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Abstract
At the end of 1988 a research project was started to develop a practical, high-resolution imaging method for ultrasonic NDI, based on the Synthetic Aperture Focusing Technique (SAFT). This project was initiated by the TNO Institute of Applied Physics and two major companies in The Netherlands. As conventional single-transducer measurements do not provide enough ultrasonic information on different types of defects, one is forced to make use of separate sources and receivers. The tandem configuration, where both source and receiver (separated by some distance) are scanned, is capable of producing useful measurements, but the application of two moving transducers makes the tandem set-up generally too complicated in practice. The single-transducer LLT-technique (Walte, Gebhardt, et al. [1]) is also very well applicable for imaging in practice. However, for this technique, which utilizes indirectly reflected and mode-converted waves, the imaging zone is very limited.
In this paper we present a SAFT-imaging method that makes use of one fixed source position, illuminating a defined region of interest, and a scanning receiver. The source position and the receiver aperture have to be chosen carefully, as will be shown with the aid of modeled results. It is demonstrated that multiple reflecting waves, interacting with a defect via the object boundaries, have to be included in the SAFT-algorithm. The improved imaging method is referred to as Multi-SAFT. A fast calculation scheme to compute indirect wavepaths in cylindrical objects is presented. The data-acquisition method will be described and a number of modeled and experimental imaging results will be given, illustrating the possibilities and improvements of the images when applying Multi-SAFT.