## 2 Ultrasonic Testing of Materials  
### Studies of Areas with Different Surface Roughness  
The results of ultrasonic testing can be influenced by both the acoustic properties of the tested material and the surface condition, which serves as a reflector for the ultrasonic wave. While in laboratory testing, it is possible to have access to both sides of the tested element and measure the thickness and surface roughness, in field conditions, especially when inspecting single-access elements, such possibilities are often limited. Therefore, the aim of this work was to investigate the extent to which the surface condition and hardness of the tested element affect the results of ultrasonic testing. In the first part of the study, a sample was prepared, containing 13 different areas with varying surface roughness. The sample was intentionally prepared from a single element to avoid variations in acoustic properties for each area. Various surface preparation techniques were used in the study, reflecting those commonly employed in workshop practices. The techniques included electropolishing with grit sizes of 40, 60, and 100; grinding with emery cloth of grit sizes 20, 40, 60, 100, and 150; shot blasting; and milling with depths of 0.1, 0.3, 0.5, and 0.7 mm. The sample with the measurement points marked in the roughness area is depicted in Figure 1. These are typical technologies used in surface preparation for painting, anti-corrosion protection, bonding, or regeneration using methods like metallization or welding. One of the basic parameters of roughness is Ra, which is the arithmetic mean deviation of the profile from the mean line. In this study, thirteen areas with differential surface preparations were separated, and the values of the roughness parameter Ra are shown in Table 1. A view of two example areas with different roughness is shown in Figure 2.  
A graphical representation of the roughness over the entire surface is shown in Figure 3. Due to the large differences between the areas, the surfaces were divided into two parts, the first with a roughness of less than 3 \(\upmu\)m (Figure 3a), and the second with a roughness of more than 50 \(\upmu\)m (Figure 3b). Figure 1: View of the underside of the sample, with thirteen different fields of varying roughness and the coordinates of the measurement points plotted. \begin{table}
\begin{tabular}{c c} \hline
**Type of Processing** & **Ra** \\ \hline Sandpaper 150 & 0.34 \\ Sandpaper 100 & 1.06 \\ Sandpaper 60 & 1.30 \\ Sandpaper 40 & 1.36 \\ Sandpaper 20 & 2.57 \\ Sandblasting EX100 & 0.33 \\ Sandblasting EX60 & 1.59 \\ Sandblasting EX40 & 2.27 \\ Blasting & 2.59 \\ Milling 0.1 & 50.62 \\ Milling 0.3 & 149.15 \\ Milling 0.5 & 250.73 \\ Milling 0.7 & 348.69 \\ \hline \end{tabular}
\end{table} 
Table 1: Overview of types of surface preparation of wave reflection and the corresponding roughness parameter Ra. Figure 2: _Cont._  
The measurement setup, illustrating the application of the ultrasonic transducer to the tested sample, is shown in Figure 4.  
Frequency ultrasoc probe 20 MHz  
Figure 4: The measurement setup—ultrasonic probe and sample with varying (regular and irregular) roughness of the reflecting surface; R\({}_{\bullet}\)—surface roughness, \(\lambda\)—ultrasonic wavelength for 20 MHz wave frequency. Figure 3: Roughness distribution on the surface of the sample with the coordinates plotted according to Figure 1; (**a**) area with roughness less than 3 \(\upmu\)m; (**b**) area with roughness greater than 50 \(\upmu\)m.  
Figure 2: View of example areas with different roughness; (**a**) 0.1 mm milling, (**b**) zoomed view of milled area, (**c**) processing with sandpaper 100, and (**d**) zoomed view of area roughened with sandpaper 100. The attenuation of longitudinal ultrasonic waves was measured using two digital ultrasonic defectoscopes: the laboratory UMT15 and the industrial USM35 XS.