Imperial College London

DrChristophSchwingshackl

Faculty of EngineeringDepartment of Mechanical Engineering

Reader in Mechanical Engineering
 
 
 
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Contact

 

+44 (0)20 7594 1920c.schwingshackl Website

 
 
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Location

 

559City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

131 results found

Lasen M, Dini D, Schwingshackl CW, 2024, Experimental control of frictional contact behaviour via piezoelectric actuation, Mechanical Systems and Signal Processing, Vol: 211, ISSN: 0888-3270

Assembled structures in complex machinery usually have many joints, used to reduce assembly time and manufacturing complexity, to facilitate maintenance, ensure sealing and provide overall structural stiffness. Joints in these structures can change the stiffness and introduce friction damping and hence they impact the dynamic behaviour of the overall assembly. Currently, the joint impact on the structure is mainly considered only as defined at the design stage; however, attempts are underway to use the frictional interfaces to improve and control the overall dynamic performance during operation. This paper explores the possibility of actively manipulating the interface geometry of a frictional joint to influence its stiffness and energy dissipation capabilities. A proof of concept experimental campaign, of a novel concept that changes the hysteretic behaviour and frequency responses in dry friction contacts by means of a series of piezoelectric actuators will be presented and discussed. The experimental results are compared against simulation results obtained using a finite element model. The investigation shows that the concept is feasible and that it effectively changes the contact conditions, by changing the hysteresis loops and influencing the frequency responses from hammer test.

Journal article

Bhattu A, Jamia N, Hermann S, Müller F, Scheel M, Özgüven HN, Schwingshackl C, Krack Met al., 2024, The TRChallenge: Experimental Quantification of Nonlinear Modal Parameters and Confrontation with the Predictions, Pages: 133-136, ISSN: 2191-5644

In recent years, the prediction of the behavior of structures with high-level nonlinearities has been a challenging area of research. In 2021, the Tribomechadynamics Research Challenge was proposed to evaluate the current state of the art in modeling in the community of jointed structures: the task was a blind prediction of the nonlinear dynamic response of a system including a frictional and a geometric nonlinearity. Participants of the challenge were given only the technical drawings, including material and surface specifications required to manufacture and assemble the system and were asked to predict the frequency and damping ratio of the lowest-frequency elastic mode as function of the amplitude. The behavior of the real system was experimentally characterized during the Tribomechadynamics Research Camp 2022. This contribution presents the experimental work performed during the research camp. As the nature of the structure requires a base excitation, two recently developed nonlinear testing techniques have been explored to extract the modal parameters: the response-controlled testing method and the phase-resonant testing method. The results obtained with the different methods are compared and the blind predictions are confronted with the experimental results in order to assess their accuracy.

Conference paper

Szydlowski MJ, Schwingshackl C, Renson L, 2023, Modeling nonlinear structures using physics-guided, machine-learnt models, 41st IMAC, A Conference and Exposition on Structural Dynamics 2023, Publisher: Springer Nature Switzerland, Pages: 71-74, ISSN: 2191-5644

The constant drive to improve the performance of aeronautic structures is leading to new designs where nonlinearity is ubiquitous. Accurately predicting the dynamic behavior of nonlinear systems is very challenging because they can exhibit a wide range of behaviors that have no linear equivalent and are very sensitive to parameter changes. In this work, we consider a physics-based model to capture the underlying linear behavior of the system. This linear model is then augmented with a data-driven, machine-learnt model that captures the nonlinearities present in the system. Standard ML models have, however, several important shortcomings from an engineering point of view. They often require large training datasets, do not generalize well to unseen conditions, and can even be physically inconsistent. To overcome these limitations, we investigate the use of Lagrangian Neural Networks (LNNs) where a neural network is used to directly model the Lagrangian function of the system. To enforce physical consistency, the Euler-Lagrange equations of motion of the system are obtained by differentiating this neural network using automatic differentiation techniques. The potential of this modeling approach is numerically and experimentally shown on a range of systems with stiffness and damping nonlinearities.

Conference paper

Wei T, Fantetti A, Cegla F, Schwingshackl Cet al., 2023, An optical method to monitor transparent contact interfaces during high frequency shear vibration cycles, Wear, Vol: 524-525, Pages: 1-12, ISSN: 0043-1648

Contacting interfaces provide frictional damping in jointed structures subjected to high dynamic loads. Predicting this frictional damping during vibration cycles is highly important since it strongly affects the dynamic response of the assembly and hence the lifetime of parts. Since frictional damping is heavily influenced by the contact condition at the interface, the most direct and insightful approach is thereby to actively monitor the contact interface. Although several methods have already been proposed to monitor the contact interfaces quasi-statically or in pre-sliding, such as digital image correlation, X-Ray and ultrasound, only limited data is available of the frictional interface behaviour during high frequency vibration.To provide a better insight into the contact interface behaviour during high frequency cyclic motion, an optical method is here developed based on transparent friction specimens and total internal reflection, and applied to an existing friction test rig. The resulting measurements across the whole interface show a large variation in the real area of contact during each vibration cycle, which could be linked to the kinematics of the contact interface. This large variation is observed for the first time in high frequency oscillating contacts and is attributed to ageing effect and fracture of asperities. These two effects dominate the contact mechanism at different sliding velocities and induce variations in the real area of contact during each vibration cycle. These results suggest that the mechanisms behind high frequency contact behaviour are more complex than what commonly assumed in dynamics simulations.

Journal article

Fantetti A, Setchfield R, Schwingshackl C, 2023, Nonlinear dynamics of turbine bladed disk with friction dampers: Experiment and simulation, International Journal of Mechanical Sciences, Vol: 257, Pages: 1-20, ISSN: 0020-7403

Accurately predicting the nonlinear dynamic response of aero-engine components is critical, as excessive vibration amplitudes can considerably reduce the operational lifespan. This paper compares experimental and numerical nonlinear dynamic responses of an industrial aero-engine, specifically focusing on the first stage turbine bladed disk with under-platform dampers (UPDs). The friction forces between UPDs and blades result in a strongly nonlinear dynamic response, influenced by stick, slip and separation contact states at the interfaces. These contact states, and the resulting global dynamic responses, are predicted with an advanced industrial modelling approach for nonlinear dynamics. The predictions are compared, updated and validated against measurement data from an operational engine test. Results highlight the importance to validate models against industrial data and show that realistic contact pressure distributions are required for increased prediction reliability. The novelty of this work includes (1) the use of unique industrial experimental data from a fully operational aero-engine, (2) the observation, at the end of engine testing, of real contact conditions in blade/UPD interfaces, (3) detailed modelling of these contact conditions with high-fidelity finite element representations in nonlinear dynamic solvers. Based on this unique industrial validation work, guidelines are proposed to improve the state-of-the-art modelling of nonlinear dynamics in structures with friction contacts.

Journal article

Yuan J, Salles L, Nowell D, Schwingshackl Cet al., 2023, Influence of mesoscale friction interface geometry on the nonlinear dynamic response of large assembled structures, Mechanical Systems and Signal Processing, Vol: 187, ISSN: 0888-3270

Friction interfaces are unavoidable components of large engineering assemblies since they enable complex designs, ensure alignment, and enable the transfer of mechanical loads between the components. Unfortunately, they are also a major source of nonlinearities and uncertainty in the static and dynamic response of the assembly, due to the complex frictional physics occurring at the interface. One major contributor to the nonlinear dynamic behavior of the interface is the mesoscale geometry of a friction interface. Currently, the effects of the interface geometry on the nonlinear dynamic response is often ignored in the analysis due to the high computational cost of discretizing the interface to such fine levels for classical finite element analysis. In this paper, the influence of mesoscale frictional interface geometries on the nonlinear dynamic response is investigated through an efficient multi-scale modeling framework based on the boundary element method. A highly integrated refined contact analysis, static analysis, and nonlinear modal analysis approach are presented to solve a multi-scale problem where mesoscale frictional interfaces are embedded into the macroscale finite element model. The efficiency of the framework is demonstrated and validated against an existing dovetail dogbone test rig. Finally, the effects of different mesoscale interface geometries such as surface waviness and edge radius, are numerically investigated, further highlighting the influence of mesoscale interface geometries on the nonlinear dynamics of jointed structures and opening a new research direction for the design of friction interfaces in friction involved mechanical systems.

Journal article

Tatar A, Schwingshackl CW, Friswell MI, 2023, Modal sensitivity of three-dimensional planetary geared rotor systems to planet gear parameters, Applied Mathematical Modelling, Vol: 113, Pages: 309-332, ISSN: 0307-904X

A parameter study is presented to determine effects of planet gear design parameters on the global modal behaviour of planetary geared rotor systems. The modal sensitivity analysis is conducted using a three-dimensional dynamic model of a planetary geared rotor system for the number of planet gears, planet mistuning, mass of planet gears, gear mesh stiffness and planet gear speed. These parameters have varying impacts on both natural frequencies and mode shapes, therefore the sensitivity of the planetary geared rotor vibration modes to the planet gear parameters is determined by computing the frequency shifts and comparing the mode shapes. The results show that the mass and mesh stiffness of planet gears have a larger influence on the global dynamic response. Torsional modes and coupled torsional-axial modes are more sensitive to gear mesh stiffness whereas lateral vibration modes are more sensitive to gearbox mass. Planet mistuning results in coupling between lateral and torsional vibrations. The planetary gearbox becomes more rigid in the torsional-axial modes and more flexible in the lateral modes with an increase in the number of planet gears. Planet gears are also found to be having significant gyroscopic effects inside the planetary gearbox. The main original findings in this study can be directly used as initial guidelines for planetary geared rotor design.

Journal article

Lasen M, Salles L, Dini D, Schwingshackl CWet al., 2023, Tribomechadynamics Challenge 2021: A Multi-harmonic Balance Analysis from Imperial College London, 40th Conference and Exposition on Structural Dynamics (IMAC), Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 79-82, ISSN: 2191-5644

Conference paper

Cabana DJA, Yuan J, Schwingshackl CW, 2023, A Novel Test Rig for the Validation of Non-linear Friction Contact Parameters of Turbine Blade Root Joints, 40th Conference and Exposition on Structural Dynamics (IMAC), Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 215-226, ISSN: 2191-5644

Conference paper

Lasen M, Dini D, Schwingshackl CW, 2023, Experimental Proof of Concept of Contact Pressure Distribution Control in Frictional Interfaces with Piezoelectric Actuators, 40th Conference and Exposition on Structural Dynamics (IMAC), Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 83-86, ISSN: 2191-5644

Conference paper

Mace T, Taylor J, Schwingshackl CW, 2022, Measurement of the principal damping components of composite laminates, Composite Structures, Vol: 302, ISSN: 0263-8223

The prediction of material damping in composite laminate components may allow for the development of lighter component designs, and can be achieved via several techniques. One such example, the modal strain energy (MSE) technique requires a combination of effective strain energy modelling and accurate principal loss factor parameters. Measuring principal loss factors can be very challenging for lightly damped components due to parasitic or ‘extraneous’ damping sources introduced by the test set-up itself. To ensure high quality input data to the strain energy models an novel damping test procedure has been developed, which allows for mono-harmonic damping parameter extraction via free decay and logarithmic decrements, whilst minimising extraneous dissipation. Improvements over the established impact hammer testing technique could be shown during the validation of the approach.

Journal article

Yang Z, Pan J, Chen J, Zi Y, Oberst S, Schwingshackl CW, Hoffmann Net al., 2022, A novel unknown-input and single-output approach to extract vibration patterns via a roving continuous random excitation, ISA TRANSACTIONS, Vol: 129, Pages: 675-686, ISSN: 0019-0578

Journal article

Tuzzi G, Schwingshackl CW, Green JS, 2022, Cross-disc coupling in a flexible shaft–disc assembly in presence of asymmetric axial–radial bearing supports, Journal of Sound and Vibration, Vol: 527, Pages: 1-19, ISSN: 0022-460X

In a flexible shaft–disc assembly supported by linear bearings, the disc 1 Nodal Diameter (ND) modes are known to couple with the shaft lateral (bending) modes, whilst the 0ND modes can couple with the shaft axial modes. In addition to these well known coupling phenomena, a previous work by the authors has shown that, in presence of an asymmetric axial–radial bearing supporting structure, shaft axial and lateral modes can interact and lead to a coupling with a single flexible disc 0 and 1 ND modes simultaneously. Given that in most circumstances a shaft carries more than one disc, this work extends the previous findings to a shaft carrying two flexible discs and particularly investigates the mechanisms of cross disc coupling due to an asymmetric supporting structure. A full 3D FEM model of the assembly has been developed to model its dynamic behaviour. New classes of coupled modes involving the shaft and the two discs have been identified and a physical explanation will be provided, considering forces/moments applied at the interface amongst subcomponents and following the hypothesis that each disc acts like an independent dynamic absorber.A parametric study of the dual discs arrangement varying stiffness, thickness and position of one disc further highlighted the dynamic interaction of the subcomponents. Specific arrangements will allow an Engine Order forcing pattern applied to one disc to excite a different mode on the other disc, with the shaft and the supports acting as the vibration energy transmitter between the two discs. The industrial implications of such phenomena are also discussed throughout this work.

Journal article

Verena G, Fantetti A, Steven K, Christoph S, Daniel Ret al., 2022, Contact stiffness of jointed interfaces: a comparison of dynamic substructuring techniques with frictional hysteresis measurements, Mechanical Systems and Signal Processing, Vol: 171, ISSN: 0888-3270

The tangential contact stiffness is an important parameter used in non-linear dynamic analyses of jointed structures since it can strongly affect the prediction of resonance frequencies. Many experimental techniques are available for contact stiffness estimations, but the reliability of such estimations remains unknown due to a lack of comparative studies. This paper proposes a comparative study of contact stiffness measurements obtained with two experimental techniques: hysteresis loop measurements and Frequency Based Substructuring (FBS). Hysteresis loops are traditionally measured with dedicated friction test rigs to provide, amongst others, contact stiffness estimations through local interface measurements. The assumption with hysteresis measurements is that the measured parameters are independent of the dynamics of the test rig and can therefore be used as input for analyses of other structures, as long as loading conditions and contact interfaces are comparable. An alternative approach to identify the contact stiffness is FBS, which uses information from the overall system dynamics. FBS has the advantage that it can be applied to any structure, without the need of building ad-hoc test rigs, consequently giving a structure-specific information. Despite this advantage over hysteresis measurements, it is as of yet not well understood how accurately FBS can extract contact stiffness values. This paper presents FBS measurements and hysteresis loop measurements performed simultaneously on the same contact interface of a traditional high-frequency friction rig during vibration, thus enabling a cross-validation of the results of both techniques. This novel comparison validates FBS approaches against local hysteresis measurements and shows the strengths and limitations of both experimental methods, making it possible to improve the current understanding of the contact stiffness of jointed structures.

Journal article

Mace T, Taylor J, Schwingshackl C, 2022, Simplified low order composite laminate damping predictions via multi-layer homogenisation, Composites Part B: Engineering, Vol: 234, Pages: 1-14, ISSN: 0961-9526

The increased adoption of composite laminates in modern engineering requires advancement in the prediction of their dynamic behaviour. Current damping prediction techniques can be prohibitively time consuming and computationally expensive for application during early design stages, and to abstract three-dimensional geometries. A novel, lower order methodology for damping prediction is proposed, which uses a higher-level of homogenisation than established composite damping prediction techniques to provide a reasonable damping prediction without requiring a detailed model of a laminate’s internal structure. Principal loss factor components are harvested from a set of base layup specimens and used to predict the modal loss factors and frequency response of a set of geometrically abstract single layup validation specimens. A numerical study shows the low-order approach to produce approximately equivalent strain energy distributions to a well-established ‘layered’ approach at reduced computational cost and for a third of the CPU time. Furthermore, the damping and amplitude predictions produced by novel methodology are shown to closely match experimental measurements, providing scope to expand the application of this approach to more complex, multi-layup laminate components.

Journal article

Wang X, Szydlowski M, Yuan J, Schwingshackl Cet al., 2022, A Multi-step Interpolated-FFT procedure for full-field nonlinear modal testing of turbomachinery components, Mechanical Systems and Signal Processing, Vol: 169, Pages: 108771-108771, ISSN: 0888-3270

Model updating for lightweight structures featuring geometrical nonlinearities has long been a goal in the aerospace industry, which requires spatially detailed measurement of the structure vibrating at large amplitudes. Performing such a measurement for lightweight structure is an extremely challenging task due to its low mass-to-area ratio, complex spatial deformation shapes, and geometrically nonlinear behaviours. Indeed, the current full-field measurements of nonlinear structural dynamics are mostly limited to flat, small-scale, academic structures such as beams or plates. To enable full-field measurement of nonlinear responses of large-scale industrial structures, a procedure based on the Three-Dimensional Scanning Laser Doppler Vibrometry (3D SLDV) is developed in this paper, in which full-field, multi-harmonic operating deflection shapes are measured when the structure is vibrating at its resonance. More specifically, a super-short sampling interval is used for each scan point to achieve a significant reduction in measurement duration. A novel Multi-step Interpolated-Fast Fourier Transform (Multi-step Interpolated-FFT) procedure is proposed to refine the coarse frequency resolution and suppress the severe spectral leakage of the signal spectra. In the procedure, the instantaneous driving frequency is first interpolated using the force signal and then used to perform a fixed-frequency interpolation for each harmonic of the response signals. In such a way, it allows accurate estimations of the frequencies, magnitudes and phase lags of the constituent harmonics in the measured signal sets. Numerical validations of the proposed procedure are carried out to investigate its accuracy and robustness with regard to different signal frequencies and noise levels before it is applied to experimental data of an industrial-scale fan blade. Results have shown that it allows, for the first time, to capture full-field, multi-harmonic operating deflection shapes of a large-sca

Journal article

Yuan J, Sun Y, Schwingshackl C, Salles Let al., 2022, Computation of damped nonlinear normal modes for large scale nonlinear systems in a self-adaptive modal subspace, Mechanical Systems and Signal Processing, Vol: 162, Pages: 1-16, ISSN: 0888-3270

The concept of nonlinear modes has been proved useful to interpret a wide class of nonlinear phenomena in mechanical systems such as energy dependent vibrations and internal resonance. Although this concept was successfully applied to some small scale structures, the computational cost for large-scale nonlinear models remains an important issue that prevents the wider spread of this nonlinear analysis tool in industry. To address this challenge, in this paper, we describe an advanced adaptive reduced order modelling (ROM) technique to compute the damped nonlinear modes for a large scale nonlinear system with frictional interfaces. The principle of this new ROM technique is that it enables the nonlinear modes to be computed in a reduced self-adaptive modal subspace while maintaining similar accuracy to classical reduction techniques. The size of such self-adaptive subspace is only proportional to the number of active slipping nodes in friction interfaces leading to a significant reduction of computing time especially when the friction interface is in a micro-slip motion. The procedure of implementing this adaptive ROM into the computation of steady state damped nonlinear mode is presented. The case of an industrial-scale fan blade system with dovetail joints in aero-engines is studied. Damped nonlinear normal modes based on the concept of extended periodic motion is successfully calculated using the proposed adaptive ROM technique. A comparison between adaptive ROM with the classical Craig-Bampton method highlights the capability of the adaptive ROM to accurately capture the resonant frequency and modal damping ratio while achieving a speedup up to 120. The obtained nonlinear modes from adaptive ROM are also validated by comparing its synthesized forced response against the directly computed ones using Craig-Bampton (CB) method. The study further shows the reconstructed forced frequency response from damped nonlinear modes are able to accurately capture reference for

Journal article

Yuan J, Salles L, Schwingshackl C, 2022, Effects of the geometry of friction interfaces on the nonlinear dynamics of jointed structure, Proceedings of the 40th IMAC, A Conference and Exposition on Structural Dynamics 2022, Publisher: Springer International Publishing, Pages: 67-74, ISSN: 2191-5644

Conference paper

Fantetti A, Mariani S, Pesaresi L, Nowell D, Cegla F, Schwingshackl Cet al., 2021, Ultrasonic monitoring of friction contacts during shear vibration cycles, Mechanical Systems and Signal Processing, Vol: 161, ISSN: 0888-3270

Complex high-value jointed structures such as aero-engines are carefully designed and optimized to prevent failure and maximise their life. In the design process, physically-based numerical models are employed to predict the nonlinear dynamic response of the structure. However, the reliability of these models is limited due to the lack of accurate validation data from metallic contact interfaces subjected to high-frequency vibration cycles. In this study, ultrasonic shear waves are used to characterise metallic contact interfaces during vibration cycles, hence providing new validation data for an understanding of the state of the friction contact. Supported by numerical simulations of wave propagation within the material, a novel experimental method is developed to simultaneously acquire ultrasonic measurements and friction hysteresis loops within the same test on a high-frequency friction rig. Large variability in the ultrasound reflection/transmission is observed within each hysteresis loop and is associated with stick/slip transitions. The measurement results reveal that the ultrasound technique can be used to detect stick and slip states in contact interfaces subjected to high-frequency shear vibration. This is the first observation of this type and paves the way towards real-time monitoring of vibrating contact interfaces in jointed structures, leading to a new physical understanding of the contact states and new validation data needed for improved nonlinear dynamic analyses.

Journal article

Zhu Y-P, Yuan J, Lang ZQ, Schwingshackl CW, Salles L, Kadirkamanathan Vet al., 2021, The data-driven surrogate model-based dynamic design of aeroengine fan systems, Journal of Engineering for Gas Turbines and Power: Transactions of the ASME, Vol: 143, Pages: 1-8, ISSN: 0742-4795

High-cycle fatigue failures of fan blade systems due to vibrational loads are of great concern in the design of aeroengines, where energy dissipation by the relative frictional motion in the dovetail joints provides the main damping to mitigate the vibrations. The performance of such a frictional damping can be enhanced by suitable coatings. However, the analysis and design of coated joint roots of gas turbine fan blades are computationally expensive due to strong contact friction nonlinearities and also complex physics involved in the dovetail. In this study, a data-driven surrogate model, known as the Nonlinear in Parameter AutoRegressive with eXegenous input (NP-ARX) model, is introduced to circumvent the difficulties in the analysis and design of fan systems. The NP-ARX model is a linear input–output model, where the model coefficients are nonlinear functions of the design parameters of interest, such that the Frequency Response Function (FRF) can be directly obtained and used in the system analysis and design. A simplified fan-bladed disc system is considered as the test case. The results show that using the data-driven surrogate model, an efficient and accurate design of aeroengine fan systems can be achieved. The approach is expected to be extended to solve the analysis and design problems of many other complex systems.

Journal article

Jin M, Kosova G, Cenedese M, Chen W, Singh A, Jana D, Brake MRW, Schwingshackl CW, Nagarajaiah S, Moore KJ, Noel J-Pet al., 2021, Measurement and identification of the nonlinear dynamics of a jointed structure using full-field data; Part II- Nonlinear system identification, Mechanical Systems and Signal Processing, Vol: 166, Pages: 1-20, ISSN: 0888-3270

The dynamic responses of assembled structures are greatly affected by the mechanical joints, which are often the cause of nonlinear behavior. To better understand and, in the future, tailor the nonlinearities, accurate methods are needed to characterize the dynamic properties of jointed structures. In this paper, the nonlinear characteristics of a jointed beam is studied with the help of multiple identification methods, including the Hilbert Transform method, Peak Finding and Fitting method, Dynamic Mode Decomposition method, State-Space Spectral Submanifold, and Wavelet-Bounded Empirical Mode Decomposition method. The nonlinearities are identified by the responses that are measured via accelerometers in a series of experiments that consist of hammer testing, shaker ringdown testing, and response/force-control stepped sine testing. In addition to accelerometers, two high-speed cameras are used to capture the motion of the whole structure during the shaker ringdown testing. Digital Image Correlation (DIC) is then adopted to obtain the displacement responses and used to determine the mode shapes of the jointed beam. The accuracy of the DIC data is validated by the comparison between the identification results of acceleration and displacement signals. As enabled by full-field data, the energy-dependent characteristics of the structure are also presented. The setup of the different experiments is described in detail in Part I (Chen et al., 2021) of this research. The focus of this paper is to compare nonlinear system identification methods applied to different measurement techniques and to exploit the use of high spatial resolution data.

Journal article

Chen W, Jana D, Singh A, Jin M, Cenedese M, Kosova G, Brake MRW, Schwingshackl CW, Nagarajaiah S, Moore KJ, Noel J-Pet al., 2021, Measurement and identification of the nonlinear dynamics of a jointed structure using full-field data, Part I: Measurement of nonlinear dynamics, Mechanical Systems and Signal Processing, Vol: 166, Pages: 1-21, ISSN: 0888-3270

Jointed structures are ubiquitous constituents of engineering systems; however, their dynamic properties (e.g., natural frequencies and damping ratios) are challenging to identify correctly due to the complex, nonlinear nature of interfaces. This research seeks to extend the efficacy of traditional experimental methods for linear system identification (such as impact testing, shaker ringdown testing, random excitation, and force or amplitude-control stepped sine testing) on nonlinear jointed systems, e.g., the half Brake–Reuß beam, by augmenting them with full-field data collected by high-speed videography. The full-field response is acquired using high-speed cameras combined with Digital Image Correlation (DIC), which enables studying the spatial–temporal dynamic characteristics of the system. As this is a video-based experiment, additional constraints are attached to the beam at the node points to remove the rigid body motion, which ensures that the beam is in the view of the camera during the entire test. The use of a video-based method introduces new sources of experimental error, such as noise from the high-speed camera’s fan and electrical noise, and so the measurement accuracy of DIC is validated using accelerometer data. After validating the DIC data, the measurements are recorded for several types of excitation, including hammer testing, shaker ringdown testing, fixed sine testing, and stepped sine testing. Using the DIC data to augment standard nonlinear system identification techniques, modal coupling and the mode shapes’ evolution are investigated. The suitability of videography methods for nonlinear system identification of nonlinear beams is explored for the first time in this paper, and recommendations for techniques to facilitate this process are made. This article focuses on developing an accurate data collection methodology as well as recommendations for nonlinear testing with DIC, which paves the way for video-based i

Journal article

Yuan J, Fantetti A, Denimal E, Bhatnagar S, Pesaresi L, Schwingshackl C, Salles Let al., 2021, Propagation of friction parameter uncertainties in the nonlinear dynamic response of turbine blades with underplatform dampers, Mechanical Systems and Signal Processing, Vol: 156, Pages: 1-19, ISSN: 0888-3270

Underplatform dampers are widely used in turbomachinery to mitigate structural vibrations by means of friction dissipation at the interfaces. The modelling of such friction dissipation is challenging because of the high variability observed in experimental measurements of contact parameters. Although this variability is not commonly accounted for in state-of-the-art numerical solvers, probabilistic approaches can be implemented to include it in dynamics simulations in order to significantly improve the estimation of the damper performance. The aim of this work is to obtain uncertainty bands in the dynamic response of turbine blades equipped with dampers by including the variability observed in interfacial contact parameters. This variability is experimentally quantified from a friction rig and used to generate uncertainty bands by combining a deterministic state-of-the-art numerical solver with stochastic Polynomial Chaos Expansion (PCE) models. The bands thus obtained are validated against experimental data from an underplatform damper test rig. In addition, the PCEs are also employed to perform a variance-based global sensitivity analysis to quantify the influence of contact parameters on the variation in the nonlinear dynamic response via Sobol indices. The analysis highlights that the influence of each contact parameter in vibration amplitude strongly varies over the frequency range, and that Sobol indices can be effectively used to analyse uncertainties associated to structures with friction interfaces providing valuable insights into the physics of such complex nonlinear systems.

Journal article

Ondra V, Sever IA, Schwingshackl CW, 2021, Identification of complex non-linear modes of mechanical systems using the Hilbert-Huang transform from free decay responses, Journal of Sound and Vibration, Vol: 495, Pages: 1-26, ISSN: 0022-460X

Modal analysis is a well-established method for analysis of linear systems, but its extension to non-linear structures has proven to be much more problematic. Several competitive definitions of non-linear modes and a variety of experimental methods have been introduced. In this paper, the definition of complex non-linear modes (CNMs) of mechanical systems is adopted and the possibility of their identification from experimental free decay responses using the Hilbert-Huang transform (HHT) is explored. It is firstly discussed that since there are similarities in the definition of intrinsic mode functions obtained using the HHT and reduced order model of slow-flow dynamics based on the CNMs, there is a reason to believe that the HHT can indeed extract the CNMs. This paper, however, presents a new insight into the use of the Hilbert-Huang transform by showing that the amplitude-dependent frequency and damping extracted from a free decay response are only suitable for detection and characterisation of non-linearities, but they cannot be used to quantify the non-linear behaviour by fitting the CNMs even if a model of the system is known. The analytical proof of the HHT cannot be currently formulated due to a limited understanding of its empirical nature. Instead, this unconventional conclusion is supported by a series of numerical studies of conservative and non-conservative non-linear systems with a wide range of parameters. In all cases, a special care is taken to apply the basic HHT only on such signals for which mode separation is possible (no mode-mixing occurs). This eliminates the need for more sophisticated HHT versions and clearly demonstrates the inability of the HHT to extract CNMs even for the simplest cases. In addition to numerical studies, the identification of several non-linear modes is demonstrated experimentally using the free decay responses obtained from the ECL benchmark. It is shown that the HHT is able to successfully extract several non-linear mode

Journal article

Yuan Y, Jones A, Setchfield R, Schwingshackl CWet al., 2021, Robust design optimisation of underplatform dampers for turbine applications using a surrogate model, Journal of Sound and Vibration, Vol: 494, Pages: 1-15, ISSN: 0022-460X

Underplatform dampers (UPD) represent an effective way to limit blade vibration in turbomachinery via frictional energy dissipation, leading to a wide range of applications. The design of an effective and reliable UPD is highly challenging, due to the inherently nonlinear nature of the contact forces, the associated computational cost for high fidelity simulation, and the manufacturing uncertainties in damper geometry. This paper presents a novel UPD optimisation approach that combines high-order, detailed nonlinear modelling of the damper interfaces with a surrogate model optimisation technique. The nonlinear dynamic behaviour of the UPD is predicted using the existing explicit damper model in combination with an ‘in-house’ multi-harmonic balance solvers, which enables capture of the damper kinematics and local contact conditions. A radial basis function based surrogate model will be used to address the computational requirement of the high fidelity simulations for alternative designs. The objective function takes into account the damping performance, resonance frequency stability and robustness due to possible uncertain variations of design parameters with manufacture tolerance. The feasibility of the proposed approach is demonstrated on a cottage roof UPD by comparing the proposed optimisation method with conventional parametric simulation method. A significantly improved solution with considerable reduction in computational effort is achieved by the current method.

Journal article

Yuan J, Schwingshackl C, wong C, Salles Let al., 2021, On an improved adaptive reduced order model for the computation of steady state vibrations in large-scale non-conservative system with friction joints, Nonlinear Dynamics, Vol: 103, Pages: 3283-3300, ISSN: 0924-090X

Joints are commonly used in many large-scale engineering systems to ease assembly, and ensure structural integrity and effective load transmission. Most joints are designed around friction interfaces, which can transmit large static forces, but tend to introduce stick-slip transition during vibrations, leading to a nonlinear dynamic system. Tools for the complex numerical prediction of such nonlinear systems are available today, but their use for large-scale applications is regularly prevented by high computational cost. To address this issue, a novel adaptive reduced-order model (ROM) has recently been developed, significantly decreasing the computational time for such high fidelity simulations. Although highly effective, significant improvements to the proposed approach is presented and demonstrated in this paper, further increasing the efficiency of the ROM. An energy-based error estimator was developed and integrated into the nonlinear spectral analysis, leading to a significantly higher computational speed by removing insignificant static modes from the stuck contact nodes in the original reduced basis, and improving the computational accuracy by eliminating numerical noise. The effectiveness of the new approach was shown on an industrial-scale fan blades system with a dovetail joints, showing that the improved adaptive method can be 2–3 times more computationally efficient than the original adaptive method especially at high excitation levels but also effectively improve the accuracy of the original method.

Journal article

Lasen M, Sun Y, Schwingshackl CW, Dini Det al., 2021, Analysis of an Actuated Frictional Interface for Improved Dynamic Performance, Nonlinear Structures & Systems, Publisher: Springer

Conference paper

Tuzzi G, Schwingshackl CW, Green JS, 2021, Shaft Bending to Zero Nodal Diameter Disc Coupling Effects in Rotating Structures Due to Asymmetric Bearing Supports, Pages: 379-382, ISSN: 2191-5644

In a flexible shaft-disc assembly, coupled shaft-disc vibration modes are likely to occur, provided that the natural frequencies of the two components are close. It is well known that the shaft axial and bending modes can couple with the zero and one Nodal Diameter (ND) modes of the disc, respectively. In a previous work, it has been shown that in presence of asymmetric axial-radial bearing supports, combined axial-bending shaft modes can occur, which are further impacted by gyroscopic forces when the system is rotating. Extending the previous findings, the impact of disc flexibility on this new coupling family has been investigated in more detail. The obtained results show the emergence of shaft whirling modes with an axial component, that can couple with 0ND or 1ND disc modes. As a result, a 0ND disc mode can possibly be excited by an out of balance mass on the shaft, leading to a previously unobserved vibration behaviour.

Conference paper

Szydlowski MJ, Schwingshackl CW, Rix A, 2021, Distributed Acquisition and Processing Network for Experimental Vibration Testing of Aero-Engine Structures, Pages: 209-212, ISSN: 2191-5644

Detailed vibration testing of large assembled structures, such as aeroengines, leads to significant requirements on data acquisition and processing. This can lead to high system cost and long post processing times, which often limit the amount of data that can be acquired. A novel hardware-software acquisition system combination is proposed here to overcome some of the challenges of large scale data acquisition, based on the idea to distribute the acquisition and data processing load between a network of specialized acquisition nodes. The nodes work in parallel and are independent of each other, while sharing a synchronization clock. Each node has the capability to process the data being acquired on-line. The network allows for testing of novel data analysis methods and its modular nature enables an easy expansion of the system when required.

Conference paper

Lasen M, Sun Y, Schwingshackl CW, Dini Det al., 2021, Analysis of an actuated frictional interface for improved dynamic performance, Pages: 227-230, ISSN: 2191-5644

Friction in assembled structures is of great interest due to its ability to reduce the vibration amplitude of critical components. The nonlinear behaviour of a structure depends on a variety of physical parameters. Among these parameters, the contact pressure distribution and the contact area have shown to be critical for the behaviour of the joint and the responses of assembled structures. In most application cases the impact of the interface geometry is not considered as a design parameter, although some attempts have been reported to shape the interface geometry for a specific dynamic response. Taking this idea of designing an interface geometry for a better dynamic performance a step further, the concept presented here propose an actively controlled interface geometry and contact pressure distribution, to change the joint behaviour during a vibration cycle. The concept consists of a device capable of manipulating the shape and pressure of a flexible membrane in contact with a rigid punch, subjected to a normal load and a tangential excitation, via a row of piezoelectric actuators.

Conference paper

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