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 in metals with low electrical conductivity is challenging. To achieve 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 interacts electrically 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 the detection of 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 the detection of small defects in a range of challenging materials at eddy current frequencies up to more than 15 MHz.