Abstract
A portable impedance analyzer (PIA) was developed based on a TiePie-HS3 device to provide the comparable impedance measurement accuracy of the Agilent 4294a impedance analyzer in the frequency range of 0~250 kHz. Then the PIA was applied to monitor the tensile stress-induced variation of the eddy current sensor’s impedance in a medium-carbon steel sample. A model of equivalent magnetic field induced by the elastic stress and the number of pinning sites indicated that the inductance of the eddy current loop firstly increased with the increase in the tensile stress and then decreased at the yield point of the material. The experimental results testified that the variation of impedance amplitude, the variation of phase angle, and the shift of two featured frequencies demonstrated opposite variation trends before and after the yield point, as predicated by the model. A new parameter, which combined the impedance variation information of the selected two frequencies, was found to exhibit nearly monotonous dependency on the tensile stress in elastic and plastic stages. The new parameter together with the developed portable impedance analyzer provided the solution to identify the elastic and plastic behaviors in ferromagnetic materials in practical applications with an eddy current technique.
Highlights
Eddy current techniques are typically applied for nondestructive evaluation of metals [1, 2]
To investigate the effects of elastic and plastic strains on eddy current response in metals by experimental tools, uniaxial tensile loading is commonly applied in the tensioned samples and the impedance variation of the eddy current sensor (ECS), which is placed near the sample surface, is recorded during the entire tension process [14]
The portable impedance analyzer was developed based on a TiePie-HS3 device and a connected laptop equipped with LabVIEW software
Summary
Eddy current techniques are typically applied for nondestructive evaluation of metals [1, 2]. Eddy current techniques have the potential to evaluate the microstructure change [5,6,7], state of applied/residual stress [8, 9], and plasticity [10, 11] in ferromagnetic materials. Dahia et al [19] alternatively measured the magnetic permeability of the Journal of Sensors ferromagnetic iron-cobalt alloy under tensile elastic stress. Both the permeability and the ECS impedance amplitude were found to monotonously increase with the increase in the elastic stress. The effects of elastic and plastic deformations in medium carbon steel on the impedance shift of an eddy current sensor were experimentally investigated. In order to identify the elastic or plastic stage in the steel, the new parameter, which combines the impedance variation information of the two frequencies, is introduced to roughly figure out the deformation types
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