A benchmark is organised to quantify the variability relative to structure dynamics computations. The chosen demonstrator is a pump in service in thermal central units, which is an engineered system with not well-known parameters, considered in its work environment. The blind modal characterisation of the separate pump components shows a 5%–12% variability on eigenfrequency values and a less than 15% frequency error in comparison with experimental values. The numerical-experimental MAC numbers reach 0.7 at the maximum, even after updating. An example of modal results on the pump assembly fixed is presented, which shows a larger discrepancy with measurement values, essentially due to the modelling of the interfaces and boundary condition, and to the possible simplification of the main components F.E. models to reduce their size. Though a significant frequency error, the first overall modes are correctly identified. If this tendency can be confirmed from all the participants’ results, the conclusion to be drawn is that, if the predictive capability of F.E. models to represent the dynamical behaviour of sub-structures is satisfactory, the one relative to structures that are built-up of several components does not allow their confident use. Additional information issued from measurements is needed to improve their accuracy.

Cet article est dédié à l’étude des vibrations libres et forcées d’une structure à symétrie cyclique, soumise à des non-linéarités géométriques, par la méthode de la balance harmonique (HBM). Dans le but d’étudier l’influence des non-linéarités un modèle simplifié a été développé. Après ajustement des paramètres du modèle, les équations du mouvement se présentent sous la forme d’équations différentielles du second ordre, linéairement couplées, où les non-linéarités se traduisent par des termes polynomiaux d’ordre deux et trois. Les solutions périodiques de ces équations sont recherchées grâce à la méthode de la balance harmonique couplée avec une procédure de continuation. Dans le cas libre, en plus des modes non-linéaires similaires et non similaires, on met en évidence des modes non-linéaires localisés. Dans le cas forcé, plusieurs types d’excitations sont considérées (excitation sur le premier mode propre linéaire et excitation detunée) et on étudie particulièrement l’influence du niveau d’excitation sur la structure des réponses dynamiques. Pour une excitation suffisamment perturbée, on montre que plusieurs solutions peuvent coexister, certaines d’entre elles étant représentées par des courbes fermées dans le plan fréquence-amplitude.

Le Forage Grande Vitesse Vibratoire (FGVV) est un nouveau procédé de perçage qui permet de tripler la productivité des perçages profonds en supprimant les cycles d’évacuation des copeaux ainsi que la lubrification. Pour cela un porte-outil spécifique aété développé, qui entre en vibrations axiales auto-entretenues lors de la coupe et permet le fractionnement du copeau, facilitant ainsi sonévacuation. Pour anticiper uneéventuelle diminution de la durée de vie des roulements à billes de l’électrobroche résultant de ces vibrations, un modèle global du comportement dynamique du système tête vibratoire-électrobroche est présenté. Ce modèle est basé sur la modélisation du comportement des entités structurales par deséléments-finis de type « poutre-rotor» prenant en compte les effets dynamiques associés aux hautes fréquences de rotations de l’électrobroche, couplé à des modèles d’interfaces identifiés par la méthode de couplage de réceptance. Le modèle est validé à l’aide d’expérimentations qui montrent une bonne correspondance entre les résultats expérimentaux et numériques. Le modèle est ensuite utilisé pour calculer la durée de vie des roulements à billes de l’électrobroche lors de différentes opérations de perçage. L’influence de la fréquence de rotation et de l’avance sur la durée de vie des roulements estétudiée, permettant ainsi de faire ressortir des plages de fonctionnement optimal.

The paper presents the experimental work developed by the authors for the identification of rotordynamic coefficients of air bearings. Air bearings work at very high rotation speeds and are known to have nonlinear dynamic characteristics depending at least on the excitation frequency. The paper presents two very similar test rigs, the testing procedure and the algorithm for identifying the rotordynamic coefficients. The test rigs consist of a rigid rotor guided by fixed bearings and driven by a spindle. The dynamic loads are applied by two orthogonally mounted shakers applying two linear independent excitations. Two air bearings are analysed in the present paper. The test procedure is first developed for a simple circular bearing with easily predictable dynamic characteristics. Its rotordynamic coefficients are identified by using a least square procedure based on rational functions. The coefficients are compared to theoretical results in order to underline the limits of the identification algorithm. The procedure is next applied to a first generation foil bearing. Rotordynamic coefficients are presented for different working conditions (static loads and rotation speeds) and are discussed comparing them to circular air bearings.

Rolling bearing is probably the most widely used component in rotating mechanical equipments and its condition monitoring and fault diagnosis to prevent the occurrence of breakdown is growing in interest since many years. Vibration signal based methods are the most popular and have been adopted in many kinds of condition monitoring systems. Starting in the early 60, an immense range of different methods has been proposed on this basis, to perform diagnosis, fault identification and classification of bearing faults. Among the others, one typical approach consists in deep analysis of the most informative frequency range output of the system under test; the identification of this band is not straightforward because the fundamental task consists in finding out the band which is the most informative in contents which, in turn, might not be corresponding to that one of the maximum response, as claimed by some authors. In this paper, Spectral Kurtosis and Support Vector Machine are analysed and compared and it is shown that they typically reach similar results, in spite of their totally different approach. A brief description of both methods is given and laboratory data are analysed from a lab rig which uses spare parts of a full size power transmission gearbox, designed by AVIO. By taking advantage of these comparisons, the analyses are conducted using classical indicators applied to the specific bands suggested by previous analysis such as the RMS and other statistical quantities. Multi dimensional graphs are reported to show the reliability of the obtained results.

An emerging research topic in civil engineering is the dynamic interaction between crowds and structures. Structures such as footbridges, which oscillate due to the crossing of a group of pedestrians, or stands within stadia or concert halls, which vibrate due to the rythmic movement of the audience are of particular interest. The objective of this study is twofold: modelling the movement of pedestrians with consideration of pedestrian-pedestrian, and pedestrian-obstacle interactions, and the incorporation of a pedestrian-structure coupling in the previous model. Frémond’s model, which allows us to simulate the movement of an assembly of particles and accounts for collisions among considered rigid particles, is presented and adapted to the crowd by giving a willingness to the circular particles, which allows each pedestrian to move according to a given target. To handle the crowd-structure interaction in the case of lateral oscillations of footbridges, the Kuramoto differential equation governing the time evolution of the lateral motion of each pedestrian is implemented in the previous model. Preliminary results obtained from numerical simulations are presented and discussed.

This paper approaches a novel technique to estimate cable tension simply based on its vibration response. The vibration response has been quite extensively adopted in the past due to its simplicity and, mainly, because the inverse approach allows the tension estimation with the cable in its original site. A first tentative approach consists in using a certain number of experimentally measured natural frequencies to be introduced in the theoretical vibration formula; this formula, however, involves also the cable length, the cable mass per unit length and its flexural rigidity. Unfortunately, some problems arise in its application to real structures, such as the case of suspended and cable-stayed bridges, because the exact cable length cannot be measured (it appears at the fourth exponent in the vibration formula); moreover section and weight can be estimated within a certain degree of accuracy, whilst the boundary conditions are often defined with difficulty. A novel extension of the method is here proposed, which takes advantage from a moving mass travelling on the cable. This is the case occurring when cables are verified with magnetic-based technology to detect rope faults and cross section reduction. In this way, the extracted natural frequencies are varying with time due to the moving load, and hence they have to be extracted adopting a time-varying approach. Although some approximation linked to the shape modification must be introduced, a simple iterative procedure can be settled, by considering the cable length as an unknown. An estimation of the equivalent length is given, and successively this value is used to obtain an estimation of the cable tension.

For a controlled structure, the changes of physical characteristics like natural frequencies, modal damping coefficients, and what’s more on mode shapes will destabilize the controlled system. To keep the control performance and guarantee its robustness, a new modal control method using real-time identification is proposed to overcome the instability of controlled structure, especially induced by the inversion of mode shape. In this paper, a modal model of time-varying mechanical structure is supplied by an identifier. The choice of the kind of control permits to use this time-varying model for optimizing and self-adapting the controller. In order to supply the controller, a modal observer is used and also updated. For illustrative purpose, this proposed methodology is carried out on a simple but representative time-varying mechanical structure. On this example, an inertia modification leads not only to low modal frequency shifts but also to inversion of a mode shape. This mode shape inversion is shown to destabilize the controlled structure when the control system is not updated. The overall procedure will be described through simulations and performed for different operating conditions, which will prove that mode shapes have to be precisely determined and updated in the controller and observer to guarantee a robust modal control with high performances in spite of the changes of structure.

In order to overcome the loss of performances issue when scaling resonant sensors down to NEMS, it proves extremely useful to study the behavior of resonators up to large displacements and hence high nonlinearities. A comprehensive nonlinear multiphysics model based on the Euler-Bernoulli equation which includes both mechanical and electrostatic nonlinearities in the case of a capacitive doubly clamped beam is presented. This purely analytical model captures all the nonlinear phenomena present in NEMS resonators electrostatically actuated including bistability, multistability which can lead to several physical limitations such as noise mixing, frequency stability deterioration as well as dynamic pull-in. Moreover, close-form expressions of the critical amplitudes and pull-in domain initiation amplitude are provided which can potentially serve for NEMS designers as quick design rules.