A thickness reduction testing method for ferromagnetic materials based on variable intensity DC magnetization

Abstract

The assessments of internal thickness reduction of ferromagnetic components are extremely important. Traditional nondestructive testing (NDT) methods are usually non-linear and sometimes the electromagnetic parameters of components are required. In this paper, a thickness reduction testing method based on variable intensity DC magnetization (VIDC-MAG) is proposed. Both theoretical modeling analysis and simulation modeling analysis show that a pole point of magnetic permeability μm appears with the increase of DC magnetization intensity, which corresponds to an inflection point magnetizing current im. The im decreases with the increase of the defect depth of the wall thickness reduction and shows a linear relationship. The experimental results show that the method performs well for the inner wall thickness thinning and the relative error of the measurement results is small. Additionally, the measurement results are related to the width of the thickness reduction, the probe excitation frequency, the magnetic properties of ferromagnetic material and the lift-off distance of the magnetizer. The method is applicable to the thickness variation of the cover layer and does not need to know the electromagnetic parameters of the components in advance, which has greater practical application value.