Development of novel air-coupled ultrasonic methods for investigation of properties of sheet composite structures

Speaker:
Sestoke, Justina; Kaunas University of Technology; Lithuania

Authors:
Kazys, R.J.; Kaunas University of Technology; Lithuania
Sliteris, R.; Kaunas University of Technology; Lithuania
Sestoke, J.; Kaunas University of Technology; Lithuania

ID: ECNDT-0553-2018
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Ultrasonic non- destructive techniques based on application of guided waves are already used for inspection of plate- type structures made of various materials including composite materials. Applicability of such methods may be widened by employing air-coupled techniques. The main currently encountered problem is big losses of ultrasonic signals caused by the attenuation and significant mismatch of acoustic impedances of ultrasonic transducers and air. The objective of this work is to develop novel air-coupled ultrasonic techniques based on excitation of guided waves by means of improved ultrasonic arrays and to apply them for a non-destructive evaluation of the properties of sheet composite structures.
For excitation of guided waves in composite plates and polymer films an air- coupled phased linear array was employed. For this purpose novel air-coupled array with PMN-32%PT piezoelectric crystals were developed. In order to reduce losses due to attenuation low frequency 50 kHz ultrasonic signals were selected. For transmission and reception of ultrasonic signals in such frequency range strip- like piezoelectric elements vibrating in a transverse extension mode were used. The good performance of the array is obtained due high electromechanical coupling coefficient which for the transverse extension mode is k32 > (0.84 – 0.90). It was found that due to the high electromechanical coupling, operation of piezoelectric elements in transmitting and receiving modes is very different, therefore special designs for transmitting and receiving arrays were proposed.
Experimental investigation of air- coupled excitation of guided waves in CFRP plates and polymer films by the developed arrays was performed. It was shown that by employing the proposed excitation and reception algorithms it is possible to generate various guided waves including waves with velocity slower than the velocity of ultrasonic waves in air. It opens new possibilities in non-destructive testing of composite structures.