**1. Introduction**

Residual stress is the totality of internal forces per area acting upon a boundary surface of a volume within a material, thereby maintaining the current size and shape of a given object under the absence of external loads [1, 2]. Depending on the range of constant stress value and direction, residual stress in polycrystals is divided into first kind (constant across larger distance, e.g., across several crystals/grains), second kind (constant across smaller distance, e.g., within a crystal/grain) and third kind (varies on the submicroscopic scale, e.g., from atom to atom) [2].

Although best expressed as a tensor, industrial practice often demands for simplified residual stress values regarding one or a few relevant directions, such as those most critical under the expected service load.

non-magnetized ferromagnetic material, the magnetic flux density B increases in its surroundings. If the field strength is reduced to zero again, the magnetic flux density remains at a value unequal to zero. The ferromagnetic material retains some residual magnetism (remanence). With periodic magnetization, a ferromagnetic material reacts with a hysteretic relation of flux density B and magnetic field H (see **Figure 1**) [4, 5]. With continuous periodic magnetization, the hysteresis curve

The reason for the occurrence of hysteretic behavior is the interaction of Bloch walls and microstructure. The Bloch wall motion is discontinuous throughout the magnetization process, as the Bloch walls are pinned by existing lattice defects, and each pinning has to be overcome by increasing applied magnetic field. Lattice defects in the microstructure such as grain boundaries, dislocations and precipitates represent energetic minima for the Bloch walls. This causes the Bloch walls to tear

Stresses affect the magnetic hysteresis behavior of ferromagnetic materials. In magnetostrictively positive materials (such as most ferromagnetic steels), tensile stress causes an increase of the differential susceptibility and a narrower hysteresis (HC shift toward smaller values) and vice versa for compressive stresses. Magnetostrictively negative materials behave contrary to positive materials [4, 5, 7].

The analysis of magnetic Barkhausen noise (MBN) is based on a discovery by Barkhausen [8]: Abrupt changes in local magnetization caused by the discontinuous Bloch wall motion can be picked up inductively as a series of voltage pulses (noise) induced into a coil that is placed close to the material surface. The location of the highest density of these impulses is in relation to the magnetic hysteresis near the coercive force HC, since this is where most Bloch wall jumps occur. The voltage pulses can be amplified, filtered and rectified for evaluation. The envelope of the filtered and rectified noise signal can be recorded as a function of the magnetization field strength, resulting in a so-called Barkhausen noise profile curve M(H) (see **Figure 2**). The magnetic Barkhausen noise has a characteristic dependence on

loose abruptly and move until a pinning site of higher strength is reached.

is traversed once with each magnetization period.

*DOI: http://dx.doi.org/10.5772/intechopen.90740*

*Nondestructive Characterization of Residual Stress Using Micromagnetic…*

*2.1.3 Magnetic Barkhausen noise*

**Figure 1.**

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*Hysteresis curve (HC, coercivity; BR, remanence) [6].*

Residual stress has significant impact on product quality and useful service life, as it affects the product geometry and processability (strip steel being just one example), the tendency to crack appearance (e.g., after hardening or forming) as well as the fatigue lifetime. The present contribution discusses micromagnetic and ultrasonic techniques that are industrially applied for nondestructive stress measurement.
