http://popups.lib.uliege.be/2684-6500/index.php?id=134 Publications of Auteurs Malte Krack fr 0 http://popups.lib.uliege.be/2684-6500/index.php?id=206 The Tribomechadynamics Research Challenge (TRC) was a blind prediction of the vibration behavior of a thin plate clamped on two sides using bolted joints. Specifically, the natural frequency and damping ratio of the fundamental bending mode were requested as function of the amplitude, starting from the linear regime until high levels, where both frictional contact and nonlinear bending-stretching coupling become relevant. The predictions were confronted with experimental results in a companion paper; the present article addresses the experimental analysis of this benchmark system. Amplitude-dependent modal data was obtained from phase resonance and response controlled tests. In the phase resonance test, a resonant phase lag between response and excitation was ensured via feedback control, and the excitation level was step-wise in-/decreased. In the response controlled test, the response level was kept fixed via feedback control. An original variant of response controlled testing is proposed: Instead of a fixed frequency interval, a fixed phase interval is analyzed. This way, the high excitation levels required outside resonance, which could activate unwanted exciter nonlinearity, are avoided. The consistency of the nonlinear modal testing methods, with each other, and with conventional linear tests at low amplitudes, is carefully analyzed. Comparisons of nonlinear-mode based predictions with direct frequency response curve measurements (at fixed excitation level) serve as additional cross-validation. Overall, these measures have enabled a high confidence in the acquired modal data. The different sources of the remaining uncertainty were further analyzed. A low reassembly-variability but a moderate time-variability were identified. The latter is attributed to some thermal sensitivity of the system. Two nominally identical plates were analyzed, which both have an appreciable initial curvature, and a significant effect on the vibration behavior was found depending on whether the plate is aligned/misaligned with the support structure. Further, a 1:2 nonlinear modal interaction with the first torsion mode was observed, which only occurs in the aligned configurations. All data (time series of nonlinear tests; linear modal properties) and post-processing methods are publicly available. Mon, 07 Oct 2024 10:32:51 +0200 Fri, 08 Nov 2024 11:39:05 +0100 http://popups.lib.uliege.be/2684-6500/index.php?id=206 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 Mon, 24 Mar 2025 15:52:46 +0100 http://popups.lib.uliege.be/2684-6500/index.php?id=180 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 Fri, 10 Jan 2025 13:39:14 +0100 http://popups.lib.uliege.be/2684-6500/index.php?id=126