The time and Fourier domain Synthetic Aperture Focusing Technique (SAFT and FSAFT) have been used to improve both sensitivity and lateral resolution of ultrasonic NDT. Following acquisition of signals over the surface of the tested piece, SAFT identifies a defect by summing all the signals within a certain area (the aperture) after giving proper time-delays. FSAFT, although using the same data, is based instead on the angular spectrum approach and utilizes a backpropagation algorithm to find the field in any plane inside the material. F-SAFT provides an accurate and time-efficient algorithm for 3-D reconstruction. We have introduced several improvements to the F-SAFT algorithm and we have coupled it to laser-ultrasonics. Improvements include temporal deconvolution to enhance both axial and lateral resolutions, control of the aperture to improve signal-to-noise ratio, as well as spatial interpolation in each sub-surface plane. The actual performance of F-SAFT is investigated for imaging simulated and real defects in several applications such as the detection of inclusions in steel slabs, visualization of stress corrosion cracks and delaminations along curved interfaces.
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