Micromagnetic simulation of Barkhausen noise origin in non-destructive testing applications
Haavisto, Valtteri (2025)
2025
Master's Programme in Science and Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2025-06-24
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202506247411
https://urn.fi/URN:NBN:fi:tuni-202506247411
Tiivistelmä
Barkhausen noise (BN) is a phenomenon that is observed when a ferromagnetic material is under a changing external magnetic field. The magnetic field causes the domain walls (DW) inside the material to move to minimize its energy. The movement of these DWs is discontinuous and occurs in a jerky manner as the DWs get stuck on different imperfections in the material. This type of movement causes the magnetization of the material to change abruptly, which can be observed as BN.
BN is one method of doing non-destructive testing (NDT). It is used to test the integrity of materials from its surface layers, such as finding grinding burns in steels. For NDT, a sensor consisting of a pickup and a magnetization unit is needed. The magnetization unit creates an external magnetic field, whereas the pickup unit detects changes in the magnetization of the sample which can be characterized as BN. Samples with altered material sections produce higher BN signal root mean square (RMS) values. In this way, defective samples can be identified and removed from use.
The aim of this thesis is with micromagnetic simulations to model the BN sensor and gain further understanding into microstructures of the material that cause BN in NDT applications. The modeled BN in this thesis is characterized as a time derivative of the magnetization's out-of-plane component. This is done since real-world NDT sensors are placed on top of the sample so that changes in the sample's out-of-plane magnetization cause BN. More precisely, a change in the out-of-plane magnetization of the sample causes the magnetic flux to change inside the sensor, inducing a voltage signal characterized as BN. In-plane magnetizations are not considered because they do not change the magnetic flux inside the sensor.
With micromagnetic simulations, an iron sample of relatively small size is studied due to numerical limitations. An in-plane external magnetic field is applied to the sample. The different microstructures that are studied are DWs, non-magnetic carbides, and crystallographic orientations. Simulations are run for a few different sensor sizes to also gain an understanding of the difference in the observed BN for varying sensor sizes.
The main result was that out-of-plane magnetization, and thus BN, is observed mainly from DWs. The magnetic domains themselves contributed little to the observed out-of-plane component and BN, regardless of the orientation of the lattice. It was also found that the decrease in sensor size increased the BN RMS value. When non-magnetic carbides were added to the system, they increased the noise in the BN signal resulting in higher RMS values.
BN is one method of doing non-destructive testing (NDT). It is used to test the integrity of materials from its surface layers, such as finding grinding burns in steels. For NDT, a sensor consisting of a pickup and a magnetization unit is needed. The magnetization unit creates an external magnetic field, whereas the pickup unit detects changes in the magnetization of the sample which can be characterized as BN. Samples with altered material sections produce higher BN signal root mean square (RMS) values. In this way, defective samples can be identified and removed from use.
The aim of this thesis is with micromagnetic simulations to model the BN sensor and gain further understanding into microstructures of the material that cause BN in NDT applications. The modeled BN in this thesis is characterized as a time derivative of the magnetization's out-of-plane component. This is done since real-world NDT sensors are placed on top of the sample so that changes in the sample's out-of-plane magnetization cause BN. More precisely, a change in the out-of-plane magnetization of the sample causes the magnetic flux to change inside the sensor, inducing a voltage signal characterized as BN. In-plane magnetizations are not considered because they do not change the magnetic flux inside the sensor.
With micromagnetic simulations, an iron sample of relatively small size is studied due to numerical limitations. An in-plane external magnetic field is applied to the sample. The different microstructures that are studied are DWs, non-magnetic carbides, and crystallographic orientations. Simulations are run for a few different sensor sizes to also gain an understanding of the difference in the observed BN for varying sensor sizes.
The main result was that out-of-plane magnetization, and thus BN, is observed mainly from DWs. The magnetic domains themselves contributed little to the observed out-of-plane component and BN, regardless of the orientation of the lattice. It was also found that the decrease in sensor size increased the BN RMS value. When non-magnetic carbides were added to the system, they increased the noise in the BN signal resulting in higher RMS values.