Novel Rotating Current Probe With GMR Array Sensors for Steam Generate Tube Inspection

This paper presents a novel probe design that uses a rotating current excitation scheme and giant magnetoresistance (GMR) sensors for steam generator tube inspection. Rotating eddy currents in the tube wall under test is induced using a pair of axial and circumferential excitation coils. A circumferential array of GMR sensors measures the radial component of the magnetic field. The perturbation in eddy current flow caused by a defect produces a radial component field that is measured by the GMR sensors. A C-scan image of the induced magnetic field obtained with the axial motion of probe shows defect location and orientation. The probe is sensitive to both the axial and circumferential defects and offers the advantages of high sensitivity over a wide frequency range. A finite-element model was developed to mimic the underlying physical process and predict the output images of defects with different orientations and sizes. A prototype unit consisting of excitation coils and GMR sensor array along with a data acquisition system was developed and tested. A steam generator tube sample with machined axial and circumferential notches was inspected using the prototype probe. Experimental results demonstrating the feasibility of the concept are presented.