2015 Volume 3 Issue 1 Pages 268-281
In ultrasonic testing, SAFT (Synthetic Aperture Focusing Technique) is a signal processing technique used to reconstruct flaw images by the superposition of A-scan waveforms. However, an additional signal processing technique may also be integrated into SAFT algorithms to get clearer ultrasound images. Wave theory is one of the most significant keys in SAFT improvement because it can lead to a better understanding of the wave phenomenon. In this paper, approximate wave solution (AWS) based on a stationary phase method is used to calculate beam radiations inside a target of inspection. Multiplying A-scan waveforms by the ultrasonic beam radiations computed from the AWS, wave diffraction characteristics that are spatial functions between a transducer and a target are incorporated into the SAFT. Moreover, the AWS can be used to predict the Effective Region (ER) where high imaging capability can be obtained. The performance of the improved SAFT, namely AWS-SAFT, and the ER technique are experimentally tested in ultrasonic imaging for a side-drilled hole (SDH) in a steel plate. As a visualizing aid, Structural Similarity (SSIM) index is introduced and Structural Similarity Index Map (SSIM-MAP) is proposed instead of a SAFT image. It is shown that SSIM-MAP can improve a flaw image, especially in that the image is more focused on the location of a flaw, and artifacts due to a longitudinal wave mode are eliminated in a transverse wave mode SAFT image. The ER, where a high contrast flaw image can be definitely obtained, is defined quantitatively in advance of SAFT imaging. The preliminary study for the use of the proposed technique in a real problem is demonstrated using ultrasound imaging of a bottom defect of a steel plate to demonstrate the application of AWS-SAFT and ER.