1,721,028 research outputs found
Dynamic behaviour of inhomogeneous multifunctional power structures
No full textEThOS - Electronic Theses Online ServiceGBUnited Kingdo
Honeycomb elastic properties: a review of some existing theories and a new dynamic approach
No full textThe influence of the nine orthotropic material properties of honeycomb on the dynamic response of a finite
element model of a simple supported sandwich plate are examined. Fifteen available theories from the
literature for the material properties of honeycomb are reviewed and their values calculated for a HexWeb
5.2-1/4-25(3003) Aluminium core. The agreement between the theoretical material properties and the major
ASTM(American Society for Testing andMaterials) standard test methods is investigated. A new and simple
technique is described for measuring the dynamic shear moduli of honeycomb materials and its values are
compared with those presented in the literature
Influence of mesoscale friction interface geometry on the nonlinear dynamic response of large assembled structures
Full text linkFriction 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
A multi-step interpolated-FFT procedure for full-field nonlinear modal testing of turbomachinery components
Full text linkModel 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-scale, geometrically-nonlinear structure vibrating at its resonance. These spatially-detailed operating deflection shapes are advantages in describing local deformation patterns of a nonlinear structure, allowing essential ingredients in model updating algorithms, such as the Modal Assurance Criterion (MAC) values, to be correlated with exceptionally high quality
Computation of damped nonlinear normal modes for large scale nonlinear systems in a self-adaptive modal subspace
No full textThe 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 forced response over a wide range of excitation levels with the maximum error less than 1% at nearly zero computational cost
Full-field measurement of lightweight nonlinear structures using 3D SLDV
No full textModern non-contact measurement techniques such as the Three-dimensional Scanning Laser Doppler Vibrometry 12 (3D SLDV) are advantageous in measuring vibrations of lightweight, thin-walled aerospace structures, which were 13 conventionally deemed as difficult or not feasible to apply using attached transducers. Nevertheless, the full-field 14 measurements using 3D SLDV are still limited to extracting modal properties of linear structures, while measurements 15 of complex nonlinear structures are rarely reported. This paper aims to extend the full-field measurement capability of 3D SLDV and combines it with a Multiple-Input-Single-Output (MISO) vibration controller to deal with nonlinear 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 structures. An advanced test strategy is introduced, which is capable of obtaining amplitude-dependent resonant frequencies, modal damping ratios and full-field, multi-harmonic mode shapes of nonlinear normal modes (NNMs). Conflicting parameters such as the frequency resolution and measurement time are optimised by combining phase separation and phase resonance testing techniques in a coherent strategy. The capabilities of the proposed nonlinear modal testing strategy are demonstrated on a realistic, large-scale fan blade that exhibits softening behaviours. Two of its NNMs were investigated at larger vibration amplitudes. Its nonlinear modal parameters were successfully extracted and validated, highlighting the time efficiency and data accuracy of the proposed strategy for measuring industrial-scale, lightweight nonlinear structures
Guidelines to use input contact parameters for nonlinear dynamic analysis of jointed structures: Results of a round robin test
Full text linkTurbomachinery and other jointed structures are carefully designed to optimise their dynamic response and prevent unwanted high-cycle fatigue failures due to vibration. Advanced numerical models are employed to predict the often nonlinear dynamic responses, but their reliability is partially limited by the lack of understanding of the friction mechanisms between the vibrating contact interfaces. Although several high-frequency friction rigs have been developed at different institutions to measure contact parameters such as friction coefficient and contact stiffness, a lack of direct comparisons prevents a throughout understanding. To address this issue, a comparison of these contact parameters has been performed by employing the high-frequency friction rigs of Imperial College London and Politecnico di Torino. A test plan was designed to cover a wide experimental space by testing the friction rigs to their limits and measuring hysteresis loops under a range of normal loads and displacement amplitudes at room temperature. Measurements from the two very different experimental setups are compared, showing a good level of agreement for the friction coefficient, but also highlighting some differences, especially for the contact stiffness. New insights are provided into the physics of these contact parameters and specific guidelines are given to improve contact models used for nonlinear dynamic analysis
A novel test rig for the validation of nonlinear friction contact parameters of turbine blade root joints
Full text linkThe assembly of components into a large-scale engineering system naturally leads to the presence of joints with frictional interfaces. The degree of agreement between numerical models and their experimental counterparts decreases when assemblies based in this kind of interfaces are studied due to the nonlinear dynamic behaviour that joints introduce. This is, for example, the case in turbine blade root joints. The main cause for these deviations are the friction-related nonlinear damping and stiffness effects influencing the dynamic behaviour of the assembly. The experimental measurement of these damping effects poses a challenge due to the presence of the excitation rig itself, which can introduces significant parasitic damping in the system. A free decay measurement is consequently the ideal way to extract the nonlinear behaviour, however, the exciter must be initially in physical contact with the test fixture in order to reach the high excitation amplitudes that lead to macro-slip friction in the fixture joints. The test setup proposed in this paper is developed for a beam on which two blade root designs have been machined at both ends (dog bone). This beam is fitted between two clamps equipped with dovetail roots and pulled into tension to simulate rotational centrifugal loading, thus creating a blade root contact joint at either end of the beam. The novel excitation method excites the beam harmonically with a rigidly connected shaker to macro-slip deflection amplitudes before decoupling from the beam to release it into free decay. This test procedure allows the contactless measurement of the variation in vibrational decay in the beam and the subsequent extraction of the resulting nonlinear frictional behaviour associated with the joints
Experimental dataset from a round robin test of contact parameters and hysteresis loops for nonlinear dynamic analysis
Full text linkThis data article describes the extensive experimental dataset of friction hysteresis measured during the round robin test of the original research article [1]. The round robin test was performed on the two different fretting rigs of Imperial College London and Politecnico di Torino, and consisted of recording comparable friction hysteresis loops on specimen pairs manufactured from the same batch of raw stainless steel. The reciprocating motion of the specimens was performed at room temperature under a wide range of test conditions, including different normal loads, displacement amplitudes, nominal areas of contact and excitation frequencies of 100 Hz and 175 Hz. Friction forces and tangential relative displacements for each specimen pair were recorded and stored as hysteresis raw data. Each hysteresis loop was post-processed to extract friction coefficient, tangential contact stiffness and energy dissipated, whose evolution with wear was thus obtained and stored as well. MATLABⓇ scripts for post-processing and plotting data are included too.The dataset can be used by researchers as a benchmark to validate theoretical models or numerical simulations of friction hysteresis models and wear mechanisms, and also to study the physics of friction hysteresis and its contact parameters. This friction data can also be used as input in models for nonlinear dynamics applications as well as to provide information on the contact measurement uncertainty under fretting motion. Other applications include using this data as a training set for machine learning applications or data-driven models, as well as supporting grant applications
Reduced order method based on an adaptive formulation and its application to fan blade system with dovetail joints
Full text linkLocalized nonlinearities due to the contact friction interfaces are widely present in the aero-engine structures. They can significantly reduce the vibration amplitudes and shift the resonance frequencies away from critical operating speeds, by exploiting the frictional energy dissipation at the contact interface. However, the modelling capability to predict the dynamics of such large-scale systems with these nonlinearities is often impeded by the high computational expense. Component mode synthesis (CMS) based reduced order modelling (ROM) are commonly used to overcome this problem in jointed structures. However, the computational efficiency of these classical ROMs are sometimes limited as their size is proportional to the DOFs of joint interfaces resulting in a full dense matrix. A new ROM based on an adaptive formulation is proposed in this paper to improve the CMS methods for reliable predictions of the dynamics in jointed structures. This new ROM approach is able to adaptively switch the sticking contact nodes off during the online computation leading to a significant size reduction comparing to the CMS based models. The large-scale high fidelity fan blade assembly is used as the case study. The forced response obtained from the novel ROM is compared to the state-of-the-art CMS based Craig-Bampton method. A parametric study is then carried out to assess the influence of the contact parameters on the dynamics of the fan assembly. The feasibility of using this proposed method for nonlinear modal analysis is also characterised
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