Eddy Current Probes Based on Magnetoresistive Array Sensors as Receivers

Abstract : Eddy Current (EC) Technique is a powerful method for detection of surface notches and of buried flaws during inspection of metallic parts. This technique is used for inspection at different industrial domains like aeronautics and nuclear one. Classical winding coils are the most commonly used EC sensors. Nevertheless, when the size of flaws decreases or the defect is rather buried deep inside the material, traditional winding coil probes turn out to reach their limits. For this reason, other technologies are investigated to improve this technique. Magnetoresistive sensors present the advantages of flat frequency response and dimensions at the micron size. These sensors are hence very attractive for the detection of buried defects that require low frequencies because of skin depth effect. Also, they are suitable for small surface defects due to high spatial resolution because of their manufacturing down to one hundred µm without losing their field sensitivity. An optimization of such probes based on magnetoresistive sensors (GMR - giant magnetoresistive and MTJ – magnetic tunnel juntions) specially designed to be integrated into an eddy current probe has been experimentally studied. Measurements using MR array probes consisting of 32 GMR- and MTJ-elements, an ASIC, subsequent readout components, and emitters for EC generating inside the material under test are shown. These probes have been developed in the IMAGIC-project1 for detection and imaging of surface breaking defects and buried flaws. The performances of developed probes have been investigated for several mock-ups.