http://popups.lib.uliege.be/2684-6500/index.php?id=116 Issues fr Tue, 17 Jan 2023 10:32:46 +0100 Wed, 06 May 2026 14:25:01 +0200 http://popups.lib.uliege.be/2684-6500/index.php?id=116 0 http://popups.lib.uliege.be/2684-6500/index.php?id=126 The Vibro-Impact Nonlinear Energy Sink (or impact damper) is well-known for its ability to engage into transient resonance captures with arbitrary frequencies and thus has inherent broad-band effectiveness. Its working principle relies on (recurrent) energy localization and local dissipation within the contact region. Dissipative (inelastic) collisions are inevitably associated with damage and challenging to predict. Recently, it has been shown theoretically that the device is effective even for purely elastic collisions when the energy is (almost) irreversibly transferred from the critical low-frequency modes to high frequencies.In that case, the device is more properly termed Impact Energy Scatterer (IES). In the present work, we experimentally validate, for the first time, the working principle of the IES. To this end, we design a test rig consisting of a cantilevered beam, hosting a spherical impactor inside a cavity at its tip. The resonant vibrations of the lowest-frequency bending mode are reduced by a factor of 10-20. Given that the IES weighs less than 1% of the host structure, this corresponds to a paramount vibration mitigation capability. We achieve excellent agreement between the measurements and the numerical predictions obtained by modeling the impacts as perfectly elastic. We also demonstrate that the dissipation in the contact region is negligible, while a substantial amount of energy is scattered to higher frequencies, validating the theoretically proposed working principle. Tue, 17 Jan 2023 10:43:42 +0100 http://popups.lib.uliege.be/2684-6500/index.php?id=126 http://popups.lib.uliege.be/2684-6500/index.php?id=135 In Lamb Waves based Structural Health Monitoring (LWSHM) of composite aeronautic structures, Deep Learning (DL) methods have proven to be promising to monitor damage using the signals collected by piezoelectric sensors (PZTs). However, those data driven algorithms are strongly problem dependent: any structural change dramatically impacts the accuracy of the predictions and the generalization of the learnt algorithms to other structures within the fleet is impossible. Transfer Learning (TL) promises to face that issue by capitalizing on the knowledge acquired on a given structure to transfer it on another from the fleet. An original TL approach based on the Optimal Transport (OT) theory is proposed here to handle this issue. OT provides a rigorous mathematical framework for TL that can be practically implemented using Input Convex Neural Networks modelling Kantorovich potentials but that has never been used for LWSHM. Using OT, the knowledge acquired on a rich LW database is transferred to poorer LW databases collected on different structures with rising structural divergences. A Structural Index (SI) is defined and used to compute the gap between those different structures and can be used to estimate a priori the necessity of the use of TL methods. The proposed OT based TL method for LWSHM manages to reduce by almost 50\% the predictions errors between numerical structures with strong differences (bias in mechanical properties and erroneous PZT position) in comparison with standard approaches. That leads to a promising approach to combine rich numerical database with poorer database in order to build robust algorithms for LWSHM of a fleet of aeronautical composite structures. Mon, 17 Apr 2023 13:21:31 +0200 http://popups.lib.uliege.be/2684-6500/index.php?id=135 http://popups.lib.uliege.be/2684-6500/index.php?id=145 Corotational formulations play an important role for flexible multibody dynamics systems, because they reflect the nature of many technical systems undergoing arbitrarily large rigid body motions but small deformations within each body. This paper defines flexible multibody dynamics and corotational formulations in this context. Furthermore, the «ingredients» and workflow of a flexible multibody dynamics simulation are briefly addressed for the reader less familiar with the topic. This part also points to major review papers and textbooks in the field, and embeds the unified formulations in the literature. The paper's main part presents state-of-the-art corotational flexible multibody dynamics formulations in a systematic and unified way. In this formulation part, the standard integral-based floating frame of reference formulation with modal reduction and with the conventionally employed lumped mass approximation is presented, and its drawbacks highlighted. Then, the so-called nodal-based, i.e., space-wise discretized, equations of motion are presented for several up-to-date nodal-displacement-based formulations within a unified framework. This approach clearly shows the equivalence of the presented formulations, and highlights the fact that the formulations differ only in the choice of degrees of freedom. Moreover, this contribution also intends to reduce the information and complexity within the scientific literature, since this unified framework allows the derivation of these formulations with significantly less effort.. Mon, 12 Jun 2023 13:12:18 +0200 http://popups.lib.uliege.be/2684-6500/index.php?id=145 http://popups.lib.uliege.be/2684-6500/index.php?id=154 The Masing conditions establish a criterion to relate the loading curve of a hysteretic system (e.g., systems with friction or plasticity) to its complete hysteresis loop. For the field of joint mechanics, where hysteretic models are often used to describe the dissipative, tangential behavior within an interface, the Masing conditions allow for significant computational savings when the normal load is constant. In practice, though, jointed systems experience time varying normal forces that modify the tangential behavior of the system. Consequently, the hysteretic behavior of jointed structures do not adhere to the Masing conditions. In this work, this discrepancy between the Masing conditions and behavior exhibited by jointed structures is explored, and it is hypothesized that if the Masing conditions accounted for variations in normal force, then they would more accurately represent jointed structures. A new set of conditions is introduced to the original set of Masing conditions, yielding a « Masing manifold » that spans the tangential displacement-tangential force-normal force space. Both a simple harmonic oscillator and a built-up structure are investigated for the case of elastic dry friction, and the results show that the hysteresis of both of these systems conforms to the three dimensional Masing manifold exactly, provided that a set of constraints are satisfied, even though the hysteresis does not conform with the original Masing conditions. Tue, 07 Nov 2023 09:24:21 +0100 http://popups.lib.uliege.be/2684-6500/index.php?id=154 http://popups.lib.uliege.be/2684-6500/index.php?id=161 Honeycomb sandwich panels are widely used in the industry because of their outstanding stiffness to mass ratios. The dynamic response of such structures is known to be relatively complex especially in the mid-frequency domain where a strong orthotropy can be observed. Several equivalent models are available in the literature to understand and predict this behavior, all relying on an assumption that the geometry of the honeycomb core is periodic and perfectly known. The effect of imperfection inevitably introduced at the manufacturing stage are characterized in this work using X-ray measurements. A simple homogenization approach is proposed, allowing the prediction of the shearing properties of the core from statistics extracted from its geometry. These properties are used to predict the dynamic behavior of the structure using a multi-layer analytical model. Results are compared to Laser Doppler Vibrometer measurements, showing a very good agreement with the predictions based on X-ray pictures. Fri, 15 Dec 2023 11:47:36 +0100 http://popups.lib.uliege.be/2684-6500/index.php?id=161 http://popups.lib.uliege.be/2684-6500/index.php?id=180 It is known that isolated frequency response branches (isolas) can occur near primary resonances under modal interactions or nonlinear damping. The present work demonstrates how emergence and vanishing of such isolas can be systematically analyzed in an experiment. Feedback control of the phase is employed to track the phase-resonant backbone curve. In addition, the amplitude of the response is controlled, as the excitation level undergoes turning points in the presence of an isola. The acquired data indicates what excitation levels lead to the formation of an isola / its merging with the main branch. Some further analysis of the data permits to characterize possible internal resonances and amplitude-dependent damping. To assess the proposed method, a test rig is considered which involves two similar cantilevered beams undergoing soft collisions via a unilateral spring. A simplified model, which relies on linear modal damping and a massless unilateral spring, suggests that isolas should appear as a result of internal resonances. In contrast, the experiment shows an isola due to nonlinear damping. More specifically, the damping ratio first increases substantially due to the frictional dissipation in inevitable joints. Subsequently, the unilateral interactions scatter energy to other modes (in a non-resonant way), which have much lower damping. This leads to a sudden drop of the effective damping ratio with the amplitude, and leads to the formation of an isola. Tue, 30 Jan 2024 14:50:52 +0100 http://popups.lib.uliege.be/2684-6500/index.php?id=180 http://popups.lib.uliege.be/2684-6500/index.php?id=190 A very compact weighted residual formulation is proposed for the construction of periodic solutions of oscillators subject to frictional occurrences. Coulomb's friction is commonly expressed as a differential inclusion which can be cast into the complementarity formalism. When targeting periodic solutions, existing algorithms rely on a procedure alternating between the frequency domain, where the dynamics is solved, and the time domain, where friction is dealt with. In contrast, the key idea of the present work is to express all governing equations including friction as equalities, which are then satisfied in a weak integral sense through a weighted residual formulation. The resulting algebraic nonlinear equations are solved numerically using an adapted trust region nonlinear solver and basic integral quadrature schemes. To increase efficiency, the Jacobian of the friction forces is calculated analytically in a piecewise linear fashion. The shape functions considered in this work are the classical Fourier functions. It is shown that periodic solutions with clear multiple sticking and sliding phases can be found with a high degree of accuracy. The equality-based formulation is shown to be effective and efficient, convergence being achieved in all cases considered with low computational cost, including for large numbers of harmonics. Importantly, this new friction formulation does not suffer from the typical limitations or hypotheses of existing frequency-time domain methods for non-smooth systems, such as regularization, penalization, or massless frictional interfaces. Mon, 04 Mar 2024 09:34:54 +0100 http://popups.lib.uliege.be/2684-6500/index.php?id=190