Showing papers in "The Shock and Vibration Digest in 1998"

A summary review of vibration-based damage identification methods

Abstract: This paper provides an overview of methods to detect, locate, and characterize damage in structural and mechanical systems by examining changes in measured vibration response. Research in vibration-based damage identification has been rapidly expanding over the last few years. The basic idea behind this technology is that modal parameters (notably frequencies, mode shapes, and modal damping) are functions of the physical properties of the structure (mass, damping, and stiffness). Therefore, changes in the physical properties will cause detectable changes in the modal properties. The motivation for the development of this technology is presented. The methods are categorized according to various criteria such as the level of damage detection provided, model-based versus non-model-based methods, and linear versus nonlinear methods. The methods are also described in general terms including difficulties associated with their implementation and their fidelity. Past, current, and future-planned applications of this technology to actual engineering systems are summarized. The paper concludes with a discussion of critical issues for future research in the area of vibration-based damage identification.

Vibration damping and control using shunted piezoelectric materials

Abstract: Research on shunted piezoelectric materials, conducted mainly over the past decade, provides new options to engineers who are responsible for solving structural vibration control problems. The general method is made possible by the relatively strong electromechanical coupling exhibited by modern piezoelectric materials. If a piezoelectric element is attached to a structure, it is strained as the structure deforms and converts a portion of the energy associated with vibration into electrical energy. The piezoelectric element (which behaves electrically as a capacitor), in combination with a network of electrical elements connected to it (a shunt network), comprises an electrical system that can be configured to accomplish vibration control through its treatment of electrical energy. Four basic kinds of shunt circuits are typically used: resistive, inductive, capacitive, and switched. Each of these kinds of shunts results in characteristically different dynamic behavior: a resistive shunt dissipates energy through Joule heating, which has the effect of structural damping. An inductive shunt results in a resonant LC circuit, the behavior of which is analogous to that of a mechanical vibration absorber (tuned mass damper). A capacitive shunt changes the effective stiffness of the piezoelectric element, which can be used to advantage in, for example, a tunable mechanical vibration absorber. A switched shunt offers the possibilities of controlling the energy transfer to reduce frequency-dependent behavior, or perhaps the conversion of energy to a usable form. This paper reviews recent research related to the use of shunted piezoelectric materials for vibration damping and control.

Controlling Buildings: A New Frontier in Feedback

Abstract: The protection of civil structures, including their material contents and human occupants, is without doubt a worldwide priority of the most serious current importance. Such protection may range from reliable operation and comfort, on the one hand, to survivability on the other. Examples of such structures leap to one's mind, and include buildings, offshore rigs, towers, roads, bridges, and pipelines. In like manner, events that cause the need for such protective measures are earthquakes, winds, waves, traffic, lightning, and-today, regrettably-deliberate acts. Indications are that control methods will be able to make a genuine contribution to this problem area, which is of great economic and social importance. In this article, we review the rapid recent developments which have been occurring in the area of controlled civil structures, including full-scale implementations, actuator types and characteristics, and trends toward the incorporation of more modern algorithms and technologies.

Structural and Mechanical Damage Detection Using Wavelets

Abstract: Damage detection has become an essential part of various research and maintenance activities in mechanical, aerospace, and civil engineering Many of the last advances in mechanical and structural damage detection are related to new developments in the area of advanced signal processing Over the past 10 years, wavelet theory in particular has been one of the emerging and fast-evolving mathematical and signal processing tools for vibration analysis This paper is an attempt to collate in one place some of the recent advances in wavelet analysis for damage detection

Fault diagnosis of rotating machinery

Abstract: This paper aims to provide a broad review of the state of the art in fault diagnosis techniques, with particular regard to rotating machinery. Fault diagnosis is a subject too wide ranging to allow a comprehensive coverage of all of the areas associated with this field to be undertaken, and it is not the authors' intention to do so. However, a general overview of the broader issues of fault diagnosis is provided, and several of the various methodologies are discussed. A detailed review of the subject of fault diagnosis in rotating machinery is then presented. Special treatment is given to the areas of mass unbalance, bowed shafts, and cracked shafts, these being among the most common rotor-dynamic faults. Vibration response measurements yield a great deal of information concerning any faults within a rotating machine, and many of the methods using this technique are reviewed.

Laser Doppler vibrometry : A review of advances and applications

Abstract: The use of laser Doppler vibrometry (LDVi) offers great potential for the improvement of the investigation capability of experimental vibration testing. Because of this, this technique is studied and applied with increasing interest in several industrial and scientific areas of research, including biomedical engineering. In particular, the traditional fields of vibration testing, such as damage detection, system identification, and model updating, benefit from the use of these novel techniques. In fact, they significantly extend measurement capabilities with respect to traditional accelerometers, as they allow remote, nonintrusive, high-spatial resolution measurements with reduced testing time and increased performances (bandwidth up to 200 kHz, velocity range of ±10 m/s, resolution of about 8 nm in displacement and 0.5 μm/s in velocity). In this work, the state of the art in LDVi technique is addressed, and the new instrument configurations, such as those for in-plane and rotational vibration measurements, are described. A review of the most innovative LDVi advances is presented with reference to recent publications, and the different methodologies are categorized according to their relative fields of application. Interesting results achieved by continuously controlling the movement of scanning LDVi mirrors are shown: measurements in tracking mode on rotating objects or continuous scanning for operational mode shapes determination are examples of these activities. Also, actual limits and fields of future research are discussed. The main limitations in LDVi instrumentation are actually represented by speckle effects and poor signal-to-noise ratio when measuring on low diffusive surfaces. For these problems, the research is continuing to develop, and important improvements are expected within the next few years.

Vibration- and friction-induced instability in disks

Abstract: The simple shape and common occurrence of disks as machine elements belies a complexity of behavior that can be an important factor in the improvement of modern machine performance. The purpose of this paper is to give a thorough account of the understanding gained from recent research investigations into the dynamics of disks. Friction-induced vibration and parametric resonance phenomena are areas of research activity that have received considerable attention. The approach taken here is to build up to the discussion of complex topics such as these by first establishing the basic principles of critical speed instability, forward and backward traveling waves, and destabilization by a transverse elastic system.

Multifield Transducers, Devices, Mechatronic Systems, and Structronic Systems with Smart Materials

Abstract: Smart materials are increasingly applied to not only traditional sensors but to actuators, precision systems, adaptive or smart structures, mechatronic systems, structronic systems, and so on. The objective of this paper is to introduce the shock and vibration community to novel transducer technologies applied to complicated multifield vibration problems involving elastic, temperature, electric, magnetic, and light interactions. The active materials include piezoelectrics, electro- and magnetostrictive materials, shape memory alloys, electro- and magnetorheological fluids, polyelectrolyte gels, superconductors, pyroelectrics, photostrictive materials, photoferroelectrics, and magnetooptical materials. This paper provides an overview of these popular active materials and their applications to transducers (sensors/actuators), devices, precision mechatronic systems, and structronic systems. Note that the emphasis is placed on their fundamental characteristics, histories, material varieties, patents, and engineering applications.

Reliability of vibrating structures with uncertain inputs

Abstract: 10 6 Summary 12 Abstract We evaluate the reliability of vibrating systems subject to severely deficient information about the dynamic loads. We stress non-probabilistic information-gap models of uncertainty, which are adapted to severe lack of information. When some probabilistic information is avail- able, we show how it can be incorporated in a hybrid probabilistic/info-gap analysis. We outline the theory of robust reliability, which replaces probabilistic reliability in those situations where prior information is insufficient to verify the choice of a probability density. We also illustrate a hybrid probabilistic/info-gap reliability analysis. Finally, we use the "gambler's theorem" and the idea of aversion to risk to provide an overall quantitative assessment of the performance of a system in an uncertain environment. 1 Modelling the Unknown "Prediction", said Niels Bohr, "is always difficult, especially of the future." But we act all the time on suppositions extrapolated from incomplete information. From coin-flips to international conflicts, we predict outcomes based on partial information. When we have extensive experience, like in ambient vibrations under known and controlled conditions, we can make reliable asser- tions. But in unique and unfamiliar circumstances we have severely limited prior knowledge, so we must be much more circumspect. In analyzing the reliability of critical components and systems with respect to rare and extraordinary events, about which we know very little, we must avoid unverifiable assumptions as much as possible. In particular, we must represent the uncertainties as reliably as possible, without extraneous assumptions. In this paper we discuss a method of reliability analysis which is developed for this purpose. There is no free lunch, informationally speaking, so an analysis based on limited prior information will be able to make only modest predictions. However, the crucial point is that the analysis itself be reliable and not illusory.

A Survey of Nonlinear Analysis Tools for Structural Systems

Abstract: After a brief introduction to nonlinear behavior in structural dynamics, a number of strategies are listed for dealing with such situations. The methods employed for the detection, identification, and quantification of structural nonlinearities are surveyed by outlining several analytical and experimental procedures. These procedures are discussed in terms of their range of usefulness and ease of application. Nonlinear modal analysis techniques that use measured nonlinear frequency response data are studied in some detail, and their potential suitability to deal with practical cases is illustrated by means of an example. Throughout the article, special emphasis is placed on compatibility with existing linear analysis techniques and finite element methodology. Finally, current requirements and future trends are also discussed.

A wave based prediction technique for vibro-acoustic systems with cylindrical shell components

Abstract: In a coupled vibro-acoustic analysis, the structural finite element model and acoustic finite or boundary element model must be solved simultaneously. This results in a very large, nonsymmetrical coupled model, which requires a computationally expensive solving procedure. To reduce the size of a coupled model, a wave based modelling technique has been developed. Instead of dividing the structural and acoustic domain into small elements and solving the dynamic equations within each element using simple approximating shape functions, the entire structural and acoustic domain are described by wave functions, which are exact solutions of the structural and acoustic homogeneous wave equation. To these wave functions, particular solutions of the inhomogeneous wave equations are added, so that the governing dynamic equations are exactly satisfied. The contributions of the wave functions to the coupled vibro-acoustic response are determined by applying the boundary conditions in a weighted residual formulation. In this paper, the method is applied to the two-dimensional cases of an acoustic cavity, of which the whole boundary surface consists of a force excited cylindrical shell structure, as well as a cavity, of which only a part of the boundary surface consists of a force excited cylindrical shell section.