KEYWORDS: Microsoft Foundation Class Library, Transducers, Aluminum, Energy harvesting, Electrodes, Sensors, Epoxies, Composites, Structural health monitoring, Nickel
Over the last decade, cantilever energy harvesters gained immense popularity owing to the simplicity of the design and piezoelectric energy harvesting (PEH) using the cantilever design has undergone considerable evolution. The major drawback of a vibrating cantilever beam is its vulnerability to fatigue over a period of time. This article brings forth an experimental investigation into the phenomenon of fatigue of a PEH cantilever beam. As there has been very little literature reported in this area, an effort has been made to scrutinize the damage due to fatigue in a linear vibrating cantilever PEH beam consisting of an aluminum substrate with a piezoelectric macro-fiber composite (MFC) patch attached near the root of the beam and a tip mass attached to the beam. The beam was subjected to transverse vibrations and the behavior of the open circuit voltage was recorded with passing time. Moreover, electro-mechanical admittance readings were obtained periodically using the same MFC patch as a Structural health monitoring (SHM) sensor to assess the health of the PEH beam. The results show that with passing time the PEH beam underwent fatigue in both the substrate and MFC, which is observed in a complimentary trend in the voltage and admittance readings. The claim is further supported using the variation of root mean square deviation (RMSD) of the real part of admittance (conductance) readings. Thus, this study concludes that the fatigue issue should be addressed in the design of PEH for long term vibration energy harvesting.
Lamb wave based Structural Health Monitoring (SHM) has received much attention during the past decades for its broad coverage and high sensitivity to damage. Lamb waves can be used to locate and quantify damage in static structures successfully. Nonetheless, structures are usually subjected to various external vibrations or oscillations. Not many studies are reported in the literature concerning the damage detecting ability of Lamb wave in oscillating structures which turns out to be a pivotal issue in the practical application of the SHM technique. For this reason in this study, the propagating capability of Lamb waves in a vibrating thin aluminum plate is examined experimentally. Two circular shaped piezoelectric wafer active transducers are surface-bonded on the aluminum plate where one acted as an actuator and another as a sensor. An arbitrary waveform generator is connected to the actuator for the generation of a windowed tone burst on the aluminum plate. An oscilloscope is connected to the sensor for receiving the traveled waves. An external shaker is used to generate out-of-plane external vibration on the plate structure. Time of flight (TOF) is a crucial parameter in most Lamb wave based SHM studies, which measures wave traveling time from the actuator to sensor. In the present study the influence of the external vibrations on the TOF is investigated. Experiments are performed under different boundary conditions of the plate, such as free-free and fixed by gluing. The effects of external vibrations in the frequency range between 10 Hz to 1000 Hz are analyzed. Comparisons are carried out between the resulting Lamb wave signals from the vibrating plate for different boundary conditions. Experimental results show that the external vibrations in relatively low frequency range do not change the TOF during the application of Lamb wave based SHM.
KEYWORDS: Microsoft Foundation Class Library, Energy harvesting, Resistors, Energy efficiency, Superposition, Electromagnetism, Prototyping, Aluminum, Civil engineering, Mechanical engineering
Vibration energy harvesting using piezoelectric material is a promising solution for powering small electric devices, which has attracted great research interest in recent years. Numerous efforts have been done by researchers to improve the efficiency of vibration energy harvesters and to broaden their bandwidths. In most reported literature, harvesters are designed to harvest energy from vibration source with a specific excitation direction. However, a practical environmental vibration source may include multiple components from different directions. Thus, it is an important concern to design a vibration energy harvester to be adaptive to multiple excitation directions. In this article, a novel piezoelectric energy harvester with frame configuration is proposed to address this issue. It can work either in its vertical vibration mode or horizontal vibration mode. Therefore, the harvester can capture vibration energy from arbitrary directions in a twodimensional plane. Experimental studies are carried out to prove the feasibility for multiple-direction energy harvesting using such harvester. The development of this two-dimensional energy harvester indicates its promising potential in practical vibration scenarios.
KEYWORDS: Energy harvesting, Magnetism, Complex systems, Oscillators, Environmental sensing, Transducers, Electromagnetism, Microsoft Foundation Class Library, Civil engineering, Electronics
Vibration energy harvesting using piezoelectric material has received great research interest in the recent years. To enhance the performance of piezoelectric energy harvesters, one important concern is to increase their operating bandwidth. Various techniques have been proposed for broadband energy harvesting, such as the resonance tuning approach, the frequency up-conversion technique, the multi-modal harvesting and the nonlinear technique. Usually, a nonlinear piezoelectric energy harvester can be easily developed by introducing a magnetic field. Either mono-stable or bi-stable response can be achieved using different magnetic configurations. However, most of the research work for nonlinear piezoelectric energy harvesting has focused on the SDOF cantilever beam. A recently reported linear 2-DOF harvester can achieve two close resonant frequencies with significant power outputs. However, for this linear configuration, although a broader bandwidth can be achieved, there exists a deep valley in-between the two response peaks. The presence of the valley will greatly deteriorate the performance of the energy harvester. To overcome this limitation, a nonlinear 2-DOF piezoelectric energy harvester is proposed in this article. This nonlinear harvester is developed from its linear counterpart by incorporating a magnetic field using a pair of magnets. Experimental parametric study is carried out to investigate the behavior of such harvester. With different configurations, both mono-stable and bi-stable behaviors are observed and studied. An optimal configuration of the nonlinear harvester is thus obtained, which can achieve significantly wider bandwidth than the linear 2-DOF harvester and at the same time overcome its limitation.
KEYWORDS: Energy harvesting, Magnetism, Complex systems, Transducers, Sensors, Bessel functions, Aluminum, MATLAB, Instrument modeling, Microsoft Foundation Class Library
The need for long-term solutions to power various wireless sensor systems has been driving the research in the area of energy harvesting for the past decade. The present paper brings forth an investigation into the realm of piezoelectric energy harvesting (PEH) using nonlinear vibrations. A piezoelectric cantilever beam with a magnetic tip mass interacting with additional magnets around it forms a multi-stable nonlinear PEH configuration. The study indicates that the multistable configuration provides a widened bandwidth as compared to the conventional linear PEH devices and an increased voltage output as compared to many other PEH devices. An experimental parametric study is conducted to arrive at an optimal configuration for the performance enhancement of the harvester along with a glimpse into the enhanced magnetostatic interactions equations and various possible magnetic nonlinear configurations for the given conditions.
KEYWORDS: Sensors, Actuators, Wave propagation, Microsoft Foundation Class Library, Transducers, Structural health monitoring, Wave plates, Ultrasonics, Inspection, Composites
In cylindrical structures such as pipelines and pressure vessels, cracks are most likely to occur along the longitudinal
(axial) direction and they can be fatal to the serviceability of the structures. Unfortunately, the conventional ultrasonic
crack detection techniques, which usually use longitudinal wave, are not very sensitive to this type of cracks. This paper
focuses on the detection and monitoring of axial cracks in cylindrical structures using torsional wave generated by
piezoelectric macro-fiber composite (MFC). The first order torsional wave is a kind of non-dispersive pure shear wave
which propagates at a fixed wave speed. Torsional wave is utilized in this work because, intuitively, it is more sensitive
to axial cracks than the family of longitudinal waves. Numerical simulation has been performed using ANSYS to show
the effectiveness of torsional wave in detecting and monitoring axial cracks. The time of flight (TOF) of the waves is
used to determine the crack position, while the crack propagation is monitored by measuring the variation in the crack
induced disturbances. Experiments have also been conducted to investigate the feasibility of the proposed method. MFC
transducers oriented at 45° against the axis of the specimen are used to generate and receive torsional waves. The
experimental results demonstrated that the crack position can be indentified and its growth can be well monitored with
the presented approach using torsional wave.
Fatigue cracks often initiate at the weld toes of welded steel connections. Usually, these cracks cannot be identified by
the naked eyes. Existing identification methods like dye-penetration test and alternating current potential drop (ACPD)
may be useful for detecting fatigue cracks at the weld toes. To apply these non-destructive evaluation (NDE) techniques,
the potential sites have to be accessible during inspection. Therefore, there is a need to explore other detection and
monitoring techniques for fatigue cracks especially when their locations are inaccessible or cost of access is
uneconomical.
Electro-mechanical Impedance (EMI) and Lamb wave techniques are two fast growing techniques in the Structural
Health Monitoring (SHM) community. These techniques use piezoelectric ceramics (PZT) for actuation and sensing.
Since the monitoring site is only needed to be accessed once for the instrumentation of the transducers, remote
monitoring is made possible. The permanent locations of these transducers also translate to having consistent
measurement for monitoring.
The main focus of this study is to conduct a comparative investigation on the effectiveness and efficiency of the EMI
technique and the Lamb wave technique for successful fatigue crack identification and monitoring of welded steel
connections using piezoelectric transducers. A laboratory-sized non-load carrying fillet weld specimen is used in this
study. The specimen is subjected to cyclic tensile load and data for both techniques are acquired at stipulated intervals. It
can be concluded that the EMI technique is sensitive to the crack initiation phase while the Lamb wave technique
correlates well with the crack propagation phase.
Fatigue is a progressive and localised damage that occurs when a material is subjected to cyclic loading. Historical cases
have shown that undetected fatigue cracks often lead to catastrophic failure, including loss of lives and assets. It is
therefore important to have a robust Structural Health Monitoring (SHM) technique to detect and monitor these cracks.
The Lamb Wave technique for SHM is promising due to its ability to interrogate a large area of the structure from only a
few locations. The feasibility of fatigue crack detection in wide specimens, where the effect of boundary reflections is
not significant in the signal processing and damage quantification process, have been investigated by other researchers7-9.
However, in a narrow structural component, the boundary reflection has a significant role in the sensor signal and the
damage quantifier from available literatures cannot be applied readily.
The main focus of this study is to investigate the feasibility of monitoring fatigue crack growth in a narrow structural
component using the Lamb Wave technique. Experimental study conducted on lab-sized aluminum beam finds that as
crack propagates amplitude of the sensor signal decreases. A damage index is proposed, and a linear relationship
between the damage index and the crack length is identified. With the proposed damage index, a crack length can be
estimated from the acquired sensor signals through a correlation factor.
KEYWORDS: Wave propagation, Microsoft Foundation Class Library, Actuators, Numerical simulations, Ultrasonics, Sensors, Composites, Structural health monitoring, Nondestructive evaluation, Dispersion
In cylindrical structures such as pipelines, cracks are more likely to occur along the longitudinal (axial) direction and
they are usually fatal to the serviceability of the structures. Unfortunately, the conventional ultrasonic crack detection
methods are not very sensitive to this type of cracks. This paper focuses on using piezoelectric macro-fiber composite
(MFC) to generate torsional wave for health monitoring of cylindrical structures. Numerical simulations are performed
using ANSYS. Nodal release method is used to model the crack. Experimental verifications are also presented. Four
pieces of MFC oriented at 45° against the axis of the specimen are used to generate both longitudinal wave and torsional
wave. The numerical results and the experimental results show that the axial-direction crack propagation in cylindrical
structures can be well monitored using the presented wave propagation approach.
Fatigue induced damage is often progressive and gradual in nature. Structures subjected to large
number of fatigue load cycles will encounter the process of progressive crack initiation, propagation
and finally fracture. Monitoring of structural health, especially for the critical components, is therefore
essential for early detection of potential harmful crack.
Recent advent of smart materials such as piezo-impedance transducer adopting the
electromechanical impedance (EMI) technique and wave propagation technique are well proven to be
effective in incipient damage detection and characterization. Exceptional advantages such as
autonomous, real-time and online, remote monitoring may provide a cost-effective alternative to the
conventional structural health monitoring (SHM) techniques.
In this study, the main focus is to investigate the feasibility of characterizing a propagating fatigue
crack in a structure using the EMI technique as well as estimating its remaining fatigue life using the
linear elastic fracture mechanics (LEFM) approach. Uniaxial cyclic tensile load is applied on a
lab-sized aluminum beam up to failure. Progressive shift in admittance signatures measured by the
piezo-impedance transducer (PZT patch) corresponding to increase of loading cycles reflects
effectiveness of the EMI technique in tracing the process of fatigue damage progression. With the use
of LEFM, prediction of the remaining life of the structure at different cycles of loading is possible.
This paper presents an easy method of embedding piezoceramic transducer (PZT) and its implementation in
electromechanical (EM) impedance based health monitoring of concrete structures. The basic principle
used in this monitoring is to record EM admittance signatures acquired from the actuations of PZT
transducer in the presence of electric field. Any deviations in these signatures during the monitoring period
indicate disturbance/ damage in the structure. The PZT can be either surface bonded or embedded, however
the important features of embedding PZT inside the host structure are durability and protection from
surface finish, vandalism and environment attacks. The embedment of PZT in the structure is not as simple
as surface bonding because there are several issues such as bonding between PZT and host structure.
Moreover, it should withstand the curing pressures and temperatures of the host material. This paper is also
expected to be useful for monitoring embeddable composite structures.
Piezoceramic (PZT) transducers are extensively used in the electromechanical impedance (EMI) based structural health
monitoring (SHM) of engineering structures. In the EMI technique, the PZT transducers are generally surface bonded to
the host structure and subjected to electric actuation, so as to interrogate the structure for the desired frequency range.
The interrogation results in the prediction of PZT electromechanical (EM) admittance signatures, from which the
mechanical impedance of the structure can be extracted. These signatures serve as indicator to predict the health/integrity
of the structure, as any change in the signature is indication of crack or damage or degradation in the structure. However
in real life, the structural components such as slabs, beams and columns are constantly subjected to some forms of
external loading. Thus, the application of EMI technique for damage assessment of structures should take into account
the influence of imposed loads since the EM admittance signature obtained for such a constantly loaded structure is
different from the one obtained when damages are present in the structure. That is, the properties of the EM admittance
signature obtained due to the imposed loads are different from that obtained due to damages. This paper presents an
experimental investigation to show their differences. The objective is to investigate the influence of loading on the EM
admittance signature. In addition, the effects of structural stiffness and EM properties of the PZT transducer on the EM
admittance signature are also studied. In the experimental test, an impedance analyzer is employed to actuate the PZT
transducer and simultaneously record the EM admittance signature. Different lab-sized specimens are loaded for various
magnitudes of external loading. The results are expected to be useful for the non-destructive evaluation of engineering
structures with imposed loads.
Piezoceramic (PZT) transducers are extensively used for damage detection in electromechanical impedance (EMI) based structural health monitoring (SHM) of engineering systems. In the EMI methods, the PZT transducers are generally surface bonded or embedded inside the host structure, and then subjected to actuation so as to interrogate the structure for the desired frequency range. The interrogation results in the prediction of electro-mechanical admittance signatures. These signatures serve as indicator of the health/integrity of the structure. The existing PZT-structure interaction models consider the PZT transducer to be negligible in mass and thus ignored it. However, for multiple PZT-structure interaction, influence of the PZT mass becomes significant as there is significant increase in the number of PZT transducers. This paper presents a novel semi-analytical multiple PZT-structure interaction model which considers the mass influence of the multiple PZT transducers. The model involves numerical modelling, modal analysis and analytical formulation of admittance signature, and thus, is semi-analytical in nature. The numerical analysis serves to obtain the structure response for use as input to the analytical equations, so as to finally predict the admittance signature. The transducers and their host structure without restricting the PZT transducers to square shaped and electrically isotropic ones. Hence, it is expected to be applicable for the non destructive evaluation (NDE) of most engineering systems. The derived model is then experimentally verified using lab sized aluminium plate.
Nondestructive evaluation (NDE) is essential in civil and building engineering. Impedance-based method uses the electro-mechanical coupling effect of piezoceramic lead-zirconate-titanate (PZT) materials to measure the force impedance of the structure. By comparing the impedance spectra of the damaged structure with the baseline (the impedance spectra for the pristine structure), the damage in the structure can be assessed. The impedance-based method has shown some advantages over the traditional NDE methods. However, it is not able to identify the location and quantity of the damage simultaneously. This paper presents a technique to overcome this limitation. The technique first measures the variations of the electro-mechanical impedance of the structure, which is similar to the impedance-based method, so that it can inherit the advantage of convenience in operation from the impedance-based method. The damage is then identified by a system identification technique which is generally employed in the vibration-based method. Due to the numerous local optima in the search space, the traditional optimization strategies may not be able to find the correct solution. This paper selects evolutionary programming (EP) as the system identification technique for its robustness in finding the global optimum. Thus, the location and the quantity of the damage can be simultaneously identified. In order to enhance the feasibility of the integrated EP and impedance-based (inEPIB) technique, a fitness function, which can be generally applied to other methods, is proposed to discriminate the variations caused by damages from the discrepancies caused by modeling errors. Experiments are carried out on beams and plates to verify the damage detection results. The results demonstrate that both the location and extent of damage can be simultaneously identified.
KEYWORDS: Transducers, Ferroelectric materials, Adhesives, Structural health monitoring, System identification, Bismuth, Systems modeling, Optical simulations, Smart materials, Analytical research
The electro-mechanical (EM) impedance method is gradually emerging as a widely accepted technique for structural health monitoring and systems identification. The method utilizes smart piezoceramic (PZT) transducers intimately bonded to the surface of a structural substrate. Through the unique electro-mechanical properties of the PZT transducers, the presence of damage, as well as the dynamical properties of the host structure are captured and reflected in the electrical admittance response. In the present work, the effect of the bond layer on the electro-mechanical response of a smart system is being studied. Experiments with the EM impedance method were performed on laboratory-sized beams. Consequently, the effects of shear lag due to the finite thickness bond layer were successfully identified. This was followed by the theoretical analysis of shear lag effects. It was found that the induced strain behavior of the structural specimen in question is inevitably modified by the presence of shear lag between the PZT transducer and the structural substrate. Subsequently, the EM admittance response of the beam specimens were simulated based on the results gathered from the theoretical analysis. Incidentally, it was found that the theoretical model clearly depicts the trends of the measured response.
In recent years, the piezoelectric-ceramic (PZT) patches are increasingly been used as impedance transducers for non-destructive evaluation (NDE) of structures. In this application, the electrical admittance of a PZT patch surface bonded to the structure is utilized as a diagnostic signature of the structure. The operating frequency is typically maintained in the kHz range for optimum sensitivity in damage detection. The electro-mechanical interaction between the host structure and the bonded patch is key to the detection of damage in this NDE technique. Although the method is well established for a wide variety of structures and material types, very little research has focused on the fundamental structure-PZT interaction. This paper reviews the fundamental electro-mechanical coupling between the structure and the PZT patch and introduces a new concept of 'active' signatures, whereby it is possible to utilize the direct interactive component of the signature for NDE afte filtering the 'inert' component. Consequences of this concept, which include increased sensitivity to damage and reduced influence of temperature fluctuations on signatures are highlighted.
Modal analysis based damage detection techniques using only first few modes are not sensitive for damage identification. The sensitivity of the modal parameters to damage is greater at the higher modes of vibration. Yet, actuation of structures at high frequencies is very difficult with the conventional modal testing methods. In this paper, a new technique that uses smart piezoelectric (PZT) material to extract the modal frequencies for higher modes of vibration is presented. A PZT transducer possesses simultaneous actuating and sensing capabilities. The electromechanical (e/m) impedance method exploits this feature of the PZT transducer to measure its drive-point impedance characteristics when bonded to a structure. Damage location is identified using the natural frequency shifts obtained from the structural impedance signatures and the corresponding undamaged state modes shapes. This technique is superior to other methods, which rely only on statistical quantification of changes in the measured structural signatures. The damage locations were successfully identified by this method for a finite element simulated beam model. The natural frequencies obtained experimentally for longitudinal and bending modes were fairly consistent with the analytical predictions. However, the modeling of damage as merely a source of stiffness reduction proves insufficient to accurately estimate its location, experimentally.
In the beginning, the electro-mechanical (EM) impedance method for structural health monitoring was recognized as a means of structural in-situ stress monitoring and measurement. Consequently, theoretical analysis based on the EM impedance method as a tool for in-situ stress identification in structural members was presented. A dynamic impedance model derived from the Euler-Bernoulli beam theory was developed to investigate the influence of in-situ stress on the dynamic and electro-mechanical response of a smart beam interrogated by a pair of symmetrically bounded, surface-bonded piezoceramic (PZT) transducers. Numerical simulation was performed for a laboratory sized smart beam subjected to a multitude of axial loads at the ends. It was found that natural frequency shifts takes place in the presence of in-situ stress. Furthermore, these shifts, which are linearly related to the magnitude of applied load, is directly reflected in the point-wise dynamic stiffness response. However, in terms of the electro-mechanical response, which can be measured directly, the shift of peaks of the EM admittance signature is not directly indicative of the natural frequency shifts. This arises as an inverse problem in engineering, which cannot be deciphered using direct approach. Back calculation of the in-situ stress using genetic algorithm (GA) was proposed.
The electro-mechanical impedance (EMI) technique, which utilizes "smart" piezoceramic (PZT) patches as collocated actuator-sensors, has recently emerged as a powerful technique for diagnosing incipient damages in structures and machines. This technique utilizes the electro-mechanical admittance of a PZT patch surface bonded to the structure as the diagnostic signature of the structure. The operating frequency is typically maintained in the kHz range for optimum sensitivity in damage detection. However, there are many impediments to the practical application of the technique for NDE of real-life structures, such as aerospace systems, machine parts, and civil-infrastructures like buildings and bridges. The main challenge lies in achieving consistent behavior of the bonded PZT patch over sufficiently long periods, typically of the order of years, under "harsh" environment. This necessitates protecting the PZT patch from environmental effects. This paper reports a dedicated investigation stretched over several months to ascertain the long-term consistency of the electro-mechanical admittance signatures of PZT patches. Possible protection of the patch by means of suitable covering layer as well as the effects of the layer on damage sensitivity of the patch are also investigated. It is found that a suitable cover is necessary to protect the PZT patch, especially against humidity and to ensure long life. It is also found that the patch exhibits a high sensitivity to damage even in the presence of the protection layer. The paper also includes a brief discussion on few recent applications of the EMI technique and possible use of multiplexing to optimize sensor interrogation time.
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