Using Image Processing and A new SAFT Algorithm for Ultrasonic Flaw Imaging in Concrete

Abstract

The Synthetic Aperture Focusing Technique (SAFT) is well established for ultrasonic imaging purposes, for example in nondestructive testing (NDT) of materials. The ultrasonic inspection of concrete is notoriously difficult however, due to its combined attenuating and scattering characteristics, especially at high frequencies. Since low frequency transducers must be used, inspection has traditionally been limited to rather simple measurements conducted in the time domain, such as the ultrasonic pulse velocity. Since the propagation velocity is related to the modulus and density, and hence indirectly to the strength of concrete, the pulse-velocity method has been established as possibly the only standard technique of nondestructive testing of concrete. It is of limited value when attempting to establish the internal condition of the material. For cases where the interrogation frequency is very low (below 250kHz), normally the transducers do not have a broadband frequency response, which causes prolonged ringing in the time domain. Recently however, advances in transducer technology and signal processing have enabled the direct visualization of internal defects both by using pulse-echo systems and SAFT the development and application of an enhanced time-domain SAFT algorithm that uses a correlation technique to perform the coherence summation. Not only is this method faster, by an order of magnitude, than equivalent time-domain SAFT algorithms, it produces images of defects within the concrete that are unobtainable by other means of processing the ultrasonic signals.