Depth profiling of defects in stainless steel using electric potential sensors and a non-contact a.c. potential drop method

A new technique is described a non-contact version of the well-known AC potential drop method. Electric potential sensor technology is capable of measuring, non-contact, the spatial electric potential associated with a sample via the displacement current, through capacitive coupling. This can be achieved due to the extremely high input impedance of the sensors, resulting in many cases in no loss of signal when compared with a contact voltage measurement. For laboratory samples, an AC current is passed through the sample and the spatial potential above the surface monitored as a function of position. For a full non-contact implementation this current would be induced inductively as in eddy current testing systems. Previous work on stainless steel using this new technique was able to detect 1 mm of material loss from the inside of a 24 mm-thick sample. The work described here extends this by applying multiple frequencies simultaneously to a sample and analysing the data for each frequency component separately. In this way, depth information about faults within a material may be obtained, due to the skin depth effect. This enables the location and characterisation of faults in three dimensions to be achieved while only performing a single scan.