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This paper presents an experimental validation of an analytical method called complex mode expansion with vector projection (CMEP), which is used to calculate the scattering coefficients (amplitude of the out-of-plane velocity) of Lamb wave modes from geometric discontinuities. For a test case, a plate with a thickness step change type geometric discontinuity is considered in this paper. The scattered wave fields from the discontinuity are expanded in terms of complex Lamb wave modes with unknown scatter coefficients. These unknown coefficients are obtained by projecting the stress or displacement boundary conditions on the displacement or stress boundary conditions utilizing the power expression. In the analytical analysis, complex-valued scatter coefficients are calculated with frequency-thickness product from 50 to 1500 kHz mm for A0 incident wave. A parametric study was conducted using CMEP to find the optimized step depth ratio for the experiment. For incident A0 mode at step depth ratio of 0.6, the scattering coefficients of reflected and transmitted S0 modes are maximum. A plate of thickness 4.86 mm with a step depth ratio of 0.6 was chosen for experimental study. Long piezoelectric wafer active sensors (PWAS) were used to create straight crested Lamb wave modes. Antisymmetric Lamb wave mode selectively excited by using two PWAS in out of phase on opposite sides of the plate. Scanning laser Doppler vibrometer was used to measure the out-of-plane velocity of scattered Lamb wave fields on the plate. Scatter coefficients were calculated from Fourier transform of the time domain signal. The obtained experimental results agree well with the CMEP analytical predictions.

Piezoelectric wafer active sensors (PWAS) used in structural health monitoring (SHM) applications are able to detect structural damage using Lamb waves. PWAS are small, lightweight, unobtrusive and inexpensive. They achieve direct transduction between electric and elastic wave energies. PWAS are charge mode sensors and can be used as both transmitters and receivers. The focus of this paper is to find a suitable in situ piezoelectric active sensor for sending and receiving Lamb waves to be used in the SHM of structures with a curved surface. Current SHM technology uses brittle piezoceramic (PZT) wafer active sensors. Since piezoceramics are brittle, this approach could only be used on flat surfaces. The motivation of our research was to explore the use of flexible piezoelectric materials, e.g. piezoelastic polymers such as PVDF. However, PVDF stiffness is orders of magnitude lower than the PZT stiffness, and hence PVDF Lamb wave transmitters are much weaker than PZT transmitters. Thus, our research proceeded in two main directions: (a) to model and understand how piezoelectric material properties affect the behaviour of piezoelectric wafer active sensors; and (b) to perform experiments to test the capabilities of the flexible PVDF PWAS in comparison with those of stiffer but brittle PZT PWAS. We have shown that, with appropriate signal amplification, PVDF PWAS can perform the same Lamb wave transmission and reception functions currently performed by PZT PWAS. The experimental results of PZT-PWAS and PVDF-PWAS have been compared with a conventional strain gauge. The theoretical and experimental results in this study gave a basic demonstration of the piezoelectricity of PZT-PWAS and PVDF-PWAS.

The acoustic emission (AE) waveforms from a fatigue crack advancing in a thin metallic plate possess diverse and complex spectral signatures. In this article, we analyze these waveform signatures in coordination with the load level during cyclic fatigue. The advancing fatigue crack may generate numerous AE hits while it grows under fatigue loading. We found that these AE hits can be sorted into various groups based on their AE waveform signatures. Each waveform group has a particular time-domain signal pattern and a specific frequency spectrum. This indicates that each group represents a certain AE event related to the fatigue crack growth behavior. In situ AE-fatigue experiments were conducted to monitor the fatigue crack growth with simultaneous measurement of AE signals, fatigue loading, and optical crack growth measurement. An in situ microscope was installed in the load-frame of the mechanical testing system (MTS) to optically monitor the fatigue crack growth and relate the AE signals with the crack growth measurement. We found the AE signal groups at higher load levels (75%?85% of maximum load) were different from the AE signal groups that happened at lower load levels (below 60% of load level). These AE waveform groups are highly related to the fatigue crack-related AE events. These AE signals mostly contain the higher frequency peaks (100 kHz, 230 kHz, 450 kHz, 550 kHz). Some AE signal groups happened as a clustered form that relates a sequence of small AE events within the fatigue crack. They happened at relatively lower load level (50%?60% of the maximum load). These AE signal groups may be related to crack friction and micro-fracture during the friction process. These AE signals mostly contain the lower frequency peaks (60 kHz, 100 kHz, 200 kHz). The AE waveform based analysis may give us comprehensive information of the metal fatigue.

This paper presents an experimental and analytical study of irreversible change in piezoelectric wafer active sensor (PWAS) electromechanical (E/M) impedance and admittance signature under high temperature exposure. After elevated to high temperatures, change in the material properties of PWAS can be quantified through irreversible changes in its E/M impedance and admittance signature. For the experimental study, circular PWAS transducers were exposed to temperatures between 50 °C and 250 °C at 50 °C intervals. E/M impedance and admittance data were obtained before and after each heating cycle. Irreversible temperature sensitivity of PWAS resonance and anti-resonance frequency was estimated as 0.0246 kHz °C−1 and 0.0327 kHz °C−1 respectively. PWAS transducer material properties relevant to impedance or admittance signature such as dielectric constant, dielectric loss factor, mechanical loss factor, and in plane piezoelectric constant were determined experimentally at room temperature before and after the elevated temperature tests. The in-plane piezoelectric coefficient was measured by using optical-fiber strain transducer system. It was found that the dielectric constant and in-plane piezoelectric coefficient increased linearly with temperature. Dielectric loss also increases with temperature but remains within 0.2% of initial room temperature value. Change in dielectric properties and piezoelectric constant may be explained by depinning of domains or by domain wall motion. The piezoelectric material degradation was investigated microstructurally and crystallographically by using scanning electron microscope and x-ray diffraction method respectively. There were no noticeable changes in microstructure, crystal structure, unit cell dimension, or symmetry. The degraded PWAS material properties were determined by matching impedance and admittance spectrums from experimental results with a closed form circular PWAS analytical model. Analytical results showed that impedance and admittance strongly depend on elastic coefficient, dielectric constant, mechanical loss factor, dielectric loss tangent and in plane piezoelectric constant. These properties were found to be susceptible to change after high temperature exposure.

Among novel non-destructive evaluation techniques for structural health monitoring, the magnetostrictive (MS)-tagged fiber-reinforced composites stands out as especially suitable due to: (a) distributed sensory properties, (b) non-contact damage detection, and (c) straightforward manufacturing implementation. Experimental data and mathematical modeling of a MS-tagged fiber reinforced composite beam under bending (flexural) loading are presented. A brief review of the state of the art identifies previous work on axially loaded MS composites, but finds no previous work on bending. Description of bending beam design and fabrication is followed by theoretical analysis and by the description of the experimental set-up and equipment used. Several analysis models were used. Test data, with and without applying magnetic bias field between loading cycles, is presented and results are discussed. Numerical values for the stress and strain versus magnetic flux density coefficients are given for both annealed and non-annealed cases. Piezomagnetic coefficients for the MS composite are calculated. The correlation between the results developed in the present paper for bending and previously published results for axial loading is found to be within 10% after correction factors depending on the quantity of the MS material.

A resonant piezo-optical ring sensor with both piezoelectric and fiber Bragg grating (FBG) sensing elements was assessed for ultrasonic wave detection. The ring sensor is an existing device that has been shown experimentally to exhibit a number of sensing features: omnidirectionality, mode selectivity, and frequency tunability. The present study uses finite element modeling to understand these features as a means to characterize and optimize the sensor. A combined vibration-wave propagation modeling approach was used, where the vibrational modeling provided a basis for understanding sensing features, and the wave propagation modeling provided predictive power for sensor performance. The sensor features corresponded to the fundamental vibrational mode of the sensor, particularly to the base motion of this mode. The vibrational modeling was also used to guide sensor optimization, with an emphasis on the FBG and piezoelectric sensing elements. It was found that sensor symmetry and nodes of extraneous resonance modes could be exploited to provide a single-resonance response. A series of pitch-catch guided wave experiments were performed on a thin aluminum plate to assess the optimized sensor configuration. Tuning curves showed a single-frequency response to a Lamb wave and mechanical filtering away from the dominant frequency; the sensor capability for mechanical amplification of a Lamb wave and mechanical amplification of a pencil-lead-break acoustic emission event were also demonstrated.

The effect of radiation, temperature, and vacuum (RTV) on piezoelectric wafer active sensors (PWASs) is discussed. This study is relevant for extending structural health monitoring (SHM) methods to space vehicle applications that are likely to be subjected to harsh environmental conditions such as extreme temperatures (hot and cold), cosmic radiation, and interplanetary vacuums. This study contains both theoretical and experimental investigations with the use of electromechanical impedance spectroscopy (EMIS). In the theoretical part, analytical models of circular PWAS resonators were used to derive analytical expressions for the temperature sensitivities of EMIS resonance and antiresonance behavior. Closed-form expressions for frequency and peak values at resonance and antiresonance were derived as functions of the coefficients of thermal expansion, the Poisson ratio, and its sensitivity, the relative compliance gradient and the Bessel function root, and its sensitivity, In the experimental part, tests were conducted to subject the PWAS transducers to RTV conditions. In one set of experiments, several RTV exposure, cycles were applied with EMIS signatures recorded at the beginning and after each of the repeated cycles. In another set of experiments, PWAS transducers were subjected to various temperatures and the EMIS signatures were recorded at each temperature after stabilization. The processing of measured EMIS data from the first set of experiments revealed that the resonance and antiresonance frequencies changed by less than 1% due to RTV exposure, whereas the resonance and antiresonance amplitudes changed by around 15%. After processing an individual set of EMIS data from the second set of experiments, it was determined that the relative temperature sensitivity of the antiresonance frequency ( is approximately and the relative temperature sensitivity of the antiresonance amplitude (ReZ) is approximately A tentative statistical analysis and comparative plots of the data from sets of PWAS transducers revealed that the trends observed on an individual PWAS are also observed on the entire set of PWAS transducers. The article concludes with a summary, conclusions, and suggestions for further work.

This study aimed to develop theoretical models to accurately predict the in-plane (longitudinal) and out-of-plane (thickness-wise) modes of the electromechanical impedance spectroscopy (EMIS) of a piezoelectric wafer active sensor (PWAS). Two main electrical assumptions are applied for both in-plane and thickness mode PWAS-EMIS in one-dimensional simplified analytical models. These assumptions are 1) constant electrical field assumption and 2) constant electrical displacement assumption. The analytical models with two assumptions are compared with one another to understand the prediction accuracy of the models in different vibration modes. Coupled field finite element analysis (CF-FEA) is also conducted with 2D PWAS model under stress-free boundary conditions. The simulations of the simplified analytical models for free PWAS-EMIS under these two assumptions are carried out. The analytical models are validated by corresponding finite element simulations as well as experimental measurements.

This paper addresses theoretical and experimental work on thickness-mode electromechanical (E/M) impedance spectroscopy (EMIS) of proof-mass piezoelectric wafer active sensors (PMPWAS). The proof-mass (PM) concept was used to develop a new method for tuning the ultrasonic wave modes and for relatively high frequency local modal sensing by the PM affixed on PWAS. In order to develop the theoretical basis of the PMPWAS tuning concept, analytical analyses were conducted by applying the resonator theory to derive the EMIS of a PWAS constrained on one and both surfaces by isotropic elastic materials. The normalized thickness-mode shapes were obtained for the normal mode expansion (NME) method to eventually predict the thickness-mode EMIS using the correlation between PMPWAS and the structural dynamic properties of the substrate. Proof-masses of different sizes and materials were used to tune the system resonance towards an optimal frequency point. The results were verified by coupled-field finite element analyses (CF-FEA) and experimental results. An application of the tuning effect of PM on the standing wave modes was discussed as the increase in PM thickness shifts the excitation frequency of the wave mode toward the surface acoustic wave (SAW) mode.

This paper presents a novel method for the detection of ultrasonic waves from acoustic emission events using piezoelectric wafer ac3tive sensors (PWAS) and optical fiber Bragg grating (FBG) sensing combined with mechanical resonance amplification principles. The method is best suited for detecting the out-of-plane motion of the AE wave with preference for a certain frequency that can be adjusted by design. Several issues are discussed: (a) study the mode shapes of the sensors under different resonance frequencies in order to understand the behavior of the ring in a frequency band of interest; (b) comparison of analytical results and mode shapes with FEM predictions; (c) choice of the final piezo-optical ring sensor shape; (d) testing of the piezo-optical ring sensor prototype; (e) discussion of the ring-sensor test results in comparison with conventional results from PWAS and FBG sensors mounted directly on the test structure. The paper ends with summary, conclusions, and suggestions for further work.

This article discusses shear horizontal (SH) guided-waves that can be excited with shear type piezoelectric wafer active sensor (SH-PWAS). The paper starts with a review of state of the art SH waves modelling and their importance in non-destructive evaluation (NDE) and structural health monitoring (SHM). The basic piezoelectric sensing and actuation equations for the case of shear horizontal piezoelectric wafer active sensor (SH-PWAS) with electro-mechanical coupling coefficient are reviewed. Multiphysics finite element modelling (MP-FEM) was performed on a free SH-PWAS to show its resonance modeshapes. The actuation mechanism of the SH-PWAS is predicted by MP-FEM, and modeshapes of excited structure are presented. The structural resonances are compared with experimental measurements and showed good agreement. Analytical prediction of SH waves was performed. SH wave propagation experimental study was conducted between different combinations of SH-PWAS and regular in-plane PWAS transducers. Experimental results were compared with analytical predictions for aluminium plates and showed good agreement. 2D wave propagation effects were studied by MP-FEM. An analytical model was developed for SH wave power and energy. The normal mode expansion (NME) method was used to account for superpositioning multimodal SH waves. Modal participation factors were presented to show the contribution of every mode. Power and energy transfer between SH-PWAS and the structure was analyzed. Finally, we present simulations of our developed wave power and energy analytical models.

Piezoelectric wafer active sensors (PWAS) are lightweight and inexpensive enablers for a large class of damage detection and structural health monitoring (SHM) applications. This paper starts with a brief review of PWAS physical principles and basic modelling and continues by considering the various ways in which PWAS can be used for damage detection: (a) embedded guided-wave ultrasonics, i.e., pitch-catch, pulse-echo, phased arrays, thickness mode; (b) high-frequency modal sensing, i.e., the electro-mechanical (E/M) impedance method; (c) passive detection, i.e., acoustic emission and impact detection. An example of crack-like damage detection and localization with PWAS phased arrays on a small metallic plate is given. The modelling of PWAS detection of disbond damage in adhesive joints is achieved with the analytical transfer matrix method (TMM). The analytical methods offer the advantage of fast computation which enables parameter studies and carpet plots. A parametric study of the effect of crack size and PWAS location on disbond detection is presented. The power and energy transduction between PWAS and structure is studied analytically with a wave propagation method. Special attention is given to the mechatronics modeling of the complete transduction cycle from electrical excitation into ultrasonic acoustic waves by the piezoelectric effect, the transfer through the structure, and finally reverse piezoelectric transduction to generate the received electric signal. It is found that the combination of PWAS size and wave frequency/wavelength play an important role in identifying transduction maxima and minima that could be exploited to achieve an optimum power-efficient design. The multi-physics finite element method (MP-FEM), which permits fine discretization of damaged regions and complicated structural geometries, is used to study the generation of guided waves in a plate from an electrically excited transmitter PWAS and the capture of these waves as electric signals at a receiver PWAS. Wave diffraction from a hole damage is illustrated through time-frame snapshots. The paper ends with conclusions and suggestions for further work.