TN32 casks are multi-layer cylindrical structures used for storage of nuclear spent fuel. The National Center for Physical Acoustics at the University of Mississippi has manufactured a scaled down model of the TN32 cask. To identify the most relevant nondestructive evaluation parameters, which will be useful while doing experiments on real TN32 casks, a series of experiments have been conducted on TN32 cask model. This paper discusses the data analysis of the experiments conducted on the cask model and the conclusions based on those experiments. Elastodynamic waves are generated in the cask model by pencil lead break and hammer hit excitation and the waves in the cask at certain locations are sensed using piezoelectric wafer active sensors (PWAS). The waveforms and frequency spectrums of waveforms arriving at PWAS are studied. There are two types of joints on the cask model: structures joined using adhesives and structures joined using press fit. The effects of various joints in the structure on elastodynamic wave propagation are also studied. Pitch catch experiments on the cask was also done using in plane excitation using PWAS. The most sensitive frequency for the cask model was identified from the frequency response spectrum obtained from a wide band chirp excitation. The influence of various joints on the frequency response spectrum is also studied. Analytical modeling of cask geometry for a given excitation is done using Normal Mode Expansion (NME) technique. Prediction of wave propagation through the scaled down model is done based on the theoretical expression derived.
Structural health monitoring (SHM) is in urgent need and must be integrated into the nuclear-spent fuel storage systems to guarantee the safe operation. The dry cask storage system (DCSS) is such storage facility, which is licensed for temporary storage for nuclear-spent fuel at the independent spent fuel storage installations (ISFSIs) for certain predetermined period of time. Gamma radiation is one of the major radiation sources near DCSS. Therefore, a detailed experimental investigation was completed on the gamma radiation endurance of piezoelectric wafer active sensors (PWAS) transducers for SHM applications to the DCSS system. The irradiation test was done in a Co-60 gamma irradiator. Lead Zirconate Titanate (PZT) and Gallium Orthophosphate (GaPO4) PWAS transducers were exposed to 40.7 kGy gamma radiation. Total radiation dose was achieved in two different radiation dose rates: (a) slower radiation rate at 0.1 kGy/hr for 20 hours (b) accelerated radiation rate at 1.233 kGy/hr for 32 hours. The total cumulative radiation dose of 40.7 kGy is equivalent to 45 years of operation in DCSS system. Electro-mechanical impedance and admittance (EMIA) signatures and electrical capacitance were measured to evaluate the PWAS performance after each gamma radiation exposure. The change in resonance frequency of PZT-PWAS transducer for both in-plane and thickness mode was observed. The GaPO4-PWAS EMIA spectra do not show a significant shift in resonance frequency after gamma irradiation exposure. Radiation endurance of new high-temperature HPZ-HiT PWAS transducer was also evaluated. The HPZ-HiT transducers were exposed to gamma radiation at 1.233 kGy/hr for 160 hours with 80 hours interval. Therefore, the total accumulated gamma radiation dose is 184 kGy. No significant change in impedance spectra was observed due to gamma radiation exposure.
Nuclear dry cask storage systems are being used for extended periods of time. Structural health monitoring of these casks has grown out of concern that the radioactive waste could jeopardize the casks’ structural health as time progresses. Ultrasonic guided waves offer a potential solution for monitoring the nuclear casks structural health without opening the containers. This paper explores sensing techniques on small-scale mockup and full scale dry cask storage systems. Methods include acoustic emission (AE) as well as active sensing. Results showed accuracies in localizations, differences in sensing techniques, structural responses, and the capabilities of ultrasonic guided waves in dry cask storage systems.
There is considerable demand for structural health monitoring (SHM) at locations where there are substantial radiation fields such as nuclear reactor components, dry cask storage canister, irradiated fuel assemblies, etc. Piezoelectric wafer active sensors (PWAS) have been emerged as one of the major SHM sensing technologies. In order to use PWAS to perform SHM in nuclear environment, radiation influence on sensor and sensing capability needs to be investigated to assure the reliability of the PWAS based method. Radiation may cause degradation or even complete failure of sensors. Gamma radiation is one of the major radiation sources near the nuclear source. Therefore, experimental investigation was completed on the gamma radiation endurance of piezoelectric sensors. The irradiation test was done in a Co-60 Gamma Irradiator. Lead Zirconate Titanate (PZT) and Gallium Orthophosphate (GaPO4) PWAS were exposed under gamma radiation at 100 Gy/hr rate for 20 hours. Electro-mechanical (E/M) admittance signatures and electrical capacitance were measured to evaluate the PWAS performance before and after every 4 hours exposure to gamma radiation. PWAS were kept at room temperature for 6 days after each 4 hours radiation exposure to investigate the effect of time on PWAS by gamma radiation. It was found that, PZT-PWAS show variation in resonance frequency for both in plane and thickness mode E/M admittance. Where, the changes in resonance amplitudes are larger for PZT-PWAS. GaPO4-PWAS E/M impedance/admittance spectra don’t show any reasonable change after gamma irradiation. A degradation behavior of electrical properties in the PZT-PWAS was observed. Capacitance value of PZT-PWAS decreases from 3.2 nF to 3.07 nF after exposing to gamma radiation for 20 hours at 100Gy/hour. This degradation behavior of electrical properties may be explained by the pinning of domain walls by some radiation induced effect. GaPO4-PWAS doesn’t show reasonable degradation in electrical properties. GaPO4 has good radiation endurance, although amplitude sensitivity is relatively low.
The increasing number, size, and complexity of nuclear facilities deployed worldwide are increasing the need to maintain readiness and develop innovative sensing materials to monitor important to safety structures (ITS). In the past two decades, an extensive sensor technology development has been used for structural health monitoring (SHM). Technologies for the diagnosis and prognosis of a nuclear system, such as dry cask storage system (DCSS), can improve verification of the health of the structure that can eventually reduce the likelihood of inadvertently failure of a component. Fiber optical sensors have emerged as one of the major SHM technologies developed particularly for temperature and strain measurements. This paper presents the development of optical equipment that is suitable for ultrasonic guided wave detection for active SHM in the MHz range. An experimental study of using fiber Bragg grating (FBG) as acoustic emission (AE) sensors was performed on steel blocks. FBG have the advantage of being durable, lightweight, and easily embeddable into composite structures as well as being immune to electromagnetic interference and optically multiplexed. The temperature effect on the FBG sensors was also studied. A multi-channel FBG system was developed and compared with piezoelectric based AE system. The paper ends with conclusions and suggestions for further work.
This paper presents guided waves based damage detection by using a hybrid PZT actuator and optic fiber Bragg grating (FBG) sensors. In the hybrid sensing, a piezoelectric wafer (PZT) is used to generate incident guided waves based on the piezoelectric principle. Meanwhile, multiple fiber Bragg grating sensors (FBG) are adopted as receivers to measure the high-frequency small-strain guided waves base on the full width half maximum (FWHM) principle. If the inspected structure has damage such as hole, crack and notch, the incident guided waves will be reflected or scattered by the damage. Through multiple FBG sensors at different locations, the damage induced waves can be acquired and further processed for damage detection. In this research, two configurations are explored, the rosette and line arrangements of multiple sensors. The sensing and wave source localization on aluminum plate are demonstrated. The results show that wave source can be successfully detected by using both the FBG rosette and the FBG array.
In US, there are over 1482 dry cask storage system (DCSS) in use storing 57,807 fuel assemblies. Monitoring is necessary to determine and predict the degradation state of the systems and structures. Therefore, nondestructive monitoring is in urgent need and must be integrated into the fuel cycle to quantify the “state of health” for the safe operation of nuclear power plants (NPP) and radioactive waste storage systems (RWSS). Innovative approaches are desired to evaluate the degradation and damage of used fuel containers under extended storage. Structural health monitoring (SHM) is an emerging technology that uses in-situ sensory system to perform rapid nondestructive detection of structural damage as well as long-term integrity monitoring. It has been extensively studied in aerospace engineering over the past two decades. This paper presents the development of a SHM and damage detection methodology based on piezoelectric sensors technologies for steel canisters in nuclear dry cask storage system. Durability and survivability of piezoelectric sensors under temperature influence are first investigated in this work by evaluating sensor capacitance and electromechanical admittance. Toward damage detection, the PES are configured in pitch catch setup to transmit and receive guided waves in plate-like structures. When the inspected structure has damage such as a surface defect, the incident guided waves will be reflected or scattered resulting in changes in the wave measurements. Sparse array algorithm is developed and implemented using multiple sensors to image the structure. The sparse array algorithm is also evaluated at elevated temperature.
A mechanical resonant piezo-optical ring sensor was studied, designed to selectively enhance the response of piezoelectric wafer active sensors (PWAS) and fiber Bragg grating (FBG) sensors. The frequency characteristics of the ring sensor were modeled through modal and harmonic analyses. The models were used to guide the experimentation, serving as a basis for comparison and implementation. Pitch-catch, resonance, and acoustic emission (AE) experiments were performed to compare the performance of the ring sensor to plate-mounted PWAS and FBG. Factors relating to optimal in-service implementation, particularly symmetric placement of FBG and PWAS, were investigated. It was found that the ring sensor was capable of amplifying an incoming Lamb wave signal. This was applied to AE experiments, where selective frequencies were amplified such that the time-domain signal had a larger amplitude response.
Interim storage of spent nuclear fuel from reactor sites has gained additional importance and urgency for resolving waste-management-related technical issues. In total, there are over 1482 dry cask storage system (DCSS) in use at US plants, storing 57,807 fuel assemblies. Nondestructive material condition monitoring is in urgent need and must be integrated into the fuel cycle to quantify the “state of health”, and more importantly, to guarantee the safe operation of radioactive waste storage systems (RWSS) during their extended usage period. A state-of-the-art nuclear structural health monitoring (N-SHM) system based on in-situ sensing technologies that monitor material degradation and aging for nuclear spent fuel DCSS and similar structures is being developed. The N-SHM technology uses permanently installed low-profile piezoelectric wafer sensors to perform long-term health monitoring by strategically using a combined impedance (EMIS), acoustic emission (AE), and guided ultrasonic wave (GUW) approach, called "multimode sensing", which is conducted by the same network of installed sensors activated in a variety of ways. The system will detect AE events resulting from crack (case for study in this project) and evaluate the damage evolution; when significant AE is detected, the sensor network will switch to the GUW mode to perform damage localization, and quantification as well as probe "hot spots" that are prone to damage for material degradation evaluation using EMIS approach. The N-SHM is expected to eventually provide a systematic methodology for assessing and monitoring nuclear waste storage systems without incurring human radiation exposure.
This paper presents theoretical predictive modeling and experimental evaluation of the structural health monitoring capability of piezoelectric wafer active sensors (PWAS) at elevated temperatures. Electromechanical impedance spectroscopy (EMIS) method is first qualified using circular PWAS resonators under traction-free boundary condition and in an ambience with increasing temperature. The theoretical study is conducted regarding temperature dependence of the electrical parameters, the capacitance C0, d31 and g31; and the elastic parameters, the in-plane compliance s11 and Young’s modulus c11, of piezoelectric materials. The Curie transition temperature must be well above the operating temperature; otherwise, the piezoelectric material may depolarize under combined temperature and pressure conditions. The material degradation is investigated by introducing the temperature effects on the material parameters that are obtained from experimental observations as well as from related work in literature. The preliminary results from the analytical 2-D circular PWAS-EMIS simulations are presented and validated by the experimental PWAS-EMIS measurements at elevated temperatures. Temperature variation may produce pyro-electric charges, which may interfere with the piezoelectric effect. Therefore, analytical simulations are carried out to simulate the pyro-electric response from the temperature effects on a free circular PWAS-EMIS in in-plane mode. For the experimental validation, PWAS transducers are placed in a fixture that provides the traction-free boundary condition. The fixture is then located in an oven integrated with PID temperature controller. The EMIS measurement is conducted during the temperature increase and the first resonance frequency peak in admittance and impedance spectra was acquired.
This paper presents the development of optical equipment that is suitable for ultrasonic guided wave detection for active
SHM in the hundreds of kHz range. In recent years, fiber Bragg grating (FBG) sensors have been investigated by many
researchers as an alternative to piezoelectric sensors for the detection of ultrasonic waves. FBG have the advantage of
being durable, lightweight, and easily embeddable into composite structures as well as being immune to electromagnetic
interference and optically multiplexed. However, there is no commercially available product that uses this promising
technology for the detection of ultrasonic guided waves because: (a) the frequency is high (hundreds of kHz); (b) the
strains are very small (nano-strain); (c) the operational loads may also induce very large quasi-static strains (the
superposition of very small ultrasonic strains and very large quasi-static strain presents a very significant challenge).
Although no turn-key optical system exists for ultrasonic guided wave detection, we developed optical ultrasonic guided
wave equipment using a tunable laser device. The measurement resolution and sampling speed were considered as the
most important criteria in our test. We achieved high sensitive (nano-strain) and high sampling rate. Comparative
measurements of low-amplitude ultrasonic waves have been done including FBG, strain gauge, and piezoelectric wafer
active sensors (PWAS). Calibration and performance improvements for the optical interrogation system are also
developed and discussed. The paper ends with conclusions and suggestions for further work.
This paper presents the application and validation of optical equipment suitable for high frequency guided wave and
acoustic emission detection with fiber Bragg grating (FBG) sensors. Guided wave and acoustic emission (AE)
measurements were compared between piezoelectric wafer active sensors (PWAS) and fiber Bragg grating (FBG)
sensors embedded onto isotropic plates and beams with an emphasis on testing FBG ultrasonic wave propagation
frequency characteristics.
The use of an acousto-ultrasonic FBG ring sensor to eliminate FBG directional dependence is also discussed. Since
FBG sensors only detect strain longitudinal to the fiber, unlike PWAS they cannot serve as omnidirectional guided wave
and AE sensors. To overcome this limitation, the use of an acousto-ultrasonic ring sensor, designed to augment and
enhance the performance of FBG sensors, is tested. The ring sensor uses mechanical amplification principles to force in-plane
vibration of the ring to occur at a specific resonance frequency. In this study, a ring sensor is bonded onto an
isotropic plate; incoming guided wave and AE measurements from an FBG bonded to the ring sensor were compared to
measurements from an FBG bonded to the plate. Preliminary results show the use of the ring sensor nearly eliminated the
directional dependence of the FBG; concurrently the FBG on the ring sensor sensed incoming guided waves and AE
events near its resonance frequency and rejected phenomenon occurring at other frequencies.
This paper discusses theoretical analysis of electro-mechanical impedance spectroscopy (EMIS) of piezoelectric wafer active sensor (PWAS). Both free and constrained PWAS EMIS models are developed for in-plane (lengthwise) and outof plane (thickness wise) mode. The paper starts with the general piezoelectric constitutive equations that express the linear relation between stress, strain, electric field and electric displacement. This is followed by the PWAS EMIS models with two assumptions: 1) constant electric displacement in thickness direction (D3) for out-of-plane mode; 2) constant electric field in thickness direction (E3) for in-plane mode. The effects of these assumptions on the free PWAS in-plane and out-of-plane EMIS models are studied and compared. The effects of internal damping of PWAS are considered in the analytical EMIS models. The analytical EMIS models are verified by Coupled Field Finite Element Method (CF-FEM) simulations and by experimental measurements. The extent of the agreement between the analytical and experimental EMIS results is discussed. The paper ends with summary, conclusions, and suggestions for future work.
This paper discusses shear horizontal (SH) guided waves that can be excited with shear type piezoelectric wafer active
sensors (PWAS). The paper starts with a review of the state of the art in SH waves modeling and their importance in
non-destructive evaluation (NDE). This is followed by basic sensing and actuation equations of shear-poled PWAS
transducers with appropriate electro-mechanical coupling coefficients. The electro-mechanical impedance of the SHPWAS
transducer is studied. The equations for shear stress transfer between PWAS and the structure are developed. The
amplitudes of shear horizontal wave modes are normalized with respect to the wave power; normal mode expansion
(NME) method is used to account for superpositioning multimodal SH waves. Modal participation factors are presented
to show the contribution of every mode. Model assumption includes: (a) straight crested guided wave propagation; (b)
evanescent waves are ignored; and (c) ideal bonding between PWAS and structure with shear load transfer concentrated
at PWAS tips. Power and energy transfer between PWAS and the structure is analyzed in order to optimize sensor size
and excitation frequency for maximum wave energy production for a given source. The paper ends with summary,
conclusion and suggestion of future work.
This paper presents a novel ultrasonic guided wave based inspection methodology for detecting and evaluating gas
accumulation in nuclear cooling pipe system. The sensing is in-situ by means of low-profile permanently installed
piezoelectric wafer sensors to excite interrogating guided waves and to receive the propagating waves in the pipe
structure. Detection and evaluation is established through advanced cross time-frequency analysis to extract the phase
change in the sensed signal when the gas is accumulating. A correlation between the phase change and the gas amount
has been established to provide regulatory prediction capability based on measured sensory data.
This paper presents a theoretical modeling of power and energy transduction of structurally-bonded piezoelectric wafer
active sensors (PWAS) for structural health monitoring (SHM). After a literature review of the state of the art, we
developed a model of power and energy transduction between the PWAS and a structure containing multimodal
ultrasonic guided waves. The use of exact Lamb waves modes for power modeling is an extension of our previously
presented simplified model that considered axial and flexural waves with low frequency approximation. The model
assumptions include: (a) straight-crested multimodal ultrasonic guided wave propagation; (b) ideal bonding (pin-force)
connection between PWAS and structure; (c) ideal excitation source at the transmitter PWAS and fully-resistive
external load at the receiver PWAS. Frequency response functions are developed for voltage, current, complex power,
active power, etc. Multimodal ultrasonic guided wave, normal mode expansion, electromechanical energy
transformation of PWAS and structure were considered. The parametric study of PWAS size and impedance match
gives the PWAS design guideline for PWAS sensing and power harvesting applications
This paper presents an investigation of 2-D power and energy transduction in piezoelectric wafer active sensors
(PWAS) for structural health monitoring (SHM). After a literature review of the state of the art, we developed a model
of 2-D power and energy transduction of PWAS attached to structure. The model is an extension of our previously
presented 1-D model. It allows examination of power and energy flow for a circular crested wave pattern. The model
assumptions include: (a) 2-D axial and flexural wave propagation; (b) ideal bonding (line-force) connection between
PWAS and structure; (c) ideal excitation source at the transmitter PWAS and fully-resistive external load at the receiver
PWAS (d) crested wave energy spread out. Wave propagation method for an infinite boundary plate, electromechanical
energy transformation of PWAS and structure, and wave propagation energy spread out in 2-D plate were considered.
The parametric study of PWAS size, impedance match gives the PWAS design guideline for PWAS sensing and power
harvesting applications. In pitch-catch PWAS application, the frequency response functions of a circular PWAS are
developed for voltage in consideration with the receiver capacitance and external resistive loads.
This paper presents a systematic investigation of power and energy transduction in piezoelectric wafer active sensors
(PWAS) for structural health monitoring (SHM). After a literature review of the state of the art, the paper develops a
simplified pitch-catch model of power and energy transduction of PWAS attached to structure. The model assumptions
include: (a) 1-D axial and flexural wave propagation; (b) ideal bonding (pin-force) connection between PWAS and
structure; (c) ideal excitation source at the transmitter PWAS and fully-resistive external load at the receiver PWAS.
Frequency response functions are developed for voltage, current, complex power, active power, etc.
First, we examined PWAS transmitter and determined the active power, reactive power, power rating of electrical
requirement under harmonic voltage excitation. It was found that the reactive power is dominant and defines the power
requirement for power supply / amplifier for PWAS applications. The electrical and mechanical power analysis at the
PWAS structure interface indicates all the active electrical power provides the mechanical power at the interface. This
provides the power and energy for the axial and flexural waves power and energy that propagate into the structure. The
sum of forward and backward wave power equals the mechanical power PWAS applied to the structure. The parametric
study of PWAS transmitter size shows the proper size and excitation frequency selection based on the tuning effects.
Second, we studied the PWAS receiver structural interface acoustic and electrical energy transduction. The parametric
study of receiver size, receiver impedance and external electrical load gives the PWAS design guideline for PWAS
sensing and power harvesting applications.
Finally we considered the power flow for a complete pitch-catch setup. In pitch-catch mode, the power flows from
electrical source into piezoelectric power at the transmitter; the piezoelectric conduction converts the electrical power
into the mechanical interface power at the transmitter PWAS and then into the acoustic wave power travelling in the
structure. The wave power arrives at the receiver PWAS and is captured at the mechanical interface between the
receiver PWAS and the structure; the captured mechanical power is converted back into electrical power at the receiver
PWAS and measured by the receiver electrical instrument. Our numerical simulation and graphical chart show the
trends in the power and energy flow behavior with remarkable peaks and valleys that can be exploited for optimum
design.
Structural health monitoring (SHM) is an emerging field in which smart materials interrogate structural components
to predict failure, expedite needed repairs, and thus increase the useful life of those components. Piezoelectric wafer
active sensors (PWAS) have been previously adhesively-bonded to structures and demonstrate the ability to detect and
locate cracking, corrosion, and disbonding through use of pitch-catch, pulse-echo, electro/mechanical impedance, and
phased array technology. The present research considers structurally-integrated PWAS that can be fabricated directly to
the structural substrate using thin-film nano technologies (e.g., pulsed-laser deposition, sputtering, chemical vapor
deposition, etc.) Because these novel PWAS are made up of nano layers they are dubbed nano-PWAS. Nano-PWAS
research consists of two parts, thin-film fabrication and nano-PWAS construction. The first part is how to fabricate the
piezoelectric thin-film on structure materials. In our research, ferroelectric BaTiO3 (BTO) thin films were successfully
deposited on structure material Ni and Ti by pulsed laser deposition under the optimal synthesis conditions.
Microstructural studies revealed that the as-grown BTO thin films have the nanopillar structures and the good interface
structures with no inter-diffusion or reaction. The dielectric and ferroelectric property measurements exhibit that the
BTO films have a relatively large dielectric constant, a small dielectric loss, and an extremely large piezoelectric
response with a symmetric hysteresis loop. The second part is nano-PWAS construction and how they are related to the
active SHM interrogation methods. Nano-PWAS architecture achieved through thin-film deposition technology and its
potential application for SHM were discussed here. The research objective is to develop the fabrication and optimum
design of thin-film nano-PWAS for structural health monitoring applications.
Piezoelectric wafer active sensors (PWAS) have been proven a valuable tool in structural health monitoring.
Piezoelectric wafer active sensors are able to send and receive guided Lamb/Rayleigh waves that scan the structure and
detect the presence of incipient cracks and structural damage. In-situ thin-film active sensor deposition can eliminate the
bonding layer to improve the durability issue and reduce the acoustic impedance mismatch. Ferroelectric thin films have
been shown to have piezoelectric properties that are close to those of single-crystal ferroelectrics but the fabrication of
ferroelectric thin films on structural materials (steel, aluminum, titanium, etc.) has not been yet attempted. In this work,
in-situ fabrication method of piezoelectric thin-film active sensors arrays was developed using the nano technology
approach. Specification for the piezoelectric thin-film active sensors arrays was based on electro-mechanical-acoustical
model. Ferroelectric BaTiO3 (BTO) thin films were successfully deposited on Ni tapes by pulsed laser deposition under
the optimal synthesis conditions. Microstructural studies by X-ray diffractometer and transmission electron microscopy
reveal that the as-grown BTO thin films have the nanopillar structures with an average size of approximately 80 nm in
diameter and the good interface structures with no inter-diffusion or reaction. The dielectric and ferroelectric property
measurements exhibit that the BTO films have a relatively large dielectric constant, a small dielectric loss, and an
extremely large piezoelectric response with a symmetric hysteresis loop. The research objective is to develop the
fabrication and optimum design of thin-film active sensor arrays for structural health monitoring applications. The short
wavelengths of the micro phased arrays will permit the phased-array imaging of smaller parts and smaller damage than
is currently not possible with existing technology.
Structural health monitoring (SHM) is important for reducing maintenance costs while increasing safety and reliability. Piezoelectric wafer active sensors (PWAS) used in SHM applications are able to detect structural damage using Lamb waves. PWAS are small, lightweight, unobtrusive, and inexpensive. PWAS achieve direct transduction between electric and elastic wave energies. PWAS are essential elements in the Lamb-wave SHM with pitch-catch, pulse-echo, phased array system and electromechanical impedance methods.
This paper starts with the state of the art on the impedance method for PWAS applications. Then, finite element impedance model for free and bonded PWAS with different sizes and shapes will be given. Experiments showed that the real part and imaginary part of PWAS had different usage. Applications of impedance-based structural health monitoring indicate impedance method as a good candidate for damage detection and sensor durability verification for SHM smart sensor.
This paper describes work performed in the development of a set of specification for the construction of an integrated electronic system for piezoelectric wafer active sensor (PWAS). The paper starts with a comprehensive review of the PWAS material properties, dimensions, and electrical characteristics. PWAS of various shapes and sizes are considered. Two boundary conditions were examined: free PWAS and PWAS attached to actual structures. For both, the PWAS immittance and the allowable dc and ac voltages were considered. The predicted values were compared with measurements performed over a wide frequency range (10 kHz to 2 MHz). Next, the electronic-equipment specifications were considered. The PWAS can be used in a number of different ways to actively detect damage in structures. Our aim was to develop electronic-equipment specifications that would extract the optimum performance from the PWAS, i.e., maximize the coupling with the structure and obtain large-amplitude Lamb wave transmission and reception. Analytical predictions were compared with measurements made using current laboratory equipment. The comparative analysis revealed that the current electronic equipment does not fully exploit the PWAS capabilities. Hence, the PWAS equipment specifications were divided into two categories: “existing” and “desired”. The former category designates integrated electronic equipment that would offer the same PWAS performance as the existing lab equipment, but be of a lower volume/weight/cost. The latter category refers to advanced electronic equipment that will exploit the full potential of PWAS transducers while being of lower volume/weight/cost than the lab equipment. Both categories are presented and discussed in the paper.
Structural health monitoring (SHM) is important for reducing maintenance costs while increasing safety and reliability. Piezoelectric wafer active sensors (PWAS) used in SHM applications are able to detect structural damage using Lamb waves. PWAS are small, lightweight, unobtrusive, and inexpensive. PWAS achieve direct transduction between electric and elastic wave energies. PWAS are essential elements in the Lamb-wave SHM with pitch-catch, pulse-echo, and electromechanical impedance methods. Traditionally, structural integrity tests required attachment of sensors to the material surface. This is often a burdensome and time-consuming task, especially considering the size and magnitude of the surfaces measured (such as aircraft, bridges, structural supports, etc.). In addition, there are critical applications where the rigid piezoceramic wafers cannot conform to curved surfaces. Existing ceramic PWAS, while fairly accurate when attached correctly to the substance, may not provide the long term durability required for SHM. The bonded interface between the PWAS and the structure is often the durability weak link. Better durability may be obtained from a built-in sensor that is incorporated into the material. An in-situ fabricated smart sensor may offer better durability. This paper gives a review of the state of the art on the in-situ fabrication of PWAS using different approaches, such as piezoelectric composite approach; polyvinylidene fluoride (PVDF) approach. It will present the principal fabrication methods and results existing to date. Flexible PVDF PWAS have been studied. They were mounted on a cantilever beam and subjected to free vibration testing. The experimental results of the composite PWAS and PVDF PWAS have been compared with the conventional piezoceramic PWAS. The theoretical and experimental results in this study gave the basic demonstration of the piezoelectricity of composite PWAS and PVDF PWAS.
Structural health monitoring (SHM) is currently using piezoelectric wafer active sensors (PWAS) permanently attached to the structure with adhesives. This is often a burdensome and time-consuming task, especially for large structures such as aircraft, bridges, etc. In addition, there are critical applications where the rigid piezoceramic wafers cannot conform to curved surfaces. Another important issue is the long term durability of the bonded interface between the PWAS and the structure, which is often the durability weak link. An in-situ fabricated smart sensor may offer better durability. This paper considers the possibility of fabricating the PWAS directly to the substrate structure in order to alleviate these problems. The paper starts with a review of the state of the art in active composite fabrication. Then, two concepts are considered: the piezomagnetic composite sensor and the piezoelectric composite PWAS. The piezomagnetic composite was fabricated using Terfenol-D magnetostrictive powder in a fiber reinforced composite beam. The strain-induced magnetic field was detected with a Lakeshore gaussmeter. The piezoelectric composite sensor was prepared by mixing lead zirconate titanate (PZT) particles in an epoxy resin. The mixture was applied onto the structural surface using a mask. After curing, the piezo composite was sanded down to the desired thickness and poled under a high electric field. The resulting in-situ composite PWAS was utilized as a sensor for dynamic vibration and impact. Characterization of the in-situ composite PWAS on aluminum structure have been recorded and compared with ceramic PWAS before and after poling. To evaluate the performance of the in-situ composite PWAS, both vibration and impact tests were conducted. Both experiments indicated that in-situ fabrication of active materials composites poses itself as a good candidate for reliable low-cost option for SHM smart sensor fabrication.
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