Shock and Vibration - Volume 17, issue 4-5

Article Type: Other

DOI: 10.3233/SAV-2010-0529

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 329-329, 2010

Authors: Elishakoff, I. | Pentaras, D.

Article Type: Research Article

Abstract: This review paper deals with two problems in structural engineering dynamics; one is deterministic, the other is of stochastic nature. One problem is linear, the other is nonlinear. Authors have a biased preferential view on these problems because of their active involvement in the discussed research topics. Still, these two problems reflect, at least in a small manner, some developments in this vast and fascinating field. The first part deals with deterministic linear vibrations of double-walled …carbon nanotubes either in classical or refined setting; the second part is devoted to the nonlinear random vibrations of structures. Show more

Keywords: Deterministic linear vibrations, nonlinear random vibrations

DOI: 10.3233/SAV-2010-0530

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 331-348, 2010

Authors: Halevi, Y. | Peled, I.

Article Type: Research Article

Abstract: Absolute Vibration Suppression (AVS) is a control method for flexible structures governed by the wave equation. Such system may be a rotating shaft, a rod in tension or a crane for which the cable mass is not negligible. First an accurate, infinite dimension, transfer function, relating arbitrary actuation and measurement points, with general boundary conditions, is derived. The transfer function consists of time delays, due to the wave motion, and low order rational terms, which correspond …to the reflection from the boundary. Thus the compact mathematical representation has a clear physical interpretation. Furthermore, it leads to the special structure of the AVS controller which is a collocated, rate to force feedback that completely eliminates the vibration. The paper investigates the properties of the AVS controller and its robustness to modeling errors, in particular small non-collocation. Show more

Keywords: Control, wave equation, vibration suppression

DOI: 10.3233/SAV-2010-0531

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 349-357, 2010

Authors: Gaul, Lothar | Becker, Jens

Article Type: Research Article

Abstract: Friction in joints significantly contributes to the observed overall damping of mechanical structures. Especially if the material damping is low, the frictional effects in joints and clamping boundary conditions dominate the structural damping. The damping and the stiffness of the structure are nonlinear functions of the system states and consequently of the excitation signal and amplitude. If these nonlinear effects should be incorporated in the design process, transient simulations must be employed in order to predict …and analyze the damping for a given excitation, though they need excessive computation power due to the nonlinear constitutive laws and the high contact stiffnesses. As one approach to alleviate transient simulations, the application of component mode synthesis (CMS) methods to structures with friction is investigated exploiting the linearity of the jointed substructures. The friction and the nonlinear normal contact is modeled by constitutive laws that are implemented in node-to-node finite elements. The necessary considerations for accurate damping prediction by the reduced models, the accuracy and the computational times for transient simulations are discussed. The developed model reduction techniques allow a strong reduction of the computation time which in turn makes it a promising tool for model updating and predictive parameter studies. As an application example, a beam-like structure with attached friction damper is investigated in simulations and the obtained numerical results after model updating are compared to experiments. Show more

DOI: 10.3233/SAV-2010-0532

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 359-371, 2010

Authors: Cirkl, David

Article Type: Research Article

Abstract: Article deals with measurement and modelling of damping force of polyurethane foam specimen being compressed. The specimens were taken from car seats and differ by material density. For modelling of damping force during harmonical kinematical excitation the explicit formula is used. Its parameters are found by means of method based on evaluation of energy dissipated during one loading cycle and also on evaluation of position of damping force extreme.

Keywords: Passengers comfort, car seat, polyurethane foam, damping force

DOI: 10.3233/SAV-2010-0533

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 373-381, 2010

Authors: Rainieri, Carlo | Fabbrocino, Giovanni | Cosenza, Edoardo

Article Type: Research Article

Abstract: Experimental estimation of damping is currently not a comprehensively solved problem. Although modal frequencies and mode shapes can be measured confidently and quite easily by means of dynamic tests on civil structures, an accurate identification of damping ratios needs further development. Experimental values can be characterized by large error bounds mainly because damping is strongly influenced by the magnitude of the dynamic response of a structure. In this paper, the main issues concerning …identification of modal damping ratios are discussed in order to define methods for their reliable estimation, to reduce uncertainties and characterize error bounds. According to this aim, a procedure for optimized modal parameter estimation via Stochastic Subspace Identification is also proposed. Then data collected from an extensive literature review are analyzed depending on the type of structure in order to point out the main factors affecting damping. Moreover, a comparison with provisions about structural damping given in National and International seismic codes for structural design of civil structures is reported. This process demonstrates that the development of a damping database based on homogeneous and reliable estimates is required for calibration of predictive formulations to be used in the framework of performance-based seismic design. Show more

Keywords: Dynamic tests, damping estimation, operational modal analysis, performance-based design

DOI: 10.3233/SAV-2010-0534

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 383-395, 2010

Authors: Miller, U. | Bograd, S. | Schmidt, A. | Gaul, L.

Article Type: Research Article

Abstract: A vibration analysis of a structure with joints is performed. The simulation is conducted with finite element software capable of performing a numeric modal analysis with hysteretic damping assumption. The joints are modeled with thin layer elements, representing dissipation and stiffness of the joints. The matrices describing the system consist of the mass, as well as real and complex-valued stiffness matrices. If the eigenvalues of this system are found in one step, due to the mode …crossing occurring for the closely spaced modes, it is difficult and time consuming to assign calculated modal damping factors to the corresponding undamped eigenvalues. In order to avoid this problem, an eigenvalue following method is used. The outcome of the solution is the graphical presentation of continuous eigenvalue paths, showing the change in the eigenvalues from the undamped to the fully damped case. For every undamped eigenvalue exists its equivalent eigenfrequency and damping factor that can be used for further numerical analysis. In scope of this article a Predictor-Corrector and a Rayleigh-Quotient Iteration algorithms are applied to the problem. The algorithms are tested specifically on the type of matrices resulting from the weakly damped hysteretic formulation arising from the simulation of metallic structures with joints. Show more

Keywords: Hysteretic damping, complex eigenvalues, path following, complex stiffness matrix

DOI: 10.3233/SAV-2010-0535

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 397-405, 2010

Authors: Moreira, R.A.S. | Corte-Real, J.D. | Rodrigues, J. Dias

Article Type: Research Article

Abstract: The design of passive damping treatments using viscoelastic materials requires both an accurate numerical analysis approach, usually making use of the finite element method, and a realistic means of material description. While the former has been widely studied and several and valuable approaches have become available during the last years, the latter is still an issue requiring additional efforts. The experimental characterization, the data modeling and finally the constitutive models able to be directly used along …analytic and numerical analysis, are still important research areas. Several viscoelastic models, able to be directly applied into a finite element analysis either in a time or in a frequency domain analysis, are available and have been widely used during the last years in most of the published work. Despite the general description and straightforward use that such modeling approaches may provide, temperature effect is usually disregarded and isothermal analysis are usually performed. Moreover, this temperature effect is naturally not directly considered as an input parameter for most of the viscoelastic material models and isothermal conditions are also considered in the experimental characterization data analysis. This work presents an extended viscoelastic model, based on well known isothermal models, where the temperature-frequency superposition effect is directly considered. The extended model is applied to the analysis of the experimental data using a data fitting procedure to identify a set of global parameters able to represent the effect of the frequency and the temperature. Show more

DOI: 10.3233/SAV-2010-0536

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 407-418, 2010

Authors: Schmidt, A. | Lotsch, M. | Gaul, L.

Article Type: Research Article

Abstract: The damping properties of materials, joints, and assembled structures can be modeled efficiently by the use of fractional derivatives in their constitutive equations. The respective models describe the damping behavior accurately over broad ranges of time or frequency where only few material parameters are needed. They assure causality and pure dissipative behavior. The concept of fractional derivatives can be implemented into discretization methods such as the finite element method, the boundary element …method, or the finite difference method. Due to the non-local character of fractional derivatives the whole deformation history of the structure under consideration has to be considered in time-domain computations. This leads to increasing storage requirements and high computational costs. A new concept for an effective numerical evaluation makes use of the equivalence between the Riemann-Liouville definition of fractional derivatives and the solution of a partial differential equation (PDE). The solution of the PDE is found by applying the method of weighted residuals where the domain is split into finite elements using appropriate shape functions. This approach leads to accurate results for the calculation of fractional derivatives where the numerical effort is significantly reduced compared with alternative approaches. Finally, this method is used in conjunction with a spatial finite element discretization and a simple rod structure is calculated. The results are compared to those obtained from alternative formulations by means of accuracy, storage requirements, and computational costs. Show more

Keywords: Fractional derivatives, damping, finite element method

DOI: 10.3233/SAV-2010-0537

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 419-428, 2010

Authors: de Lima, A.M.G. | Rade, D.A. | Bouhaddi, N.

Article Type: Research Article

Abstract: Engineering structures incorporating viscoelastic materials are characterized by inherent uncertainties affecting the parameters that control the efficiency of the viscoelastic dampers. In this context, the handling of variability in viscoelastic systems is a natural and necessary extension of the modeling capability of the present techniques of deterministic analysis. Among the various methods devised for uncertainty modeling, the stochastic finite element method has received major attention, as it is well adapted for applications …to complex engineering systems. In this paper, the stochastic finite element method applied to a structural three-layer sandwich plate finite element containing a viscoelastic layer, with random parameters modelled as random fields, is presented. Accounting for the dependence of the behaviour of the viscoelastic materials with respect to frequency and temperature, using the concepts of complex modulus and shift factor, the uncertainties are modelled as homogeneous Gaussian stochastic fields and are discretized according to the spectral method, using Karhunen-Loève expansions. The modeling procedure is confined to the frequency domain, and the dynamic responses are characterized by frequency response functions (FRF's). Monte Carlo Simulation (MCS) combined with Latin Hypercube Sampling is used as the stochastic solver. The typically high dimensions of finite element models of viscoelastic systems combined with the large number of Monte Carlo samples to be computed make the evaluation of the FRF's variability computer intensive. Those difficulties motivate the use of condensation methods specially adapted for viscoelastic systems, in order to alleviate the computational cost. After the presentation of the underlying formulation, numerical applications of moderate complexity are presented and discussed aiming at demonstrating the main features and, particularly, the computation cost savings provided by the association of MCS with the suggested condensation procedure. Show more

Keywords: Uncertainty propagation, viscoelasticity, model condensation, stochastic finite elements

DOI: 10.3233/SAV-2010-0538

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 429-444, 2010

Authors: Luczak, Marcin | Vecchio, Antonio | Peeters, Bart | Gielen, Ludo | Van der Auweraer, Herman

Article Type: Research Article

Abstract: This paper presents a novel approach in the field of experimental and numerical investigation of mechanical properties of composite structures. It takes into account test data variability resulting from structural dynamic properties measurement and uses them to quantify uncertainties in model parameters updating. The main goal of the conducted research is to investigate the dynamic properties of fibre reinforced composite structures. Non-destructive experimental and numerical simulation methods are used hereto. In the …experimental part, different test configurations were taken into account. The excitation was performed by means of random and harmonic, single and multi point stimuli while the response measurement was done through contact and non-contact acceleration, velocity and dynamic strain sensing. The test results are applied in two ways: for the structural identification of the object and for non-deterministic updating of the numerical model according to a range of experimental models obtained from test. The sources of the test data variabilities were related to the excitation and measurement technique applied for the investigated object. Non – deterministic model updating and verification & validation included uncertainties of its parameters by means of interval and stochastic methods. A number of variable test modal models were statistically assessed to investigate impact of variability source onto modal model parameters. The presented research was conducted in the context of the FP6 Marie Curie project UNVICO-2. Show more

Keywords: Modal testing, uncertainty, updating, strain

DOI: 10.3233/SAV-2010-0539

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 445-459, 2010

Authors: Koziol, Piotr | Mares, Cristinel

Article Type: Research Article

Abstract: This paper analyses theoretically the response of a solid for fast moving trains using models related to real situations: a load moving in a tunnel and a load moving on a surface. The mathematical model is described by Navier's elastodynamic equation of motion for the soil and Euler-Bernoulli equation for the beam with appropriate boundary conditions. Two modelling approaches are investigated: the model with half space under the beam and the model with finite thickness of …supporting medium. The problem of singularities for displacements calculation is discussed in relation with boundary conditions and types of considered loads: harmonic and constant, point and distributed moving loads. The analysis in frequency-time and frequency-velocity domains is presented and discussed with regard to critical velocities. A wavelet approximation method based on application of coiflet filters is used for the derivation of displacements in physical domain. A new, modified filter is used in numerical calculations allowing to alleviate numerical difficulties related to the form of approximating sequences based on classical filters, formulated in previous publications. The effectiveness of the coiflet approach is discussed for filter coefficients with different order of accuracy. This methodology is very efficient for the analysis in the range of relatively high and low load frequencies (treated as an approximation of a constant load) which are especially important for the analysis of vibrations generated by trains moving with velocities near critical values. Results of numerical simulations are presented, demonstrating their utility for modelling and preliminary analysis of complex models. Show more

Keywords: Moving load, critical velocity, vibrations, wavelet method

DOI: 10.3233/SAV-2010-0540

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 461-472, 2010

Authors: Radeş, M.

Article Type: Research Article

Abstract: Mode Indicator Functions (MIFs) are real-valued frequency-dependent scalars that exhibit local minima or maxima at the modal frequencies of the system. This paper presents an overview of the currently used and some recently developed MIFs, revealing their features and limitations. Eigenvalue or singular value based MIFs use rectangular frequency response function (FRF) matrices calculated in turn at each excitation frequency. Their plots have as many curves as the number of references. Recently developed MIFs do the …simultaneous analysis of all FRF information organized in a compound FRF (CFRF) matrix. The left singular vectors or the Q-vectors obtained from the pivoted QLP decomposition of this matrix contain the frequency information and are used to construct MIFs. The number of curves in such a MIF plot is equal to the effective rank of the CFRF matrix. If the number of response coordinates is larger than this rank, a single point excitation can locate even double modes. The condition to use as many input points as the multiplicity of modal frequencies is no more imposed. Show more

Keywords: MMIF, CMIF, CoMIF, UMIF, QCoMIF, QRMIF

DOI: 10.3233/SAV-2010-0541

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 473-482, 2010

Authors: Marchesiello, S. | Bellino, A. | Garibaldi, L.

Article Type: Research Article

Abstract: Many engineering structures, such as cranes, traffic-excited bridges, flexible mechanisms and robotic devices exhibit characteristics that vary with time and are referred to as time-varying or nonstationary. In particular, linear time-varying (LTV) systems have been often dealt with on a case-by-case basis. Many concepts and analytic methods of linear time-invariant (LTI) systems cannot be applied to LTV systems, as for example the conventional definition of modal parameters. In fact, LTV systems violate one of the …assumptions of the conventional modal analysis, which is stationarity. Subspace-based identification methods, proposed in the 1970s, have been attracting much attention due to their affinity to the modern control theory, which is based on the state space model. These methods are now successfully applied to many industrial cases and may be considered reference methods for identifying LTI systems. In this paper the use of a subspace-based method for identifying LTV systems is discussed and applied to both numerical and experimental systems. More precisely a modified version of the SSI method, referred to here as ST-SSI (Short Time Stochastic Subspace Identification) is introduced as well as a method for predicting time-varying stochastic systems using the angle variation between the subspaces; the latter is able to predict the system parameter in the "near" future. Show more

Keywords: Time-varying, frozen technique, subspace rotation

DOI: 10.3233/SAV-2010-0542

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 483-490, 2010

Authors: Viet Ha, Nguyen | Maxime, Peeters | Jean-Claude, Golinval

Article Type: Research Article

Abstract: An efficient approach to Structural Health Monitoring of dynamical systems based on the Wavelet Transform (WT) and the concept of subspace angle is presented. The objective is to propose a detection method that is sensitive to the onset of nonlinear behaviour in a dynamic system. For this purpose, instantaneous frequencies are identified first from output-only vibration signals using the Wavelet Transform. Time varying deformation shapes are then extracted by analyzing the whole measurement data set on …the structure. From this information, different dynamic states of the structure may be detected by inspecting time variations of 'modal' features. The experimental structure considered here as application example is a clamped beam with a geometric nonlinearity. Detection of nonlinearity is carried out by means of the concept of subspace angles between instantaneous deformation modes extracted from measurement data using the continuous Wavelet Transform. The method consists in controlling the angular coherence between active subspaces of the current and reference states respectively. The proposed technique, which shows a good sensitivity to small changes in the dynamic behaviour of the structure, may also be used for damage detection. Show more

Keywords: Identification, nonlinearity, detection, subspace angle

DOI: 10.3233/SAV-2010-0543

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 491-506, 2010

Authors: Borges, R.A. | de Lima, A.M.G. | Steffen Jr., V.

Article Type: Research Article

Abstract: Dynamic vibration absorbers are discrete devices developed in the beginning of the last century used to attenuate the vibrations of different engineering structures. They have been used in several engineering applications, such as ships, power lines, aeronautic structures, civil engineering constructions subjected to seismic induced excitations, compressor systems, etc. However, in the context of nonlinear dynamics, few works have been proposed regarding the robust optimal design of nonlinear dynamic vibration absorbers. In …this paper, a robust optimization strategy combined with sensitivity analysis of systems incorporating nonlinear dynamic vibration absorbers is proposed. Although sensitivity analysis is a well known numerical technique, the main contribution intended for this study is its extension to nonlinear systems. Due to the numerical procedure used to solve the nonlinear equations, the sensitivities addressed herein are computed from the first-order finite-difference approximations. With the aim of increasing the efficiency of the nonlinear dynamic absorber into a frequency band of interest, and to augment the robustness of the optimal design, a robust optimization strategy combined with the previous sensitivities is addressed. After presenting the underlying theoretical foundations, the proposed robust design methodology is performed for a two degree-of-freedom system incorporating a nonlinear dynamic vibration absorber. Based on the obtained results, the usefulness of the proposed methodology is highlighted. Show more

Keywords: Nonlinear dynamics, dynamic vibration absorber, robust optimization, sensitivity analysis

DOI: 10.3233/SAV-2010-0544

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 507-520, 2010

Authors: Policarpo, H. | Neves, M.M. | Ribeiro, A.M.R.

Article Type: Research Article

Abstract: This article presents a study on the use of the dynamical response of multi-laminated periodic bars to create resonance band gaps within useful frequency ranges. The objective is to control, in a passive form, the longitudinal vibration transmissibility in specific and wide enough frequency ranges of interest. This is achieved by the separation of two adjacent eigenfrequencies. A relation between the modal analysis, the harmonic analysis and the Bloch wave theory is proposed, for which no …reference was found in the searched literature. As shown, the selection of appropriate material pairs is essential to obtain useful frequency ranges. The use of pairs of steel and cork agglomerate is proposed, since it allows the design of attenuators at lower frequencies through a prediction based on finite element analysis (FEA). This approach requires the storage modulus of cork for which analytical and numerical FEA models were verified and validated. A methodology to obtain experimentally the storage modulus of cork is presented. Regarding the structural improvement problem, we discuss a methodology to design periodic bars for a specific location of the first attenuation's frequency range and illustrate the main results through several examples. Show more

Keywords: Passive isolation, vibration, multi-laminated bar, cork agglomerates, structural modification

DOI: 10.3233/SAV-2010-0545

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 521-535, 2010

Authors: Uhl, T. | Lisowski, W.

Article Type: Research Article

Abstract: One of the important challenges present nowadays in the automotive industry is minimizing of a car components design time. Traditional manufacturing of a prototype is usually a time and a cost consuming process. Alternatively, rapid prototyping techniques can be used in such a case. In the reported research a brake caliper was investigated, since it is an example of an element, which should have very strictly defined structural dynamic properties. As a technique of rapid prototyping …of the considered caliper the 3D printing of a mould was selected. A process of the caliper casting with the use of the "prototype" mould is different than the one with the use of the metal form. Thus it is very likely that the both considered types of the caliper would possess different properties from the point of view of structural dynamics. Structural dynamic properties can be analyzed both numerically and experimentally. Simulation of the caliper FE model with uncertain parameters was used to analyze influence of various caliper parameters on its natural frequency values. Modal testing of the caliper was performed with the aim of investigation of applicability of Experimental Modal Analysis for determination of variability of natural frequencies resulting from the manufacturing process. In the course of this research, the natural frequencies of the prototype caliper and the standard caliper were compared. Show more

Keywords: Structural dynamics, finite element modelling, experimental modal analysis, variability of structural properties, rapid prototyping

DOI: 10.3233/SAV-2010-0546

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 537-550, 2010

Authors: Chen, Shen-En | Dai, Kaoshan

Article Type: Research Article

Abstract: Unique conductor-pole couplings complicate the dynamic behaviors of electric transmission pole line systems. Finite element modeling is performed on two typical transmission poles used in southeastern USA – a steel pole and a prestressed concrete pole. The two poles are representative of unique structure types: a heavy rod-like structure and a lightweight, shell-type structure. Since coupling issues between the pole and the cable introduce great complexities for modeling the pole line system, simplified numerical models are …used. Limited full-scale modal test results are presented to verify the numerical models. The prestressed concrete pole is shown to be easier for mode identification than the steel pole – but both numerical models show complicated coupled vibration modes. This study is part of a larger study to establish an understanding of the dynamic response analyses of power grid under ground vibrations. Show more

Keywords: Finite element modeling, prestressed concrete poles, vibration characteristics

DOI: 10.3233/SAV-2010-0547

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 551-561, 2010

Authors: Almeida, R.A.B. | Urgueira, A.P.V. | Maia, N.M.M.

Article Type: Research Article

Abstract: When deriving an experimental model from Frequency Response Functions (FRFs), it may happen that the measurement of certain FRFs is impossible. This may be an important issue, mainly in the field of condition monitoring and damage detection, since some points of interest may become inaccessible in operational conditions. In this circumstance, it is useful to have some tools that can provide the prediction of such dynamic information. The transmissibility concept, extended to a general multiple degree-of-freedom …system, can play an important role to circumvent these situations. The authors have shown in previous works that the estimation of such FRFs can be made possible by invoking important properties associated with the transmissibility function. The objective of this work is to evaluate different sets of FRFs, estimated by using the transmissibility concept and its associated properties, in an actual continuous structure to which different patterns of structural modification are applied. A supplementary study in this work shows that some of the sets for applied forces/known responses can better estimate the FRF data. Show more

Keywords: Transmissibility, structural modification, Frequency Response Functions

DOI: 10.3233/SAV-2010-0548

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 563-577, 2010

Authors: Friedmann, A. | Mayer, D. | Kauba, M.

Article Type: Research Article

Abstract: Structural health monitoring methods based on modal properties have been proven to be well suited for infrastructure objects, e.g. bridges, buildings or wind turbines. The considerable size of these structures leads to long distances between the sensors and the signal processing units and a large number of sensors. To save cabling effort and lower the amount of data which has to be transmitted, the structural analysis may be decentralized with a network of smart sensors. …In this paper, a strategy for decentralized signal analysis with the Random Decrement method is discussed. This method allows for the estimation of auto- and cross-correlation functions simply from triggered averaging of time histories. These serve as a basis for the modal decomposition by operational modal analysis methods. While the Random Decrement signatures can be processed in a decentralized way on the small microcontrollers commonly used, e.g. in wireless sensing applications, the deeper analysis of resonance frequencies and mode shapes is done by a central unit. After a short introduction into the theory of the Random Decrement method, its application in decentralized data acquisition is illustrated with a numerical example. Afterwards, a simple experimental structure exposed to actual wind loads is instrumented and the decentralized signal processing strategy is tested with the acquired data. A first algorithm for the modal decomposition of the estimated correlation functions is implemented and tested. Show more

Keywords: Modal analysis, modal testing, structural health monitoring

DOI: 10.3233/SAV-2010-0549

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 579-588, 2010

Authors: Manson, G. | Barthorpe, R.J.

Article Type: Research Article

Abstract: The paper is concerned with adopting a data-driven approach to damage detection and location on an aerospace structure without recourse to an artificial neural network. Five advanced features are selected, each detecting the removal of only one of five inspection panels on the structure. The features give perfect classification for damage location for single-site damage and 98.1% correct classification for multi-site damage scenarios, using a statistically calculated threshold. However, if the threshold values for two of …the five features are altered slightly, 100% correct classification would be possible for single- and multi-site damage. Show more

Keywords: Feature selection, damage location, structural health monitoring, neural networks

DOI: 10.3233/SAV-2010-0550

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 589-599, 2010

Authors: Rucevskis, S. | Wesolowski, M.

Article Type: Research Article

Abstract: During the last decades a great variety of methods have been proposed for damage detection by using the dynamic structure characteristics, however, most of them require modal data of the structure for the healthy state as a reference. In this paper the applicability of the mode shape curvature squares determined from only the damaged state of the structure for damage detection in a beam structure is studied. To establish the method, two aluminium beams containing different-size …mill-cut damage at different locations are tested by using the experimentally measured modal data. The experimental modal frequencies and the corresponding mode shapes are obtained by using a scanning laser vibrometer with a PZT actuator. From the mode shapes, mode shape curvatures are obtained by using a central difference approximation. With the example of the beams with free-free and clamped boundary conditions, it is shown that the mode shape curvature squares can be used to detect damage in the structures. Further, the extent of a mill-cut damage is identified via modal frequencies by using a mixed numerical-experimental technique. The method is based on the minimization of the discrepancy between the numerically calculated and experimentally measured frequencies. Show more

Keywords: Damage detection, dynamic response, mode shape curvature, scanning laser vibrometer

DOI: 10.3233/SAV-2010-0551

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 601-610, 2010

Authors: Cichowicz, Artur

Article Type: Research Article

Abstract: The response spectrum generally provides a good estimate of the global displacement and acceleration demand of far-field ground motion on a structure. However, it does not provide accurate information on the local shape or internal deformation of the response of the structure. Near-field pulse-like ground motion will propagate through the structure as waves, causing large, localized deformation. Therefore, the response spectrum alone is not a sufficient representation of near-field ground motion features. Results show …that the drift-response technique based on a continuous shear-beam model has to be employed here to estimate structure-demand parameters when structure is exposed to the pulse like ground motion. Conduced modeling shows limited applicability of the drift spectrum based on the SDOF approximation. The SDOF drift spectrum approximation can only be applied to structures with smaller natural periods than the dominant period of the ground motion. For periods larger than the dominant period of ground motion the SDOF drift spectra model significantly underestimates maximum deformation. Strong pulse-type motions are observed in the near-source region of large earthquakes; however, there is a lack of waveforms collected from small earthquakes at very close distances that were recorded underground in mines. The results presented in this paper are relevant for structures with a height of a few meters, placed in an underground excavation. The strong ground motion sensors recorded mine-induced earthquakes in a deep gold mine, South Africa. The strongest monitored horizontal ground motion was caused by an event of magnitude 2 at a distance of 90 m with PGA 123 m/s^{2} , causing drifts of 0.25%–0.35%. The weak underground motion has spectral characteristics similar to the strong ground motion observed on the earth's surface; the drift spectrum has a maximum value less than 0.02%. Show more

Keywords: Pulse-type ground motion, drift spectrum, response spectrum

DOI: 10.3233/SAV-2010-0552

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 611-617, 2010

Authors: Hedayat, Amir A. | Yalciner, Hakan

Article Type: Research Article

Abstract: Inadequate attention during design and construction of some of reinforced concrete (RC) buildings in Cyprus has raised questions about the performance level of these existing buildings under future earthquakes. This study aimed to assess the seismic structural response of a four story existing RC building. For this purpose, first, the weak structural elements (e.g.\ the not safety column-beam joints and weak columns) were detected using linear static procedure (LSP) analyses on the basis of Turkish earthquake …code. Then, two different strengthening methods were examined. In the first method which is common in Cyprus, the existing building was strengthened based on LSP, using column jacketing to satisfy seismic code requirements to remove the weak elements. The second strengthening method was carried out using nonlinear static procedures (NSP) to achieve the basic safety objective (BSO) performance level described in FEMA 356. For existing and both strengthened structures, pushover curves were obtained and following FEMA 356, performance points were calculated and compared. The seismic responses of existing and strengthened buildings were also assessed using incremental dynamic analyses (IDA). Nonlinear dynamic time history analyses have been performed by using SDOF models of these buildings under action of different scales of 20 ground motion records. Then IDA curves for each earthquake have been constructed. Limit – states at each performance level have been defined and summarizing the multi – record IDA curves, 16%, 50% and 84% fractile curves were obtained. Since selected structure represents common existing buildings in Cyprus, probabilistic structural damage estimation fragility curves were also obtained in terms of peak ground acceleration (PGA) for each considered performance level. Results showed that the strengthening method based on the NSP to satisfy the BSO requirements is much more effective than the one based on the LSP to improve the building performance and to reduce the probability of exceeding of limit states IO, LS and CP at any seismic zone. Show more

Keywords: Incremental dynamic analysis, performance assessment, jacketing, pushover analysis

DOI: 10.3233/SAV-2010-0553

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 619-629, 2010

Authors: Setareh, M. | Lovelace, M.

Article Type: Research Article

Abstract: Annoying building floor vibrations have become a serious serviceability issue. This is mainly due to decrease in the system mass resulting from the use of higher strength materials; use of computer-assisted design and the Load and Resistance Factor Design Method to optimize the structure based on the strength requirements; fewer partitions and more innovative designs by architects achieving long, column free spans resulting in a reduction in the natural frequency and damping. This paper provides details …of the vibration analysis and design of a novel office building. Three-dimensional computer models of the structure were created and various modifications were made to the original structure, designed based on static loads, to reduce the possible excessive floor vibrations when subjected to walking excitations. Tuned mass dampers were also designed as a back-up vibration control system. A series of dynamic tests were conducted on the building floor to identify the dynamic properties of the structure and these were then used to update the original computer model. Finally, various forcing functions representing human walks and the updated computer model of the structure were used to evaluate the accuracy of the walking excitation force models to predict the structural response. Conclusions are made on the validity of each forcing function studied here. Show more

Keywords: Serviceability problems, floor vibrations, tuned mass damper, human walk force model

DOI: 10.3233/SAV-2010-0554

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 631-639, 2010

Authors: Cardoso, J.B. | Moita, P.P. | Valido, A.J.

Article Type: Research Article

Abstract: A sensitivity analysis and multicriteria control optimization formulation is derived for mechanical systems. This formulation is implemented into an interactive optimum design code and it is applied to optimize protection systems for the prevention of injuries. The limiting isolation capabilities of the systems are determined. The effect of pre-acting control is investigated. Control forces as well as the time at which the control should act before the instant of impact are considered as design variables. The …same idea used by the authors in previous articles for minimum time control problems is applied here to find the preview time. Dynamic response index, maximum acceleration, rattlespace, or maximum power of the resisting force among others can be used as performance criteria. In order to handle the multicriteria problem, both the reduced feasible region method and a min-max upper bound method are utilized. The adjoint system approach is used to calculate the sensitivities. The dynamic response of the systems and its sensitivity are discretized via space-time finite elements. The equations of motion and the sensitivity equations are integrated at-once as it is typical for the static response. This way, displacement, velocity or acceleration control conditions can be imposed easily at any point in time. Also, adjoint system response is obtained without needing primary response memorization. Mathematical programming is used for the optimal control process. Show more

Keywords: Optimum control, preview control, multicriteria, space-time finite elements

DOI: 10.3233/SAV-2010-0555

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 641-649, 2010

Authors: Devriendt, Christof | Presezniak, Flavio | De Sitter, Gert | Vanbrabant, Katy | De Troyer, Tim | Vanlanduit, Steve | Guillaume, Patrick

Article Type: Research Article

Abstract: This article uses frequency domain transmissibility functions for detecting and locating damage in operational conditions. In recent articles numerical and experimental examples were presented and the possibility to use the transmissibility concept for damage detection seemed quite promising. In the work discussed so far, it was assumed that the operational conditions were constant, the structure was excited by a single input in a fixed location. Transmissibility functions, defined as a simple ratio between two measured …responses, do depend on the amplitudes or locations of the operational forces. The current techniques fail in the case of changing operational conditions. A suitable operational damage detection method should however be able to detect damage in a very early stage even in the case of changing operational conditions. It will be demonstrated in this paper that, by using only a small frequency band around the resonance frequencies of the structure, the existing methods can still be used in a more robust way. The idea is based on the specific property that the transmissibility functions become independent of the loading condition in the system poles. A numerical and experimental validation will be given. Show more

Keywords: Operational conditions, health monitoring, transmissibility

DOI: 10.3233/SAV-2010-0556

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 651-675, 2010

Authors: Rocha, Bruno | Silva, Carlos | Suleman, Afzal

Article Type: Research Article

Abstract: The paper presents a structural health monitoring system based on propagation of tuned Lamb waves and their interference with discontinuities. The dispersion curves are studied to determine the appropriate type and dimension of transducers and to select the optimum scanning frequencies and relevant propagation modes. A piezoelectric sensor network was implemented in an aluminum plate in order to generate and to sense the wave propagation and associated reflections. The algorithm developed for damage detection relies on …the comparison of undamaged and damaged responses of the structure. Combinations of filters and statistical methods were applied to detect differences in the sensor signals acquired for the two different states (damaged and undamaged), corresponding to damage reflections. In order to eliminate the false positives due to noise, a probability analysis is performed to obtain the final damage position. The software designed for the current application allows the automatic calculation of dispersion curves, it executes the scans, performs data processing, executes the detection algorithm and presents the probable damages and their positions in a graphical form. Experiments were performed with the introduction of cumulative damages in the plate such as surface and through-the-thickness holes and cuts, ranging from 7 mm to 1 mm in diameter. Additionally, a stringer was attached to the plate by a single rivet line to simulate an aircraft skin structure. Cuts originating from rivet holes and connecting adjacent rivets, as well as loosened rivets were detected by the system. The introduction of the stringer resulted in a loss of precision in the determination of the radial position of the damages near it. Also, the network revealed significant difficulties in the detection of damages beyond the stringer. Show more

Keywords: Structural health monitoring, non-destructive testing and evaluation, lamb waves, piezoelectric networks

DOI: 10.3233/SAV-2010-0557

Citation: Shock and Vibration, vol. 17, no. 4-5, pp. 677-695, 2010