## 3 Results and Discussion
### Influence of Laser Treatment on Mechanical Properties
#### 3.2.1 Tensile-Stress Analyses  
Figure 4 shows the variation in the measured engineering stress (MPa) vs. the engineering strain for laser-treated (a) and untreated (b) specimens, with \(\beta\) = 0\({}^{\circ}\) (i.e., 90\({}^{\circ}\) with respect to LIPSS direction, green points), \(\beta\) = 45\({}^{\circ}\) (blue points) and \(\beta\) = 90\({}^{\circ}\) (orange, yellow and red points, same direction as LIPSS orientation). Except for \"\(\beta\) = 0\({}^{\circ}\), laser treated\", \begin{table}
\begin{tabular}{c c c} \hline \hline
**E (\(\upmu\)J)** & **\(\Lambda\) (\(\upmu\)m)** & \(\theta\) (\({}^{\circ}\)) \\ \hline
260 & 889 & 24 \\
130 & 837 & 22 \\
105 & 835 & 21 \\
65 & 837 & 15 \\
**52** & **819** & **11** \\
32.5 & 839 & 16 \\
26 & 822 & 18 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Spatial period (\(\Lambda\)) of LIPSS and angular dispersion (\(\theta\)) obtained by FFT analyses. The selected parameters are highlighted in bold. Figure 3: SEM images and related FFT analyses of LIPSS generated by different sets of parameters couples (RR, E): (1 MHz, 260 \(\upmu\)J) (**a**), (2 MHz, 130 \(\upmu\)J) (**b**), (2.5 MHz, 105 \(\upmu\)J) (**c**), (4 MHz, 65 \(\upmu\)J) (**d**), (5 MHz, 52 \(\upmu\)J) (**e**), (8 MHz, 32.5 \(\upmu\)J) (**f**), (10 MHz, 26 \(\upmu\)J) (**g**). In the red box are highlighted the optimum parameters values. three different samples were fabricated and tested for each condition. It can be observed that all specimens showed the same trend with comparable values, with a stabilization of the stress values in the range of 600-700 MPa for strains above 30%. Figure 5 summarises the average values obtained for key mechanical parameters like YS (orange bars), UTS (blue bars) (see Figure 5 left), UE (orange bars), EL (blue bars) (see Figure 5 centre), and RA (blue bars) (see Figure 5 right). YS expresses the limit of the elastic behaviour of the material and identifies the stress value at the beginning of plastic behaviour. The UTS defines the maximum stress the material can withstand. Both are expressed in MPa (force/area). UE is the value of elongation in correspondence of UTS. EL corresponds to the value of strain at the material break (excluding the elastic part of the stress-strain curve). RA is a ratio of minimal and original cross-sections of the same specimen during the test. UE, EL and RA values are expressed in %. For each graph in Figure 5, values relative to the laser-textured batch (T) are plotted on the left side whilst those relative to untreated specimens (NT) are plotted on the right side. It is easy to observe that values relative to the two batches (T) and (NT) are largely compatible if not equivalent when considering the error bars. This means that the surface texturing generated using nearly P = 260 W has no impact on the mechanical properties of the foil. This can be explained considering two key aspects. On one hand, the use of a fast polygon scanner prevents heat accumulation phenomena and unwanted thermo-mechanical degradations (see SEM analysis below) expected also for fs laser when considering high RR (MHz) and high P values [23; 24]. On the other hand, although the variation in S\({}_{\text{a}}\) has a bearing on the mechanical properties of a metallic sheet [25], in our case the S\({}_{\text{a}}\) contribution induced by LIPSS could be neglected. In fact, we estimated a value of S\({}_{\text{a}}\) = 253 nm for the pristine foil and S\({}_{\text{a}}\) = 272 after the texturing, corresponding to a relative variation of \(\approx\)7%. Going in deeper, Figure 5 (left) shows that both strength values (YS and UTS) were not affected by the value of \(\beta\) (specimen orientation). On the contrary, elongation parameters (Figure 5 centre and right) like UE and EL increased when \(\beta\) passed from 0\({}^{\circ}\) to 90\({}^{\circ}\). As already mentioned here below, this anisotropic behaviour is not related to laser texturing.