An air-coupled ultrasonic array scanning system for rapid through transmission NDT
Within the aerospace industry there is an increasing requirement to investigate the structural integrity of the new composite materials that are now being used frequently in the manufacture of aircraft. The complexity of the material manufacture necessitates that evaluation is required prior to final production and it is the development of a novel approach to this testing that constitutes the focus for the work of this Thesis. Existing techniques frequently utilise ultrasonic signals to interrogate the sample under investigation, however, these are cumbersome and scan speed is invariably slow when testing of large samples is considered. This is because large samples are normally tested using a through transmission approach, where narrow jets of water are used to couple the ultrasonic signal through the propagation channel. The fundamental basis of the proposed approach is the removal of the water couplant, enabling a receiver array to be employed, and thus scan large areas more quickly. Flexibility would also be increased with this technique due to a capability to scan moisture sensitive parts. In order to achieve this, however, the considerable problem of the acoustic impedance mismatch at each solid/air boundary would have to be overcome. Firstly, a narrowband, relatively low frequency approach is selected. It is concluded however, that in order to maximise the scan speed benefit, parallel data acquisition from the receiver array elements must be achieved and no signal averaging must be performed. A small array element pitch and focussing are deemed necessary in the pursuit of adequate defect detection resolution. It is important to select the most appropriate transducer technology for coupling in air and a comprehensive comparison of two relevant technologies (piezocomposite and electrostatic) is carried out. Piezocomposites are found to be superior in terms of sensitivity, robustness and focusing capability. A novel acoustic matching layer is developed to improve coupling from the transducers to the air load. This is investigated microscopically and acoustically and a linear model is developed to enable the design for the most successful operation. Prototype air-coupled systems are produced and scan results compared favourably with the results using water-coupled techniques.