Buli Xu

Development and Testing of High-temperature Piezoelectric Wafer Active Sensors for Extreme Environments

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.

Durability and Survivability of Piezoelectric Wafer Active Sensors on Metallic Structure

Piezoelectric wafer active sensors are small, inexpensive, noninvasive, elastic wave generators/receptors that can be easily affixed to a structure. Piezoelectric wafer active sensor installation on the health-monitored structure is an important step that may have significant bearing on the success of the health monitoring process. The purpose of this paper is to explore the durability and survivability issues associated with various environmental conditions on piezoelectric wafer active sensors for structural health monitoring. The durability and survivability of the piezoelectric wafer active sensor transducers under various exposures (cryogenic and high temperature, temperature cycling, outdoor environment, operational fluids, large strains, fatigue load cycling) were considered over a long period of time. Both free piezoelectric wafer active sensors and bonded piezoelectric wafer active sensors on metallic structural substrates were used. Different adhesives and protective coatings were compared to find the candidate for piezoelectric wafer active sensor application in structural health monitoring. In most cases, piezoelectric wafer active sensors survived the tests successfully. The cases when piezoelectric wafer active sensors did not survive the tests were closely examined and possible causes of failure were discussed. The test results indicate that lead zirconate titanate piezoelectric wafer active sensors can be successfully used in a cryogenic environment; however, it does not seem to be a good candidate for high temperature. Repeated differential thermal expansion and extended environmental attacks can lead to piezoelectric wafer active sensor failure. This emphasizes the importance of achieving the proper design of the adhesive bond between the piezoelectric wafer active sensor and the structure, and of using a protective coating to minimize the ingression of adverse agents. The high-strain tests indicated that the piezoelectric wafer active sensors remained operational up to at least 3000 microstrain and failed beyond 6000 microstrain. In the fatigue cyclic loading, conducted up to 12 millions of cycles, the piezoelectric wafer active sensor transducers sustained at least as many fatigue cycles as the structural coupon specimens on which they were installed.

Development of a field-portable small-size impedance analyzer for structural health monitoring using the electromechanical impedance technique

Electromechanical (E/M) impedance method is emerging as an effective and powerful technique for structural health monitoring. The E/M impedance method utilizes as its main apparatus an impedance analyzer that reads the in-situ E/M impedance of piezoelectric wafer active sensors (PWAS) attached to the monitored structure. Laboratory-type impedance analyzers (e.g. HP4194) are bulky, heavy, and expensive. They cannot be easily carried into the field for on-site structural health monitoring. To address this issue, means of to reduce the size of the impedance analyzer making the impedance analyzer more compact and field-portable are explored. In this paper, we present a systematic approach to the development of a field-portable small-size impedance analyzer for structural health monitoring using the electromechanical impedance technique. Our approach consists of several developmental stages. First, we perform a simulation of the E/M Impedance technique and develop the software tools for analyzing the signal in a fast and efficient way while maintaining the desired accuracy. The objective of this signal processing part is to obtain the complex impedance, ZR+iZI)=|Z| angle arg Z, at a number of frequencies in a predetermined range. Several signal processing methods were explored such as: (a) integration method; (b) correlation method; (c) Discrete Fourier transform (DFT) method. Second, we discuss the hardware issues associated with the implementation of this approach. The hardware system architecture consists of several blocks: (a) reference signal generation; (b) voltage and current measurements; and (c) digital signal acquisition and processing. Practical results obtained during proof-of-concept experiments are presented and comparatively examined.

Lamb wave dispersion compensation in piezoelectric wafer active sensor phased-array applications

Lamb-wave testing for structural health monitoring is complicated by the dispersion nature of the wave modes. The dispersion effect will result in a propagated wave with longer time duration, deformed envelop shape as compared to its excitation counterpart, and hard to be interpreted. This paper first reviews the dispersion compensation and removal algorithms. Second, it compares these two methods by applying them to two widely used low-frequency Lamb wave modes: S0 and A0. Numerical simulations are compared in parallel with experimental results. Finally, the dispersion compensation algorithm is applied to 1-D PWAS phased array and demonstrated to improve the phase arrays spatial resolution.

Lamb Waves Decomposition and Mode Identification Using Matching Pursuit Method

Matching pursuit (MP) is an adaptive signal decomposition technique and can be applied to process Lamb waves, such as denoising, wave parameter estimation, and feature extraction, for health monitoring applications. This paper explored matching pursuit decomposition using Gaussian and chirplet dictionaries to decompose/approximate Lamb waves and extract wave parameters. While Gaussian dictionary based MP is optimal for decomposing symmetric signals, chirplet dictionary based MP is able to decompose asymmetric signals, e.g., dispersed Lamb wave. The extracted parameter, chirp rate, from the chirplet MP can be used to correlate with two Lamb wave modes, S0 and A0.

Efficient electromechanical (E/M) impedance measuring method for active sensor structural health monitoring

Electro-mechanical impedance method is emerging as an important and powerful technique for structural health monitoring. The E/M impedance method utilizes as its main apparatus an impedance analyzer that reads the in-situ E/M impedance of the piezoelectric wafer active sensors (PWAS) attached to the monitored structure. Present-day impedance analyzer equipments (e.g. HP4194) are bulky, heavy and expensive laboratory equipment that cannot be carried into the field for on-site structural health monitoring. To address this issue, several investigators have explored means of miniaturizing the impedance analyzer making the impedance analyzer more compact and field-portable. In this paper we present an improved algorithm for efficient measurement of the E/M impedance using PWAS transducers. Instead of using a sine wave as the excitation signal to the PWAS and slowly changing its frequency, our method utilizes a chirp signal which is abundant in frequency components. By applying Fast Fourier Transform (FFT) to both the input and response signals, the impedance spectrum of the PWAS is acquired. The algorithm was implemented and tested in a real-time system, which consists of excitation signal generation module, voltage and current measurement module and digital signal acquisition module. The size and the implementation of the overall system using either a laptop or a digital signal processor (DSP) are also discussed. Finally, practical results are presented and comparatively examined.

Development of DSP-based electromechanical (E/M) impedance analyzer for active structural health monitoring

The electromechanical (E/M) impedance method is a powerful technique in active structural health monitoring (SHM). E/M impedance method utilizes as its main apparatus an impedance analyzer that reads the in-situ E/M impedance of piezoelectric wafer active sensors (PWAS) attached to the monitored structure. Present-day impedance analyzer equipments (e.g.HP4194) are bulky, heavy and expensive laboratory equipment that cannot be carried into the field for on-site structural health monitoring. This paper presented the development of a compact and low-cost impedance analyzer system. First, two types of impedance measurement approaches were evaluated in a PC-based simplified impedance analyzer system. It was found that the first approach, which measures impedance frequency by frequency, is very accurate but is not time-efficient and needs more efforts to be implemented. As for the second approach, which measures impedance using broad-band excitation and transfer function method, provides a good compromise among the measurement time-efficient, accuracy and implementation efforts. Experimental results show that this approach can be used for E/M impedance method for structural health monitoring. Second, to eliminate the PC in the measurement system, a DSP-based impedance analyzer system is proposed for further miniaturization. The system hardware configuration and design, software state flow for impedance measurement, and preliminary testing were presented in details.

Lamb waves time-reversal method using frequency tuning technique for structural health monitoring

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 to thin-wall structures 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 Lamb wave time reversal has not yet been fully studied. This paper addresses this problem by developing a theoretical model for the analysis of Lamb wave time reversal in thin-wall structures based on the exact solutions of the Rayleigh-Lamb wave equation. The theoretical model first used to predict the existence of single-mode Lamb waves. Then the time reversal behavior of single-mode and two-mode Lamb waves is studied numerically. The advantages of single-mode tuning in the application of time reversal damage detection are highlighted. The validity of the proposed theoretical model is verified through experimental studies. In addition, a similarity metric for judging time invariance of Lamb wave time reversal is presented. It is shown that, under certain condition, the use of PWAS-tuned single-mode Lamb waves can greatly improve the effectiveness of the time-reversal damage detection procedure.

OPTIMIZED EXCITATION SIGNAL SOURCE FOR EFFICIENT MEASUREMENT OF E/M IMPEDANCE

This paper presented an improved algorithm using digitally synthesized excitation signal sources for efficient and accurate measurement of the E/M impedance for structural health monitoring with PWAS (piezoelectric wafer active sensor) transducers. Instead of using a sinusoidal excitation to measure the structural E/M impedance at a single frequency at a time, the digitally synthesized signal sources get the entire structural E/M impedance spectrum immediately at one step through permanently attached PWAS. Digitally synthesized signal source is a time efficient way for E/M impedance spectrum measurement. Firstly, introduction of E/M impedance measurement and the concept of using transfer function to achieve admittance and impedance of PWAS were provided. Secondly, two ways of synthesizing signal sources were discussed. Thirdly, the characteristics and performance of the two signal sources for E/M impedance measurement were compared in simulation and examined by laboratory experiment of measuring a free PWAS impedance spectrum over a wide frequency range (100k~1MHz). Finally, the hardware implementation and measurement precision of the new impedance methods were discussed and concluded.

LAMB WAVE TUNING FOR PIEZOELECTRIC WAFER ACTIVE SENSOR APPLICATIONS IN IN‐SITU STRUCTURAL HEALTH MONITORING

An analytical and experimental investigation of the Lamb wave mode tuning with piezoelectric wafer active sensors (PWAS) is presented. The analytical investigation assumes shear lag transfer of tractions and strains. Analytical solution using the space‐wise Fourier transform is reviewed and closed form solutions are presented for the case of ideal bonding (i.e., load transfer mechanism localized at the PWAS boundary). Experimental tests are run to verify the predicted tuning curves. The concept of “effective PWAS dimension” is introduced to account for the discrepancies between the ideal bonding hypothesis and the actual shear‐lag load transfer mechanism. Two applications of PWAS transducers are presented to illustrate that the capability to excite only one desired Lamb wave mode is critical for practical structural health monitoring applications, including PWAS phased array technique (e.g., the embedded ultrasonics structural radar, EUSR) and the baseline‐free time reversal process (TRP).