## 2 Materials and Methods
### Mechanical Testing Procedures  
Three types of mechanical resistance tests were carried out on laser-treated and untreated 200 \(\upmu\)m thick stainless-steel foils: standard tensile tests (i), fatigue tests (ii) and cruciform specimen tests (iii). Tensile tests (i) were performed using specimens shown in Figure 2a to evaluate basic mechanical parameters (yield strength YS, ultimate tensile strength UTS, uniform elongation UE, total plastic elongation EL and reduction of area RA) and the material's anisotropy.  
For the latter, three specimens were cut from the untreated and laser-treated foils in three different directions \(\beta\) with respect to the rolling orientation (see Figure 2d): \(\beta\) = 0\({}^{\circ}\) (perpendicular to LIPSS direction), \(\beta\) = 45\({}^{\circ}\) and \(\beta\) = 90\({}^{\circ}\) (parallel to LIPSS direction). The tests were performed under quasi-static conditions at room temperature according to standard procedures reported in [14]. For the sake of repeatability, each test was carried out in the same conditions three times using a Mayes universal testing machine (designed and developed in house by COMTES, Dobfany, Czech Republic) equipped with hydraulic grips and a crosshead velocity of 1.2 mm/s.  
Figure 1: A schematic of the R2R pilot line.  
Figure 2: (**a**) Specim geometry for standard tensile testing; (**b**) cruciform specimen geometry dedicated for the mechanical testing performed in three loading modes (plane strain, uniaxial and biaxial); distance between slits \(S\) was 300 \(\upmu\)m; (**c**) cruciform specimen testing setup equipped with a Digital Image Correlation system for deformation tracking; (**d**) direction of cutting for specimens in three different directions with respect to LIPSS orientation; (**e**) hourglass specimens for the fatigue tests. All values are given in mm.  
The fatigue strength (ii) was evaluated in the high cycle fatigue (HCF) regime, following the recommendations reported in [15]. At least 10 hourglass specimens with a stress concentration factor (K\({}_{\text{t}}\)) of 1.02 (Figure 2e) machined in the \(\beta\) = 90\({}^{\circ}\) direction were tested for each condition. The tests were conducted at room temperature using a servo-hydraulic testing machine (Landmark by MTS, Berlin, Germany) equipped with a 1 kN load cell and mechanical grips. A load frequency of 30 Hz was applied, and the stress ratio (R = Min stress/Max stress) was set at 0.1 to prevent specimen buckling. The fatigue limit was defined at 2 million cycles.  
The tests (iii) performed using cruciform specimens (see Figure 2b) were carried out to investigate the effect of different pre-strain conditions on both untreated and laser-treated foils. Three different loading modes were applied to pre-strain the specimens: plane strain (zero strain in the direction perpendicular to loading), uniaxial (loading applied in one direction) and biaxial (loading applied in two directions) tension. Each specimen was firstly tested until failure (designated as 100%), then pre-strained at 75%, 50% and 25% of the failure elongation. Figure 2c illustrates the testing setup for cruciform specimens equipped with a digital image correlation (DIC) system for tracking deformations. Finally, samples of tests (iii) were characterised via SEM.