High-frequency eddy current measurements using sensor-mounted electronics

Abstract

Eddy current techniques are used widely for the detection of surface breaking cracks in metal samples and the detection of such defects on metals with low electrical conductivity is challenging. For good sensitivity to small surface cracks, the electromagnetic skin-depth of the eddy current needs to be small, which often means operating at MHz frequencies. One of the major challenges in high frequency eddy current testing is that the capacitance of the cable between the instrument electronics and the sensor head becomes significant in the MHz range, making the system unstable and introducing noise into the system as the cable moves and electrically interacts with objects close to it. There are significant benefits to locating the electrical circuitry directly behind the eddy current sensor coils, reducing issues with cable induced electrical noise, enabling one to detect smaller defects at earlier stages of growth. Materials such as nickel based super-alloys, titanium, austenitic steel and carbon fibre composites are often used in safety critical applications, where the ability to detect surface cracks at the earliest possible stage is vital. Examples are presented that show detection of small defects on a range of challenging materials at eddy current frequencies to over 15 MHz.

Characterisation of small defects using miniaturised EMAT system

“…These can require the removal of any coating prior to inspection, which is potentially costly and environmentally unfriendly, and the inspection may contaminate the sample. An alternative is to use eddy current inspection, which is well-suited to detecting surface defects on conductive materials and Email address: r.s.edwards@warwick.ac.uk (R. S. Edwards) components, including through thin non-conductive coatings [1,4,5]. Inspection is limited near sample edges, and magnetic permeability changes as well as lift-off variations are sources of noise [6][7][8][9].…”

Destructive and non-destructive testing methods for characterization and detection of machining-induced white layer: A review paper

“…At high frequencies, the capacitance of the cable connecting the ECT probe to the electrical circuitry can be affected by interaction with the surroundings and results in additional noise to the impedance measurement. A system has been developed that mounts the electrical circuitry in the test probe (Hughes et al, 2016) and this facilitated the detection of smaller artificial slots than a conventional system due to an improved signal to noise ratio. Currently, the only solution to the issue of surface roughness is to polish the surface being inspected to produce an ideal flat surface.…”