Yongrae Roh

PZT-based active damage detection techniques for steel bridge components

This paper presents the results of experimental studies on piezoelectric lead-zirconate–titanate (PZT)-based active damage detection techniques for nondestructive evaluations (NDE) of steel bridge components. PZT patches offer special features suitable for real-time in situ health monitoring systems for large and complex steel structures, because they are small, light, cheap, and useful as built-in sensor systems. Both impedance and Lamb wave methods are considered for damage detection of lab-size steel bridge members. Several damage-sensitive features are extracted: root mean square deviations (RMSD) in the impedances and wavelet coefficients (WC) of Lamb waves, and the times of flight (TOF) of Lamb waves. Advanced signal processing and pattern recognition techniques such as continuous wavelet transform (CWT) and support vector machine (SVM) are used in the current system. Firstly, PZT patches were used in conjunction with the impedance and Lamb waves to detect the presence and growth of artificial cracks on a 1/8 scale model for a vertical truss member of Seongsu Bridge, Seoul, Korea, which collapsed in 1994. The RMSD in the impedances and WC of Lamb waves were found to be good damage indicators. Secondly, two PZT patches were used to detect damage on a bolt-jointed steel plate, which was simulated by removing bolts. The correlation of the Lamb wave transmission data with the damage classified by in and out of the wave path was investigated by using the TOF and WC obtained from the Lamb wave signals. The SVM was implemented to enhance the damage identification capability of the current system. The results from the experiments showed the validity of the proposed methods.

Finite element analysis of underwater capacitor micromachined ultrasonic transducers

A simple electromechanical equivalent circuit model is used to predict the behavior of capacitive micromachined ultrasonic transducers (cMUT). The equivalent circuit model of the cMUT lacks important features such as coupling to the substrate and the ability to predict crosstalk between elements of an array of transducers. To overcome these deficiencies, a finite element model of the cMUT is constructed using the commercial code ANSYS(R). Calculation results of the complex load impedance seen by single capacitor cells are presented, then followed by a calculation of the plane wave real load impedance seen by a parallel combination of many cells that are used to make a transducer. Crosstalk between 1-D array elements is found to be due to two main sources: coupling through a Stoneley wave propagating at the transducer-water interface and coupling through Lamb waves propagating in the substrate. To reduce the crosstalk level, the effect of structural variations of the substrate are investigated, which includes a change of its thickness and etched trenches or polymer walls between array elements.

Characterization of all the elastic, dielectric, and piezoelectric constants of uniaxially oriented poled PVDF films

Polyvinylidene fluoride (PVDF), a piezoelectric material, has many useful applications, for example, as sensors, transducers, and surface acoustic wave (SAW) devices. Models of performance of these devices would be useful engineering tools. However, the benefit of the model is only as accurate as the material properties used in the model. The purpose of this investigation is to measure the elastic, dielectric and piezoelectric properties over a frequency range, including the imaginary part (loss) of these properties. Measurements are difficult because poled material is available as thin films, and not all quantities can be measured in that form. All components of the elastic stiffness, dielectric tensor, and electromechanical coupling tensor are needed in the models. The material studied here is uniaxially oriented poled PVDF that has orthorhombic mm2 symmetry. Presented are the frequency dependence of all nine complex elastic constants, three complex dielectric constants, and five complex piezoelectric constants. The PVDF was produced at Raytheon Research Division, Lexington, MA. Measurements were made on thin films and on stacked, cubical samples. The elastic constants c/sub 44//sup D/ and c/sub 55//sup D/, the dielectric constants e/sub 11//sup T/ and e/sub 22//sup T/, as well as the piezoelectric constants g/sub 15/ and g/sub 24/ reported here have not been published before. The values were determined by ultrasonic measurements using an impedance analyzer and a least square data-fitting technique.

Development of a saw gas sensor for monitoring so2 gas

Abstract We have developed a new type of SO 2 gas sensor by applying a particular inorganic thin film on SAW (surface acoustic wave) devices. The sensor consists of twin SAW oscillators with a centre frequency of 54 MHz fabricated on LiTaO 3 piezoelectric single crystals. One delay line of the sensor is coated with a CdS thin film that selectively adsorbs and desorbs SO 2 , while the other is uncoated for use as a stable reference. Deposition of CdS thin film has been carried out by the spray pyrolysis method using an ultrasonic nozzle. Mass loading and electric-field changes induced by the SO 2 gas adsorbed onto the CdS film result in corresponding frequency shifts directly proportional to the gas concentration. The relative change in the frequency of the two oscillators is monitored with a digital signal-processing circuit. SO 2 sensor properties investigated include sensitivity, response time and repeatability. The sensor shows promising performance as a microsensing tool and is capable of measuring concentrations in air less than 200 parts per billion of SO 2 .

Design and fabrication of a new traveling wave-type ultrasonic linear motor

This paper describes the development of a new traveling wave-type ultrasonic linear motor. The motor consists of a metallic stator and two piezoelectric ceramic plates bonded to the stator, thereby eliminating the need for external vibrators. The two piezoceramic plates excite two ultrasonic standing waves with a certain phase difference, which convert to a traveling wave when combined together. The operation principle of the motor is presented along with the theoretical equations, and the feasibility is verified using the finite element method. Based on the finite element analyses, detailed dimensions and boundary conditions are determined for the proper generation of a traveling wave. Experimental fabrication and characterization of a sample motor is performed to confirm the numerical results and prove the practical applicability of the new structure. The new linear motor presented in this work offers a relatively simple structure compared to traditional traveling wave-types while maintaining all the characteristic features of an ultrasonic motor.

Design and fabrication of piezoceramic bimorph vibration sensors

A bimorph-type piezoceramic vibration sensor is developed that can make up for the shortcomings of current mass loaded type sensors, such as high price, low sensitivity, and complex structures. For the design, in conjunction with piezoelectric constitutive equations, we derive full analytic response equations of the bimorph sensor to external forces. As the external forces, two representative cases are considered, step and sinusoidal excitation. Based on the results, actual sensors are fabricated and tested for verification of the theoretical results. Thermal stability of the bimorph structure is also checked through experiments. Further, a comparison of performance of the developed sensor is made with that of a commercial reference sensor so that quantitative evaluation of its sensitivity can be made. The sensor developed in this work shows excellent sensitivity and thermal stability in addition to the merits of simple structure and ease of fabrication. The design procedure with the sensor voltage response equations derived in this paper can be applied to develop piezoelectric bimorph sensors to meet any practical specifications.

Impedance-based Damage Detection for Civil Infrastructures

The objective of this study is to investigate the feasibility of an impedance-based damage detection technique using piezoelectric (PZT) transducers for civil infrastructures such as steel bridges. The basic concept of the technique is to monitor the changes in the electrical impedance to detect structural damages. Those changes in the electrical impedance are due to the electro-mechanical coupling property of piezoelectric materials and the host structure. In this study, at first, a numerical analysis was performed to understand the basics of this technique through a simple 1-D electro-mechanical system. The experimental studies on three kinds of structural members were carried out to detect the locations of cracks and loosened bolts. It was that cracks or loosened bolts near the PZT sensors could be effectively detected by monitoring the shifts of the resonant frequencies of the impedance functions.

Optimization of structural variables of a flextensional transducer by the statistical multiple regression analysis method.

The performance of an acoustic transducer is determined by the effects of many structural variables, and in most cases the influences of these variables are not linearly independent of each other. To achieve optimal performance of an acoustic transducer, we must consider the cross-coupled effects of its structural variables. In this study, with the finite-element method, the variation of the operation frequency and sound pressure of a flextensional transducer in relation to its structural variables is analyzed. Through statistical multiple regression analysis of the results, functional forms of the operation frequency and sound pressure of the transducer in terms of the structural variables were derived, with which the optimal structure of the transducer was determined by means of a constrained optimization technique, the sequential quadratic programming method of Phenichny and Danilin. The proposed method can reflect all the cross-coupled effects of multiple structural variables, and can be extended to the design of general acoustic transducers.

Simple expressions of the reflection and transmission coefficients of fundamental Lamb waves by a rectangular notch.

The scattering of Lamb waves by a two-dimensional rectangular notch is investigated for rapid inspection of defects in a structure. To derive the reflection and transmission coefficients of the scattered waves in a simple way, the scattering caused by the notch is analyzed through the composition of individual scattering processes. Linear equations corresponding to the reflection and transmission coefficients are constructed along with scattering graphs. For an illustration of the efficacy of the presented method, the scattering of fundamental symmetric and anti-symmetric modes are inspected according to the depth and width of a notch in a plate. Validity of these expressions is demonstrated by the comparison of the theoretical analysis results with those from the finite element analysis.

PZT-induced Lamb waves and pattern recognitions for on-line health monitoring of jointed steel plates

This paper presents a non-destructive evaluation (NDE) technique for detecting damages on a jointed steel plate on the basis of the time of flight and wavelet coefficient, obtained from wavelet transforms of Lamb wave signals. Probabilistic neural networks (PNNs) and support vector machines (SVMs), which are tools for pattern classification problems, were applied to the damage estimation. Two kinds of damages were artificially introduced by loosening bolts located in the path of the Lamb waves and those out of the path. The damage cases were used for the establishment of the optimal decision boundaries which divide each damage class’s region from the intact class. In this study, the applicability of the PNNs and SVMs was investigated for the damages in and out of the Lamb wave path. It has been found that the present methods are very efficient in detecting the damages simulated by loose bolts on the jointed steel plate.