TRANSDUCER TECHNOLOGY FOR ULTRASONIC NDT APPLICATIONS zyx Ultrasonic transducers for high temperature applications zyxwv A. McNab K.J.Kirk A.Cochran zyxwvutsrqpon Indexing terms: Ultrasonic transducers, High temperature, Nondestructive testing zyxwvutsr Abstract: zyxwvutsrqpo There is an increasing demand for ultrasonic transducers to work at high temperatures. Transducers operating in the range from 400 to 1000°C are being applied in the power, process, automotive and aeroengine industries, as well as in materials research. To date, the majority of high temperature transducer designs have been based on adaptations of conventional single and dual-element probes for nondestructive testing (NDT). Such designs are outlined. It is noted, however, that the attempt to construct high temperature versions of conventional transducers has led to overcomplicated, expensive structures. In particular, they are modelled on probes used for scanning at ambient temperatures, although scanning is unlikely to be possible at high temperatures. An alternative approach is therefore described, based on the monolithic ultrasonic array structure. This offers not only lower transducer costs, but also full electronic control of the ultrasonic beam angle and emission point, thus facilitating inspection of a region within a test component using only one or two devices in fixed positions. Typical results demonstrating the beam-steering performance of the array, as well as those from B-scan time-of- flight testing, are presented. It is concluded that the problems of transducer development for ultrasonic, high-temperature NDT are not yet completely solved, and that further effort is required in the key areas of materials science and transducer structure. Moreover, a simple design with the minimum of bond lines is most likely to succeed on the grounds of cost and reliability. 1 Introduction A major challenge in ultrasonic transducer design is the development of transducers for applications at high zyxwvu 0 IEE, 1998 IEE Proceedings online no. 19982210 Paper first received 4th Decmber 1997 and in revised form 15th May 1998 A. McNab and A. Cochran zyxwvutsrqp are with the Centre for Ultrasonic Engineer. ing, Department of Electronic and Electrical Engineering, Umversity of Strathclyde, 204 George Street, Glasgow G1 lXW, UK K.J. Kirk is zyxwvutsrqpo with the Department of Physics and Astronomy, University of Glasgow, Glasgow G12 SQQ, UK zyxwvutsrqpo IEE Proc-Sei. Meas. zyxwvutsrqpon Technol., Vol. 145, No. zyxwvutsrqpo 5, September 1998 temperatures, since present commercial transducers are limited typically to 50°C. Some applications require only a relatively small increase in transducer operating temperature, to about 200"C, for example, to monitor manufacturing processes, characterise hot liquids and make down-hole measurements in the oil and gas industries. Others require much higher temperatures, from 400 to 1000"C, for example, in the power and process industries (the temperature of superheated steam is 565°C) and in aero-engine manufacture and operation. Further potential applications exist in mate- rials research and the automotive sector. It is devices operating in the range 400-1000°C that are principally considered in this paper. The potential annual market for nondestructive testing (NDT) in the UK alone has been estimated at several hundred trans- ducers [l]. They will be used for three possible reasons: to reduce operating costs by avoiding outages for con- ventional testing at low temperatures; to monitor proc- esses which take place at high temperature; and to detect flaws such as cracks which are revealed only at operating temperatures. In ultrasonic nondestructive testing, measurement of wall thickness and detection and characterisation of cracks can make use of a number of noncontacting technologies such as electromagnetic-acoustic transduc- ers (EMATs) [2], laser generation and detection of ultrasound [3], and airborne ultrasound based on con- ventional piezocomposite [4] and electrostatic [5] trans- ducers. These noncontacting methods may be particularly important for online monitoring of materi- als and welding processes [6]. However, because of problems of access, instrumentation sensitivity, testing costs and inspection resolution, there is also a clear need for piezoelectric transducers that can be mounted directly onto components at high temperatures, and utilised either for a short time for spot measurements, or continuously over a long period for in situ monitor- ing. Early work in this area relied upon insulating the probes from the effects of high temperature, either by using momentary contact [7], or by interposing a buffer material between the hot component and the cooled transducer. The problem of coupling the ultrasound into the test material still remained, and both dry cou- pling [8] and liquids which evaporated within a given period, were used. For both spot measurement and monitoring, trans- ducers which can withstand elevated temperatures over an extended period are advantageous. However, condi- tions placed on their construction are severe. The devices must withstand heating from the ambient tem- 229