Simulation tool for the Eddy current magnetic signature (EC-MS) non-destructive method

Abstract

Magnetism has been used for years as a subsidiary method to identify the level of mechanical residual stress through ferromagnetic steels. It is admitted that residual stress strongly modifies the magnetic behavior and consequently the monitored magnetic signatures. However, some of them like the Eddy Current Magnetic Signature (EC-MS) happen to be especially sensitive. EC-MS exhibits drastic changes, strong enough to predict and anticipate the presence of elastic or plastic deformations. The EC-MS signature is obtained by plotting the imaginary part versus the real part of an Eddy current probe impedance, controlling locally a tested sample under the influence of a slowly varying high amplitude magnetic excitation. The superimposition of the weak amplitude, high-frequency magnetic excitation of the Eddy current probe to the quasi-static contribution creates minor hysteresis loop situations where the reversible magnetic contribution is the main provider. EC-MS is, therefore, an exceptional way to observe and characterize this reversible contribution and this over different average magnetic states. This contribution tends to be particularly sensitive to residual stresses and strains, and so is EC-MS. In this article, the Jiles-Atherton-Sablik theory and an improved Dodd and Deeds analytical solution for the simulation of a pancake-type Eddy current coil are combined to simulate EC-MS. From the simulation results, we illustrate and understand why EC-MS is so sensitive to residual stress. Eventually, we justify this method as probably the most indicated magnetic method for the control and evaluation of residual stresses through structural and construction steels.