Synthetic Focusing Aperture Techniques for Non-destructive Testing of Materials

Synthetic Aperture Focusing Technique imaging has became a popular tool in the field of non-destructive testing in the past years. The acquisition procedure consists in collecting the data after activating elements one by one. The reconstructed object is then computed by adding the contribution of each channel, selecting the proper time delay values. It is the principle of the well-known delay and sum processing. An extension of this conventional process is possible by using several elements to receive the ultrasonic field. This article proposes to study the two methods and to compare them to the regular real-time focusing technique. Experimental results show that SAFT techniques give improvements in term of lateral resolution. Moreover, the extension of the conventional SAFT method enables to significantly increase the signal-to-noise ratio. We finally propose to use multiple paths for the delay computation and apply this approach to vertical slit inspection. It leads to a noise reduction because of the averaging of the uncorrelated noise. INTRODUCTION Synthetic Aperture Focusing Technique (SAFT) is a reconstruction process used in many multi-element imaging systems (radar [7], sonar [8], ultrasound [1,2]). The principle is to sequentially acquire the data and to post-process them by delaying and summing the contributions [3,4]. The application of this technique is not obvious in the context Non-Destructive Testing (NDT) because of the specific acquisition procedure and the weakness of the processing time. Nevertheless, current systems and specifically ultrasonic phased array probes allows to easily use these techniques. The purpose of this communication is to present the SAFT principle in the context of NDT of industrial parts. In the first section, the conventional SAFT and an extended version are presented. Then, we will compare the performances trough several experimental results. We also propose to use multi-paths in the case of slit inspection. Conclusions finally arise and perspectives are proposed. SAFT PRINCIPLE AND RECONSTRUCTION In conventional or Mono-static Synthetic Aperture (MSA) imaging [1,2], a single element of the transducer is employed in both transmission and reception. The transmission/reception is repeated over all the elements (see Figure 1). The emitted wave is supposed to be spherical and propagates in all directions of the material. The data is then composed of the received signals of each element. If N elements are considered, the data has N A-scans, forming a B-scan image Y(t,i) where t is the time and i is the index of the transmitting/receiving element going from 1 to N. A single A-scan corresponding to the element i is noted ri(t).