*4.1. Testing Details*

The tests were performed on plate specimens made of S235 steel (according to Standard EN 10025) and DC01 steel (according to Standard EN 10130). The steel grades properties are listed in Table 1.


**Table 1.** Steel chemical composition and strength properties.

The specimens were loaded on a Galdabini Sun 10P tensile testing machine (Galdabini, Cardano al Campo (VA), Italy). They were repeatedly subjected to uniaxial tension in such a manner that with every loading cycle, there was a slight increment in plastic strain. When the ultimate strength value was exceeded, a narrowing started to form in the specimen. The loading process was discontinued the moment that the narrowing was visible enough to detect through a visual inspection. Due to the material hardening, a change in hardness occurs in the plastically deformed area. Non-uniform plastic strain leads to the creation of a specific hardness profile on the specimen surface.

For the measuring apparatus described in Section 2, the details of the configurations used during the BN measurements are presented in Table 2.


**Table 2.** Details of measuring configurations.

At the preliminary stage of the testing, measurements were carried out changing the magnetization direction every 30◦; angles 0◦ and 180◦ correspond to the load direction. Example results are presented in Figure 6. They confirm the literature data [27,28] that prove that the signal maximum and minimum values are obtained for the maximum strain direction and for the normal direction, respectively.

**Figure 6.** Polar diagram illustrating average Barkhausen noise energy *EBN* in the place of the specimen narrowing (0–180◦—direction parallel to the tensile load direction, 90–270◦—direction normal to the tensile load direction).

In the engineering practice, diagnostic correlations should be independent of the directions of principal residual stresses in the material, which are usually unknown. One proposal for a simplified solution is the absolute value of the Barkhausen noise properties:

$$V = \sqrt{V\_X^2 + V\_Y^2} \tag{3}$$

where *VX* and *VY*, respectively, are the property values for two directions of the magnetizing field application (e.g., *EBN* or *NoETOT*) for which the most extreme values are obtained.

Considering the above, the Barkhausen noise was measured only in two directions perpendicular to each other, i.e., in direction *X*, parallel to the load direction, and in direction *Y*, normal to the load direction.

After the Barkhausen noise was measured in the same measuring points, HV5 hardness measurements were performed using the Krautkramer TIV hardness tester (General Electric, Boston, MA, USA). The measurements were carried out along a measurement line being the specimen axis of symmetry. Example testing results for plastically deformed specimens made of S235 and DC01 steel are shown in Figures 7 and 8, respectively.

**Figure 7.** Distribution of HV5, root mean square (RMS), and *NoETOT* along the S235 steel specimen length.

**Figure 8.** Distribution of HV5, RMS, and *NoETOT* along the DC01 steel specimen length.
