Development of a Novel TMR-Based Eddy Current Probe With Double-Cylinder Magnetic Shielding for Inspection of Nonferromagnetic Metals

To improve detection sensitivity and reduce edge effects, a novel eddy current (EC) probe including a single excitation coil with double-cylinder magnetic shielding is developed based on a tunnel magnetoresistive (TMR) sensor. The outer-layer and inner-layer silicon steel shields are used to constrain the magnetic field. A simulation study is performed to optimize the probe design using a 2-D axisymmetric finite element model. The simulation shows that the EC density of up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8.92 \times 10^{{6}}$ </tex-math></inline-formula> A/m2 for the probe with shielding is more than eight times higher than that without shielding, with the former having a relative change in magnetic-filed strength (~44.7%) approximately four times than that of the latter. The shielding efficiency and probe sensitivity are verified experimentally by detecting surface defects, edge defects, and deformation damage in nonferromagnetic metallic specimens. It is found that the probe can detect magnetic fields with a strength of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-{6}}$ </tex-math></inline-formula> T. The probe successfully detected surface defects with a dimension of 2 mm (length), 0.5 mm (width), and 1 mm (depth) on an aluminum alloy specimen, edge defects in the region around the circumference of the hole on a nickel-based alloy specimen, and deformation damage occurred in an SS304 flat specimen with a width of 15 mm during the conventional tensile test possibly due to dislocation, slip, and martensitic transformation. These results proved that the developed TMR-based EC probe with magnetic shielding can detect defects and damage in nonferromagnetic metals.