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

120 results found

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, Salles L, Schwingshackl C, 2022, Effects of the Geometry of Friction Interfaces on the Nonlinear Dynamics of Jointed Structure, Pages: 67-74, ISSN: 2191-5644

Friction interfaces are commonly used in large-scale engineering systems for mechanical joints. They are known to significantly shift the resonance frequencies of the assembled structures due to softening effects and to reduce the vibration amplitude due to frictional energy dissipation between substructural components. It is also widely recognized that the geometrical characteristics of interface geometry have a significant impact on the nonlinear dynamical response of assembled systems. However, the full FE modeling approaches including these geometrical characteristics are extremely expensive. In this work, the influence of geometry of friction interfaces is investigated by using a multi-scale approach. It consists in integrating a semi-analytical contact solver into a high-fidelity nonlinear vibration solver. A highly efficient semi-analytical solver based on the boundary element method is used to obtain the pressure and gap distribution from the contact interface with different geometrical characteristics. The static pressure and gap distribution are then used as input for a nonlinear vibration solver to evaluate nonlinear vibrations of the whole assembled structure. The effectiveness of the methodology is shown on a realistic “Dogbone” test rig, which was designed to assess the effects of blade root geometries in a fan blade disk system. The friction joints with different interface profiles are then investigated. The obtained results show that the effects of the surface geometrical characteristics can have a significant impact on the damping and resonant frequency behavior of the whole assembly.

Conference paper

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

Yang Z, Pan J, Chen J, Zi Y, Oberst S, Schwingshackl CW, Hoffmann Net al., 2021, A novel unknown-input and single-output approach to extract vibration patterns via a roving continuous random excitation., ISA Trans

Operating deflection shape analysis allows investigating the dynamic behaviour of a structure during operation. It normally requires simultaneous, multi-point measurements to capture the response from an unknown excitation source (unknown-input and multiple-output), which can complicate its usage for structures without ease of access. A novel vibration pattern testing method is proposed based on a roving continuous random excitation employing a small robotic Hexbug device and a single-point measurement. The Hexbug introduces a random excitation in consecutive locations while roaming over the structure. The resulting multi-modal, time and location dependent response of the system is captured in a single location, and then analysed with a newly developed method based on empirical wavelet transform, multiscale morphological filtering and optimization to extract the excited vibration patterns. The efficiency of the proposed method is experimentally demonstrated on a free-free and a cantilevered beam with comparison to mode shapes extracted by hammer test. The validation highlights its ability to extract several vibration patterns from a long slender structure with good accuracy and robustness, with the general ability to expand the usability of an operating deflecting shape analysis.

Journal article

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

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

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

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

Kosova G, Jin M, Cenedese M, Chen W, Singh A, Jana D, Brake MRW, Schwingshackl CW, Nagarajaiah S, Moore KJ, Noël JPet al., 2021, Nonlinear system identification of a jointed structure using full-field data: Part ii analysis, Pages: 185-188, ISSN: 2191-5644

Mechanical joints have a significant influence on the dynamic response of assembled structures. Due to friction, wear, and non-idealized boundary conditions, joints introduce significant nonlinearity into the dynamics of assembled structures. To better understand and, in the future, tailor the nonlinearities, accurate methods are needed to characterize the dynamic properties of jointed structures. In this research, the response analysis for a beam with a bolted lap joint is studied with the help of several available identification techniques. The experimental setup and data capture are described in Part I of this work, providing high spatial resolution data for a variety of excitation methods. The nonlinear identification of the data is the focus of this paper, aiming to perform nonlinear modal analysis and to localize the nonlinear characteristics of the structure with a series of different approaches.

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

Brøns M, Kasper TA, Chauda G, Klaassen SWB, Schwingshackl CW, Brake MRWet al., 2020, Experimental investigation of local dynamics in a bolted lap joint using digital image correlation, Journal of Vibration and Acoustics, Transactions of the ASME, Vol: 142, ISSN: 1048-9002

The dynamics of structures with joints commonly show nonlinearity in their responses. This nonlinear behavior can arise from the local dynamics of the contact interfaces. The nonlinear mechanisms at an interface are complicated to study due to the lack of observability within the contact interface itself. In this work, digital image correlation (DIC) is used in combination with a high-speed camera to observe the local motion at the edge of the interface of a bolted lap joint. Results demonstrate that it is possible to use this technique to monitor the localized motion of an interface successfully. It is observed that the two beam parts of the studied lap joint separate when undergoing bending vibrations and that there is a clear asymmetry in the response of the left and the right end of the interface. Profilometry indicates that the asymmetry in the response is due to the mesoscale topography of the contact interface, highlighting the importance of accounting for surface features in order to model the nonlinearities of a contact interface accurately.

Journal article

Smith SA, Brake MRW, Schwingshackl CW, 2020, On the Characterization of Nonlinearities in Assembled Structures, Journal of Vibration and Acoustics, Transactions of the ASME, Vol: 142, ISSN: 1048-9002

This work refines a recently formalized methodology proposed by D.J. Ewins consisting of ten steps for model validation of nonlinear structures. This work details, through a series of experimental studies, that many standard test setup assumptions that are made when performing dynamic testing are invalid and need to be evaluated for each structure. The invalidation of the standard assumptions is due to the presence of nonlinearities, both known and unrecognized in the system. Complicating measurements, many nonlinearities are currently characterized as constant properties instead of variables that exhibit dependency on system hysteresis and actuation amplitude. This study reviews current methods for characterizing nonlinearities and outlines gaps in the approaches. A brief update to the CONCERTO method, based on the accelerance of a system, is derived for characterizing a system’s nonlinearities. Finally, this study ends with an updated methodology for model validation and the ramifications for modeling assemblies with nonlinearities are discussed.

Journal article

Brake MRW, Krack M, Schwingshackl CW, 2020, Special Issue: Tribomechadynamics, Journal of Vibration and Acoustics, Transactions of the ASME, Vol: 142, ISSN: 1048-9002

Journal article

Tuzzi G, Schwingshackl CW, Green JS, 2020, Study of coupling between shaft bending and disc zero nodal diameter modes in a flexible shaft-disc assembly, JOURNAL OF SOUND AND VIBRATION, Vol: 479, ISSN: 0022-460X

Journal article

Heller D, Sever IA, Schwingshackl CW, 2020, A method for multi-harmonic vibration analysis of turbomachinery blades using Blade Tip-Timing and clearance sensor waveforms and optimization techniques, Mechanical Systems and Signal Processing, Vol: 142, ISSN: 0888-3270

A novel concept of investigating blade vibration in turbomachinery is presented on the basis of Blade Tip-Timing (BTT) and clearance sensor waveform analysis methods (BLASMA), with which vibration parameters are determined by global optimization. It is shown that the modulation of the sensor output by blade vibration can offer additional information compared with under-sampled time-of-arrival (TOA) data from traditional BTT applications. The sensor data can not only improve the validity of statements on blade vibration but also lessen the dependence on contact-based strain gauges measurements to produce reference data. A study was conducted to evaluate the merit of sensor waveform analysis with regard to determining asynchronous and synchronous single-harmonic and multi-harmonic blade vibration parameters. At first, waveforms were recorded with capacitive sensors during an experiment conducted on a research compressor. The experimentally measured waveforms were afterwards replicated in a simulator for imitating passing events of rotating and vibrating blades along a single virtual capacitive sensor. Finally, vibration properties, such as amplitudes, frequencies, and phases, are extracted from these waveforms with the help of global optimization methods. An investigation into the error proneness of the methodology is attached.

Journal article

Tufekci M, Mace T, Özkal B, Dear J, Schwingshackl C, Salles Let al., 2020, Nonlinear dynamic behaviour of a nanocomposite: epoxy reinforced with fumed silica nanoparticles, XXV ICTAM

This study focuses on identification and modelling of vibration characteristics of a nanocomposite; an epoxy resin as thematrix and fumed silica as the reinforcement. The resin alone is manufactured and characterised. Using the same methodology,the manufacturing and characterisation of the silica-reinforced nanocomposite are performed. Following the manufacturing and theexperimental characterisation process, a nonlinear model is built to represent characterised behaviour. The model is validated by aseparate test case which is also an experimental technique to extract the damping characteristics of a structure.

Conference paper

Haslam AH, Schwingshackl CW, Rix AIJ, 2020, A parametric study of an unbalanced Jeffcott rotor supported by a rolling-element bearing, Nonlinear Dynamics, Vol: 99, Pages: 2571-2604, ISSN: 0924-090X

Rolling-element bearings are widely used in industrial rotating machines, and hence there is a strong need to accurately predict their influence on the response of such systems. However, this can be challenging due to an interaction between the dynamics of the rotor and the bearing nonlinearities, and it becomes difficult to provide a physical explanation for the nonlinear response. A novel approach, combining a Jeffcott rotor supported by a detailed bearing model with the generalised harmonic balance method, is presented, enabling an in-depth study of the complex rotor–stator interaction. This allows the quasi-periodic response of the rotor, due to variable compliance, to be captured, and the impact of clearance, ring and stator compliance, and centrifugal loading of the bearing on the response to be investigated. A strongly nonlinear response was observed due to the bearing, leading to large shifts in frequency as the excitation amplitude was increased, and the emergence of stable and unstable operating regions. The variable compliance effect generated sub-synchronous forcing, which led to sub-resonances when the ball pass frequency coincided with the frequency of one of the modes. Radial clearance in the bearing had by far the largest influence on the unbalance response, the self-excitation due to variable compliance, and the stability. Introducing outer ring compliance was found to slightly soften the system, and centrifugal loading on the bearing elements marginally increased the system’s region of instability, but neither of these effects had a significant impact on the response for the investigated bearing. When the bearing was mounted on a sufficiently compliant stator, the system was found to behave linearly.

Journal article

Schwingshackl CW, Nowell D, 2020, The Measurement of Tangential Contact Stiffness for Nonlinear Dynamic Analysis, Pages: 165-167, ISSN: 2191-5644

Nonlinear dynamic models for frictional interfaces require a number of input parameters to allow a realistic representation of the contact interface. Interface geometry and static pressure distributions can be obtained reliably from numerical analysis. However, it is also necessary to measure the friction coefficient, and the tangential and normal contact stiffness. The tangential contact stiffness plays a significant role in the dynamic response, but is very challenging to measure. In this paper quasi-static and dynamic experiments developed at the University of Oxford and at Imperial College London respectively, will be compared and discussed. Of particular interest is the dependence of the stiffness on the static normal load and the overall contact area.

Conference paper

Rojas E, Punla-Green S, Broadman C, Brake MRW, Pacini BR, Flicek RC, Quinn DD, Schwingshackl CW, Dodgen Eet al., 2020, A Priori Methods to Assess the Strength of Nonlinearities for Design Applications, Pages: 243-246, ISSN: 2191-5644

One of the greatest challenges to the optimization of assembled systems is a lack of understanding of how jointed interfaces augment system dynamics. Thus, a design tool that can assess the nonlinearity of a joint prior to manufacturing and experimentation will lead to significant savings in qualification testing and improved performance in terms of dynamic properties and failure rates. This paper explores the a priori metric hypothesis for jointed structures, which states that the strength of a nonlinearity (SNL) can be estimated by a metric derived from both the magnitude and uniformity of contact pressure within an interface and the modal strain energy at an interface’s location.

Conference paper

Fantetti A, Pennisi C, Botto D, Zucca S, Schwingshackl Cet al., 2020, Comparison of contact parameters measured with two different friction rigs for nonlinear dynamic analysis, International Conference on Noise and Vibration Engineering (ISMA) / International Conference on Uncertainty in Structural Dynamics (USD), Publisher: KATHOLIEKE UNIV LEUVEN, DEPT WERKTUIGKUNDE, Pages: 2165-2174

Conference paper

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