Detection of damage in non-magnetic steel by magnetism

Wear, corrosion, material fatigue, these signs of degradation are common to most materials. This makes the early detection of damage all the more important, preferably at a micro-scale. Magnetic test methods are often used for this purpose, which was previously not possible with non-magnetic steel. Researchers from Kaiserslautern and Mainz have now developed a process in which they apply a thin magnetic layer to the steel. Changes in the microstructure can thus be detected by changes in the magnetic effects. Materials such as aluminum can also be tested in this way. The study was published in the Journal of Magnetism and Magnetic Materials.

Steel is one of the most frequently used materials. It is used in many variations including stainless steel, high strength quenched and tempered steel, and low cost structural steel. Steels can be magnetic or non-magnetic. They are used in cutlery, in vehicle components or in steel beams of buildings and bridges. Sometimes steel is exposed to high temperatures or stress. “This can lead to microstructural changes, cracks or component failure”, explains Dr Marek Smaga, researcher at the Department of Materials Science under the direction of Professor Dr Tilmann Beck at the Technische Universität Kaiserslautern (TUK). Experts speak in this context of material fatigue. This damage is initially only visible at the micro level. Even with magnetic test methods, it is not yet possible to detect changes of this scale in non-magnetic steel at an early stage.

Engineers from TUK and physicists from Johannes Gutenberg University in Mainz (JGU) present a solution in their current study. Their technique uses magnetic effects, although it is applied to a non-magnetic material. “With magnetic steel it is possible to find changes in the structure very early on,” explains Kaiserslautern doctoral student Shayan Deldar. “Even tiny deformations change the magnetic properties. This can be measured with special sensor technology.”

Researchers coated non-magnetic steel with magnetic films, each 20 nanometers thick, made of terfenol-D, an alloy comprising the chemical elements terbium, iron, and dysprosium, or permalloy, a nickel-iron compound. The researchers then used a so-called Kerr microscope to check whether stresses in the steel could be detected in the microscopic range. “This is achieved using what is called the Kerr effect,” explains Smaga, “which allows magnetic microstructures, so-called domains, to be imaged by rotating the direction of polarization of light.”

Scientists examined magnetically coated steel plates that were previously exposed to mechanical load. “We observed a characteristic change in the structure of the magnetic domain,” explains Dr Martin Jourdan of the Institute of Physics at the Johannes Gutenberg University in Mainz. “The microscopic stress in non-magnetic steel causes the direction of magnetization of the thin film to change.”

Compared to conventional testing methods, this method has the advantage of detecting signs of fatigue much earlier at the micro level. The researchers’ method could be used in new testing techniques in the future. In addition, it is not only interesting for non-magnetic steel, other materials such as aluminum, titanium and certain composite materials could also be provided with such a layer.

Provided by Technische Universität Kaiserslautern