Air coupled NDE-constraints and solutions for industrial implementation

There is increasing industrial interest in the use of non-contact, gas-coupled ultrasound for a range of non-destructive evaluation (NDE) applications. Ultrasonic generation and detection via piezoelectric or electrostatic transducers have attracted particular interest and a number of related publications have appeared in the recent technical literature. However, these have centred largely on laboratory experiments since extrapolation of the technology to the real industrial environment is not straightforward. Questions still remain with regard to the practical robustness and reliability of such systems. This paper addresses the major difficulties associated with industrial implementation of gas coupled NDE. The influence of the gas channel on ultrasonic wave propagation is described with regard to system operating frequency, ultrasonic beam characteristics and possible environmental variables such as temperature and pressure. Methods for predicting and stabilising ultrasonic system performance are proposed, in conjunction with transducer design, where emphasis is placed on efficiency for gas operation and the requirement for minimisation of electronic intrinsic noise. Techniques for implementing sufficient improvement to enable practical, real time operation are then discussed, including instrumentation design and signal conditioning requirements. Finally, examples of robust NDE systems are presented, embracing through transmission scanning, array based imaging and multi-frequency Lamb wave scanning