Spin+X Researchers Develop An Approach For Strain Detection In Non-magnetic Materials

Abrasion, corrosion, material fatigue - most materials underlie these ageing processes. Because of that, it is even more important to discover defects early on, at best at the microscopic stage. Often, magnetic test procedures are used for this. In non-magnetic steel, this was not possible until now. SPIN+Xresearchers from Kaiserslautern and Mainz developed a technique, in which they apply a thin magnetic layer onto steel. Like this, changes in the microstructure can be detected via changes of magnetic effects and even materials as aluminum can be tested. The study was published in the Journal of Magnetism and Magnetic Materials.

Steel is one of the most widely used materials. It is used in many variants, such as stainless steel, high-strength tempered steel or low-cost structural steel. Steels can be magnetic or nonmagnetic. They are used in cutlery, in components of vehicles or in steel beams of buildings and bridges. Sometimes steel is exposed to high temperatures or stresses. "This can lead to microstructural changes, cracks or component failure," says Dr. Marek Smaga, who works at the Department of Materials Science under Professor Tilmann Beck at the University of Kaiserslautern (TUK). Experts speak of material fatigue in this context. Such damageis initially visible only at the micro level. However, magnetic testing methods do not yet serve for detecting changes in nonmagnetic steel at an early stage.
This is exactly what SPIN+X engineers from the TUK and physicists at the Johannes Gutenberg University Mainz (JGU) are working on, and they are presenting a solution in their current study. The special feature: They make use of magnetic effects, although it is non-magnetic material. "With magnetic steel, you can find changes in the structure early on," explains PhD student Shayan Deldar from Kaiserslautern. "Even tiny deformation changes the magnetic properties. This can be measured with special sensor technology."
Researchers have coated a non-magnetic steel with different 20-nanometer magnetic films made of terfenol-D, an alloy of the chemical elements terbium, iron and dysprosium, or permalloy, a nickel-iron compound. In order to subsequently check whether strains of the steel at the microscopic level can be detected, the researchers used a so-called Kerr microscope. "Here, the so-called Kerr effect is used," Smaga explains the method "with which the magnetic microstructures, the so-called domains, can be imaged by the rotation of the light polarization direction."
The scientists have studied magnetically coated steel plates of a few millimeters of size, which were previously exposed to mechanical stress. "We have observed that there is a characteristic change in the magnetic domain structure," explains lecturer Dr. Martin Jourdan from the Institute of Physics of Johannes Gutenberg University Mainz. "The microscopic strains in non-magnetic steel cause the direction of the magnetization of the thin layer to change."Compared to conventional testing methods, the method offers the advantage of detecting signs of fatigue much earlier on the micro level. The researchers' method could in the future be used for new testing techniques. In addition, it is not only interesting for non-magnetic steel, also other materials such as aluminum, titanium and certain composites could be provided with such a layer.
The study was published in the renowned journal "Journal of Magnetism and Magnetic Materials": "Strain detection in non-magnetic steel by Kerr-microscopy of magnetic tracer layers". M. Jourdan, M.M.B. Krämer, M. Kläui, H.-J. Elmers, S. Deldar, M. Smaga, T. Beck. DOI:https://doi.org/10.1016/j.jmmm.2018.05.081