Synthetic Magnetic Field Imaging With Triangle Excitation Coil for Inspection of Any Orientation Defect

Detection of any orientation defect that is deeply embedded under rivet head in multilayer riveted structure is a challenging task for eddy current testing (ECT). In this paper, a novel probe with right-angle triangle excitation coil and two tunneling magnetoresistance (TMR) sensor arrays is proposed to address this problem. The two TMR sensor arrays, which are capable of obtaining magnetic field images with spatial resolution of 0.5 mm, are placed on top of the two right-angle sides of the excitation coil measuring the normal component magnetic field. The probe moves along the direction of the long side of the triangular coil during the inspection. Therefore, the two sensor arrays scan the structure under test sequentially and two images are recorded. An image processing algorithm including data shifting, rotating, and multifrequency mixing is developed to fuse the two images, which results in a synthetic image. A prototype probe is built and tested to demonstrate the feasibility of the design. Three-layer aluminum samples with carbon steel rivet and defect located in the second and third layers are evaluated by the prototype probe. Defects with different sizes and orientations are imaged. Experiment results show that the proposed probe is capable of detecting defects in any direction. The signal processing algorithm highlights the defect information. The orientation angle of the defect can be estimated from the synthetic image.