http://popups.lib.uliege.be/2684-6500/index.php?id=197 Issues fr Mon, 26 Aug 2024 10:40:39 +0200 Wed, 06 May 2026 14:25:48 +0200 http://popups.lib.uliege.be/2684-6500/index.php?id=197 0 http://popups.lib.uliege.be/2684-6500/index.php?id=198 Many examples of dynamic analysis require modelling of dry friction. Often this is represented by a so-called Jenkins element or multiple Jenkins elements in parallel, which is sometimes termed a parallel-series Iwan element. This study considers the case where a system that includes these representations of dry friction are loaded dynamically with a combination of deterministic harmonic and random excitation. This paper presents a new efficient method for predicting the response of systems subject to combined deterministic and random excitation. The method is based on equivalent linearisation and involves averaging across both an ensemble of random responses as well as over a harmonic excitation period. The key novelty in the approach is the use of an auxiliary harmonic term to facilitate an analytical representation of the nonlinear force. This overcomes the challenge of analysing a discontinuous hysteretic nonlinearity in the presence of random excitation. Analytical solutions are presented and it is shown that the proposed approach can predict the approximate response at a significantly reduced computational cost. Tue, 03 Sep 2024 16:38:02 +0200 http://popups.lib.uliege.be/2684-6500/index.php?id=198 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 http://popups.lib.uliege.be/2684-6500/index.php?id=206 http://popups.lib.uliege.be/2684-6500/index.php?id=219 Experiments and physics-based modeling efforts both show that the features within a jointed interface can have an outsized influence on the nonlinear dynamics of a large-scale structure. The interfacial features, including asperities and meso-scale topology, are often six to ten orders of magnitude smaller in scale than the structure itself, yet can significantly change the natural frequencies and damping of a structure and can lead to the premature failure due to wear if not properly designed. A significant amount of recent research has been invested in understanding and predicting the nonlinear dynamics of structures with jointed interfaces; however, there are many challenges that still remain before accurate predictions of a jointed structure's nonlinear dynamics and wear properties becomes accessible to design engineers. This article is a reflection of the outcomes of the 2023 International Workshop on the Mechanics of Jointed Structures in which the state of the art of joints modeling was assessed and future directions for research on jointed structures were identified. As such, this paper makes several recommendations for new research thrusts to improve the understanding of jointed structures in addition to highlighting the current state of the art and recent advances in modeling and experimentally characterizing jointed structures. Mon, 07 Oct 2024 10:50:33 +0200 http://popups.lib.uliege.be/2684-6500/index.php?id=219 http://popups.lib.uliege.be/2684-6500/index.php?id=228 The tightness of a flange depends on the current deformation state of the structure. For obvious reasons, this could be a critical and important information. In this paper, numerical preliminary investigations are presented on the question whether the deformation state inside a contact surface can be estimated in a very short time with a few strain measurements outside the contact surface.The presented theory is numerically evaluated using a flange with 12 bores. It turns out, that the deformation inside the contact area, and therefore the tightness, can be computed out of strain data in milliseconds. The magnitude of the strains are in a measurable range and the numerical results show robustness against noise. Even if the presented approach may not be applicable one-to-one for measurements, this numerical preliminary investigation shows that mechanical strains could in principal be used to observe the state or the tightness within a joint. Mon, 04 Nov 2024 10:14:26 +0100 http://popups.lib.uliege.be/2684-6500/index.php?id=228 http://popups.lib.uliege.be/2684-6500/index.php?id=239 This paper is the completion of a previous work in which an algorithm was developed for direct calculation of the hysteresis cycle of Underplatform Dampers (UDs) used for vibration damping of turbine blades. While the previous paper examined the scalability of a given damper shape as a function of its size, platform size and contact parameters, this paper instead shows how the method can be used to evaluate the advantages and disadvantages of different damper shapes. Only the asymmetric contact damper is studied, in its simplest version with three pad-located contacts, and its merits are discussed in comparison with the wedge or cottage-roof type dampers, whose crucial difficulties are pointed out. A family of five dampers obtained by distortion of an isosceles damper of 60° vertex aperture is studied as an example of application of the method. A criterion is defined for positioning the single contact to avoid lifting when the coefficient of friction is maximum at =0.7, then performance is studied for two values in the range expected for normal operation,  =0.5 and =0.3. For each of the five dampers, the different problems that arise in calculating the hysteresis cycle at the onset of the total full-slip, called Base-Cycle, are examined, first on the damper in terms of the “force Base-Cycle,” otherwise known as the “equilibrium trace diagram,” then of the “moment Base-Cycle” on the platform. The “moment Base-Cycle” is employed in the context of the Platform Centered Reduction (PCR) technique, and diagrams of the real and imaginary components of the complex moment-rotation stiffness of the platform representing the mutual damper-platform actions are defined. Finally, the values of the energies dissipated on the contact pads of the dampers are determined, the sum of which is checked against the total energy dissipated by the moment of the contact forces acting on the platform, consistent with the Platform Centered Reduction. The parameters and diagrams that characterize the shape of a damper, regardless of its subsequent coupling with a specific turbine blade, are useful tools for characterizing its performance with greater insight than can be obtained from the usual purely numerical approach. Fri, 06 Dec 2024 09:32:33 +0100 http://popups.lib.uliege.be/2684-6500/index.php?id=239 http://popups.lib.uliege.be/2684-6500/index.php?id=269 Non-metallic materials display frequency and temperature dependent dynamic properties which must be characterised for use in computer simulations. The characterisation of such properties is important as most modern structures utilise these materials. Hence, a novel test method has been developed, which combines vibration testing with finite element analysis, to yield dynamic modulus of elasticity and damping. Material properties have been measured in the frequency range 2Hz - 2000Hz. The test method involves a cantilever beam. Two samples of the test material sandwich the root of the beam and are held in place between inertial masses. Experimental modal analysis techniques, where an instrumented hammer vibrates the beam, are used to exercise the material. The modulus of elasticity of the material is found by constructing a finite element model of the test setup and tuning the simulated response with that of the experiment. Damping properties are extracted by applying data fitting techniques to the time histories and spectrum. These are then converted into a material damping property by simulating and accounting for the energy balance between the samples and test setup. Doing so also improves damping simulation accuracy by eliminating the need to estimate Rayleigh damping values at each frequency, as now the material damping property at each frequency can be directly used. For test accuracy, it is important that the sample is gripped in a manner that exercises it effectively and is simple to simulate. Eliminating slipping, and thus the difficult to model friction, is a key concern and has been investigated in depth. The best solution found is an axisymmetric bolting arrangement which holds the samples in place. Additionally, the test setup utilises suspension, together with inertial masses and an orthogonal layout to isolate against external vibrations. Polymers and rubbers, which exhibit complex frequency dependent behaviour, have been reliably characterised using this method. The damping material Sorbothane has also been re-characterised and produced results that aligned with manufacturer specifications. This method proves to be a reliable procedure for dynamic material property testing. Mon, 07 Apr 2025 14:17:21 +0200 http://popups.lib.uliege.be/2684-6500/index.php?id=269