Victor Giurgiutiu

Tuned Lamb Wave Excitation and Detection with Piezoelectric Wafer Active Sensors for Structural Health Monitoring

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 Embedded Active Sensors for Aging Aircraft Structural Health Monitoring

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.

Embedded Self-Sensing Piezoelectric Active Sensors for On-Line Structural Identification

The benefits and limitations of using embedded piezoelectric active sensors for structural identification at ultrasonic frequency are highlighted. An analytical model based on structural vibration theory and theory of piezoelectricity was developed and used to predict the electro-mechanical (E/M) impedance response, as it would be measured at the piezoelectric active sensors terminals. The model considers one-dimension structures and accounts for both axial and flexural vibrations. Experiments were conducted on simple specimens in support of the theoretical investigation, and on realistic turbine blade specimen to illustrate the methods potential. It was shown that E/M impedance spectrum recorded by the piezoelectric active sensor accurately represents the mechanical response of a structure. It was further proved that the response of the structure is not modified by the presence of the sensor, thus validating the latters noninvasive characteristics. It is shown that such sensors, of negligible mass, can be permanently applied to the structure creating a nonintrusive sensor array adequate for on-line automatic structural identification and health monitoring. The sensor calibration procedure is outlined. Numerical estimation of the noninvasive properties of the proposed active sensors in comparison with conventional sensors is presented. Self-diagnostics capabilities of the proposed sensors were also investigated and methods for automatic self-test implementation are discussed. The paper underlines that the use of piezoelectric wafer active sensors is not only advantageous, but, in certain situations, may be the sole investigative option, as in the case of precision machinery, small but critical turbine-engine parts, and computer industry components.

Characterization of Piezoelectric Wafer Active Sensors

In the beginning, the classical one-dimensional analysis of piezoelectric active sensors is reviewed. The complete derivation for a free-free sensor is then extended to cover the cases of clamped and elastically constrained sensors. An analytical model based on structural vibration theory and theory of piezoelectricity was developed and used to predict the electromechanical (E/M) impedance response, as it would be measured at the piezoelectric active sensor’s terminals. The model considers one-dimensional structures and accounts for both axial and flexural vibrations. The numerical analysis was performed and supported by experimental results. Experiments were conducted on simple beam specimens to support the theoretical investigation, and on thin gauge aluminum plates to illustrate the method’s potential. It was shown that E/M impedance spectrum recorded by the piezoelectric active sensor accurately represents the mechanical response of a structure. It was further proved that the response of the structure is not modified by the presence of the sensor, thus validating the sensor’s non-invasive characteristics. The sensor calibration procedure is outlined and statistical analysis was presented. It was found that PZT active sensors have stable and repeatable characteristics not only in as-received condition, but also while mounted on 1-D or 2-D host structure. It is shown that such sensors, of negligible mass, can be permanently applied to the structure creating a non-intrusive sensor array adequate for on-line automatic structural identification and health monitoring.

Electro-Mechanical Impedance Method for Crack Detection in Thin Plates

This paper describes the utilization of Electro-Mechanical (E/M) impedance method for structural health monitoring of thin plates. The method allows the direct identification of structural dynamics by obtaining its E/M impedance or admittance signatures. The analytical model for two-dimensional structure was developed and verified with experiments. Good matching of experimental results and calculated spectra was obtained for axial and flexural components. The ability of the method to identify the presence of damage was investigated by performing an experiment where the damage in the form of crack was simulated with an EDM slit placed at various distances from the sensor. It was found that the crack presence dramatically modifies the E/M impedance spectrum and this modification decreases as the distance between the sensor and the crack increases. Several overall-statistics damage metrics, which may be used for on-line structural heath monitoring, were investigated. Among these candidate damage metrics, the αth power of the correlation coefficient deviation, CCDα,3< α <7, used in the high frequency band 300-450 kHz, was found to be most successful. Careful selection of the high frequency band and proper choice of the appropriate damage metric were found to be essential for successful damage detection and structural health monitoring.

Embedded non-destructive evaluation for structural health monitoring, damage detection, and failure prevention

In this paper we review the state of the art in an emerging new technology: embedded ultrasonic non-destructive evaluation (NDE). Embedded ultrasonic NDE permits active structural health monitoring, i.e. the on-demand interrogation of the structure to determine its current state of structural health. The enabling element of embedded ultrasonic NDE is the piezoelectric wafer active sensor (PWAS). We begin by reviewing the guided wave theory in plate, tube, and shell structures, with special attention to Lamb waves. The mechanisms of Lamb wave excitation and detection with embeddable PWAS transducers is presented. It is shown analytically and verified experimentally that Lamb wave mode tuning can be achieved by the judicious combination of PWAS dimensions, frequency value, and Lamb mode characteristics. Subsequently, we address in turn the use of pitch-catch, pulse-echo, and phased array ultrasonic methods for Lambwave damage detection. In each case, the conventional ultrasonic NDE results are contrasted with embedded NDE results. Detection of cracks, disbonds, delaminations, and diffuse damage in metallic and composite structures are exemplified. Other techniques, such as the time reversal method and the migration technique, are also presented. The paper ends with conclusions and suggestions for further work.

In situ 2-D piezoelectric wafer active sensors arrays for guided wave damage detection

This paper presented development work of an in situ method for damage detection in thin-wall structures using embedded two-dimensional ultrasonic phased arrays. Piezoelectric wafer active sensors were used to generate and receive guided Lamb waves propagating in the plate-like structure. The development of a generic beamforming algorithm that does not require parallel ray assumption through using full wave propagation paths is described. A virtual beam steering method and device, the embedded ultrasonic structural radar, was implemented as a signal post-processing procedure. Several two-dimensional configurations were investigated and compared with beamforming simulation. Finally, rectangular shape arrays were developed for verifying the generic formulas and omnidirectionality. The rectangular arrays yield good directionality within the 360 degrees full range and are able to detect damage anywhere in the entire plate.

Embedded-ultrasonics Structural Radar for In Situ Structural Health Monitoring of Thin-wall Structures:

Embedded-ultrasonics structural radar (EUSR) is a new concept and methodology for in situ nondestructive evaluation (NDE) and structural health monitoring (SHM) of thin-wall structures. EUSR consists of: (a) an array of piezoelectric wafer active sensors (PWAS) embedded into the structure; and (b) electronic modules for signal transmission/reception, processing, and interpretation. EUSR utilizes guided elastic waves (Lamb waves) generated omnidirectionally into the thin-wall structure by surface-mounted permanently attached PWAS. The paper starts with the general concepts of the EUSR algorithm: transmission beamforming, reception beamforming, and time-of-fight (TOF) determination. Next, details of the Lamb wave generation with PWAS, verification of group-velocity dispersion curves, identification of optimal excitation frequency, and confirmation of wave front omnidirectionality are discussed. In the third part of the paper, the actual implementation of the EUSR method in a proof-of-concept demonstration is presented. The construction of the PWAS-phased array is described, and detection of cracks located broadside and offside of the PWAS array is illustrated. The method is shown to be easy to use through a visually interactive LabView™ interface. Very good detection accuracy is observed. The proof-of-concept experiments presented in this paper were illustrated on metallic structures; however, the EUSR concept may also work on composite and hybrid structures, although the range of detection may be reduced by the medium attenuation.

DAMAGE IDENTIFICATION IN AGING AIRCRAFT STRUCTURES WITH PIEZOELECTRIC WAFER ACTIVE SENSORS

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-R 2)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.

Damage Detection in Thin Plates and Aerospace Structures with the Electro-Mechanical Impedance Method

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.