The Application of Ultrasonics in Non-Destructive Testing: A Review of Some of the Research at University College London

This paper presents a review of some of the research projects at University College London which use ultrasonic waves in non-destructive testing. The projects covered include a low-frequency scanning acoustic microscope, a new dark-field acoustic microscope, an update on finite difference model studies of wave propagation and scattering, developments,in mode-conversion techniques for defect characterisation, a new high-resolution acoustic field sampling probe and a resume of the work on ZnO transducers, INTRODUCTION The need for quantitative non-destructive testing, and in particular defect characterisation, has been recognised for many years, It is clear that the achievement of quantatitive NOT must now assume a high priority if the full benefits of the advances in.fracture mechanics and related areas of materials science, are to be realised, This has been appreciated for several years in both the USA and the UK and, as a result, there is now considerable interest in this research area in British Universities and industrial laboratories, as well as in their counterparts in the USA. This paper, together with Ameri et al, (1), (to be presented later), reviews the current work in ultrasonic NDT in the Department of Electronic and Electrical Engineering at University College London. The presentation also extends the work on model studies of wave propagation and scattering given by Bond, (2), at last year's meeting: this work was formerly at The City University, London, and has now moved to join the long-established research in acoustic imaging and microscopy, SAW signal processing and ultrasound-orientated thin film technology, at UCL. Current studies can be divided into six related groups of projects: (i) use of a laser probe (1,3) to measure material properties such as SAW velocity, elastic constants or crystal orientation, (ii) investigation of the interaction between a propagating wave and a defect using the scattered and mode-converted waves to identify characteristics of the defect by use of the laser probe (3) in an imaging mode to examine cracks or overlay defects, (iii) use of the acoustic microscopy (1} to produce 'acoustic-contrast' images of small structures, (iv) the theoretical investigation of fundamental wave propagation scattering using numerical (finite difference) models (2), together with experimental investigations using short pulse probes, (v) the development of the new family of mode-conversion techniques for defect characterisation, reported at last year's meeting, (2,5}, (vi) engineering support for the above activities; typically the development of single crystal and thin film transducers, acoustic lenses and electronic instrumentation. The following paragraphs consider some specific aspects of these projects the application of low-frequency (12MHz) scanning acoustic microscope to NDT, a new dark-field acoustic microscope, an update on numerical model studies of wave propagation and scattering (2), developments in mode-conversion techniques for defect characterisation, a new high-resolution acoustic-field sampling probe and a resume of the work on ZnO transducers. TRANSMISSION SCANNING ACOUSTIC MICROSCOPY FOR DEFECT DETERMINATION IN METALLIC.OBJECTS The Scanning Acoustic Microscope (SAM) has been successfully used to examine a wide range of biological state materials (6). Recent work has tended to concentrate upon the reflection configuration of the instrument which eases alignment constraints in high resolution systems (7), while transmission imaging at lower resolutions has received less attention (8). Both configurations have applications in NDT in the location and detection of flaws and voids within an opaque material. Experimental results from a transmission system will be presented that demonstrate the detection of such defects.* The essential elements of a transmission SAM are shown in Fig. 1. An acoustic lens produces a diffraction limited focal spot at the object plane. A second lens collimates the energy transmitted through the object. An image is built up by mechanically scanning the object through the focussed beam and using the measured transmissivity at each image point to modulate the intensity of a synchronously scanned display. 165 The results presented here were obtained using a transmission SAM operating at 12MHz, employing 16mm radius, 25mm aperture fused quartz lenses. The resolution in a steel sample was approximately 650)lm. * Transmission Scanning Acoustic Microscopy for Determination in Thick Objects. D. Sinclair & E.A. Ash (to be published). Piezoelectric Lens transducer surface