Barkhausenrauschen Analyse

Um den Barkhauseneffekt zu verstehen, ist die Entstehung der Rauschimpulse im Barkhausensignal notwendig. Zur Erzeugung eines Barkhausensignales (BN) magnetisiert ein Magnetfeld das Werkstück – daher ist das Verfahren nur für ferromagnetische Werkstoffe geeignet. Ferromagnetische Materialien verfügen im submikroskopischen Bereich über eine Vielzahl von magnetischen Domänen (Weiss´sche Bezirke), die untereinander durch Domänenwände (Blochwände) getrennt sind. Jede Domäne verfügt über eine magnetische Vorzugsrichtung.

Solange kein magnetisches Feld anliegt, sind die magnetischen Domänen in chaotischer Unordnung, das Werkstück ist in Summe feldfrei. Wirkt ein äußeres Magnetfeld auf das Werkstück, versuchen sich die magnetischen Domänen entlang der Feldlinien des äußeren Feldes auszurichten.

Der Vorgang der Ausrichtung der Domänen erzwingt eine Bewegung der Domänenwände. Die Domänen, deren Vorzugsrichtung am ehesten dem äußeren Magnetfeld entsprechen, wachsen auf Kosten der entgegengesetzt orientiertn Domänen. Hiebei verschiebt sich die Domänenwand. Wird das Magnetfeld kontinuierlich verstärkt, tritt schliesslich Sättigung ein – im sättigungszustand sind alle Domänen parallel zum äußeren Magnetfeld ausgerichtet.

Geht das äußere Feld wieder zurück auf Null, versuchen die magnetischen Domänen wieder ihre ursprüngliche Lage einzunehmen. Jedoch wir diese Rückbewegung durch verschiedene Hindernisse wie Ausscheidungen, Korngrenzen, Einschlüsse, Versetzungen im Kristallgitter und kleine Bereiche von anderen Materialphasen behindert. Während der Bewegng der Domänenwand wird Energie verbraucht, um die Hindernisse zu überwinden. Die dabei auftretenden abrupten Sprünge beim Übergang über das Hindernis führen zu plötzlichen, lokalen Änderungen der Magnetisierung des Werkstücks. Selbst bei stufenloser Änderung des äußeren Magnetfelds sind diese sogenannten Barkhausensprünge der Grund für die Unstetigkeit dieser Vorgänge, was sich direkt in der Hysteresekurve beobachten lässt.

Barkhausen noise provides information on the surface and a very close area beneath the surface. A Barkhausen noise signal has a wide power spectrum starting from the adjusted magnetizing frequency and ending above 2 MHz in most of the ferromagnetic materials. The effective depth of signal penetration is between 0.01 mm and 1 mm. The Barkhausen noise signal 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.

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

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

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. As an example, hard magnetic materials have wider and soft magnetic materials have narrower BN signal envelope shapes. Characteristic of the BN signal are amplitude, peak and width shapes which are affected by the microstructure of the sample and applied magnetizing field.

Barkhausen noise measurement requires

• a main analyzer (Rollscan), model depending on the main use
• a Barkhausen Noise sensor developed for the component to be tested
• optional component handling equipment (stand)
• software for data acquisition (ViewScan) and analysis (MicroScan).

During the measurement, the sensor magnetizes and demagnetizes the sample in a cycle and picks ups the induced BN signal and transfers it to the main analyzer. Rollscan (the main analyzer) is the power source and digital signal processor. ViewScan or MicroScan are the software for real time data acquisition and reporting. Stands are custom made equipment for component handling.

Viele Oberflächenbearbeitungen, wie z. B. Schleifen, Hartdrehen, Einsatz-, Induktionshärten und Kugelstrahlen verursachen erhebliche Änderungen in Eigenspannung und Mikrogefügestruktur der Randzone, die mittels Barkhausenrauschen charakterisiert werden können. Die Kontrolle von Hartfeinbearbeitungen (Prüfung auf Schleifbrand) stellt die Hauptanwendung des Barkhausenverfahrens dar. Prozesse wie Kriechen und Ermüdung ändern ebenfalls die Eigenspannung und Mikrostruktur und können mittels Barkhausenrauschen analysiert werden.

Praktische Anwendungen der magnetoelastischen Barkhausenrauschenmethode können allgemein in drei Kategorien eingeteilt werden:

• Bewertung der Eigenspannungen; solange die mikrogefügestrukturellen Variablen bekannt und konstant bleiben
• Bewertung des Mikrogefüges; solange die Eigenspannungen bekannt und konstant bleiben
• Prüfen von Randzonen auf Veränderungen in Eigenspannung und Mikrogefüge, welche durch Wärmebehandlungen und Bearbeitungsprozesse beeinflusst werden können

Barkhausen noise gives a response to the stress level of specimens. Barkhausen noise can be used for the evaluation of stress state of materials. Tensile stress increases the Barkhausen noise signal amplitude and compressive stress decreases Barkhausen noise signal amplitude.

Cold working processes which are used to create complex compressive residual stress distributions at the surface layer can be characterized by Barkhausen noise. As an example, Barkhausen noise can be used to characterize and evaluate the effectiveness of shot peening process and improve the quality control process of shot peening. With Barkhausen noise, coverage and uniformity of the shot peening process can be inspected.

Measurement of residual stresses with Barkhausen noise is not a straightforward application since Barkhausen noise does not directly produce any MPa value results for stress state determination. However, with a calibration process, it can be done successfully and nondestructively. Evaluation of welding stresses is another practical application of Barkhausen noise.

Barkhausen noise can be used for the evaluation of the hardness state of the materials. Soft materials increase the Barkhausen noise signal amplitude and hard materials decrease Barkhausen noise signal amplitude.

Barkhausen noise can differentiate soft and hard parts from each other in a production line. It can easily match the production rates of most manufacturing lines, enabling real-time hardness control.

Detection of grinding burns and grinding process control

Grinding burn is a common name for thermal damages which occur on the surface during grinding processes. Grinding burn causes local discolorations on the surface and it can soften or harden surface layers.

When the temperature is over the normal tempering range but below the austenization temperature (Ac3), slow quenching is applied to form a soft material that is called over-tempered martensite (OTM). The formation of OTM is known as retempering burn which creates tensile stress state and reduces the hardness on the surface.

When the temperature is above the austenization temperature (Ac3), rapid quenching is applied to form the untempered martensite (UTM) on the surface layer of the ground workpiece. The UTM layer is harder, prone to corrosion, sensitive to micro cracks and more brittle than the core of the workpiece and it changes the surface integrity and surface stress state into the tensile. The formation of the hard layer of martensite (UTM) is called rehardening burn which is another form of grinding burn.

Since Barkhausen noise gives the same response for both hardness and stress changes, different forms of grinding burns can be detected reliably.

Grinding processes can be controlled by monitoring the condition of the ground parts. 100% detection of grinding damage which occurs due to wheel wear, incorrect feed rate, wheel speed and other parameters can be made reliably by Barkhausen Noise Analysis.

100% detection of grinding burns is the most common industrial application of Barkhausen noise which is recognized by many organizations such as The Department of the Air Force, US Navy, SAE, FAA, AST and many more.

Detection of surface defects through Cr-coating

The detection of service-related thermal damages through chromium plate is a straightforward and reliable application for Barkhausen noise. The main application for detecting the surface defects through Cr-coating is landing gears of aircraft. During overhauling of the planes, using Barkhausen noise to detect surface defects through Cr-coating saves at least a week of time per part by preventing the unnecessary chrome stripping of parts such as axles, cylinders, and bores.