The Barkhausen effect

A novel nondestructive magnetic evaluation technique for preventive maintenance on gears and other components.

By Madhav Rao Govindaraju and John Wallace

PlantServices.com

Plants and structural components that have been in operation for long periods need special attention from plant operators to ensure safe and reliable performance. Efficient and economic operation of these plants requires regular preventive maintenance. In general, it is more effective to monitor the material condition that could lead to a subsequent failure rather than detecting a defect after its initiation. Conventional non-destructive evaluation (NDE) methods, such as ultrasonics and radiography techniques, fail to detect incipient damage caused by loading history or changing microstructural conditions that could lead to failure. Hence, improved or new NDE techniques are needed to monitor microstructural changes to estimate both remaining life of the component and extent of material degradation. The magnetic Barkhausen technique is one such advanced preventive maintenance tool for industry.

Magnetic nondestructive evaluation techniques

Nondestructive evaluation plays a significant role in pre-service and in-service inspection of plant components. Plant maintenance personnel need advanced NDE techniques to help them counter challenges such as increasing maintenance costs, shorter downtimes and stricter environmental regulations.

Primary objectives of NDE methods in plant maintenance include:

• Detection and evaluation of critical defects.
• Monitoring and assessment of loads and residual stresses.
• Evaluation of microstructure changes during service.

Magnetic NDE techniques are relatively new inspection methods gaining popularity for evaluating ferrous structural components. Magnetic properties of steels are sensitive to microstructural changes induced by mechanical and thermal treatments. Hence, changes in magnetic properties can be used to evaluate the material condition. Magnetic NDE techniques, especially Barkhausen emission measurements, show excellent sensitivity to residual stress levels and changes in the microstructure.

Barkhausen emission technique is an easy-to-use, semi-quantitative technique for monitoring changes in near surface stress distribution or microstructure of ferromagnetic machinery components.

Barkhausen emission analysis

Ferromagnetic materials are full of small magnetic regions called domains. Each domain is magnetized along a certain crystallographic easy direction of magnetization, and domains are separated from one another by boundaries called domain walls. These domain walls move under the influence of an applied magnetic field. This movement of domain walls results in a change in magnetization within the material and will induce an electrical pulse in a pick-up coil. When the electrical pulses produced by domain movement are added, a noise-like signal called the Barkhausen effect or magnetic Barkhausen emissions (MBE) is generated, named after its discoverer Heinrich Barkhausen. Amplification of these signals produces audio/radio frequency noise, which can be observed on an oscilloscope or spectrum analyzer.

MBE has a power spectrum that extends to about 2 MHz, the amplitude of which is damped exponentially as a function of depth below the surface. The damping is attributable to eddy current damping experienced by the propagating electromagnetic fields the domain wall movement creates.
Measurement depth in ferromagnetic materials depends on the frequency range of the Barkhausen emission signals and material properties, such as conductivity and permeability. The measurement range for MBE varies between 0.01 to 1.5 mm from the surface for most ferromagnetic materials.

Theoretical aspects and practical applications of Barkhausen effect can be obtained by excellent review articles by Allessandro et al., [1], Tiitto [2], Matzkanin et al. [3], and Jiles [4].

Magnetic Barkhausen emissions depend on material properties

The MBE spectrum depends on the magnetic state of the sample--including chemical composition, microstructure, magnetic and thermal history, and applied or residual stress. The characteristics of the MBE signal depend most importantly, for a given sample, on microstructure and stress.

Elastic stresses in metals are known to influence the metal's magnetic domain structure. This phenomenon of elastic stresses interacting with domain structure is called "magnetoelastic interaction". And in materials with positive magnetic anisotropy (such as iron, cobalt, and low-alloy steels), it is this interaction that causes tensile stresses to increase the intensity of Barkhausen emissions while the presence of compressive stresses decreases signal intensity. Therefore, MBE signals can be used to deduce the nature and magnitude of the stresses in the material.

In general, MBE intensity decreases continuously with an increase in hardness. Higher hardness implies an increased number of defect pinning centers, which impede the movement of both dislocations and magnetic domain walls. Hence, harder materials exhibit lower levels of magnetic Barkhausen emissions. The compressive stress state of a shot-peened surface will yield a relatively low MBE output. If the MBE signal from a sample begins to rise, it may indicate the protective compressive surface stresses induced by shot peening are being converted to a potentially dangerous tensile condition.

In general, MBE analysis can provide information about the stress-state of a sample when the microstructure is known or fixed. Conversely, it can provide information about microstructure as long as the sample remains in a known state of stress. The technique usually cannot, however, provide useful information when the stress-state and the microstructure are both unknown.