Industrial quality control measurement visualized at nanoscale

The structure of a ferromagnetic material is characterized by its microstructural and magnetic features. When a ferromagnetic material is placed in an external magnetic field, its small magnetic domains start to move. This movement causes a measurable signal called Barkhausen noise (BN). These BN-based methods are used in the quality control for example in car industry

The phenomena is studied in Tampere University’s multidisciplinary project BarFume (Magnetic Barkhausen Noise: From Fundamental Physics to Non-Destructive Testing). Recently, the researchers succeeded in mimicking and visualizing the BN-measurements in steel by the transmission electron microscope at university’s Microscopy Center.

At Tampere University the BarFume project combines the knowhow of researchers from the Materials Characterization group and Tampere Microscopy Center lead by Professor Minnamari Vippola and Complex Systems group lead by Professor Lasse Laurson. The research consortium is led by Senior Scientist Mari Honkanen from Microscopy Center. The project has received funding from Academy of Finland.

“The aim of the four-year project is to improve understanding of how the microstructural features of the sample affect the domain wall movement, thus Barkhausen noise, and how this information could be used to improve the accuracy of Barkhausen noise-based quality control methods,” says consortium principal investigator Mari Honkanen.

“In addition, novel machine learning techniques will be developed to automatically classify materials microstructure using Barkhausen noise as input,” adds consortium sub principal investigator, Prof. Lasse Laurson.

Barkhausen noise to reveal grinding burns in wind turbine gears

In 1919, German Prof. Heinrich Barkhausen noticed that when ferromagnetic material is placed in the time-varying, external magnetic field, the magnetization of the material changes with discontinuous jumps caused by the movement of the magnetic domains as the domain walls past pinning sites in material. In 1980s, the BN-based methods were started to use in industrial applications.

“The BN-based methods are sensitive to the microstructure and stresses of the sample. These days, methods are used to find for example possible grinding burns in gears used in wind turbines and transmission components in heavy-duty vehicles, tells Mari Honkanen.

The measurement itself is simple: a sample is placed in the time-varying, external magnetic field causing the movements of magnetic domain walls. Microstructural features, such as grain boundaries and carbides, hinder their motion. The domain wall may stop in these obstacles and then suddenly cross it when the magnetic field increases.

“These sudden motions change the magnetization of the sample, resulting in a BN signal,” Honkanen explains.

Mimicking industrial quality control measurement in electron microscope

The BN measurement has been studied widely, however; the interpretation of results is usually challenging because of the complexity of the phenomenon. This interdisciplinary project now combines materials characterization to BN measurements, micromagnetic modelling, and advanced statistical analysis.

The utilization of the research results is obvious in collaboration with an industrial partner, Stresstech Oy, which manufactures, as the only company in the world, commercial Barkhausen noise analysis equipment.

During preparing the project research plan, preliminary studies on the visualization of BN measurements at nanoscale in the transmission electron microscope were carried out. The information was combined with BN measurements for same materials and achieved results were published as an article “Mimicking Barkhausen noise measurement by in-situ transmission electron microscopy - Effect of microstructural steel features on Barkhausen noise” in the high-quality materials science journal Acta Materialia. The Signal and Image Restoration group lead by Prof. Alessandro Foi enabled co-registration of microscope images before constructing them to videos.

Tampere Microscopy Center (TMC) provides instruments, training, and services for researchers at Tampere University, in other research institutes and in industry.

Further information

Mari Honkanen
tel. +358 40 849 0133
mari.honkanen [at] tuni.fi

Lasse Laurson
tel. +358 50 545 5387
lasse.laurson [at] tuni.fi

Minnamari Vippola
tel. +358 40 849 0148
minnamari.vippola [at] tuni.fi