Ultrasonic transducer equipped with a magnetoelectric sensor for weld quality monitoring

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Ultrasonic transducer equipped with a magnetoelectric sensor for weld quality monitoring


Author: Cheung, Kwok-fung
Title: Ultrasonic transducer equipped with a magnetoelectric sensor for weld quality monitoring
Degree: M.Phil.
Year: 2009
Subject: Hong Kong Polytechnic University -- Dissertations.
Ultrasonic transducers.
Ultrasonic welding.
Department: Dept. of Applied Physics
Pages: xvii, 248 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2321028
URI: http://theses.lib.polyu.edu.hk/handle/200/3843
Abstract: Study of the theoretical design, fabrication and resonance characteristics of a 20 kHz bolt-clamped Langevin-type piezoelectric transducer and the investigation of the design, fabrication and evaluation of a magnetoelectric sensor were presented and discussed in this thesis. Evaluation of the fabricated transducer equipped with the magnetoelectric sensor for monitoring weld quality of welded products was performed. Conclusions and suggestions for future work were also included. Major research results are described as follows: 1) A complete process of the fabrication of an ultrasonic transducer was developed. The process was divided into four sections: theoretical design, computation analysis, assembly and evaluation. Theoretical design of the transducer followed two different paths: design of the general appearance and design of the transducer's dimensions. The appearance design followed the requirements to optimize the efficiency of the transducer and ensure good stability of the transducer without considering certain dimensions of the components. The design of transducer's dimensions was determined by different mathematical formulas, and the resonance frequency, displacement distributions and stress distributions equations were developed. So, the basic physical model of transducer was developed. Computational analysis was done by performing the finite element analysis (FEA). For this analysis, the finite element model of the transducer was developed. Results of the analysis showed the resonance characteristics of the transducer. Only the first three resonances were investigated in detail. For each of them, the corresponding resonance frequency, anti-resonance frequency and effective electromechanical coupling coefficient were obtained. Also, the vibration mode for each of these resonances and an electrical impedance spectrum which included these resonances were obtained. The technique used to build the transducer was described together with procedures for the preparation of components. The transducer was evaluated by measuring its electrical and vibrational characteristics. The measurements were compared to the results of FEA. The comparison of electrical characteristics showed fairly good agreement and that of vibrational characteristics showed close agreement. 2) A magnetoelectric sensor was integrated into the fabricated transducer to monitor the weld quality of welded products. A Terfenol-D / PMN-PT / Terfenol-D magnetoelectric laminated composite was used as the sensing element of the sensor. The basic structure and theoretical modeling of the element were presented. The method used to fabricate the element was described. The magnetic characteristics of the fabricated element were measured and the design concept and the principle of the sensor were developed. The stand-alone workability of the magnetoelectric sensor was evaluated. The results basically proved the ability of the sensor to reveal the mechanical characteristics of the transducer during a welding process. Also, the method of mounting the sensor on the transducer was determined. The performance of weld quality monitoring of the transducer equipped with the sensor was evaluated in practical welding processes. In the evaluation, laser vibrometer results were used for comparison as they could reveal the vibrational characteristics of the process. An AC current sensor was also used as another method to reveal the electrical conditions of the process. All devices were able to monitor the weld quality of welded products. The obtained results showed that magnetoelectric sensor can be used in the field of weld quality monitoring.

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