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Determination of Linear Defect Depths from Eddy Currents Disturbances

Ramos, H. ; Rocha, T. ; Ribeiro, A. L. ; Pasadas, D.

Determination of Linear Defect Depths from Eddy Currents Disturbances, Proc Review of Progress in Quantitative Nondestructive Evaluation - QNDE, Baltinore, United States, Vol. 00, pp. 00 - 0, July, 2013.

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One of the still open problems in the inspection research concerns the determination of the maximum depth to which a surface defect goes. Eddy current testing being one of the most sensitive well established inspection methods, able to detect and characterize different types of defects in conductive materials, is an adequate technique to solve this problem.
This paper reports a study concerning the disturbances in the magnetic field and in the lines of current due to a machined linear defect having different depths in order to extract relevant information that allows the determination of the defect characteristics. The image of the eddy currents (EC) is paramount to understand the physical phenomena involved.
The EC images for this study are generated using a commercial finite element model (FLUX). The used excitation produces spatially a uniform magnetic field on the plate under test in the absence of defects and the disturbances due to the defects are compared with those obtained from experimental measurements. In order to increase the limited measuring range obtained with detection coils, giant magnetoresistors (GMR) are used which also allow to lower the working frequency. The geometry of the excitation planar coil produces a uniform magnetic field on an area around the GMR sensor, inducing a uniform eddy current distribution on the plate. In the presence of defects in the material surface, the line of currents inside the material are forced to flow around or underneath the defect and the magnetic field produced by these current disturbances is sensed by the GMR sensor.
The paper describes the theoretical study of the electromagnetic system, the experiments that have been carried out to support the theory and conclusions are drawn for cracks having different depths.