Assessment of the accuracy of damping estimation for lightly damped structures (original) (raw)

Damping models for structural vibration

Cambridge University, 2000

This dissertation reports a systematic study on analysis and identification of multiple parameter damped mechanical systems. The attention is focused on viscously and non-viscously damped multiple degree-offreedom linear vibrating systems. The non-viscous damping model is such that the damping forces depend on the past history of motion via convolution integrals over some kernel functions. The familiar viscous damping model is a special case of this general linear damping model when the kernel functions have no memory.

Damping Estimation of Engineering Structures with Ambient Response Measurements

Damping plays a key role in dynamic response prediction, vibration control. Following the brief description of ambient vibration testing and measurements, three non- parametric damping estimation approaches for engineering structures are introduced: (1) time domain approach using Data Correlation (DC); (2) time-frequency analysis via Wavelet Transform (WT), and (3) a spatial domain approach based on Frequency Domain Decomposition (FDD). The major focus of the damping estimation approaches presented is on their capability in dealing with on-site full- scale testing of engineering structures using ambient response measurement. Computer simulations based on ambient response measurements and modal identification of a 15-story office building and a transmission tower were conducted to evaluate their performances of different approaches in dealing with closely spaced modes and noisy data. Major challenges in damping estimation are also discussed.

Experimental Identification of Overall Structural Damping of System

Strojniski Vestnik Journal of Mechanical Engineering, 2013

The dynamic behaviour of large and complex structures largely depends on damping resistance in the structure. A portion of the structural energy is lost to deformations in material, friction between the contact surfaces, and relative motion within the structure. Often, in an analysis of numerical models, before the dynamic analysis of transient events (transient analysis), the damping resistance is adopted on the basis of recommendations, which implies an error of transient response (introduced by frequencies, logarithmic decrements and maximal amplitudes). Decreasing amortized vibratory movement is dependent on the extent of the structural damping. This paper presents the importance of structural damping in structural analysis and shows the experimental and theoretical procedure for identifying G values of the structural damping coefficient. A model for determining the G coefficient is shown in the example of a real tower crane structure. The experimentally obtained values were then used in the transient numerical FEM analysis, as the basis for adopting the conclusions about the dynamic behaviour of this class of structures (transportation machines). The effect of the external perturbation force of trapezoidal impulse form (lifting and quickly lowering of load) is introduced and the dynamic task, as an example of the use of the G coefficient G, is solved. The experimentally determined damping (theoretically isolated for tall truss structures) can be used in similar transient analyses.

Frequency Based Spatial Damping Identification—Theoretical and Experimental Comparison

Conference proceedings of the Society for Experimental Mechanics, 2017

This research compares spatial damping identification methods, both theoretically and experimentally. In contrast to the commonly used damping methods (modal, proportional) the spatial damping information improves structural models with a known location of the damping sources. The real case robustness of full FRF matrix and local equation of motion methods were tested against: modal and spatial incompleteness, differences in viscous and hysteretic damping models and the effect of damping treatments. To obtain accurate results, a careful analysis of measurements in terms of reciprocity in the raw measurements, and in terms of how to preserve symmetry has to be done. It was found that full FRF matrix needs to be symmetrisized due to small deviations in reciprocity before the damping identification. Full frequency response function (FRF) matrix methods (e.g.: Lee-Kim) can identify the spatial damping if spatial and modal incompleteness are carefully evaluated, but the measurement effort increases with second order and, consequently, the size of the FRF matrix. Keywords Spatial damping • Inverse identification • Frequency response • Modal incompleteness • Spatial incompleteness 3.1 Introduction Numerical and analytical prediction of the structural responses depends on the identified spatial damping throughout the structure. Good damping prediction is important for validation of analytical/numerical models in civil, mechanical and aerospace engineering. Damping in linear mechanical systems is usually identified using one of the methods such as logarithmic decay [1] in time domain, continuous wavelet transform [2], the Morlet wave method [3] or the synchrosqueezed wavelet [4] in time-frequency domain or half-power point [1] and first order perturbation [5] in frequency domain. Damping identification methods form the basis of several model updating methods [6, 7], where accurate damping identification can further improve numerical models [8, 9]. Two other examples where exact damping spatial location is needed are: identification of damping sources on existing structures and precise application of damping treatment. However, typically used damping identification methods [1, 8] do not provide spatial information (damping distribution throughout the structure). An alternative approach is to use direct damping identification methods that were developed for identification of damping distribution directly from the frequency response functions (FRF) without the transformation to the modal coordinates. One of the typically used direct methods is the Lee and Kim's dynamic stiffness method [10]. The core of the method is a inverse identification of linear damping model from the complex part of the measured data which have been found to be very sensitive to real world problems in most follow-on studies, e.g. phase error [11], noise when the modal overlap is low [5] and leakage [12]. Ozgen and Kim [12] proposed a new experimental procedure with simultaneous excitation of all nodes to overcome described measurement errors, but the procedure is not practical with lots of measurement degrees of freedom because demands as many shakers as there are measurement degrees of freedom. Some direct methods, not considered in this research, are reviewed in [5, 11, 13]. This research focuses on the modal and spatial incompleteness effect on the identification of spatial damping. Modal incompleteness deals with limited frequency span over number of modes whereas spatial incompleteness covers effects of non-measured points on the structure [14]. Modal and spatial incompleteness for spatial damping have been studied numerically [5, 15] and experimentally [11] on low DOF models.

SOME OBSERVATIONS ON THE CHARACTERIZATION OF STRUCTURAL DAMPING

Journal of Sound and Vibration, 2002

This paper deals with the characterization of damping in dynamical structural systems. In particular, the problem of how the modal damping ratios change with di!erent boundary conditions is addressed. It is shown that only Rayleigh-type damping is actually independent of boundary conditions and modal damping ratios can be easily converted from one boundary condition to another. This condition applies independently to continuous, discrete and discretized systems.

Use of instantaneous estimators for the evaluation of structural damping

Journal of Sound and Vibration, 2004

This study focuses on the definition of time-frequency instantaneous estimators to be employed in the identification of structural damping from signals measured in ambient vibration conditions. The estimators discussed here are obtained from instantaneous curve fitting applied directly to time-frequency representations of dynamic response signals. One of the strong points of this technique is its flexibility: from each signal, a time function of modal damping and amplitude is extracted, providing punctual information on the stability and consistency of damping estimate. The aim of this paper is to study the implications related to the use of this method with linear time-frequency representations. Linear representations, within the inherent limits of their linear nature, still retain attractive computational advantages and lend themselves to a clear theoretical interpretation. In the last part of the paper, a series of numerical applications to simple systems is presented to support theoretical argumentations. Finally, an experimental application to the identification of the damping in a real concrete building is shown.

Experimental Damping Estimation of Material by Sinusoidal Base Excitation

2015

The material damping, which means energy dissipation in materials under cyclic loading, is an important design consideration for vibrating structures especially in the airplane and automobile engineering. The main objective of this work is to determine natural frequency and damping ratio by sweep sine test and half power bandwidth method respectively. Aluminium Beams of required size are prepared and are excited using an Electro-Dynamic Shaker excitation technique over the frequency range of 5-500 Hz. The modal analysis of the beam is also done on ANSYS platform as well as theoretically. A good concurrence for natural frequency is found in all these three methods. Based on the experimental results, damping ratio is determined by using half power band width method. The effect of three major geometrical parameters on material damping is also investigated. For this purpose, Design of Experiment (Taguchi L9 orthogonal Array) is performed to find the parameters combination to give the op...

Damping modelling and identification using generalized proportional damping

2005

ABSTRACT In spite of a large amount of research, the understanding of damping forces in vibrating structures is not well developed. A major reason for this is that, by contrast with inertia and stiffness forces, the physics behind the damping forces is in general not clear. As a consequence, modelling of damping from the first principle is difficult, if not impossible, for real-life engineering structures.

Sensitivity Studies on Damping Estimation

The need for reliable non-destructive evaluation techniques and detection of damage at the earliest possible stage has been pervasive throughout the civil engineering community in recent years. The so-called vibration-based health monitoring techniques rely on the fact that damage causes changes in the local structural damping (energy dissipation). Thus, studying the damping (and its evolution) might yield to a reasonable way of damage detection. There are several methods of damping estimation known today. In this paper, the authors compare four different approaches – half-power bandwidth, two forms of random decrement technique and stochastic subspace identification. The basis for the analyses was provided by two different types of real civil structures. Furthermore, possible problems occurring in practice and suggested solutions will be discussed. As the available measurement data lent itself to a straight comparison between damaged and undamaged structures, the authors were able ...

SIMULATION OF EXPERIMENTAL DAMPING DETERMINATION OF ELASTIC SYSTEMS

15th International Research/Expert Conference ”Trends in the Development of Machinery and Associated Technology” TMT 2011, Prague, Czech Republic, 12-18 September 2011, 2011

Vibrations analysis is important part of construction design and possible failure detection. Vibration highly depends on damping characteristics. Damping characteristics must be considered or determined experimentally, which is commonly done by analysis of vibration records. Determination of damping is important due that internal damping of elastic systems sometimes could be controlled to reduce amplitude of vibrations and increase a life of construction. In this paper, simulation of experimental damping determination of elastic systems is presented. Vibrations are simulated by numerical solution of mathematical model of damped vibrations of two d.o.f. system. Waveform obtained by simulation is imported in software for experimental data analysis and analysed as a real waveform. Damping analysis is performed in time domain and tested for different initial condition.