Time of flight diffraction and synthetic aperture focusing technique for NDT of weldments

Abstract

The limits of anomaly detection by use of nondestructive testing (NDT) methods are based on the interaction of the material with the applied probing medium. Various methods are employed for detection of anomalies in weldments. Investigations using the radiographic, ultrasonic pulse/echo, time of flight diffraction, and synthetic aperture focusing techniques were carried out on a carbon steel weld pad (thickness 18 mm) with natural anomalies introduced at predetermined locations. The anomalies were first imaged by radiography. The pulse/echo technique, using different parameters, could detect almost all the planar anomalies except a cluster of porosity. Although time of flight diffraction could recognize most of the anomalies, it was not possible to clearly resolve and characterize the cluster of pores and group of cracks. Cracks with cleft edges were almost impossible to identify. The experimental results revealed that time of flight diffraction at a minimum meets code requirements for detection of anomalies. Sizing of length and through-wall extent can be carried out using the initial time of flight diffraction data. Using the synthetic aperture focusing technique, it was possible to resolve and characterize all the anomalies on par with the radiographic information. In addition, two clusters of porosity were distinctly seen, separated by 7 mm. Hence, the reporting criteria of the "ASME Boiler and Pressure Vessel Code" (ASME, 2004) can be satisfied using ultrasonic imaging techniques such as time of flight diffraction and synthetic aperture focusing for detection, sizing and characterization of anomalies in weldments in lieu of radiography. Destructive tests were also employed at a later stage to corroborate and validate the findings of anomalies by NDT methods.