materials Article Examining Ferromagnetic Materials Subjected to a Static Stress Load Using the Magnetic Method Tomasz Chady 1, * and Ryszard Lukaszuk 2   Citation: Chady, T.; Lukaszuk, R. Examining Ferromagnetic Materials Subjected to a Static Stress Load Using the Magnetic Method. Materials 2021, 14, 3455. https:// doi.org/10.3390/ma14133455 Academic Editor: Giovanni Bruno Received: 19 May 2021 Accepted: 15 June 2021 Published: 22 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Faculty of Electrical Engineering, West Pomeranian University of Technology in Szczecin, Sikorski St. 37, 70-313 Szczecin, Poland 2 Independent Researcher, Sikorski St. 37, 70-313 Szczecin, Poland; tramwajarz.szczecinski@gmail.com * Correspondence: tchady@zut.edu.pl; Tel.: +48-91-449-41-34 Abstract: This paper discusses the experimental examination of anisotropic steel-made samples sub- jected to a static stress load. A nondestructive testing (NDT) measurement system with a transducer, which enables observation of local hysteresis loops and detection of samples’ inhomogeneity, is proposed. Local hysteresis loops are measured on two perpendicular axes, including one parallel to the rolling direction of the samples. The results confirm that the selected features of the local hysteresis loops provide important information about the conditions of ferromagnetic materials. Furthermore, it is shown that the selected parameters of the statistical analysis of the achieved measurements are beneficial for evaluating stress and fatigue changes induced in the material. Keywords: nondestructive evaluation; magnetic methods of testing; NDT 1. Introduction Steel is susceptible to the harmful effects of certain external environmental factors. For this reason, it is necessary to subject steel products to examination at the stage of both production and operation. If the internal and external structure of the object must remain intact, nondestructive testing (NDT) is performed. Detection of small inhomogeneities in the material allows us to observe degradation at an early stage, reducing the possibility of a catastrophic failure and alternative repair costs. The good electrical conductivity and high permeability of steel create possibilities to detect discontinuities in its structure using electromagnetic methods of NDT. The electromagnetic methods have high sensitivity, so apart from detecting a defect, it is also possible to pinpoint its location and assess its dimensions. In the case of steel-made sheets rolled in the direction opposite to the grain orientation, magnetic anisotropy is a particular obstacle during an examination. In addition, anisotropy can also be induced by stress. This results in the need for the inspection to be carried out in two orthogonal directions of the material. Ferromagnetic materials can be tested using several electromagnetic methods of NDT: ■ the magnetic flux leakage method is based on observation of the magnetic flux dis- tribution over the material surface [1]. The primary magnetic field source causes a magnetic flux in the material. A barrier to the secondary flux is any inhomogeneity in the material structure that has a significant reluctance value [2]. The flux leakage method allows us to assess the tested object’s surface and subsurface inhomogene- ity [3]. The main advantages are high sensitivity, easiness of signal acquisition, and the possibility of automation [2,4]. However, this method also has some disadvantages, including sensitivity to material impurities and the need to magnetize the object [5]; ■ the magnetic particle inspection method allows for the detection of both surface and subsurface heterogeneities [6]. First, the sample is exposed to an external magnetic field, whereby magnetic powder particles can be placed on the outer surface of the sample in two ways: during the magnetization or after switching the magnetic field source off. The magnetic flux dispersing on the inhomogeneities appears on the Materials 2021, 14, 3455. https://doi.org/10.3390/ma14133455 https://www.mdpi.com/journal/materials