Analysis of Eddy Current Distribution and Resulting Flaw Detection Mechanism for Self-Nulling Probe

Abstract

<p>The Self-Nulling Probe developed at NASA-LaRC has been previously shown to simplify inspections and increase detectability of surface and subsurface flaws in conductive materials [1]. The operation of the probe has been explained in terms of a redistribution of the eddy currents and associated magnetic field by the presence of inhomogeneities in the test article [1, 2]. In the present work a mathematical model is introduced to calculate the eddy current flow induced by the Self-Nulling Probe in test samples containing through notch flaws. The model combines axisymmetric finite element modeling (FEM) with con-formal mapping techniques in order to calculate the stream function of the induced current flow about arbitrarily sized and positioned notches. Physical parameters such as the induced eddy current density, magnetic field normal to the surface of the sample, and probe output voltage are then calculated from the stream function of the induced current flow. Simulation results are compared with experimental measurements taken on calibration standards with good agreement.</p>