A spatial study of the areas dissipating the most energy is performed to optimize the placement of the hybrid weaving pattern. | Download |
 H. Pinault2020 | Reduction with multi-wavelength learning for open or heterogeneous waveguides : application to railway track dynamicsAbstract[...]The main goal of this work is to propose a waveguide model representing the whole length to handlethe variability between geometrically periodic cells, suited to time-based computations to deal with non-linearities,and with reduction to allow simulations in a time sufficiently small for use in a design group. The analysis ofwaveguides is a recurring subject in the literature. Bloch-Floquet theory is often used to compute solutions definedon a reference cell at several wavenumbers. However, this formulation does not allow variability between periodiccells, is not suited for time-based computations, and may lead to a significant numerical cost if the interface betweencells is large. To address these limitations, a reduction strategy is proposed, based on the building of a learningsubspace from computed static and modal forms within a frequency range of interest, and for few wavenumbers. Amodel of the full guide is built from reduced cell models and can account for variability. By adjusting the extremecells of the model, this strategy can be adapted for both finite and infinite periodic structures.This reduction strategy is applied to the study of a heterogeneous periodic structure, generated from randomfields. The presence of frequency bandgaps and local modes is assessed. The learning phase manages to takecorrectly these phenomena into account. The strategy is extended to non-periodic heterogeneous structures bycombining several periodic samples.Another goal is to approach radiation in an open medium with absorbing PML boundaries, while maintainingthe possibility to achieve both time and frequency-based computations, which is a requirement of the reductionstrategy. To that end, a FEM implementation with 3D wave attenuation is detailed. The frequency-based analysis ofthis formulation raising contionning issues, conditions are proposed that are sufficient avoid the problem. Theapplication case of a train pass-by on a track shows another limitation : a wrong asymptotic behavior at lowfrequencies.For each of the three main topics, the proposed methodologies are applied to railway track models. They give abetter understanding of the behavior of ballasted or discontinuous slab tracks at low frequencies. | Download |
 G. Martin2017 | Calculation/Test methods for the analysis squealAbstract[...]Brake squeal is a nuisance commonly encountered in the car industry which often results in financial penalties towards brake manufacturers, even if no robust solution exists for the conception. Numerical simulation and experimental characterizations are the classical two-track approaches to analyze squeal phenomena. Numerical simulation allows a fine analysis of vibration behaviors and the evaluation of conception modification impacts, but models are not perfect and their validity domain must be defined. Measurement guarantees that every parameter is taken into account, but it is often spatially poor and not really suited for modification prediction. In this context, the thesis objectives are to make an overview of test operating methods, to provide tools allowing an easier interaction between both test and simulation teams and to enforce the developments in a business application.After a review of the hypotheses and methods for modal identification, a critical analysis of the identification results leads to the characterization of biases and the introduction of detailed criteria to evaluate the quality of the result. Calculation/test correlation allows to qualify the models and tools are integrated to facilitate its implementation. Several criteria are defined to highlight the sources of bad correlation coming from the measure on the one hand and the model on the other hand. Sources of poor correlation are also identified with the MDRE expansion algorithm, whose limitations are compared with those of static and dynamic expansions.Model updating is then processed by a systematic procedure imbricating the steps of updating of geometry, material properties and contacts between components. A contact parametrization is proposed with a multi-model reduction allowing calculation times compatible with industrial time. It also allows sensitivity studies with the introduction of the notion of component modes in an assembly.Finally, a detailed analysis of a measurement campaign on a braking system under squeal conditions is carried out. A parallel between the changes of the limit cycle and the degree of coupling of the complex modes is proposed. In the time-frequency domain, variability, repeatability, reproducibility and the ability to aggregate sequential measurements are evaluated. The result is finally extended on the finite element model, which allows useful interpretations. | Download |
 O. Vo Van2016 | Introduction of variability into pantograph-catenary dynamic simulationsAbstract[...]In railways, electrical current is generally collected by the train through a complex coupledmechanical system composed of a pantograph and a catenary. Dynamic phenomena that occur duringtheir interaction are still not fully understood. Furthermore, the system behaviour is sensitive to numerousparameters and thus highly variable. The first contribution of this thesis is a detailed analysis of thepantograph-catenary dynamic interaction separating phenomena due to the dynamic response of the pantographto the catenary geometry from wave propagations, reflections and transmissions that occur in thecatenary. The coincidence of frequencies or characteristic times is then shown to explain most variationsin the quantities of interest. Moreover, droppers surrounding the mast have been shown to be particularlyimportant in dynamic interaction. Ratio of wire impedances and sum of wave velocities also appeared tobe dimensioning quantities for catenary design. The second contribution was to reduce epistemic uncertaintylinked with model parameters such as catenary damping, contact stiffness and element size. Thefinal contribution was to use the model in a configuration with random parameters. An initial step was tostatistically characterise physical catenary parameters using available measurements. From this randommodel, ranking of uncertainties using Sobol indices on static and dynamic criteria was shown to be possible.An absence of correlation between geometric and dynamic criteria was also found, which has notableimplications for maintenance policies. The high number of sensitivity studies also gave the occasion tohighlight the maturity of simulation tool and propose directions for further work on design, maintenance orcertification of pantographs and catenaries. | Download |
 E. Arlaud2016 | Reduced dynamic models of piecewise periodic media : application to railway tracks.Abstract[...]Railway tracks have evolved over years based on empirical results and their dynamic behavior still can be difficult to assess or to quantify. A numerical model can thus provide assistance in track design by assessing the mechanical performance of new structures, or allowing the diagnostic of existing track parts, as a complement to relevant in-situ measurements.The numerical model developed in this work combines a reduction strategy with numerical techniques used to solve the propagation equations in waveguides in the frequency / wave number domains. The peculiarity of the exposed methodology is the introduction of a model based on a periodic sub structuring of the track, keeping only a few wavelengths carefully chosen to solve dispersion equations. Based on this model reduction technique and its extension to time domain models with moving contact, the computational time and storage capacity required are greatly reduced. Thus, this model is efficient and useful for engineering purposes in railway tracks studies.Numerical validation of the reduction is carried out by building a complete reference model in the frequency domain. In parallel, measurement campaigns (receptance measurement and sleeper acceleration under passing trains) were performed on a transition zone between ballasted and slab tracks on a High Speed Line. These tests are compared to simulation results in both time and frequency domains on different areas of the transition. After successful validation, the model is used to improve understanding of the role of the substructure on the dynamic behavior.The final major development of this work is the introduction of a strategy to extend the reduction to piecewise periodic structures and the development of post-processing tools to highlight the dynamic effects generated by the transition zone. | Download |
 C. Hammami2014 | Integration of dissipative joints models in the vibratory design of damped structureAbstract[...]Dissipation in mechanical systems plays an important role in limiting vibrationlevels. The present work deals with maximizing damping induced by junctions and treatsjunctions integrated into an overall structure. After introducing the physical models used inrepresenting viscoelastic behavior, a first example introduces the concept of coupling. Coupling,estimated by preliminary calculations of elastic natural frequencies sensitivity, ensures thatjunctions sufficiently contribute to the global behavior to induce damping. To study this couplingas well as obtain accurate damping predictions by complex modes calculation, model reductionmethods, adapted to parameterized problems, are needed and different strategies are detailedand validated. A structure representing aeronautic construction is then used to illustrate theproposed damping design process. The study highlights the functional importance of thecohesive zone under the bolt and the possibility of using the residual flexibility of the joint tointroduce a viscoelastic layer enabling high damping. Experimental modal tests confirm thevalidity of the design and the predicted trends, but also show the influence of manufacturingdefects which limit the induced damping. The manuscript finally addresses the need forrepresentative models of detailed joints in overall models. Although model reduction is possibleand desirable, the construction of meta-models representing the main forces transmitted by thejoints is useful in modeling and necessary for the operation of elementary tests. Generalstrategies for defining principal deformations and loads are introduced and their validity isillustrated. | Download |
 G. Vermot des Roches2011 | Frequency and time simulation of squeal instabilities. Application to the design of industrial automotive brakes.Abstract[...]Brake squeal is a common noise problem encountered in the automotive industry. Higherfriction coefficients and weight reduction recently led to higher vibration levels in the audiblefrequency range. This quality issue becomes economic due to penalties imposed to the brakesupplier although no robust design method exists. The industrial practice thus relies on costlyprototyping and adjustment phases. The evolution of computational power allows computationof large mechanical assemblies, but non-linear time simulations generally remain out of reach.In this context, the thesis objective is to provide numerical tools for squeal resolution at earlydesign stages.Parameterized reduction methods are developed, using system real modes as Rayleigh-Ritzvectors, and allow very compact reduced models with exact real modes. The proposed ComponentMode Tuning method uses the components free/free modes as explicit degrees of freedom.This allows very quick sensitivity computation and reanalyzes of an assembly as function oflocal component-wise parameters. Non-linear time simulations are made possible through twoingredients. A modified non-linear implicit Newmark scheme and a fixed Jacobian are adaptedfor contact vibrations. The brake is reduced keeping a superelement with exact real modes anda local non-linear finite element model in the vicinity of the pad/disc interaction.A set of design tools is illustrated for a full industrial brake model. First, instant stabilitycomputations and complex mode trajectories are studied. Modal interactions and non-linearphenomena inside the limit cycles are thus well understood. Time/frequency correlations areperformed using transient modal identification and space-time decomposition. A time domainmodal damping model is also shown to be very useful. The modification of a critical componentfor squeal resolution is finally tested and validated. | Download |
 T. Thenint2011 | Study of a nonlinear system with shocks under broadband excitation : application to a steam generator tubeAbstract[...]The steam generator is a heat exchanger and participates to the nuclear safety. Energy is transferred from the primary to the secondary fluid through many U-tubes maintained vertically by support plates. A sludge deposit tends to modify the boundary conditions and the secondary fluid flow. A fluid-elastic instability can then occur and lead to quick tube ruin. This thesis seeks a better understanding of the effect of contact nonlinearity on the dynamics of a tube in-air intermittently impacting the support plates and its consequences in regards with instability. The use of discretized contact conditions with circular obstacles distributed over the thickness of the plates and the use of enriched reduction bases allow quick and relevant nonlinear numerical simulations. These simulations are well correlated with experimental measurements and valid even with strong nonlinearity or negative modal damping. The evolution of power spectral densities (PSD) with growing excitation amplitude is analyzed: padding of the anti-resonances, peak shift and spread. It is then shown that an apparent stiffness associated with a permanent bilateral contact is pertinent to describe these transitions. In the case of an unstable linear system, one demonstrates that the nonlinearity keeps the responses bounded or stabilised, thus paving the way for future work with real or simulated fluid flows. | Download |
| C. Florens 2010 | Modeling of the viscoelastic honeycomb panel equipped with piezoelectric patches in view of vibroacoustic active control designAbstract[...]Active control has often been considered for low frequency control of noise radiated by trim panels inside aircraft or helicopter cabins. Trim panels are usually made of honeycomb core sandwich because of their high strength to mass ratio. Active control techniques applied to honeycomb panel have not always given results as good as expected and this thesis aims to understand these limitations based on validated mechanical models of the active panels. For the modeling of honeycomb panels, the main difficulty is to estimate equivalent properties for the core. A numerical homogenization procedure is introduced to estimate effective parameters of a shell/volume/shell model based on the correlation with periodic modes of a detailed 3D model. The use of periodic modes allows a detailed analysis of the influence of constituent properties, especially glue and skin. Tests show that the considered Nomex based honeycomb has significantly viscoelastic behavior. In the model, the viscoelastic behavior of the core is taken into account by a frequency dependence of material parameters. Piezoelectric actuators and sensors are included in the validated honeycomb model. Strategies for integration in a numerical design process are discussed. Finally, the static response to an applied voltage is shown to correspond to a blister shape with local bending of the skin rather than global bending of the panel. This behavior results in poor actuator performance, which is also found in a realistic panel configuration studied at ONERA. | Download |
 A. Sternchüss2009 | Multi-level parametric reduced models of rotating bladed disk assembliesAbstract[...]Bladed disks found in turbomachines are complex structures whose vibration characteristics are generally determined by exploiting the symmetry properties of their nominal configuration. This symmetry no longer exists either when disks are assembled to form a rotor or when discrepancies in the mechanical parameters are introduced (intentional or unintentional mistuning). Fine meshes required to correctly evaluate stress distributions would lead to prohibitive model sizes (typically a few million degrees of freedom). The objective of this thesis is to introduce model reduction techniques that rely on the combination of separate computations of acceptable size. This provides a means for in-depth studies of the behaviour of dense 3D models of multi-stage bladed rotors with possible mistuning. At first, Fourier transforms performed separately on each individual disk allows to understand the inter-harmonic coupling induced by inter-stage coupling and mistuning. From this study, a first method uses cyclically symmetric solutions plus sector modes with fixed inter-sector interfaces to build a reduced sector model. The latter is exact for target modes and very accurate for others. This method is extended to multi-stage assemblies by employing multi-stage mono-harmonic eigensolutions. Illustrations focus on the proposed methodology that enables to deal with large scale industrial models while remaining compatible with various post-processing procedures (free or forced response computations, analysis of their spatial harmonic content, energy distributions and localization effects...). This methodology is finally extended to the handling of parametric models depending on the rotation speed. The enrichment of the initial sets of target vectors with computations at three rotation speeds enables a fast and accurate recovery of the evolution of the eigenfrequencies with respect to the rotation speed in any operating range. | Download |
| B. Groult 2008 | Extending structural dynamic modification methods to the design of viscoelastic damping devicesAbstract[...]The aim of this work is to extend the use of a structural dynamic modification method to cases of damping device design. These devices are meant to improve the mechanical behaviour of some structures used by EDF (Electricité de France). The first step of the work is to give theoretical bases on the modelling of heavily damped structures and especially viscoelastic ones. One then performs a design study for an experimental testbed that is representative of structures EDF wants to treat. The performance of the damping device is evaluated experimentally and compared with the numerical results. This numerical design study highlights the limitations of classical design methods in the context of industrial exploitation of EDF (non updated models, complex structures, access to measurements limited). The second part of the work thus deals with the adaptation of a structural dynamic modification method to the case of the proposed damped modification. The chosen method is based on a modal coupling formulation that has been tested within EDF on undamped applications. But the method needs some adaptations because of the use of damping terms in equations and the fact that the mechanical behaviour is frequency dependant (a characteristic of viscoelastic materials). The robustness of the method is then analyzed through some points that are important for its application. The method is experimentally validated through the testbed. | Download |
 G. Kergourlay2004 | Vibroacoustic measurement and prediction of viscoelastic structures. Application to an acoustic chamberAbstract[...]Damping augmentation is a common approach to vibration control in structures (vibroacoustics, vibration fatigue). Viscoelastic materials can be used to design efficient damping treatments. The mechanical properties of these materials however depend on frequency, but also on prestress and temperature. Traditional representations for viscoelastic materials, finite element design and computation methods for damped structures are presented along with new reduction techniques such as the Residual Iteration process.A dynamic test bench has been designed and built to measure directly the complex stiffness of viscoelastic films of standard sandwich plates using an electrodynamic shaker in the frequency range of 1-2000 Hz, for temperatures of 0-50°C and what is new, for static shear rates of 0-300 %. The tests reveal how the temperature-frequency superposition hypothesis can be extended to prestress. The second objective of this work is to investigate and improve the vibroacoustic behaviour of an audio speaker. A finite/boundary element model is built and validated through measurements for the low frequency range (0-2000 Hz). An efficient damping treatment is proposed and its properties optimized. | Download |
 M. Corus2003 | Improvement of structural dynamic modification method using data expansion and model reduction techniquesAbstract[...]This work deals with the improvements of methods devoted to estimate the influence of modifications on the dynamic behaviour of a structure (addition of stiffeners or masses, boundary condition changes). The input data for the method are an experimentally derived model of the initial structure and a numerical model of the modification. Based on a coarse model of the structure to be modified, the proposed methodology tackles two major difficulties : efficient predictions for distributed modifications and handling of the lack of measurement points on the coupling interface. In addition, displacements bases introduced to reconstruct unmeasured behaviour of the interface limit error propagation through the process. Moreover, two indicators are introduced to select the optimal prediction. A number of industrial examples illustrate the performance of this approach, in terms of reactivity as well as quality of the predictions of dynamic behaviour. | Download |
 A. Bobillot2002 | Model reduction methods for updating. Application to the case of Ariane 5Abstract[...]This thesis addresses model reduction methods for vibration applications, with an emphasis on cases related to FEM model updating. | Download |
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