Barkhausen Noise Analysis
Barkhausen Noise Analysis (BNA) method, also referred to as the Magnetoelastic or the Micromagnetic method is based on a concept of inductive measurement of a noise-like signal, generated when magnetic field is applied to a ferromagnetic sample. After a German scientist Professor Heinrich Barkhausen who explained the nature of this phenomenon already in 1919, this signal is called Barkhausen noise.
Barkhausen Noise - the Phenomenon
Ferromagnetic materials consist of small magnetic regions resembling individual bar magnets called domains. Each domain is magnetized along a certain crystallographic easy direction of magnetization. Domains are separated from one another by boundaries known as domain walls. AC magnetic fields will cause domain walls to move back and forth. In order for a domain wall to move, the domain on one side of the wall has to increase in size while the domain on the opposite side of the wall shrinks. The result is a change in the overall magnetization of the sample.
If a coil of conducting wire is placed near the sample while the domain wall moves, the resulting change in magnetization will induce an electrical pulse in the coil. The first electrical observations of domain wall motion was made by professor Heinrich Barkhausen in 1919. He proved that the magnetization process, which is characterized by the hysteresis curve, in fact is not continuous, but is made up of small, abrupt steps caused when the magnetic domains move under an applied magnetic field. When the electrical pulses produced by all domain movements are added together, a noise-like signal called Barkhausen noise is generated.
Barkhausen noise has a power spectrum starting from the magnetizing frequency and extending beyond 2 MHz in most materials. It is exponentially damped as a function of distance it has traveled inside the material. This is primarily due to the eddy current damping experienced by the propagating electromagnetic fields that Domain wall movements create. The extent of damping determines the depth from which information can be obtained (measurement depth). The main factors affecting this depth are
- frequency range of the Barkhausen noise signal analyzed, and
- conductivity and permeability of the test material.
Measurement depths for practical applications vary between 0.01 and 1.5 mm.
Barkhausen Noise - the Properties
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, in materials with positive magnetic anisotropy (iron, most steels and cobalt), compressive stresses will decrease the intensity of Barkhausen noise while tensile stresses increase it 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 micro-structure 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.
Barkhausen Noise Analysis - the Applications
Many common surface treatments such as grinding, shot peening, carburizing and induction hardening involve some modification of both stress and microstructure and can be readily detected using the method. Various dynamic processes such as creep and fatigue similarly involve changes in stress and microstructure and can also be monitored with Barkhausen noise.
Practical applications of the magnetoelastic Barkhausen noise method can be broadly divided into three categories:
- Evaluation of residual stresses; provided microstructural variables can be reasonably controlled.
- Evaluation of microstructural changes; provided level of stress can be reasonably controlled.
- Testing of the following surface defects, processes and surface treatments that may involve changes in both stresses and microstructure:
- Detection of grinding defects and grinding process control
- Detecting surface defects through Cr-coating
- Evaluation of shot-peening effect in steel
- Measurement of residual surface stresses in steel mill rolls and steel sheet