ORBi Open Repository and Bibliography
In may 2007, the ULg's Administrative Board (joined in June 2007 by the FUSAGx) decided to create an institutional repository and defined a strong institutional self-archiving policy to increase the visibility, accessibility and impact of the University's publications (Board's decision).
This decision led to the official launch, in November 2008, of the ORBi platform including both the Academic Bibliography and the Institutional Repository of the Wallonia-Europe University Academy.
Liquid oxygen compatibility of materials for space propulsion needs - Bozet, Jean-Luc; Heyen, Georges; Dodet, Claude; Duquenne, Marc; Kreit, Patrick
Since the origin, the use of liquid oxygen as oxidizer is widespread in space propulsion. It has been the source of numerous incidents. Many materials can burn violently in an oxygen enriched environment when ignited. Consequently, it is mandatory to perform oxygen compatibility tests. In the present work, compatibility tests are impact tests and auto ignition tests. Both standardized test methods are illustrated by test results on polymeric materials (i.e. fluorinated resins, polyimides, polyetherketone) for seals and ball bearing cage. For future needs, in order to face the increase of pressure of injection of propellants in cryogenic engines, the adiabatic...
Influence of liquid oxygen environment on the tribological behaviour of PCTFE/440C steel contact - Cautain, Satia; Duquenne, Marc; Robbe-Valloire, François; Bozet, Jean-Luc
This article proposes a tribological study of contact between polychlorotrifluoroethylene (PCTFE) and 440C steel. The tests are performed on a pin-on-disc cryotribometer specifically designed to overcome the additional issues due to the use of cryogenic fluid. Test parameters such as speed, contact pressure and sliding distance are kept the same for the whole campaign, leaving the environment as the only variable. This study focuses on the analysis of friction, wear of PCTFE pin and appearance of disk after tests. The test results show the influence of the state of environment (gas or liquid), its temperature and its reactivity (neutral or...
Practical design of a nonlinear tuned vibration absorber - Grappasonni, Chiara; Habib, Giuseppe; Detroux, Thibaut; Wang, Fengwen; Kerschen, Gaëtan; Jensen, Jakob Søndergaard
The aim of the paper is to develop a new nonlinear tuned vibration absorber (NLTVA) capable of mitigating the vibrations of nonlinear systems which are known to exhibit frequency-energy-dependent oscillations. A nonlinear generalization of Den Hartog’s equal-peak method is proposed to ensure equal peaks in the nonlinear frequency response for a large range of forcing amplitudes. An analytical tuning procedure is developed and provides the load-deflection characteristic of the NLTVA. Based on this prescribed relation, the NLTVA design is performed by two different approaches, namely thanks to (i) analytical formulas of uniform cantilever and doubly-clamped beams and (ii) numerical shape...
Experimental modal analysis of a beam travelled by a moving mass using Hilbert Vibration Decomposition - Bertha, Mathieu; Golinval, Jean-Claude
In this paper the problem of modal identification of time-varying system is investigated. To do so, a technique based on the sifting process of the Hilbert Vibration Decomposition (HVD) method is presented. The key idea is to estimate the instantaneous frequency of the dominant mode, to extract its corresponding component by demodulation of the recorded signals and then to iterate with the subsequent dominant mode. In the case of multiple recorded signals, a source separation method is used as a preprocessing step to facilitate the identification of the instantaneous frequency for the following demodulation step.
To illustrate the method, an...
Experimental Validation of a Thermal Model of a LOx Flooded Ball Bearing - Servais, Christophe; Bozet, Jean-Luc; Kreit, Patrick; Guingo, Sébastien
Ball bearings of turbopumps of rocket engines work in very singular conditions; they are flooded into liquid oxygen or liquid hydrogen. Therefore, the use of any conventional lubricants (oil or grease) is proscribed. Although cryogenic fluids can cool this kind of ball bearing, they cannot separate the surfaces in contact. As a result, there is a significant increase in frictional power losses. This paper presents tests performed on a cryotechnic ball bearing flooded in liquid oxygen. The test results showed that beyond a critical loading of the bearing, a sudden increase in temperature occurred. A thermal model is also put...
Test equipments for characterizing the behavior of materials, bearings and dynamic seals in cryogenic fluid environments - Bozet, Jean-Luc; Servais, Christophe; Duquenne, Marc; Kreit, Patrick; Dodet, Claude
Test rigs available at the cryogenic test site of the University of Liège concern quite delicate high rotational speed components of cryogenic rocket engines. That is to say bearings and dynamic seals. These rigs gave successfully important results for the understanding and the improvement of the running of the so called components. Until now the test zone is equipped for handling liquid oxygen and liquid nitrogen.
Influence of tribology of cage material on ball bearing cage instability - Servais, Christophe; Duquenne, Marc; Bozet, Jean-Luc
By creating a solid lubricant thickness on both bearing races, a cage material of cryogenic ball bearing plays a significant role in the good dynamical behavior of the cage. This role is essential because of the lack of conventional lubricant into this kind of bearing.
In this paper, a method able to identify if a particular potential cage material can correctly fulfill its function is described. In other words, if it can lead to a stable movement of the cage. From the identification of fundamental tribological parameters governing the cage behavior, this method presents an example of ranking of such materials....
Robust optimal rendezvous using differential drag - Dell'Elce, Lamberto; Martinusi, Vladimir; Kerschen, Gaëtan
The practical realization of the differential drag technique for orbital relative maneuvers must cope with the several and severe uncertainty sources affecting drag modeling. Neglecting these uncertainties might yield to oversimplified solutions whose representation of a real-life scenario is questionable. The first outcome of this study consists in the synthesis of a robust optimal controller which combines differential flatness theory and the scenario approach to generate a reference path which can be easily followed. The second outcome is the characterization of the relevant uncertainties of the differential drag problem, with a special focus on the aerodynamic force. The developments are...
Résonance locale et caractérisation de défauts de délaminage par vibrothermographie - Demy, Philippe
La vibrothermographie est une méthode de thermographie active utilisée en contrôle non destructif qui présente des caractéristiques intéressantes et peu communes pour la détection de défauts. Cependant, la méthode souffre d’un manque de fiabilité des résultats lié au chaos acoustique inhérent à la nature du contact pour la stimulation. La principale innovation de cette thèse consiste à étudier et proposer une méthode alternative à la vibrothermographie classique à partir d’un couplage linéaire obtenu par collage. La mise en évidence et l’étude d’un phénomène de résonance locale de délaminage a permis la mise au point d’une technique performante et fiable pour...
Wind Tunnel Testing of a VTOL MAV Propeller in Tilted Operating Mode - Theys, Bart; Dimitriadis, Grigorios; Andrianne, Thomas; Hendrick, Patrick; De Schutter, Joris
This paper presents experimental results of the full 3-axis force vector and 3-axis moment vector acting on a propeller, commonly used for a Vertical Take Off and Landing Micro Aerial Vehicle (VTOL MAV). Measurements were carried out in a wind tunnel using a high resolution 6-axis force/moment sensor embedded in a customized test rig at several wind speeds, propeller rotational speeds and angles of the propeller shaft with respect to the air stream. Results show strong moments acting on the propeller in forward flight and unstable conditions in descending flight. Power calculations reveal a decrease in power consumption during slow...
Validation of differential drag propellantless maneuvers using 6 DoF simulations and stochastic dynamics - Dell'Elce, Lamberto; Kerschen, Gaëtan
Differential drag is regarded as a promising technique in low-Earth orbits, since it allows to turn the often-undesired drag perturbation into a control force for relative maneuvers. Nonetheless, relevant uncertainties in drag modeling make its practical realization a challenge, especially if no other propulsive means is available to accommodate them. The main outcome of this study is the identification and characterization of the uncertainty sources affecting differential-drag-based maneuvers. This characterization is carried out by using statistical methods in conjunction with mechanical modeling considerations to infer a probabilistic model of the sources from observed data. The developments are illustrated in the...
Muti-scale methods with strain-softening: damage-enhanced MFH for composite materials and computational homogenization for cellular materials with micro-buckling - Wu, Ling; Nguyen, Van Dung; Adam, Laurent; Doghri, Issam; Noels, Ludovic
In this work, multi-scale methods with strain softening are developed in the contexts of damage modeling for composite laminates and of buckling analyses in cellular materials.
First, an anisotropic gradient–enhanced continuum damage model is embedded in a mean–field homogenization (MFH) process for elasto-plastic composites. The homogenization procedure is based on the newly developed incremental secant mean-field homogenization formulation, for which the residual stress and strain states reached in the phases upon a fictitious elastic unloading are considered as starting point to apply the secant method. The mean stress fields in the phases are then computed using isotropic secant tensors, which are...