## 2 Samples and Experimental Details
### Magnetic Adaptive Testing  
The samples shown in Figure 1 were measured by MAT method. For the measurement, a magnetizing yoke was put on the surface of the sample. The size of the yoke was chosen to fit the size of samples: cross-section 10 mm \(\times\) 5 mm, total outside length 18 mm. A triangular waveform of magnetizing current, led into the 100 turns of the magnetizing coil, wounded on the bow of the yoke was applied with step-wise increasing amplitudes and with a fixed slope magnitude in all the triangles.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample No.** & & **23** & **24** & **25** & **26** & **27** \\ \hline RPM & [t/min] & 1000 & 500 & 600 & 500 & 600 \\ Feed & [mm/min] & 75 & 1200 & 1700 & 2000 & 2500 \\ Lateral offset & [mm] & 0.1 & 0.1 & 0.1 & 0.2 & 0.25 \\ \(R_{a}\) & [\(\mu\)m] & 0.13 & 0.49 & 0.33 & 0.73 & 0.61 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Manufacturing parameters and surface roughness parameters of the measured samples.  
Figure 1: The investigated 22 NiMoCr37 samples having different surface roughness conditions.  
Figure 2: Sample 24. Manufacturing grooves can be clearly seen. Longitudinal orientation is indicated.  
Such a way a triangular time-variation of the effective field, \(h_{a}(t)\), is produced in the magnetizing circuit and a signal is generated in the pick-up coil. \(h_{a}(t)\) sweeps linearly with time, \(t\), and the voltage signal \(U(h_{a},h_{b})\), in the pick-up coil is proportional to the differential permeability, \(\mu(h_{a},h_{b})\), of the magnetic circuit.  
\[\mu(h_{a},h_{b})=const*U(h_{a},h_{b})=const*\partial B(h_{a},h_{b})/\partial h _{a}*\partial h_{a}/\partial t \tag{2}\]  
Output signal was measured by a 50-turn pick-up coil, wounded around a yoke leg. A specially designed permeameter [17] was used during measurement. Therefore, the obtained triangular like waveform is composed by two linear sessions having the same slew-rate but with opposite signs, considering the whole cycle of the reverse-magnetization. In addition, the final (i.e., peak) level of the reached current in each half-period increased by fixed step, where the direction of magnetization is reversed, in order to be able to record concentric minor magnetization loops. These two things in combination result in the triangular like waveform can be seen in left side of Figure 3. Therefore, instead of describing the frequency spectrum of the exciting signal, the \"speed\" of magnetization can characterize the difference between the two triangular like waveforms are used for the comparison in this paper.  
The output signal of pick-up coil is proportional to the differential permeability of the whole magnetic circuit if the magnetizing current increases linearly with time. Recorded permeability loops can also be seen in the right side of Figure 3. MAT measurements were repeated by two different slopes of magnetizing current.  
Permeability matrices were calculated from the minor hysteresis loops, and each matrix element was normalized by the corresponding element of the reference sample 23. In this way, a large data pool can be obtained. The optimally chosen permeability matrix elements, which characterize the best the changes in material properties due to different material degradation, were selected by correlation analysis. Details of the whole magnetic adaptive testing evaluation process is described in detail in [8].