What is Barkhausen noise?

Barkhausen noise is a noise-like signal, generated when a magnetic field is applied to a ferromagnet.

History of Barkhausen noise

The nature of Barkhausen noise was explained already in 1919 by Prof. Heinrich Barkhausen.

However, Barkhausen noise started to be used in industrial applications as an analysis method at the beginning of the 1980s. Today, it is a recognized nondestructive method for materials characterization and heat treatment defect testing.

How is Barkhausen noise generated?

To generate Barkhausen noise, the material must be magnetized; hence it is applicable only for ferromagnetic materials, which are steel (except Austenitic), Nickle and Cobalt, and their alloys.

Ferromagnetic materials are composed of magnetic domains in which all magnetic dipoles are aligned in the easy axis direction. Domain walls are the borders between the domains. At the domain wall, magnetic dipoles have to reorient themselves.

Hysteresis curve and Barkhausen noise

In the absence of a magnetic field (H=0), magnetic domains are randomly oriented. If the material is subjected to a magnetic field, the magnetic domains tend to align themselves in the magnetic field’s direction.

Under the applied magnetic field, domain walls move back and forth because the domain, which has an orientation closest to the applied magnetic field, increases its size by expanding the other domains with different orientations than the applied magnetic field.

When the magnetic field is constantly increasing, all the magnetic domains become parallel to the applied magnetic field by orienting themselves. At this Bs (saturation) point, a polycrystalline material may behave like a single domain state.

When applied magnetization becomes zero again, some magnetic flux (B) will remain in the material. At this Br (remanence) point, not all the magnetic domains can go back to their initial alignments. Hence, the material has some level of residual magnetism.

When the applied magnetic field continues to increase in the opposite direction, there is a point, Hc (coercivity), in which most of the domains can go back to their initial alignments. Hence, the material has no residual magnetism.

During their motion, domain walls may spend their energy to consume the less favorable oriented domains, to move away from the pinning sites.

For small external magnetic fields in the Rayleigh regime, reversible domain wall movements still may occur.

For a strong external magnetic field in the Barkhausen regime, the energy of the domain walls overcomes these pinning sites’ energy. This is why domains may not follow the same path to go back to their initial alignments.

Pinning sites, which are precipitates, inclusions, dislocations, grain boundaries, and small volumes of second phase materials, slow down the domain wall’s movement. The domain walls may be trapped behind these sites.

Due to energy spending to overcome pinning sites, the abrupt jumps lead to sudden changes in the magnetization of the material, which is directly observable in the hysteresis loop.

Why is it called noise?

The changes in the magnetization induce electrical pulses, which generate a noise-like signal called Barkhausen noise. Barkhausen noise, the irreversible jumps of domain walls over pinning sites, is called “noise” because of the noise heard from the speaker used in the original experiment.

The intensity of the Barkhausen noise signal depends on the number of Barkhausen jumps (the count rates), which is directly related to pining sites’ presence. In practice, more Barkhausen activity (count rate, jump) leads to higher signal amplitude.

What are the properties of Barkhausen noise?

Barkhausen noise (BN) gives information from the surface and very close area beneath the surface. Barkhausen noise signal has a wide power spectrum starting from the adjusted magnetizing frequency and ending above 2 MHz in most ferromagnetic materials. The effective depth of signal penetration is between 0.01 mm and 1 mm. One possible way to have more information from beneath the surface (to increase the penetration depth) is to lower the magnetization and analyze frequencies.

However, the Barkhausen noise signal’s penetration is damped due to skin effect, which is caused by the opposing eddy currents induced by the changing magnetic field.

An estimation of the penetration depth of the BN signal can be calculated using the following formula:

Where δ denotes the penetration depth, μ represents the magnetic permeability, σ means the electrical conductivity, and ƒ denotes the frequency of the alternating magnetic field.

where

For a low-alloyed hardened and tempered steel component, if we use a magnetizing voltage frequency of 125 Hz, the applied magnetic field’s penetration depth is around 2 mm. For industrial applications, such as grinding burn detection, heat treatment defect detection, and a frequency range of 70–200 kHz. For the same steel component, this range will give an analyzing depth of around 0,1 mm.

The actual values of analyzing the depth of measurement may be somewhat (approx. 30%) higher than given in the above table due to the actual variations in g(f).

The lower the permeability and conductivity, the deeper the analyzing depth of measurement. Decreasing the frequency range of the Barkhausen noise has the same effect on the analyzing depth of measurement.

Two main material characteristics will directly affect the intensity of the Barkhausen noise signal.

One is the presence and distribution of elastic stresses, which will influence the way domains to choose and lock into their easy direction of magnetization. This phenomenon of elastic properties interacting with the material’s domain structure and magnetic properties is called a “magnetoelastic interaction.” As a result of magnetoelastic interaction, in materials with positive magnetic anisotropy (iron, most steels, and cobalt), compressive stresses will decrease Barkhausen noise intensity while tensile stresses increase it. This fact can be exploited so that by measuring the intensity of Barkhausen noise, residual stress can be determined. The measurement also defines the direction of principal stresses.

Processes as cold rolling and shot peening, which is used to create complex compressive residual stress distributions at the surface layer, can be characterized by Barkhausen noise.

The other important material characteristic affecting Barkhausen noise is the microstructure of the sample. This effect can be broadly described in terms of hardness: the noise intensity continuously decreases in microstructures characterized by increasing hardness. In this way, Barkhausen noise measurements provide information on the microstructural condition of the material. The microstructure of the sample directly affects the shape of the signal output as well. For example, hard magnetic materials have wider and soft magnetic materials with narrower BN signal envelope shapes.

As the Barkhausen noise is an inductive method, several factors influence the BN signal. Here are some of them:

Residual magnetism

Excessive residual magnetism (remanence) will prevent the correct formation of Barkhausen pulses because the displayed test signal will be lower.

Retained austenite content

The retained austenite content should not exceed 40% by volume. As the retained austenite content rises, the Barkhausen value falls. This is because the austenite is paramagnetic.

Electromagnetic fields and grounding

Strong electromagnetic fields, such as a PC monitor or a transformer, can be a source of faults in the signal.

Summary

Barkhausen noise effectively detects the hidden metallurgical defects, such as grinding burns in hardened and ground components. It has been demonstrated that it is a cost-effective and capable non-destructive testing method of identifying defects.

References

• M. Deveci, “Nondestructive Determination of Case Depth by Barkhausen Noise Method”, Tampere University of Technology, Master’s thesis, Retrieved from https://dspace.cc.tut.fi/dpub/bitstream/handle/123456789/24049/deveci.pdf
• H. Barkhausen, Zwei mit Hilfe der neuen Verstärker entdeckte Erscheinungen, Physische, Zeitschrift, (1919) Volume 20. pp. (401–403).
• S. Tiitto, “On the influence of microstructure on magnetization transitions in steel”, Acta Polytechnica Scandinavica, Applied Physics Series №119, Helsinki 1977.
• P. Weiss, “L’hypothèse du champ moléculaire et la propriété ferromagnétique” J. Phys. Theor. Appl., 1907, 6 (1), pp.661–690
• F. Bloch, G. Gentile, “Zur anisotropie der magnetisierung ferromagnetischer Ein-kristalle” Z. Phys. 1931, 70:395–408
• M. Getzlaff, 2008, “Fundamentals of Magnetism” (Springer, Berlin)
• D. Jiles, 1991, “Introduction to magnetism and magnetic materials” (Chapman and Hall, London, New York)
• C. Stefanita, 2008, “From bulk to nano the many sides of magnetism” (Springer-Verlag, Berlin, Heilderberg, Germany)
• B. D. Cullity, C. D. Graham, 2009 “Introduction to magnetic materials” 2nd ed., John Wiley & Sons, New York, 2009
• T. Miyazaki, H. Jin, 2012, “The physics of ferromagnetism” (Springer, London)
• A. Sorsa, “Prediction of material properties based on nondestructive Barkhausen noise measurement” 2013, University of Oulu, Acta Universitatis Ouluensis. C, Technica, Issue:442, ISBN: 978–952–62–0068–2, Retrieved from http://jultika.oulu.fi/files/isbn9789526200682.pdf
• B. Karpuschewski, 1996, “Introduction to micromagnetic techniques” Proceed-ings of 1st International Conference on Barkhausen Noise and Micromagnetic Testing, September 1–2, 1998, Hannover — Germany

The content of this post was originally posted at https://www.linkedin.com/pulse/properties-barkhausen-noise-murat-deveci/ and https://www.linkedin.com/pulse/barkhausen-noise-analysis-murat-deveci/. The graphics were provided by Stresstech.

Also published on https://www.murat.fi/science/material-science/barkhausen-noise/