Application of Ultrasonic Beam Modeling to Phased Array Testing of Complex Geometry Components

Abstract

INTRODUCflONFor several years, the French Atomic Energy Commission (CEA) has developed phased array techniques to improve defect characterization and adaptability to various inspection configurations [1]. Such techniques allow to steer and focus the ultrasonic beam radiated by a transducer split into a set of individually addressed elements, using amplitude and delay laws. For most conventional systems, those delay laws are extracted from geometric ultrasonic paths between each element of the array and a geometric focusing point, or experimentally deduced for a detected defect. Geometric delay laws are usually applied to perform beam-forming abilities [2] for simple geometry components (e.g. beamsteering over a plane specimen), whereas experimental delays can be supplied to the array at transmission and reception to optimally adapt the ultrasonic beam to the detected defect, in a so-called self-focusing process [3,4]. This method, relevant for complex material or geometry leading to phase distortion or complex paths that cannot be predicted by simple geometrical calculations, obviously requires the existence of a reflector and the ultrasonic beam radiated by the experimental delay law cannot be known. Therefore this technique is used to improve defect detection (optimal sensibility) rather than defect characterization. To assess complex geometry components inspection with an adaptive system, the CEA has developed new modeling devoted to predict the ultrasonic field radiated by arbitrary transducers through complex geometry and material specimen [5]. A model allows to compute optimized delay laws to preserve the characteristics of the beam through the complex surface, as well as the actual radiated field using those delays. This paper presents two applications of this model: the inspection of a misaligned specimen, and the inspection of an irregular surface. A COMPUTATION MODEL FOR NDT CONFIGURATIONS: CHAMP-SONSThe Champ-Sons model has been developed for several years [6], and validated using comparisons with other models -angular spectrum and finite elements method [7] -, and experiments [8]. This model, based on the Rayleigh integral extended to take account of refraction through a Fluid/Solid interface, gives accurate and quantitative results assuming that both field computation and source points are not close to the interface (about

Applications of Digital Signal Processing

“…On the other hand, it is important to reduce this ultrasonic channels number in order to minimize technological costs. In addition, it should be noted that the detection capability also depends on other important factors, because, from a more general point of view, still some physical limitations of the ultrasonic beams remain for a) certain angles of the scanning (Chang and Hsieh 2002), b) for certain complex geometries of the industrial components to be tested (Roy et al 1999) or c) for biological elements in medical diagnosis (Defontaine et al 2004, Reguieg et al 2006. Schemes have been preliminarily proposed in order to improve flaw detection in difficult conditions, trying to resolve these type of aspects well with two transducers and additional digital signal processing of echoes (Chang and Hsieh 2002), or well with several arrays of few elements (Engl and Meier 2002).…”