Showing papers in "Smart Materials and Structures in 2001"

Modal identification of output-only systems using frequency domain decomposition

Abstract: In this paper a new frequency domain technique is introduced for the modal identification of output-only systems, i.e. in the case where the modal parameters must be estimated without knowing the input exciting the system. By its user friendliness the technique is closely related to the classical approach where the modal parameters are estimated by simple peak picking. However, by introducing a decomposition of the spectral density function matrix, the response spectra can be separated into a set of single degree of freedom systems, each corresponding to an individual mode. By using this decomposition technique close modes can be identified with high accuracy even in the case of strong noise contamination of the signals. Also, the technique clearly indicates harmonic components in the response signals.

Ionic polymer-metal composites: I. Fundamentals

Abstract: This paper, the first in a series of four review papers, presents a brief summary of the fundamental properties and characteristics of ionic polymeric-metal composites (IPMCs) as biomimetic sensors, actuators and artificial muscles. The forthcoming three review papers, to follow this paper, will address in detail such fundamentals and, in particular, manufacturing techniques and the electronic and electromechanical characteristics of IPMCs (part II), the phenomenological modelling of the underlying sensing and actuation mechanisms in IPMCs (part III) and the potential application areas for IPMCs (part IV). This paper is a summary of all recent findings and current state-of-the art manufacturing techniques, phenomenological laws and mechanical and electrical characteristics. A number of methodologies in developing high-force-density IPMCs are also reported.

Microelectromechanical systems (MEMS):fabrication, design and applications

Abstract: Micromachining and micro-electromechanical system (MEMS) technologies can be used to produce complex structures, devices and systems on the scale of micrometers. Initially micromachining techniques were borrowed directly from the integrated circuit (IC) industry, but now many unique MEMS-specific micromachining processes are being developed. In MEMS, a wide variety of transduction mechanisms can be used to convert real-world signals from one form of energy to another, thereby enabling many different microsensors, microactuators and microsystems. Despite only partial standardization and a maturing MEMS CAD technology foundation, complex and sophisticated MEMS are being produced. The integration of ICs with MEMS can improve performance, but at the price of higher development costs, greater complexity and a longer development time. A growing appreciation for the potential impact of MEMS has prompted many efforts to commercialize a wide variety of novel MEMS products. In addition, MEMS are well suited for the needs of space exploration and thus will play an increasingly large role in future missions to the space station, Mars and beyond. (Some figures in this article are in colour only in the electronic version)

Damage diagnosis using time series analysis of vibration signals

Abstract: A novel time series analysis is presented to locate damage sources in a mechanical system, which is running in various operational environments. The source of damage is located by solely analyzing the acceleration time histories recorded from a structure of interest. First, a data normalization procedure is proposed. This procedure selects a reference signal that is `closest' to a newly obtained signal from an ensemble of signals recorded when the structure is undamaged. Second, a two-stage prediction model (combining auto-regressive (AR) and auto-regressive with exogenous inputs (ARX) techniques) is constructed from the selected reference signal. Then, the residual error, which is the difference between the actual acceleration measurement for the new signal and the prediction obtained from the AR-ARX model developed from the reference signal, is defined as the damage-sensitive feature. This approach is based on the premise that if there were damage in the structure, the prediction model previously identified using the undamaged time history would not be able to reproduce the newly obtained time series measured from the damaged structure. Furthermore, the increase in residual errors would be maximized at the sensors instrumented near the actual damage locations. The applicability of this approach is demonstrated using acceleration time histories obtained from an eight degrees-of-freedom mass-spring system.

Vibration-based damage detection in civil engineering: excitation sources and temperature effects

Abstract: This paper discusses two very relevant practical issues in the application of vibration-based health monitoring to civil engineering structures: the excitation source and the effect of temperature. The idea of vibration-based damage detection is to measure dynamic characteristics such as eigenfrequencies, damping ratios and mode shapes on a regular basis. The state, and eventually degradation, of the structure is reflected in the evolution of these characteristics. Unfortunately, it is not only the health of a structure that influences its measurable dynamics, but also the applied excitation and the changing temperature are important factors and may erode the damage detection potential. In the first part, the results of different excitation types are compared: band-limited noise generated by shakers, an impact from a drop weight and ambient sources such as wind and traffic. In the second part, the undeniable effect of temperature on measured eigenfrequencies is demonstrated and a methodology is proposed to distinguish these temperature effects from real damage events. The method could be validated on a unique data set from a bridge that was artificially damaged after a one-year monitoring period.

Micromechanical analysis of fully coupled electro-magneto-thermo-elastic multiphase composites

Abstract: A homogenization micromechanical method is employed for the prediction of the effective moduli of electro-magneto-thermo-elastic composites. These include the effective elastic, piezoelectric, piezomagnetic, dielectric, magnetic permeability and electromagnetic coupling moduli, as well as the effective thermal expansion coefficients and the associated pyroelectric and pyromagnetic constants. Comparisons between the present homogenization theory, the generalized method of cells and the Mori-Tanaka predictions are given. Results are presented for fibrous and periodically bilaminated composites.

Structural health monitoring of innovative bridges in Canada with fiber optic sensors

Abstract: This paper describes the development and application of fiber optic sensors for monitoring bridge structures. Fiber Bragg gratings (FBGs) have been used to measure static and dynamic loads on bridge decks and columns, including composite repairs for rehabilitation purposes. A new long gage concept that permits overall average strains to be measured has also been developed with gage lengths varying from 1-20 m. These gages can be bonded to the concrete structure or imbedded in the composite repair patch. Six projects undertaken by ISIS Canada to incorporate fiber optic sensing to monitor the structural health of bridges in Canada are described. Data will be presented for several bridges that indicate a measure of system reliability over several years in a hostile environment. The benefits of fiber optic sensors will be highlighted.

Attenuation and localization of wave propagation in rods with periodic shunted piezoelectric patches

Abstract: Shunted piezoelectric patches are periodically placed along rods to control the longitudinal wave propagation in these rods. The resulting periodic structure is capable of filtering the propagation of waves over specified frequency bands called stop bands. The location and width of the stop bands can be tuned, using the shunting capabilities of the piezoelectric materials, in response to external excitations and to compensate for any structural uncertainty. A mathematical model is developed to predict the response of a rod with periodic shunted piezoelectric patches and to identify its stop band characteristics. The model accounts for the aperiodicity, introduced by proper tuning of the shunted electrical impedance distribution along the rod. Disorder in the periodicity typically extends the stop bands into adjacent propagation zones and, more importantly, produces the localization of the vibration energy near the excitation source. The conditions for achieving localized vibration are established and the localization factors are evaluated for different levels of disorder on the shunting parameters. The numerical predictions demonstrate the effectiveness and potentials of the proposed treatment that requires no control energy and combines the damping characteristics of shunted piezoelectric films, the attenuation potentials of periodic structures, and the localization capabilities of aperiodic structures. The theoretical investigations presented in this paper provide the guidelines for designing tunable periodic structures with high control flexibility where propagating waves can be attenuated and localized.

A self-powered mechanical strain energy sensor

Abstract: With the growing use of sensors in various structural and mechanical systems, the powering and communication of these sensors will become a critical factor. Wireless communication electronics are becoming ubiquitous and with the decreasing electrical power requirements for these circuits it is now feasible to generate power from the conversion of mechanical energy into electrical energy. This paper focuses on the theoretical and experimental analysis of a simple mechanical strain energy sensor with wireless communication. A simple beam bending experiment is given to illustrate some of the characteristics of the self-powered strain energy sensor.

Structural health monitoring system based on diffracted Lamb wave analysis by multiresolution processing

Abstract: A health monitoring system is presented composed of integrated disc-shaped, 100 µm thick and 5 mm diameter piezoelectric transducers (PZTs) working sequentially as Lamb wave emitters and receivers. The diagnostic is based on the analysis of Lamb wave signals recorded before and after damage. In the composite, delaminations are discontinuities producing mode conversion processes generating various outgoing modes. The multiresolution processing allows the isolation of various propagation modes and their extraction in order to measure, for various propagation paths, the time delay between the arrivals of the main burst and of a specific outgoing mode. This process permits, with good accuracy, the localization of damage and the estimation of its extent. The robustness and portability of this technique is demonstrated by the fact that, after validation in our laboratory, it was successfully applied to data coming from an experiment conducted in another laboratory using its own acousto-ultrasonic health monitoring hardware system.

A novel thick-film piezoelectric micro-generator

Abstract: The use of alternative electrical energy sources to batteries is of particular significance to remote sensor systems. A vibration-powered micro-generator, based on a screen printed piezoelectric material, is proposed for this purpose. Theoretical and experimental results show that 2 µW can be generated for a vibration frequency of only 80 Hz. The device is not optimized and significant improvements are envisaged in the future.

Active-passive hybrid piezoelectric networks for vibration control: comparisons and improvement

Abstract: In this research, the passive damping and active control authority of several basic active-passive hybrid piezoelectric networks are analysed and compared. The comparison is performed in a nondimensionalized manner, throughout which the importance of the generalized electro-mechanical coupling coefficient is highlighted. It is concluded that these configurations yield very similar open-loop performance for the same electro-mechanical coupling. It is shown that larger electro-mechanical coupling leads to higher passive network damping and, depending on the design and configuration, could also derive better active authority and overall performance. A method of increasing the electro-mechanical coupling coefficient by using a negative capacitance circuit is proposed, analysed and experimentally verified.

Analysis of piezoelectric coupled circular plate

Abstract: This paper deals with the vibration analysis of a circular plate surface bonded by two piezoelectric layers, based on the Kirchhoff plate model. The form of the electric potential field in the piezoelectric layer is assumed such that the Maxwell static electricity equation is satisfied. The validation of the theoretical model is done by comparing the resonant frequencies of the piezoelectric coupled circular plate obtained by the theoretical model and those obtained by finite-element analysis. The mode shape of the electric potential obtained from free vibration analysis is generally shown to be non-uniform in the radial direction in contrast to what is commonly assumed. The piezoelectric layer is shown to have an effect on the frequencies of the host structure. The proposed model for the analysis of a coupled piezoelectric circular plate provides a means to obtain the distribution of electric potential in the piezoelectric layer. The model provides design reference for piezoelectric material application, such as an ultrasonic motor.

Diagnostic Lamb waves in an integrated piezoelectric sensor/actuator plate: analytical and experimental studies

Abstract: The objective of this study is to model the diagnostic transient waves in an integrated piezoelectric sensor/actuator plate with a view to using it as a first step towards establishing an entire structural health monitoring system and to provide experimental verification of the proposed models. PZT ceramic disks are surface mounted on an aluminum plate acting as both actuators and sensors to generate and collect A0 mode Lamb waves. Mindlin plate theory is adopted to model the propagating waves by taking both transverse shear and rotary inertia effects into account. Actuator and sensor models are both proposed. The interaction between an actuator and the host plate is modeled based on classical lamination theory. The converse piezoelectric effect of the actuator is treated as an equivalent bending moment applied to the host plate. The sensor acts as a capacitor that converts the sensed strain change into a voltage response. An analytical expression for the sensor output voltage in terms of the given input excitation signal is derived, and then experimental work is performed to verify the accuracy of the analytical model. Experimental results show that single-mode Lamb waves in the plate can be successfully generated and collected through the integrated PZT disks. The experiment also shows that the predicted sensor output for both amplitude and phase agrees well with experimentally collected data.

Impact identification of stiffened composite panels: I. System development

Abstract: An investigation was conducted to develop a real-time identification technique for the prediction of the location and force history of low-velocity impacts on composite panels with beam stiffeners. The investigation included both analysis and experiments. Measurements from built-in piezoceramic sensors were used to reconstruct the force history using the smoother-filter algorithm. Impact location was determined by minimizing the difference between modeled response and actual response. Computer codes were developed for model construction (WFMODEL) and impact identification (WIDIMPACT). The model allowed for the variation of plate structural properties (density and stiffness) due to the presence of stiffeners. Results of extensive experiments verified the model, as coded in WFMODEL, for impacts both in the bay and on the stiffener. The identification technique, as implemented in the code WIDIMPACT, demonstrated real-time capability. Good agreement was found between predicted and actual force history and location, with success for different impact masses.

Ways and options for aircraft structural health management

Abstract: The number of aircraft is increasing worldwide as well as their age. This has led to an increasing market of aging aircraft. The older an aircraft structure becomes, the more difficult it may be to predict under which conditions it will be flown and this especially when it is influenced by continuous upgrades in avionics, flight control systems or even engines, thus resulting in a possible change of flight envelopes. It is under these conditions that the aircraft structure needs increased care regarding monitoring and subsequent life management. This paper addresses the different means of how this can be done, starting with conventional inspection and gradually moving over to state-of-the-art loads and finally damage monitoring, where the latter is very much driven by ongoing smart materials and structures initiatives. Benefits either obtained or still to be expected from the different ways of monitoring are described.

A critical analysis of electric shunt circuits employed in piezoelectric passive vibration damping

Abstract: Vibration damping of an elastic structure can be obtained by bonding a piezoelectric actuator onto the vibrating structure, and by shunting the actuator to a suitable electric resonant circuit, tuned to the structural eigenmode to be damped. The achievable damping depends on the particular electric circuit used. In this paper three different electric shunt circuits are analyzed and their performances are compared. In particular, the optimal values of the electric components belonging to each shunt circuit and the corresponding exponential time decay rates of the free vibrations relevant to the controlled structural eigenmode are explicitly determined, also taking into account the inherent structural damping. Finally, some experimental results are reported, obtained by using two of the shunt circuits analyzed, which are in good agreement with the theoretical predictions.

Application of ARMAV models to the identification and damage detection of mechanical and civil engineering structures

Abstract: In this paper, the application of auto-regressive moving average vector models to system identification and damage detection is investigated. These parametric models have already been applied for the analysis of multiple input-output systems under ambient excitation. Their main advantage consists in the capability of extracting modal parameters from the recorded time signals, without the requirement of excitation measurement. The excitation is supposed to be a stationary Gaussian white noise. The method also allows the estimation of modal parameter uncertainties. On the basis of these uncertainties, a statistically based damage detection scheme is performed and it becomes possible to assess whether changes of modal parameters are caused by, e.g. some damage or simply by estimation inaccuracies. The paper reports first an example of identification and damage detection applied to a simulated system under random excitation. The `Steel-Quake' benchmark proposed in the framework of COST Action F3 `Structural Dynamics' is also analysed. This structure was defined by the Joint Research Centre in Ispra (Italy) to test steel building performance during earthquakes. The proposed method gives an excellent identification of frequencies and mode shapes, while damping ratios are estimated with less accuracy.

Vibration-based damage assessment utilizing state space geometry changes: local attractor variance ratio

Abstract: A novel feature extracted from a nonlinear time series is presented within the context of vibration-based damage detection in a system An eight-degree-of-freedom spring-mass-damper `structure' is considered with damage incurred by a stiffness degradation in one spring The system is excited with a chaotic input, and by tuning the Lyapunov exponents of the chaotic excitation to the dominant eigenvalue of the structure the dimensionality of the entire system is effectively controlled Both the input and output are viewed in state space as geometric objects, and the effect of the damage is shown to alter the geometric properties of the corresponding attractors at a local level, which may be captured in construction of the feature The utility of the feature is compared with that of a number of modal-based features and shown to be superior in resolving capability and in robustness

Wind tunnel testing of a Mach-scaled rotor model with trailing-edge flaps

Abstract: This paper presents the wind tunnel testing of a four-bladed Mach-scaled rotor model with piezoelectric bender actuated trailing-edge flaps. Correctly phased, this flap motion can be used for the active suppression of vibratory hub loads. First, the University of Maryland Advanced Rotorcraft Code was used to conduct a parametric study to determine the optimal flap sizing and location as well as the required deflection amplitudes. Next, a simplified rotor analysis which explicitly models the actuator dynamics was used to design a multi-layer actuator configuration that is capable of achieving the required flap deflections. Based on the above design studies, a matched set of four Mach-scaled rotor blades with piezo-bender actuated trailing-edge flaps were fabricated in-house. This rotor model was operated using a one-seventh scale Bell-412 Mach-scaled rotor hub. Finally, the rotor system was tested in forward flight in the Glenn L Martin wind tunnel. These tests consisted of open-loop single-frequency tests at different rotor speeds and collective settings. Some preliminary closed-loop tests using a neural network control algorithm were also conducted. Presented at the American Helicopter Society 56th Annual Forum, Virginia Beach, VA. Copyright 2000 by the American Helicopter Society, Inc. All rights reserved.

Damage identification using support vector machines

Abstract: Support vector machines have recently been established as a powerful tool for classification and regression problems Their use in the field of damage identification is illustrated here with reference to two problems which can be naturally cast in terms of classification The first is a fault classification problem for ball bearings and the second looks at locating damage within a framework structure The performance is compared to more established means of engineering pattern recognition

Damage identification on the Z24 bridge using vibration monitoring

Abstract: In the framework of developing a non-destructive damage identification technique, vibration monitoring is a useful evaluation tool that relies on the fact that the occurrence of damage in a structural system leads to changes in its dynamic properties. The damage identification techniques are based on the observed shifts in eigenfrequencies and modeshapes and relate the dynamic characteristics to a damage pattern of the structure. The presented technique makes use of the calculation of modal bending moments and curvatures to derive the bending stiffness at each location. Damage identification results are compared with results from a classical sensitivity based updating technique. The basic assumption in both techniques is that damage can be directly related to a decrease of stiffness in the structure. Damage assessment techniques are validated on the progressively damaged prestressed concrete bridge Z24 in Switzerland, tested in the framework of the BRITE-EURAM project SIMCES. A series of full modal surveys are carried out on the bridge before and after applying a number of damage scenarios.

Influence of long-term storage in cold hibernation on strain recovery and recovery stress of polyurethane shape memory polymer foam

Abstract: In this paper, the effects of long-term storage in compressed cold hibernated elastic memory (CHEM) polyurethane foam, a kind of shape memory polymer, are investigated experimentally. The foams were pre-strained at a high temperature, which was above the glass transition temperature, to 80% and 93.4%, respectively, and then cooled back to room temperature. After various periods of cold hibernation (up to two months), they were heated up at fixed length or against different constant loads. It is found that: (1) the maximum stress that the foam could exert at fixed length depends heavily on the amount of pre-strain; (2) expansion rates of 380 and 1273% from the hibernated size against a 1 N load (pre-strained by 80 and 93.4%, respectively) are achievable. However, upon further increases in load, the expansion is reduced dramatically. It appears that the tested CHEM polyurethane foam retains its shape memory properties even after being stored in a compacted state for a long period. Complete strain recovery is attainable for a hibernation period of up to two months.

Development of a piezoelectric actuator for trailing edge flap control of full scale rotor blades

Abstract: The present paper covers the development of a piezoelectric actuator for trailing edge flap control on a 34 ft diameter helicopter main rotor. The design of an actuator using biaxial stack columns, and its bench, shake, and spin testing are described. Actuator bench testing proves the basic actuator concept, but also points to required performance improvements. Actuator robustness is demonstrated in shake and spin tests simulating the full range of dynamic conditions inside the rotor blade. A series of actuator improvements are implemented, resulting in almost doubled performance. Projections using the latest stack technology show that the improved actuator will meet the performance requirements. The next steps in this program are development of the actuator and full scale rotor system for whirl tower testing and flight testing on the MD Explorer. Presented at SPIE's Symposium on Smart Structures and Materials, Paper Number 3668-104, Newport Beach, 1999.

Structural health monitoring by electrical resistance measurement

Abstract: Structural health monitoring by dc electrical resistance measurement is reviewed. The technique is valuable for evaluating composites and joints, provided that the materials involved are not all electrically insulating. The measurement pertains to either the volume electrical resistivity of a bulk material or the contact resistivity of an interface, and it can be performed in real time during loading or heating.

Vibration control of smart piezoelectric composite plates

Abstract: This study on the vibration control of smart piezoelectric composite plates investigates the effect of the stretching-bending coupling of the piezoelectric sensor/actuator pairs on the system stability of smart composite plates. Based on first-order shear theory and consistent methodology, a smart isoparametric finite element is formulated and the classical negative velocity feedback control method is adopted for the active vibration control analysis of smart composite plates with bonded or embedded distributed piezoelectric sensors and actuators. It is shown mathematically and demonstrated numerically that generally the coupling effect tends to result in system instability unless the sensor/actuator pairs are collocated or the plate simply supported. The result of this study can be used to aid the placement of piezoelectric sensor/actuator pairs of smart composite plates as well as for robust controller design.

Ship hull structure monitoring using fibre optic sensors

Abstract: We discuss the need for ship hull monitoring and the roles such a system may fill during the different stages of a ship's lifetime. We have found that fibre optic sensors are well suited for this application and present the fibre Bragg grating technology that has been employed in the composite hull embedded sensor system project (CHESS). Signal processing is the key to real-time structure monitoring, and we comment on the modular signal processing system that is being developed at FFI. Finally we present the CHESS installation on a new Norwegian naval vessel and show some results obtained with this system in systematic sea-keeping tests. The CHESS measurements on the new Norwegian fast patrol boat have been instrumental in the design verification of the vessel.

Structural health monitoring from fiber-reinforced composites to steel-reinforced concrete

Abstract: Accurate interpretation of sensor measurements in terms of physical changes in structures is a major challenge for the development of robust structural health monitoring systems. An active diagnostic system was proposed to detect embedded damage in fiber-reinforced composites and steel-reinforced concrete. Due to the inherent difference in material characteristics, changes in sensor measurements resulting from damage for the two material systems were found considerably different. For a given excitation, it was found that local delamination in fiber-reinforced composites reduced the measured signal strength of a nearby sensor, while rebar debond in concrete increased signal strength. Techniques based on the characteristics of these material responses to damage are being developed for adequate active sensing diagnostic systems for each material.

Thermal behavior of a metal embedded fiber Bragg grating sensor

Abstract: With embedded sensors it is possible to monitor structural parameters at critical locations which are not accessible to ordinary sensors. Recently, the fiber optic sensor has emerged as a promising technology to be integrated with structures. The embedding of fiber optic sensors into composites and some metals, especially those with low melting points, have been reported. However, all reported embedding techniques so far are either complicated or it is difficult to achieve coherent bonding with low residue stresses. Thus, it is of interest to pursue some economical ways to embed fiber optic sensors into metallic structures with low residue stresses. In this work, a new technique is proposed for embedding a fiber optic sensor into metallic structures, such as nickel, with minimized residue stress. Fiber Bragg grating (FBG) sensors have been embedded into nickel structures. The thermal performance of such an embedded FBG sensor is studied. Higher temperature sensitivity is demonstrated for the embedded FBG sensors. For temperature measurements, the embedded FBG sensor yields an accuracy of about 2 °C. Under rapid temperature changes, it is found that thermal stresses due to the temperature gradient in the metallic structures would be the main cause for errors.

In-flight tracking of helicopter rotor blades using shape memory alloy actuators

Abstract: This paper describes the concept, design, fabrication and control of a shape memory alloy (SMA) wire actuator for tracking helicopter rotor blades while in-flight. A NACA 0012 wing section that has a 12 in chord and span was constructed with a trailing-edge tab with a 4 in span and a 2.4 in chord. A shape memory alloy wire actuator was embedded into the wing section. The actuator consists of a wire clamp, a hinge tube and several pre-strained, 0.015 in diameter SMA (Nitinol) wires. It was shown that with SMA wires that have 3.158% initial pre-strain, a tab deflection of 29° could be obtained.