Corrosion detection in welds and heat-affected zones using ultrasonic Lamb waves

Abstract

Measurements are reported on the amplitude of ultrasonic Lamb waves reflected from a single 20 mm-diameter flat-bottomed hole located adjacent to the weld of a 6 mm-thick butt welded steel plate. The hole was introduced to simulate the presence of corrosion defects in the heat-affected zone, and served to assess the suitability of using the fundamental So Lamb waves as a means for remote detection of corrosion in fluid-filled storage tanks. Measurements were made using a prototype, omnidirectional multi-element transducer developed at Imperial College, and also using a single edge-mounted transducer. Detection of a hole was only possible when using the plate tester at oblique incidence angles, because of the inability to discriminate weld echoes from those due to the hole at normal or near-normal incidence. Detection of holes was also limited to those with a minimum depth of approximately 3 mm by the presence of coherent background noise. Measurements using a single edge-mounted transducer demonstrated that preferential waveguide propagation of the So mode occurred along the weld, in a manner coined 'weld-guided waves'. This permitted easy detection of a 2 mm deep-hole located adjacent to the weld at a distance of 1 m from the transducer. It is likely that a practical surface-mounted transducer based on the S 0 waveguide mode of operation could be developed for the inspection of corrosion defects within the heat-affected zone of fluid-filled tanks. It is probable that such a device would have a range of many metres, be simple in construction, and that the method would be relatively unaffected by the presence of fluid and inherently sensitive because of the suppression of other propagation modes.