Immersion and TOFD

Time of flight diffraction (TOFD) is a well-developed ultrasonic non-destructive testing (NDT) technique which has been applied successfully for accurate sizing of defects in thick sections. Codes of practice such as ASME now permit TOFD for routine examination as an alternative to radiography for thick weldments. However, examination of thinner sections by TOFD has its limitations. The main limitation is that as the thickness of the specimen reduces, the lateral wave, diffracted wave and the back wall echo merge together and it is very difficult to identify and size the discontinuity. Also, the size of conventional transducers limits the required probe separation. Limited success has been obtained internationally through the application of miniature probes and software for extending TOFD to lower thicknesses. In these cases, the minimum thickness that has been examined is 7 mm. A new methodology based on a simple and novel combination of TOFD and the immersion technique has been proposed by the authors that successfully extends the application of TOFD to thinner sections down to 3 mm. Immersion coupling provides a delay line, the necessary angles and probe separation making it possible to examine the thin components successfully. This paper highlights the results of detailed experimental investigations on immersion TOFD and its successful application for the evaluation of welds in hexcan used for encapsulating nuclear fuel pins