By applying an alternating magnetic field to the piece under examination, and measuring the intensity of domain movement through a coil which causes an intermittent electric sound (Barkhausen Noise) the noise level indicates the amount of stress present.

The intensity of the noise is influenced by different microstructural parameters (size of the grain, hardness, direction of the fibres etc.) as well as, obviously, the chemical composition of the material. Being sensitive to the variation of these parameters, this method also allows detection of microstructurally discontinuous areas generated, for example, by manufacturing faults, welding seams, and so forth.

However, separation of the effects of all these parameters is, at present, an open problem and their simultaneous variation is perhaps the greatest limit to applying this method. In particular, measuring the state of stress assumes importance when it is possible to control the other parameters and therefore a calibration test is needed for each material and microstructural state examined.

2. Correlation of Barkhausen Noise-residual stress using diffractometric calibration

As mentioned previously, the value of the Barkhausen Noise is influenced by both the presence of residual stress and the microstructural parameters of the sample under examination. To be able to identify areas suffering from "grinding burns" (with residual stress close to the traction or even in the traction) using Barkhausen noise, X-Ray Diffraction were applied to evaluate both the state of stress and eventual microstructural alterations. Correlating the results provided by the diffraction with the Barkhausen Noise values allows us to create a "calibration curve" so we can attribute residual stress values to the noise values. Obviously the Barkhausen/residual stress conversion is valid only for the material being examined (type of steel, heat treatment, roughness). Below we give the
calibration chart for manganese steel used for producing sprocket wheels. The surfaces analysed had been ground.