Synthetic Aperture Focussing Technique - State of the Art
V. Schmitz, S. Chakhlov, W. Müller, FhG IzfP, Saarbrücken, Germany
ABSTRACT
Detection and evaluation of defects during inservice inspection relies strongly on ultrasonic inspection techniques. The performance of such techniques can be highly improved if the high-frequency ultrasonic signals are stored and evaluated by tomographic algorithms like Synthetic Aperture Focusing Technique "SAFT". The technique has been applied in different nuclear power plants, mostly on pipe systems with circumferential welds where the scan was performed along the axis of the pipe or on pressurizer where the shape of the surface could be regarded to be quasi-plane. This technique has been drastically improved by combining the SAFT-reconstruction procedure with a powerful 3D-CAD system. With the knowledge of the shape of the component, a 3D drawing will be created by a parametric description. Without such a knowledge but with a manipulator which is able to measure the outer contour of the surface the inner dimensions can be added and a realistic description of the component will be synthesised. During the conventional evaluation of a SAFT reconstruction, the relation of the position of a weld defect towards the weld or towards the cladding or inner contour of the component has to be performed by a manual evaluation. The feature - placing the synthetic probe at the begin and end of a scan at the same coordinates into the CAD-drawing - allows to fit the coordinates of the experiment into the CAD-model. The benefit is, that all images of existing defects, seam of the weld or contour of the inner surface are positioned accurately into the component which assures an improved classification into surface connected defects, undercladding cracks or defects within a weld or not. Additionally the reconstruction process is automatically adjusted to the real surface and acoustic imaging is possible not only for plane surfaces but for cylindrical or non-regular surfaces. The features outlined above will be demonstrated by practical examples where 2 MHz and 4 MHz probes with different insonification angles have been used in the application centre of the IZFP and in the field on pressurised components.
Publication Source: First International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components , 20 - 22 October 1998, Amsterdam, Netherlands. Held by the Joint Research Centre of the European Commission. Publisher:Woodhead Publishing Limited
NDTnet
