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This paper presents vibration-based methods for detecting and quantifying delaminations in composite structures. Three different delamination sizes were considered numerically and experimentally. Our approach combines and compares finite element method (FEM) numerical analysis and experimental measurements to find the location of structural delaminations and identify their severity. First, we noticed that a low-frequency modal study comparing pristine and damaged plates could not provide local damage information. Hence, a high frequency-modal analysis was performed to obtain the local damage information. The damaged mode shapes corresponding to resonance frequencies that cause significant local vibration (high amplitude) over the damaged regions were calculated numerically. The result shows the capability of damage identification (location and size of structural delaminations) within the high-frequency modal analysis. The numerical results were validated experimentally. The experiments include electromechanical impedance spectroscopy (EMIS) and scanning laser Doppler vibrometer (SLDV) techniques. The EMIS experiment was implemented to predict the resonance frequencies which cause significant local vibration over the delamination regions. The spectra of the electromechanical response resemble the vibration response of the structure and detect the new resonances created by the presence of the delamination. The test plate’s experimental resonance peaks show a good match with the resonance frequencies of the numerical modal analysis. The SLDV experiments measured the local vibration over the investigated scanning areas. The operational vibration shapes corresponding to predicted resonance frequencies were created. These experimental mode shapes may visually measure the delaminations by observing intense local vibration over the delamination zones. Comparing the numerical mode shapes and the operational vibration shapes shows a good match between numerical and experimental results.

Delamination is one of the most common and dangerous failure modes for composites because it takes place and grows in the absence of any visible surface damage. The successful implementation of delamination detection in aerospace composite structures is always challenging due to the general anisotropic behavior of composites and multilayer delamination scenarios. This article presents a numerical and experimental investigation to detect and characterize the multilayer delaminations in carbon fiber–reinforced polymer composite plates using guided waves and wavenumber analysis. Multiphysics three-dimensional finite element simulations of the composite plate with five different delamination scenarios are conducted to provide the out-of-plane wave motion for wavenumber analysis. The out-of-plane results from finite element simulations of one delamination and two delaminations are validated by the scanning laser Doppler vibrometer measurements. It is found that the wavenumber analysis can identify the plies between which the delamination occurs and evaluate the delamination severity by comparing the new wavenumbers due to the trapped waves in the delamination regions, which is potentially related to delamination severity. Both numerical and experimental results demonstrate a good capability for the detection and characterization of multilayer delaminations in composite structures.

This paper addresses a forward problem using an analytical global–local (AGL) method based on the physics of the Lamb wave propagation in the presence of a crack in the complex structure which would be beneficial for the inverse problem of damage detection. Discontinuity in a structure acts as a damage source and interacts with a Lamb wave. The global analytical solution determines the wave generation by a transmitter, wave propagation through the structure, and detection by a receiver. The local analytical solution determines the scattering coefficients for the Lamb waves at the discontinuity location. These frequency-dependent scatter coefficients are calculated considering transmission, reflection, and mode conversion. An analytical method called “complex mode expansion with vector projection (CMEP)” is used to calculate the scattering coefficients of Lamb wave modes from geometric discontinuities. The scattered wave field from a discontinuity is expanded in terms of complex Lamb wave modes with unknown scatter coefficients. These unknown coefficients are obtained from the boundary conditions using a vector projection utilizing the power expression. Two test cases are considered in this paper: (a) a plate with a pristine stiffener and (b) a plate with a cracked stiffener. Complex-valued scattering coefficients are calculated from 50 to 350 kHz for S0 incident waves. Scatter coefficients are compared for both cases to identify the suitable frequency range to excite a Lamb wave to detect the crack. The frequency-dependent complex-valued scattering coefficients are then inserted into the global analytical model. Therefore, in combination AGL method provides the exact analytical Lamb wave solution for the simulation of Lamb wave propagation and interaction with a discontinuity. By comparing the waveforms for both pristine stiffener and cracked stiffener, the crack can be detected. An FEM transient analysis was also performed to calculate the scattered wave signals. FEM results agree well with the AGL predicted results. An experiment was also performed to validate the AGL result. The obtained experimental results match well with the CMEP analytical predictions. The present AGL method is a highly computationally efficient simulation approach which allows performing virtual experiments for structural health monitoring applications.

The guided wave technique is commonly used in the health monitoring of thin-walled structures because the guided waves can propagate far in the structures without much energy loss. However, understanding of the wave propagation in bounded layered structures is still lacking. In this study, the Lamb wave field of single- and multi-layer plates excited by surface-mounted piezoelectric wafer active sensors is theoretically analyzed using the normal mode expansion method, which is based on the elastodynamic reciprocity relation and utilizes the orthogonality relations of the Lamb wave modes. The mode participation factors of Lamb wave in single- and multi-layer isotropic plates are derived. The time domain responses are obtained through the inverse Fourier transform of the structural response spectrum, which is obtained by multiplying the transfer function with the excitation frequency spectrum. The developed normal mode expansion method is first applied to an aluminum single-layer plate. The obtained analytical tuning curves and out-of-plane velocity of the plate are in good agreement with the numerical and experimental results. Finally, the analytical wave responses of an aluminum–adhesive–steel triple-layer plate are verified through comparison with the finite element analysis and experiment. The proposed normal mode expansion method provides a reliable and accurate calculation of the wave field in single- and multi-layer plates.

In this article, estimation of crack size, shape, and orientation was investigated numerically and experimentally using Lamb waves. A hybrid global–local approach was used in conjunction with the imaging methods for the numerical simulation. The hybrid global–local approach allowed fast and efficient prediction of scattering wave signals for Lamb wave interaction with crack from various incident directions. The simulation results showed the directionality effect of the scattering wave signals and suggested an optimum transmitter–sensor configuration. Two imaging methods were used: one involves the synthetic time reversal concept and the other involves Gaussian distribution function. Both imaging methods show very good agreement during simulations. Experiments were designed and conducted based on the simulated results. A network of eight piezoelectric wafer active sensors was used to capture the scattering waves from the crack. Both the pitch-catch and pulse-echo experimental modes were used. The directionality effect of incident Lamb waves on the imaging results was studied. The effect of summation, multiplication, and combined algorithms for each imaging method was studied. It was found that both methods can successfully predict the crack size and orientation. An attempt was made to use these imaging methods for detecting and sizing smaller sized damage (1- to 3-mm-diameter hole). It was found that these methods can successfully localize the hole, but size estimation was a bit challenging because of the smaller dimensions. The scattering waves for various hole sizes were studied.

This article presents a theoretical and numerical analysis of guided wave released during an acoustic emission event using excitation potentials. Theoretical formulation showed that guided wave generated using excitation potentials follows the Rayleigh–Lamb equation. The numerical studies predict the out-of-plane displacement of acoustic emission guided wave on the plate surface at some distance away from the source. Parameter studies were performed to evaluate the effect of (1) pressure and shear potentials acting alone and in combination, (2) plate thickness, (3) source depth, (4) rise time, and (5) propagating distance away from the source. Numerical results showed that peak amplitude of S0 signal increases with increasing plate thickness, whereas the peak amplitude of A0 signal initially decreases and then increases with increasing plate thickness. Regarding the source depth, it was found that peak amplitude of S0 signal decreases and A0 signal increases with increasing source depth. Peak time showed a notable contribution to the low-frequency component of A0 signal. There were large losses in S0 and A0 peak signal amplitude over the propagation distance from 100 to 500 mm.

This paper addresses an analytical and experimental analysis based on the physics of the Lamb wave propagation and interaction with the discontinuity. An analytical method called “complex modes expansion with vector projection (CMEP)” is used to calculate the scattering coefficients (amplitude of the out-of-plane velocity) of Lamb wave modes from geometric discontinuities. Two cases are considered in this research: (a) a plate with a pristine stiffener and (b) a plate with a cracked stiffener. Complex-valued scattering coefficients are calculated from 50 kHz to 350 kHz for A0 incident waves. Scatter coefficients are compared for both cases to identify a suitable frequency range to excite Lamb waves using a piezoelectric wafer active sensor (PWAS) to detect the crack. The frequency-dependent complex-valued scatter coefficients are then inserted into the global analytical model. The global analytical solution predicts time domain scattered signal from the discontinuity. The crack can be detected by comparing the waveforms for pristine stiffener and cracked stiffener. An experiment was conducted for both pristine stiffener and cracked stiffener to compare with the analytical results. A long PWAS was placed parallel to the waveform to create straight crested Lamb wave modes in the plate. Antisymmetric Lamb wave modes were selectively excited by using two PWAS transducers placed on opposite sides of the plate and energized by out-of-phase signals. A single-point laser Doppler vibrometer (LDV) was used to measure the out-of-plane velocity of scattered Lamb waves on the plate. The obtained experimental results agree well with the analytical predictions.

Guided wave attenuation in composites due to material damping is strong, anisotropic, and cannot be neglected. Material damping is a critical parameter in selection of a particular wave mode for long-range structural health monitoring in composites. In this article, a semi-analytical finite element approach is presented to model guided wave excitation and propagation in damped composite plates. The theoretical framework is formulated using finite element method to describe the material behavior in the thickness direction while assuming analytical expressions in the wave propagation direction along the plate. In the proposed method, the Kelvin–Voigt damping model using a complex frequency-dependent stiffness matrix is utilized to account for anisotropic damping effects of composites. Thus, the existing semi-analytical finite element approach is being extended to include material damping effect. Theoretical predictions are experimentally validated using scanning laser Doppler vibrometer measurements of guided wave propagation generated by a circular piezoelectric wafer active sensor transducer in a unidirectional carbon fiber reinforced polymer composite plate. The proposed method achieves good agreement with the experimental results.

This paper presents cylindrical coordinate solutions of axis symmetric circular crested elastic waves that appear due to sudden energy release during incremental crack propagation in a plate. Axis symmetric assumption decouples the elastic wave problem to Lamb (P+SV) and shear (SH) horizontal waves. Helmholtz decomposition principle was used to decompose displacement field in to unknown scalar and vector potentials; and body force vectors to known excitation scalar and vector potentials respectively. Therefore, Navier–Lame equations yield a set of four inhomogeneous wave equations of unknown potentials Φ, Hr, Hθ, Hz and known excitation potentials A * , B r * , B θ * , B z * . There are two types of potentials exist in a plate for axis symmetric circular crested Lamb wave: pressure potentials Φ, A* and shear potentials Hθ, B θ * . Inhomogeneous wave equations for Φ and Hθ were solved due to generalized excitation potentials A* and B θ * in a form, suitable for numerical calculation. The theoretical formulation shows that elastic waves generated in a plate using excitation potentials follow the Rayleigh-Lamb equations. The resulting solution is a series expansion containing the superposition of all the Lamb wave modes existing for the particular frequency-thickness combination under consideration. In addition, bulk wave solution is also recovered due to the effect of the excitation potentials. The numerical studies modeled the two-dimensional (2D) (circular crested) AE elastic wave propagation in order to simulate the out-of-plane displacement that would be recorded by an AE sensor placed on the plate surface at some distance away from the source. Parameter studies were performed to evaluate: (a) the effect of the pressure and shear potentials; (b) the effect of the thickness-wise location of the excitation potential sources varying from mid-plane to the top surface (source depth effect); (c) the effect of peak time (d) the effect of propagating distance away from the source. A Gaussian pulse is used to model the growth of the excitation potentials during the AE event; as a result, the actual excitation potential follows the error function variation in the time domain. The numerical studies show that the peak amplitude of A0 signal is higher than the peak amplitude of S0 signal and the peak amplitude of bulk wave is not significant compared to S0, A0 peak signals.

Piezoelectric wafer active sensors have been widely used for Lamb-wave generation and acquisition. For selective preferential excitation of a certain Lamb-wave mode and rejection of other modes, the piezoelectric wafer active sensor size and the excitation frequency should be tuned. However, structural damping depends on the structure material and the excitation frequency and it will affect the amplitude response of piezoelectric wafer active sensor–excited Lamb waves in the structure, that is, tuning curves. Its influence on the piezoelectric wafer active sensor tuning reflects the effect of structural health monitoring configuration considered in the excitation. Therefore, it is important to have knowledge about the effect of structural damping on the tuning between piezoelectric wafer active sensor and Lamb waves. In this article, the analytical tuning solution of undamped media is extended to damped materials using the Kelvin–Voigt damping model, in which a complex Young’s modulus is utilized to include the effect of structural damping as an improvement over existing models. This extension is particularly relevant for the structural health monitoring applications on high-loss materials, such as metallic materials with viscoelastic coatings and fiber-reinforced polymer composites. The effects of structural damping on the piezoelectric wafer active sensor tuning are successfully captured by the improved model, with experimental validations on an aluminum plate with adhesive films on both sides and a quasi-isotropic woven composite plate using circular piezoelectric wafer active sensor transducers.

In this article, the effect of the acoustic emission sensor on the acoustic emission waveforms from fatigue crack growth in a thin aerospace specimen is presented. In situ acoustic emission fatigue experiments were performed on the test coupons made of aircraft grade aluminum plate. Commercial Mistras S9225 acoustic emission sensor and piezoelectric wafer active sensor were used to capture the acoustic emission waveforms from the fatigue crack. It has been shown that the piezoelectric wafer active sensor transducer successfully captured the fatigue crack–related acoustic emission waveforms in the thin plate. The piezoelectric wafer active sensor transducer seems to capture more frequency information of the acoustic emission waveform than the conventional acoustic emission sensor in this particular application. We have also shown the evolution of the acoustic emission waveforms as the fatigue crack grows. The signatures of the fatigue crack growth were captured by the evolution of the acoustic emission waveforms. This waveform evolution is highly related to the physical boundary conditions of the cracks as well as the fatigue crack growth mechanism. The fatigue loading and acoustic emission measurement were synchronized using the same acoustic emission instrumentation. This synchronization provided the exact load level when the acoustic emission signals had occurred during the fatigue crack growth.

In this study, we focus on analyzing the acoustic emission waveforms of the fatigue crack growth despite the conventional statistics-based analysis of acoustic emission. The acoustic emission monitoring technique is a well-known approach in the non-destructive evaluation/structural health monitoring research field. The growth of the fatigue crack causes the acoustic emission in the material that propagates in the structure. The acoustic emission happens not only from the crack growth but also from the interaction of the crack tips during the fatigue loading in the structure. The acoustic emission waveforms are generated from the acoustic emission events; they propagate and create local vibration modes along the crack faces (crack resonance). In-situ fatigue and acoustic emission experiments were conducted to monitor the acoustic emission waveforms from the fatigue cracks. Several test specimens were used in the fatigue experiments, and corresponding acoustic emission waveforms were captured. The acoustic emission waveforms were analyzed and distinguished into three types based on the similar nature in both time and frequency domains. Three-dimensional harmonic finite element analyses were performed to identify the local vibration modes. The local crack resonance phenomenon has been observed from the finite element simulation that could potentially give the geometric information of the crack. The laser Doppler vibrometry experiment was performed to identify the crack resonance phenomenon, and the experimental results were used to verify the simulated results.

Multiple approaches and programs are available to the public to predict ultrasonic guided-wave propagation dispersion curves in a material. Each approach and program will have its own advantage and disadvantage making it suitable for a specific end use and less suitable for others. This manuscript aims to compare three different guided-ultrasonic wave dispersion curves retrieval methods for multiple cases. First a single layer of isotropic, unidirectional, orthotropic and monoclinic material is examined, followed by a multi-layer case (consisting of 10 layers of the aforementioned materials) and finally four different laminates and one sandwich laminate are evaluated. The goal of this manuscript is to give a concise overview of the advantages and limitations of each approach to assist the end user in choosing the right program to use.

The manufacturing process of carbon fiber reinforced polymer (CFRP) composite structures can introduce many characteristic defects and flaws such as fiber misorientation, fiber waviness, and wrinkling. Therefore, it becomes increasingly important to detect the presence of these defects at the earliest stages of development. Eddy current testing (ECT) is a nondestructive inspection (NDI) technique that has been proven quite effective in detection of damage in metallic structures. However, NDI of composite structures has mainly relied on other methods such as ultrasonic testing (UT) and X-ray to name a few and not much on ECT. In this paper, the authors explore the possibility of using ECT in NDI of CFRP composites by conducting simulations and experiments thereafter. This research is based on the fact that the CFRP displays some low-level electrical conductivity due to the inherent conductivity of the carbon fibers. This low-level conductivity may permit eddy current pathways to cause the flow of eddy currents in the CFRP composites that can be exploited for nondestructive damage detection. An invention disclosure describing our high-frequency ECT method has also been processed. First, the multiphysics finite element method (FEM) simulation was used to simulate the detection of various types of manufacturing flaws and operational damage in CFRP composites such as fiber misorientation, waviness, wrinkling, and so on. Thereafter, ECT experiments were conducted on CFRP specimens with various manufacturing flaws using the Eddyfi Reddy eddy current array (ECA) system.

This paper presents the interaction of Lamb waves with rivet hole cracks from multiple directions of incident using the finite element approach. Lamb waves undergo scattering and mode conversion after interacting with the damage. Shear horizontal waves appear in the scattered waves because of the mode conversion. Instead of analyzing the whole large structure, the local damage area is analyzed using finite element analyses and analytical formulation is used to analyze the whole structure. The scatter fields are described in terms of wave damage interaction coefficients that involve scattering and mode conversion of Lamb waves. Lamb wave mode (S0 and A0) hit the damage from multiple directions and corresponding wave damage interaction coefficients are obtained around the damage. Harmonic analysis has been performed over the fundamental frequency domain and “scatter cubes” of complex-valued wave damage interaction coefficients are formed. The scatter cube provides the information of relative amplitude and phase of scattered waves around the damage that can be used for designing the sensor installation. An application based on real time domain signal has been illustrated for the problem of multiple-rivet-hole cracks using the scatter cubes with the analytical framework.

In this paper, an attempt has been made to conduct controlled low velocity impact experiments in quasi-isotropic carbon fiber reinforced polymer (CFRP) composites of increasing thicknesses to generate barely visible impact damage (BVID) that creates 1″ impact damage diameter. Such impacted CFRP coupons with controlled damage diameter would serve in the future for validation of structural health monitoring (SHM) and nondestructive evaluation (NDE) damage detection methodologies. Various 2-mm, 4-mm and 6-mm thick quasi-isotropic CFRP composite plates having similar stacking sequence were manufactured in a compression molding machine. These plates were cut into numerous 6″ × 4″ coupons to be impacted in accordance with ASTM D7136 standard on a Dynatup impact testing machine. The goal of the experiments was to identify the combination of impactor mass, energy and momentum at which approximately 1″ impact damage diameter could be produced for test coupons of increasing thicknesses. Interesting observations were made with respect to the size and shape of the impact damage as the thickness of the composite coupons was increased from 2-mm to 6-mm. Modified experiments were also conducted on 6-mm coupons to achieve close to 1″ impact damage diameter. The greatest challenge in conducting these experiments was in the thick 4-mm and 6-mm coupons which did not display a predictable trend. Future work should focus on overcoming this challenge towards achieving a predictable impact damage size methodology.

This paper proposes a new single-mode guided wave-based method to detect various types of damage in aerospace composites. In this method, single-mode guided wave excitation was achieved using adjustable angle beam transducers (ABT). The ABT tuning angles of various pure-mode guided waves were calculated based on Snell's law applied to the composite dispersion curves. A finite element (FE) simulation of pure S0 mode excitation in a crossply composite plate was conducted and the simulation results were validated by the experiment. For the first time, angle beam transducers were applied to generate pure shear horizontal (SH0) wave in a thick quasi-isotropic composite plate. The pure SH0 wave excitation was successfully verified by a three-dimensional (3D) FE simulation. SH0-mode wave propagation and interaction with delaminations were further conducted and strong trapped waves within the delamination regions were observed. Experiments using S0 or SH0 pure-mode guided waves were conducted to detect various types of composite damage, such as wrinkle damage in the crossply composite plate, multilayer delaminations by Teflon inserts, and actual impact damage in the thick quasi-isotropic composite plate. A significant amplitude drop was observed due to the presence of different composite damage types. In addition, a linear scanning method using pure SH0 wave was also developed to estimate the sizes of delaminations and impact damage. Both numerical and experimental results demonstrated the validity and usefulness of the proposed method for the detection of various damage types in composites.

The paper deals with the development of low-cost tools for fast computer simulation of guided wave propagation and diffraction in plate-like structures of variable thickness. It is focused on notched surface irregularities, which are the basic model for corrosion damages. Their detection and identification by means of active ultrasonic structural health monitoring technologies assumes the use of guided waves generated and sensed by piezoelectric wafer active sensors as well as the use of laser Doppler vibrometry for surface wave scanning and visualization. To create a theoretical basis for these technologies, analytically based computer models of various complexity have been developed. The simplest models based on the Euler–Bernoulli beam and Kirchhoff plate equations have exhibited a sufficiently wide frequency range of reasonable coincidence with the results obtained within more complex integral equation based models. Being practically inexpensive, they allow one to carry out a fast parametric analysis revealing characteristic features of wave patterns that can be then made more exact using more complex models. In particular, the effect of resonance wave energy transmission through deep notches has been revealed within the plate model and then validated by the integral equation based calculations and experimental measurements.

This paper presents a study to understand the physical nature of fatigue crack growth as an acoustic emission source and detectability of the crack length form the recorded acoustic emission signal in plate structures. For most of the thin walled engineering structures, the acoustic emission detection through sensor network has been well established. However, the majority of the research is focused on prediction of the acoustic emission due to fatigue crack growth using stochastic methods. Where, stochastic models are used to predict the criticality of the damage. The scope of this research is to use predictive simulation method for acoustic emission signals and extract the damage related information from acoustic emission signals based on physics of material. This approach is in contrast with the traditional approach involving statistics of acoustic emissions and their relation with damage criticality. In this article, first, we present our approach to understand fatigue crack growth as source of acoustic emission using physics of guided wave propagation in FEM. Then, using this physical understanding, we present our investigation on detectability of crack lengths directly from crack-generated acoustic emission signals. Finally, we present our method to extract fatigue crack length information from acoustic emission signals recorded during fatigue crack growth.

In this work, a predictive model of attenuated guided wave propagation in carbon fiber–reinforced polymer using Rayleigh damping is developed. After a brief introduction, this article reviews the theory of guided waves in anisotropic composite materials. It follows with a discussion of the piezoelectric wafer active sensors, which are lightweight and inexpensive transducers for structural health monitoring applications. Experiments were performed on a carbon fiber–reinforced polymer panel to measure the dispersion curves and the piezoelectric wafer active sensors tuning curves. Lamb wave damping coefficient was modeled using the multi-physics finite element method and compared with experimental results. A discussion about the capability to simulate, with multi-physics finite element method commercial software, guided wave in composite material using the Rayleigh damping is developed. This article ends with conclusion, and suggestions for further work are also presented.

This article presents the WaveFormRevealer—an analytical framework and predictive tool for the simulation of guided Lamb wave propagation and interaction with damage. The theory of inserting damage effects into the analytical model is addressed, including wave transmission, reflection, mode conversion, and nonlinear higher harmonics. The analytical model is coded into MATLAB, and a graphical user interface (WaveFormRevealer graphical user interface) is developed to obtain real-time predictive waveforms for various combinations of sensors, structural properties, and damage. In this article, the main functions of WaveFormRevealer are introduced. Case studies of selective Lamb mode linear and nonlinear interaction with damage are presented. Experimental verifications are carried out. The article finishes with summary and conclusions followed by recommendations for further work.

Fatigue cracking in sheet metal structures is a common problem during practical applications. It can cause a disastrous failure in the running condition of the structure if it is not addressed in time. A crack can potentially produce acoustic emission (AE) signals during the crack growth event as well as during rubbing/clapping of faying surfaces. All kind of AE signals from the crack provide useful information regarding the condition of the cracked structure. AE signals due to crack faying surface rubbing/clapping need to be identified and separated from AE signals due to other sources. This paper discusses the study of the AE signals generated due to rubbing/clapping of crack faying surfaces. The initial fatigue crack was generated in a sheet metal sample through fatigue loading. The sample was then vibrated at various frequencies using a vibration exciter, and AE signals were generated by vibration induced rubbing/clapping of crack faying surfaces. AE signal signature due to vibration induced rubbing/clapping was identified and studied. AE signal due to rubbing/clapping of sheet metal sample while undergoing an axial static load is also discussed in this paper. The sheet metal sample with fatigue crack was axially loaded using a test cell manufactured in-house for loading sheet metal samples. The samples were vibrated to excite AE signals, and the influence of static load on crack rubbing/clapping AE signals were studied. The vibration-induced crack faying surface motion generated a large number of AE signals. A comparison among these AE signals was performed by using the Pearson correlation of the signals. The first novelty of this research is experimental design and fabrication of an experimental setup for detecting vibration induced rubbing and clapping in fatigue cracked metallic structures. The second novelty of this research is the development of a novel correlation coefficient based approach for comparing a large number of AE signals.

This article presents an analytical model for power and energy transfer between excited piezoelectric wafer active sensors and host structure. This model is based on exact multimodal Lamb waves, normal mode expansion technique, and orthogonality of Lamb waves. Modal participation factors are presented to show the contribution of every mode to the total energy transfer. The model assumptions include the following: (1) straight-crested multimodal ultrasonic guided wave propagation, (2) propagating waves only, (3) ideal bonding (pin-force) connection between piezoelectric wafer active sensors and structure, and (4) ideal excitation source at the transmitter piezoelectric wafer active sensors. Constrained piezoelectric wafer active sensor admittance is reviewed. Electrical active power, mechanical converted power, and Lamb wave kinetic and potential energies are derived in closed-form formulae. Numerical simulations are performed for the case of symmetric and antisymmetric excitation of thin aluminum structure. The simulation results are compared with axial and flexural approximation for the case of low-frequency Lamb waves. In addition, a thick steel structure example is considered to illustrate the case of multimodal guided waves. A parametric study for different excitation frequencies and different transducer sizes is performed to show the best match of frequency and piezoelectric wafer active sensor size to achieve maximum energy transfer into the excited structure.

This paper presents a hybrid finite element and analytical method to predict the 1-D guided wave propagation interaction with damage for nondestructive evaluation (NDE) and structural health monitoring (SHM) application. The finite element mesh is used to describe the region around the damage (defects or flaws). In contrast to other hybrid models developed elsewhere, the interaction between Lamb waves and defects is computed in the time domain using the explicit solver of the commercial finite element method (FEM) software ABAQUS. Analytical methods can perform efficient modeling of wave propagation but are limited to simple geometries. Realistic structures with complicated geometries are usually modeled with the FEM. However, to obtain an accurate wave propagation solution at ultrasonic frequencies is computationally intensive and may become prohibitive for realistic structures. In response to today's complex cases not covered by the simulation tools available, we aim to develop an efficient and accessible tool for SHM applications. This tool will be based on a hybrid coupling between analytical solutions and time domain numerical codes. Lamb wave interaction with a notch is investigated by using this method, and the results obtained are with respect to transmission, reflection and mode conversion. Because of the symmetric mode shape, S0 is more sensitive to the shallow notch than A0. By making use of the fact that the reflection increases with increase in notch depth and mode conversion are maximized when the notch is around half through the thickness of the plate, the reflection and conversion coefficients can be used to characterize the depth of the notch.

This article presents a combined finite element method and analytical process to predict the one-dimensional guided- wave propagation for nondestructive evaluation and structural health monitoring application. Analytical methods can per- form efficient modeling of wave propagation but are limited to simple geometries. In response to today's most complex cases not covered by the simulation tools available, we aim to develop an efficient and accessible tool for structural health monitoring application. This tool will be based on a hybrid coupling between analytical solution and time-domain numerical codes. Using the principle of reciprocity, global analytical calculation is coupled with local finite element method analysis to utilize the advantages of both methods and obtain a rapid and accurate simulation method. The phe- nomenon of interaction between the ultrasonic wave, the defect, and the structure, leading to a complex signature, is efficiently simulated by this hybrid global-local approach and is able to predict the specific response signal actually received by sensor. The finite element mesh is used to describe the region around the defects/flaws. In contrast to other hybrid models already developed, the interaction between Lamb waves and defects is computed in the time domain using the explicit solver of the commercial finite element method software ABAQUS.

The advancement of composite materials in aircraft structures has led to an increased need for effective structural health monitoring technologies that are able to detect and assess damage present in composite structures. The study presented in this article is interested in understanding self-sensing piezoelectric wafer sensors to conduct electromechanical impedance spectroscopy in glass fiber reinforced polymer composite to perform structural health monitoring. For this objective, multi-physics-based finite element method is used to model the electromechanical behavior of a free piezoelectric wafer active sensor and its interaction with the host structure on which it is bonded. The multi-physics-based modeling permits the input and output variables to be expressed directly in electric terms, while the two-way electromechanical conversion is done internally in the multi-physics-based finite element method formulation. The impedance responses are also studied in conditions when the sensor bonding layer is subject to degradation and when the sensor itself is subjected to breakage, respectively. To reach the goal of using the electromechanical impedance spectroscopy approach to detect damage, several damage models are generated on simplified orthotropic structure and laminated glass fiber reinforced polymer structures. The effects of the modeling are carefully studied through experimental validation. A good match has been observed for low and high frequencies.

An experimental evaluation of the structural health monitoring capability of piezoelectric wafer active sensors on composite structures at cryogenic temperatures is presented. The piezoelectric wafer active sensor–based electromechanical impedance and the pitch–catch methods were first qualified for cryogenic temperatures using piezoelectric wafer active sensor–instrumented composite specimens dipped in liquid N2. Subsequently, damage detection experiments were performed on laboratory-scale composite specimens with (a) impact damage and (b) built-in Teflon patches simulating in service delaminations. Finally, a comprehensive damage detection test was performed on a full-scale specimen subjected to pressure and cryogenic temperature cycles. Based on these tests, we conclude that piezoelectric wafer active sensor–based structural health monitoring methods show promise for damage detection in composite materials even in extreme cryogenic conditions. Recommendations for further work are also included.

The modeling of the interaction between piezoelectric wafer active sensors (PWAS) and structural waves and vibration is addressed. Three main issues are discussed: (a) modeling of pitch-catch ultrasonic waves between a PWAS transmitter and a PWAS receiver by comparison between exact Lamb wave solutions and various finite element method (FEM) results; (b) analytical modeling of the power and energy transduction between PWAS and ultrasonic guided waves highlighting the tuning opportunities between PWAS and the waves; (c) the use of the transfer matrix method to model the electromechanical (E/M) impedance method for direct reading of high-frequency local structural vibration and comparison with FEM results. The paper ends with a summary and conclusions followed by recommendations for further work.

Piezoelectric wafer active sensors (PWAS) are lightweight and inexpensive enablers for a large class of structural health monitoring (SHM) applications. This paper presents and discusses the challenges and opportunities related to the use of PWAS in the structures specific to space applications. The challenges posed by space structures are often different from those encountered in conventional structures. After a review of PWAS principles, the paper discusses the multi-physics power and energy transduction between structurally guided waves and PWAS; predictive modeling results using a simplified analytical approach are presented. Experimental results on space-like specimen structures are presented. Survivability of PWAS transducers under cryogenic space-like conditions is experimentally verified. The paper ends with conclusions and suggestions for further work.

This article 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, we developed a predictive modeling of the multi-physics power and energy transduction between structurally guided waves and PWAS in closed-form analytical expressions. The model assumptions include: (a) one-dimensional axial and flexural wave propagations; (b) ideal bonding (pin-force) connection between PWAS and structure; and (c) ideal excitation source at the transmitter PWAS and fully resistive external load at the receiver PWAS. Both wave propagation method for an infinite beam and normal mode expansion method for a finite beam were considered. Frequency response functions were developed for voltage, current, complex power, active power, etc. The power and energy analysis for single PWAS transmitter, single PWAS receiver and a complete PWAS pitch-catch setup were considered. The numerical simulation and graphical charts show the trends in the power and energy flow behavior of PWAS for SHM. The peaks and valleys of power can be exploited for optimum design for PWAS SHM applications.

A time–frequency analysis-based signal processing study for detecting active corrosion in aluminum plate-like structure utilizing the broadband piezoelectric wafer active sensors is presented in this article. Tests were conducted on an aluminum plate with a network of sensors installed on one side of the plate for Lamb wave generation and reception. The corrosion was emulated as material loss of an area of 50 × 38 mm2 on the opposite side of the plate. The corroded area resulted in a thickness loss on the plate and a change in wave propagation as well. The experimental data were first evaluated by a statistical damage index (DI) based on root mean square values and then the Cohen’s class motivated cross-time–frequency analysis. The cross-time–frequency analysis was found more reliable and precise for detecting the corrosion progression when compared to the DI method. Not only can the proposed metric correctly evaluate the phase difference of specific frequency and time, it also carries useful information of phase difference, which is strongly correlated to the physics of corrosion detection using Lamb waves. Novel aspects of this study include a sensing approach that can sense corrosion damage on both external and internal surfaces of a given structure, the employment of effective tuning in corrosion detection, and using cross-time–frequency analysis to quantitatively evaluate thickness loss. Though the corrosion studied herein is an idealized and simplified situation, the subject work on phase difference and cross-time–frequency analysis is useful first-step effort and opens a new way to perform Lamb wave-based corrosion detection. The results presented in this article combine easy-to-examine corrosion assumptions together with low-frequency antisymmetric Lamb wave analysis to provide a stepping stone for more complicated analysis needed for further real life corrosion assessment.

An analytical investigation of the interaction between piezoelectric wafer active sensor (PWAS), guided Lamb waves, and host structure is presented in this paper, supported with application examples. The analytical investigation assumes a PWAS transducer bonded to the upper surface of an isotropic flat plate. Shear lag transfer of tractions and strains is assumed, and an analytical solution using the space-wise Fourier transform is reviewed, closed-form solutions are presented for the case of both ideal bonding (i.e., load transfer mechanism localized at the PWAS boundary) and not ideal bonding (i.e., load transfer mechanism localized close the PWAS boundary). The analytical solutions are used to derive Lamb wave mode tuning curves which indicate that frequencies exist at which the A0 mode or the S0 mode can be either suppressed or enhanced. The paper further shows that the capability to excite only one desired Lamb wave mode is critical for practical structural health monitoring applications such as PWAS phased array technique (e.g., the embedded ultrasonics structural radar, EUSR) and the sparse array imaging. Extensive experimental tests that verify the tuning mechanism and prediction curves are reported. Examples of correctly tuned EUSR images vs. detuned cases illustrate the paramount importance of Lamb wave mode tuning for the success of PWAS based damage detection.

Development of high-temperature piezoelectric wafer active sensors (HT-PWAS) using high-temperature piezoelectric material for harsh environment applications is of great interest for structural health monitoring of high-temperature structures such as turbine engine components, airframe thermal protection systems, and so on. This article presents a preliminary study with the main purpose of identifying the possibility of developing PWAS transducers for high-temperature applications. After a brief review of the state of the art and of candidate high-temperature piezoelectric materials, the article focuses on the use of gallium orthophosphate (GaPO4) samples in pilot PWAS applications. The investigation started with a number of confidence-building tests that were conducted to verify GaPO4 piezoelectric properties at room temperature and at elevated temperatures in an oven. Electromechanical (E/M) impedance measurements and material characterization tests (scanning electron microscopy, X-ray diffraction, energy dispersive spectrometry) were performed before and after exposure of HT-PWAS to high temperature; it was found that GaPO4 HT-PWAS maintain their properties up to 1300°F (∼705°C). In comparison, conventional PZT sensors lost their activity at around 500 F (∼260°C). Subsequently, HT-PWAS were fabricated and installed on metallic specimens in order to conduct an in situ evaluation of their high-temperature performance. A series of in situ tests were performed using the E/M impedance and pitch-catch methods; the tests were conducted in two situations: (a) before and after exposure to high temperature and (b) inside the oven. The experimental results show that the fabricated HT-PWAS can survive high oven temperatures up to 1300°F (∼705°C) and still present piezoelectric activity. The article also discusses fabrication techniques for high-temperature PWAS applications, including the wiring of the sensor ground and signal electrodes, bond layer adhesive selection, and preparation.

The paper presents an overview of the fundamental research performed in the US in the field of flight structures under the sponsorship of the Air Force Office of Scientific Research (AFOSR). After presenting a general overview of AFOSR, the paper focuses on the structural mechanics program. Three large applications areas are considered: (a) future flight structures; (b) structural sustainment; (c) structural dynamics and vibration control. These three broad areas are covered at various levels of complexity and detail. Currently supported topics are presented and major funding and results are discussed. The paper ends with summary and conclusions.

PWAS are small and non-invasive transducers that can be easily affixed to a structure to interrogate it at will. Their wideband non-resonant properties make them advantageous candidates for multiple modes structural health monitoring (SHM) and nondestructive evaluation (NDE) simply by changing the driving frequencies. This paper presents research results obtained by using PWAS transducer for SHM/NDE. The results are given in the following directions: (1) PWAS operation principles; (2) PWAS electromechanical impedance measurements; (3) PWAS ultrasonic SHM/NDE; and (4) PWAS multi-mode corrosion detection. The analytical methods and experimental results can be successfully used in the design of active SHM systems to be embedded on operational structures, such as aircraft panels.

In situ structural health monitoring aims to perform on-demand interrogation of the structure to determine the presence of service-induced damage and defects using non- destructive evaluation ultrasonic wave methods. Recently emerged piezoelectric wafer active sensors (PWAS) have the potential to significantly improve damage detection and health monitoring. PWAS are low-profile transducers that can be permanently attached onto the structure or inserted in between composite laminates, and can perform structural damage detection in thin-wall structures using guided wave methods (Lamb, Rayleigh, SH, etc.). This paper describes the analytical and experimental work of using PWAS-guided waves for in situ structural damage detection on thin-wall structures. We begin with reviewing the guided wave theory in plate structures and PWAS principles. The mechanisms of Lamb wave excitation and detection using PWAS is presented. Subsequently, we address in turn the use of PWAS to generate Lamb waves for damage (cracks and corrosion) detection in metallic structures. Pulse-echo, pitch-catch, phased array and time reversal methods are illustrated demonstrating that PWAS Lamb-waves techniques are suitable for damage detection and structural health monitoring. The last part of the paper treats analytically and experimentally PWAS excitation and tuning in composite materials. The research results presented in this paper show that in situ SHM methodologies using PWAS transducers hold the promise for more efficient, effective and timely damage detection in thin-wall structures.

Lamb wave time reversal method is a new and tempting baseline-free damage detection technique for structural health monitoring. With this method, certain types of damage can be detected without baseline data. However, the application of this method using piezoelectric wafer active sensors (PWAS) is complicated by the existence of at least two Lamb wave modes at any given frequency, and by the dispersion nature of the Lamb wave modes existing in thin-wall structures. The theory of PWAS-related Lamb wave time reversal has not yet been fully studied.

Distribution statement A: approved for public release. Distribution is unlimited Abstract: This paper presents the perspective of the Structural Mechanics program of the Air Force Office of Scientific Research on the damage assessment of structures. It is found that damage assessment of structures plays a very important role in assuring the safety and operational readiness of Air Force fleet. The current fleet has many aging aircraft, which poses a considerable challenge for the operators and maintainers. The nondestructive evaluation technology is rather mature and able to detect damage with considerable reliability during the periodic maintenance inspections. The emerging structural health monitoring methodology has great potential, because it will use on-board damage detection sensors and systems, will be able to offer on-demand structural health bulletins. Considerable fundamental and applied research is still needed to enable the development, implementation, and dissemination of structural health monitoring technology.

Nastic structures are synthetic constructs capable of controllable deformation and shape change similar to plant motility, designed to imitate the biological process of nastic movement found in plants. This paper considers the mechanics and bioenergetics of a prototype nastic structure system consisting of an array of cylindrical microhydraulic actuators embedded in a polymeric plate. Non-uniform expansion/contraction of the actuators in the array may yield an overall shape change resulting in structural morphing. Actuator expansion/contraction is achieved through pressure changes produced by active transport across a bilayer membrane. The active transport process relies on ion-channel proteins that pump sucrose and water molecules across a plasma membrane against the pressure gradient. The energy required by this process is supplied by the hydrolysis of adenosine triphosphate. After reviewing the biochemistry and bioenergetics of the active transport process, the paper presents an analysis of the microhydraulic actuator mechanics predicting the resulting displacement and output energy. Experimental demonstration of fluid transport through a protein transporter follows this discussion. The bilayer membrane is formed from 1-Palmitoyl-2-Oleoyl-sn-Glycero-3-[Phospho-L-Serine] (Sodium Salt), 1-Palmitoyl-2-Oleoyl-sn-Glycero- 3-Phosphoethanolamine lipids to support the AtSUT4 H+-sucrose cotransporter.

Summary For some time, engineers and scientists have been using the nature as an inspiration for their design of advanced robotics and mechanical systems. Today, nature has also become the inspiration for an entire new class of material systems – the adaptronic structures [1]. Using biological analogies and nature's ability to adapt a material's structure, morphology, shape, and properties to accommodate its changing environment and its aging process, present day engineers and scientists are designing material systems that can change their shape, control their vibrations, monitor their own health, and perform many other biological-inspired functions.

A project to enhance the mechatronics/microcontroller education of mechanical engineering students at the University of South Carolina is presented. First, the state of the art in Mechatronics education is presented and discussed. Then, focus is shifted to the Mechatronics education in the Department of Mechanical Engineering at the University of South Carolina. Subsequently, the paper examines the hardware and software used for mechatronics/microcontroller education. Examples are given of the MC68HC11 microcontroller and the different evaluation boards used for (a) code development; (b) embedded applications. Then, attention is given to the software used in the mechatronics/microcontroller education. The THRSim11 comprehensive simulation and interfacing software is described. Finally, the paper discusses the interfacing between the microcontroller and the various electro-mechanical sensing and actuation components used in a mechatronics project. The use of functional modules for teaching interfacing skills to mechanical engineering students is described. The paper finishes with conclusions and further work.

The use of the electro-mechanical (E/M) impedance method for health monitoring of thin plates and aerospace structures is described. As a nondestructive evaluation technology, the E/M impedance method allows us to identify the local dynamics of the structure directly through the E/M impedance signatures of piezoelectric wafer active sensors (PWAS) permanently mounted to the structure. An analytical model for 2-D thin-wall structures, which predicts the E/M impedance response at PWAS terminals, was developed and validated. The model accounts for axial and flexural vibrations of the structure and considers both the structural dynamics and the sensor dynamics. Calibration experiments performed on circular thin plates with centrally attached PWAS showed that the presence of damage modifies the high-frequency E/M impedance spectrum causing frequency shifts, peak splitting, and appearance of new harmonics. Overall-statistics damage metrics and probabilistic neural network (PNN) are used to classify the spectral data and identify damage severity. On thin-wall aircraft panels, the presence of damage influences the sensors E/M impedance spectrum. When crack damage is in the PWAS medium field, changes in the distribution of harmonics take place and when crack damage is in the PWAS near field, changes in both the harmonics and the dereverberated response are observed. These effects are successfully classified with PNN and overall-statistics metrics, respectively. This proves that permanently attached PWAS in conjunction with the E/M impedance method can be successfully used in structural health monitoring to detect the presence of incipient damage through the examination and classification of the high-frequency E/M impedance spectra.

The capability of embedded piezoelectric wafer active sensors (PWAS) to excite and detect tuned Lamb waves for structural health monitoring is explored. First, a brief review of Lamb waves theory is presented. Second, the PWAS operating principles and their structural coupling through a thin adhesive layer are analyzed. Then, a model of the Lamb waves tuning mechanism with PWAS transducers is described. The model uses the space domain Fourier transform. The analysis is performed in the wavenumber space. The inverse Fourier transform is used to return into the physical space. The integrals are evaluated with the residues theorem. A general solution is obtained for a generic expression of the interface shear stress distribution. The general solution is reduced to a closed-form expression for the case of ideal bonding which admits a closed-form Fourier transform of the interfacial shear stress. It is shown that the strain wave response varies like sin a, whereas the displacement response varies like sinc a. Maximum coupling is achieved when the PWAS length equals the half wavelength of a particular Lamb wave mode. Since Lamb wave modes wavelengths vary with frequency, the tuning of certain modes at certain frequencies can thus be achieved. Tuning curves are derived and verified against experimental results. A particular S0 mode ‘sweet spot’ is found at 300 kHz for a 7-mm PWAS attached to a 1.6-mm aluminum plate. Crack detection via the pulse echo technique using the phased array principle and tuned S0 mode Lamb waves is demonstrated as an effective structural health monitoring method.

Piezoelectric wafer active sensors can be applied to aging aircraft structures to monitor the onset and progress of structural damage such as fatigue cracks and corrosion. Two main detection strategies are considered: (a) the wave propagation method for far-field damage detection; and (b) the electro-mechanical (E/M) impedance method for near-field damage detection. These methods are developed and verified on simple-geometry specimens, and then tested on realistic aging-aircraft panels with seeded cracks and corrosion. The specimens instrumentation with piezoelectric-wafer active sensors and ancillary apparatus is presented. The experimental methods, signal processing, and damage detection algorithms, tuned to the specific method used for structural interrogation, are discussed. In the wave propagation approach, the pulse-echo and acousto-ultrasonic methods were considered. Reflections from seeded cracks were successfully recorded. In addition, acoustic emission and low-velocity impact were also detected. In the E/M impedance method approach, the high-frequency spectrum is processed using overall-statistics damage metrics. The (1- R2)3 damage metric, where R is the correlation coefficient, was found to yield the best results. The simultaneous use of the E/M impedance method in the near field and of the wave propagation method in the far field opens the way for a comprehensive multifunctional damage detection system for aging aircraft structural health monitoring.

High-field theoretical and experimental analysis of piezoelectric and magnetostrictive actuators is presented. First, the analysis of a piezoelectric stack actuator (PiezoSystems Jena PAHL 120/20) is described. A theoretical model based on the linear theory of piezoelectricity is developed. Extensive experiments were conducted, aimed at lowfrequency dynamic electro-mechanical behavior characterization. Curve fitting procedures are used to adjust the model coefficients for various load levels. Through comparison with experimental data, the model is adjusted to include nonlinear terms related to higher losses on the unloading cycle. Second, the impedance analysis of a magnetostrictive actuator (Etrema AA140J025) is described. Linear piezomagnetism is assumed, as an approximation to nonlinear magnetostrictive behavior about a bias point. Low-field and high-field impedance measurements were performed, revealing left shifting of the actuator resonance as the power is increased. Model tuning of the impedance model on the experimental data showed material parameters trends similar with those reported in the literature. Although the numerical values developed during this phenomenological study are particular for the actuators under consideration, the characterization approach can be extended to analysis of other actuators of this type.

In this paper we present the results of a systematic theoretical and experimental investigation of the fundamental aspects of using piezoelectric wafe active sensors (PWASs) to achieve embedded ultrasonics in thin-gage beam and plate structures. This investigation opens the path for systematic application of PWASs forin situ health monitoring. After a comprehensive review of the literature, we present the principles of embedded PWASs and their interaction with the host structure. We give a brief review of the Lamb wave principles with emphasis on the understanding the particle motion wave speed/group velocity dispersion. Finite element modeling and experiments on thin-gage beam and plate specimens are presented and analyzed. The axial (S 0) and flexural (A 0) wave propagation patterns are simulated and experimentally measured. The group-velocity dispersion curves are validated. The use of the pulse-echo ultrasonic technique with embedded PWASs is illustrated using both finite element simulation and experiments. The importance of using high-frequency waves optimally tuned to the sensor-structure interaction is demonstrated. In conclusion, we discuss the extension of these results toin situ structural health monitoring using embedded ultrasonics.

The use of embedded piezoelectric-wafer active-sensors for in-situ structural health monitoring of thin-wall structures is presented. Experiments performed on aircraft-grade metallic specimens of various complexities exemplified the detection procedures for near-field and far-field damage. For near-field damage detection, the electro-mechanical (E/M) impedance method was used. Systematic experiments conducted on statistical samples of incrementally damaged specimens were followed by illustrative experiments on realistic aging aircraft panels. For far-field damage detection, guided ultrasonic Lamb waves were utilized in conjunction with the pulse-echo technique. Systematic experiments conducted on aircraft-grade metallic plates were used to develop the method, while experiments performed on realistic aging-aircraft panels exemplified the crack detection procedure.

Piezoelectric wafer active sensors may be applied on aging aircraft structures to monitor the onset and progress of structural damage such as fatigue cracks and corrosion. The state of the art in piezoelectric-wafer active sensors structural health monitoring and damage detection is reviewed. Methods based on (a) elastic wave propagation and (b) the Electro-Mechanical (E/M) impedance technique are cited and briefly discussed. For health monitoring of aging aircraft structures, two main detection strategies are considered: the E/M impedance method for near field damage detection, and wave propagation methods for far-field damage detection. These methods are developed and verified on simple-geometry specimens and on realistic aging aircraft panels with seeded cracks and corrosion. The experimental methods, signal processing, and damage detection algorithms are tuned to the specific method used for structural interrogation. In the E/M impedance method approach, the high-frequency spectrum, representative of the structural resonances, is recorded. Then, overall-statistics damage metrics can be used to compare the impedance signatures and correlate the change in these signatures with the damage progression and intensity. In our experiments, the (1 R2)3 damage metric was found to best fit the results in the 300-450 kHz band. In the wave propagation approach, the pulse-echo and acousto-ultrasonic methods can be utilized to identify the additional reflections generated from crack damage and the changes in transmission phase and velocity associated with corrosion damage. The paper ends with a conceptual design of a structural health monitoring system and suggestions for aging aircraft installation utilizing active-sensor arrays, data concentrators, wireless transmission, and a health monitoring and processing unit.

A method for characterizing smart materials actuators in full-stroke static and low frequency dynamic regimes is presented. At first, static and dynamic linear models of the piezoelectric actuator response under electro-mechanical excitation are discussed. Then, a thorough static and dynamic experimental characterization on a typical large-stroke piezoelectric actuator is performed. The measurements indicated a strong dependence of the actuator stiffness and piezoelectric properties on the electromechanical loading. The comparison of the model with the measured behavior is performed and the material coefficients are tuned to locally match the observed nonlinear behavior. The comparison also allows the identification of key parameters of the induced-strain actuator model. These parameters are necessary for design optimization towards maximum mechanical energy output and minimum electrical power input. The paper provides useful basic data for the design of actuation systems incorporating active materials actuators.