## 2 Materials and Methods  
### Base Substrate, Sample Dimensions, and Mechanical Pre-Treatments  
The base material used in this study was ferritic-pearilitic steel (S235 JRG2), with its fundamental properties summarized in Table 1. The initial semi-finished product consisted of cold-rolled steel sheets measuring 2000 \(\times\) 1000 \(\times\) 1 mm, which were cut using guillotine shears to obtain final test specimens of 160 \(\times\) 65 \(\times\) 1 mm. To facilitate sample suspension, 5 mm diameter holes were punched using a manual press. Given the anisotropic properties of the steel (resulting from the rolling process), all samples were oriented uniformly, with elongated ferritic grains aligned along the longitudinal axis (Figure 1). Before undergoing further processing, the samples were degreased with technical ethanol (minimum 95 wt.% ethanol) to prevent contamination from preservation substances. The mechanical pre-treatment by blasting was conducted manually within a blasting cabinet, utilizing both synthetic corundum and steel grit as abrasive media. The equipment operated on a pressure system, where the abrasive material was stored in a pressurized vessel and delivered through a compressed air hose to a nozzle. Identical blasting parameters were maintained for both abrasives:  
* Pressure: 0.5 \(\pm\) 0.05 MPa
* Nozzle-to-sample distance: 200-250 mm
* Abrasive impact angle: 45 \(\pm\) 10\({}^{\circ}\)
* Blasting duration: 70-80 s per side \begin{table}
\begin{tabular}{c c} \hline \hline
**Mechanical Property** & **Minimum Value, MPa** \\ \hline Tensile Strength (Rm) & 360 \\ Yield Strength (Re) & 235 \\ \hline
**Chemical Composition** & **Maximum Value, wt.\%** \\ \hline Carbon (C) & 0.170 \\ Manganese (Mn) & 1.400 \\ Phosphorus (P) & 0.045 \\ Sulphur (S) & 0.045 \\ Nitrogen (N) & 0.014 \\ Silicon (Si) & – \\ Iron (Fe) & Remainder \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Fundamental parameters of steel S235 JRG2 [4]. Figure 1: Dimensions of test samples and the orientation of ferrite grains. This procedure aligns with the methodology outlined in [17]. The specific properties of the blasting media are detailed in Table 2.  
The mechanical pre-treatment by grinding was performed using P40 abrasive paper, featuring a circular section (75 mm diameter), open-coated grain structure, synthetic resin bonding, and paper backing, secured to the support plate via hook-and-loop fastening. The abrasive material (synthetic corundum) shared the same composition as specified in Table 2, except for the grain size, which ranged between 400 and 500 \(\upmu\)m [18]. The difference in grain size arises from the wider distribution typically observed in F-grit series (loose abrasives) compared to P-grit series (coated abrasives). During machining, the test specimen was clamped in a vice, and the initial developed length for grinding the first side was approximately 220 mm. After grinding, the sample was trimmed, bent at the opposite end, and ground on the reverse side (developed length: approximately 190 mm). Following the mechanical pre-treatment, final trimming to 160 mm and suspension hole creation were performed. Grinding was conducted using a vertical knee-type milling machine (FA 3 AV) equipped with a custom-designed fixture (Figure 2). The cutting conditions were as follows:  
* Tool rotational speed: n = 45 min\({}^{-1}\)
* Workpiece feed rate (cross-feed of milling table): s = 450 mm\(\cdot\)min\({}^{-1}\)  
The tool pressure was applied using a steel helical compression spring (EN 10270-1 SH material [19]), housed inside the fixture cavity. The spring characteristics were as follows:  
* Wire diameter: 1.6 mm
* Outer diameter: 12 mm
* Free length: 45 mm
* Number of coils: 11  
The spring characteristics were evaluated using a universal testing machine (ZDM 5/51). For the experimental setup, a normal force of F = 100 \(\pm\) 5 N was applied, corresponding to a spring compression of 16.4 \(\pm\) 0.3 mm.