Improvement of Defect Characterization Witu Ultrasonic Adaptive Focusinc

In order to obtain the highest performances requested by the safety authorities for ultrasonic inspections, in terms of flaw characterization, the CEA (French Atomic Energy Commission) has developped the F.A.U.S.T. (Focusing Adaptive UltraSonic Tomography ) system, supported by the french Institute for Nuclear Protection and Safety (IPSN). This technique is based on phased array transducers dynamically controlled by a multi-channel acquisition system, allowing several different delay and amplitude laws at transmission and reception. Each US channel can be stored, enabling various reconstruction and imaging procedures. This allows conventional phased array techniques, as variable depth focusing and 3D beam steering, but also beam apodization with non-uniform profiles, or tandem (separated transmission and reception elements) techniques. Current studies and experiments include defect characterization using the information received by each part of the array and 3D steering around the defect. Experiments carried out with various reflectors show the ability of the system to discriminate different types of defect and to characterize them. THE -FAUST' (FOCUSING ADAPTIVE ULTRASONIC TOMOGRAPHY) SYSTEM was developed at the French Atomic Energy Commission (CEA) to improve performances of ultrasonic non destructive testing in terms of adaptability to various control configurations and defect characterization. The focusing of an ultrasonic transducer is obtained by applying a delay to each elementary emitting source of the transmitting array so that elementary contributions constructively interfere in a given zone of the inspected component. For monolithic standard transducers, this delay is provided either by a lens or by the direct shaping of the emitting surface. The inherent design of these conventional technologies does not allow to vary the delay law so that the geometry of the focused beam is fixed. The FAUST system relies on optimized phased array transducers connected to a multi-channel acquisition system supplying amplitude and delay laws allowing to drive the ultrasonic beam. In the first part of this paper, we describe the general design of the FAUST system. A field computational model [2] developed at the CEA is used to design optimized phased array dedicated to NDE configurations (immersed transducers used to focus through Fluid/Solid interfaces). Theoretical delay laws and related ultrasonic fields are also calculated from this model. Experiments have been carried out using an optimized phased array. Comparisons between experimental and theoretical results allow us to validate both the model and the system. In addition, experiments are presented which point out the capabilities of the system to drive the ultrasonic beam using the model. Various applications of beam forming are presented as depth focusing in both transmission and reception, apodization at reception and beam steering. Finally, we investigate the capabilities of the system to use information received on each channel. The storage of the signals received by ail the elements of the array allows one to perform different reconstruction procedures. Useful information can be extracted from the received signals: experimentally measured delay laws can be determined from reflected signals to obtain an optimal imaging, while the related amplitude distribution over the array points out geometrical characteristics of the reflector. General Design of the FAUST System The FAUST system is based on analogic pulsers with digital counters allowing to adjust amplitude and delay parameters for each ultrasonic channel of a phased array. The acquisition system has been elaborated to allow different acquisition and imaging procedures. All parameters (listed in table 1) involved in beam forming can be adjusted for each acquisition channel.