Enhanced transient eddy current detection of deep corrosion

Abstract

Recent developments in the use of transient eddy current methods have demonstrated the advanced ability to detect and characterise corrosion and cracks within complex structures, using a single acquisition scan. In addition, the use of Hall sensors to measure the magnetic field directly enables measurements at greater depths in metals than is achieved using a conventional coil sensor. However, the rapid increase in effective field size with depth in the structure means that there is always a trade-off between defect sensitivity and susceptibility to the presence of nearby edges, fasteners and other sub-structure. The ability to characterise the defect in terms of metal loss, size and depth can also suffer when defect sensitivity is increased. This paper reports an investigation to determine how the sensitivity could be increased for metal-loss defects deeper than 15 mm (0.625) in the case when the proximity of structural changes is not an issue. The essential requirement is to increase the contrast between good and defective structure by either reducing noise levels or increasing the strength of the field reflected from the defect. An increase in the reflected field can be achieved by either increasing the incident field strength, or changing its spatial characteristics. Results have shown that defect sensitivity can be increased using these methods to enable a considerable improvement in detectability of metal loss deep in thick aluminium structure.