Introduction to phased array ultrasonic techniques

Non-destructive testing covers the field of activity which, in industry, consists in examining the volume or surface of an object, without damaging its integrity for subsequent use. Ultrasonic techniques are among the most commonly used, as are eddy current, radiography, dye penetrant testing... [R 1 400] . They are based on the principles of mechanical wave propagation in solids and fluids, with frequencies in the ultrasonic range. More precisely, the aim is to produce a vibration in the part to be examined, and to detect the signals after interaction with any defects present in the part. In certain specific configurations, the technique involves tracing the echoes obtained by reflection: this is literally "echo-graphy".

This technique, which has long been used in the medical field using multi-element sensors, has been deployed in the industrial sector for the past ten years. Although there are many papers on the subject, the possibilities and limitations of this innovative technique are not always clearly defined.

This article aims to provide a better understanding of the physical principles underpinning the multi-element technique, some rudimentary calculations and a brief overview of possible advanced applications.

To begin with, the general principles governing conventional ultrasound techniques are recalled, in particular the phenomenon of diffraction.

The multi-element technique is introduced after a brief transition to focused ultrasound. The physical principles governing it are described, and the various characteristic parameters of the transducers are presented, along with their impact on the beam. The principle of calculating delay laws is explained, along with possible "parasitic effects".

The possibilities offered by multi-elements in conventional configurations are presented. Returning, at the end of the article, to the question of focused ultrasound, some more advanced applications are briefly described.

This article is illustrated by the results of numerical simulations; the calculations were carried out using the CIVA software (commercial version 9.2) developed by CEA (link in the "To find out more" section).