Performance of Ultrasonic Imaging with Frequency Domain Saft (f-saft)

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. Introduction: Ultrasonic testing is widely used for detecting, locating and sizing flaws in many applications. In conventional ultrasonics, an improvement of the lateral resolution and signal-tonoise ratio (SNR) is achieved by focusing the acoustic field with lenses or curved transducers or by using a computational technique that basically consists of performing the focusing numerically. The last method is known as the Synthetic Aperture Focusing Technique [1]. SAFT is traditionally implemented by scanning a focused piezoelectric transducer over the surface of the specimen and then processing the collected data array. Originally developed in the time domain, SAFT can be advantageously implemented in the frequency domain (F-SAFT). F-SAFT is based instead on the plane wave decomposition of the measured ultrasonic field at the surface (angular spectrum approach) and then utilizes a backpropagation algorithm to find the field in any plane inside the material [2, 3]. F-SAFT provides an accurate and time-efficient algorithm for 3dimensional reconstruction. On the other hand, laser-ultrasonics has brought practical solutions to a variety of NDT problems that cannot be solved by using conventional ultrasonic techniques [4, 5]. Laserultrasonics uses two lasers, one with a short pulse for the generation of ultrasound and another one, long pulse or continuous, coupled to an optical interferometer for detection. Laserultrasonics is a technique that combines the advantages of both optical and ultrasonic sensing. Laser-ultrasonics allows sensing remotely as an optical technique and sensing inside materials as well as on their surface as an ultrasonic technique. The technique features also a large detection bandwidth, which is important for numerous applications, particularly small defect detection and material characterization. Another feature of laser-ultrasonics, particularly useful for testing parts of complex shapes, is the generation of an ultrasound wave propagating at well defined angles, independently of the shape of the part and of the incidence angle of the optical generation beam. However, these very attractive characteristics of laser-ultrasonics are often limited by the relatively poor sensitivity of laser-based detection. Recently, we have introduced several improvements to the F-SAFT algorithm and we have coupled it to laser-ultrasonics [6, 7]. 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 control of the aperture also allows reduction in the sampling requirements to further decrease both data acquisition time and processing time, therefore making the technique more attractive for industrial use. It should be noted that laserultrasonics is inherently well adapted for SAFT data acquisition since laser beams are easily scanned, far-field ultrasound is easily obtained, and the generated ultrasound is widely diverging. SAFT compensates the relatively poor sensitivity of laser-based detection by collecting and adding up many signals originating from the detected defect. In this paper, we demonstrate the capability of this combined technique for detecting and characterizing flaws in structural materials. 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 (SCCs) and delaminations along curved interfaces. F-SAFT method: We assume that the generation and detection beams are focused at the same location onto the surface. By moving the sample mounted on a stage or by scanning the laser beams, a 2-D mesh of signals at the surface of the material is obtained. As shown in Fig. 1, if a diffraction source, such as a defect tip, is present at point C located at a depth z within the sample, this particular point re-radiates the acoustic field originating at point Mj. The ultrasonic signal S(Mi,t) received at any point Mi in the measurement mesh exhibits a peak at time t = 2di / v, where v is the ultrasonic wave velocity in the material and di is the distance CMi. Consequently, the summation: