## 3 Results and Discussion  
### Tensile Test  
The use of spot-welded joints in structural design ensures that the shear force is the primary load received by the welds whenever the joints are under tension or compression. The welds may be subjected to tension loading, shear stress, or both. In the former case, the loading direction would be perpendicular to the joint plane [76]. Spot welds used in automobile structures may experience shear stress from the rotation or relative movement of adjacent sheets and tensile loads from separation forces acting perpendicular to the sheets as they work [77; 78]. Despite this, shear force at RSW joints is the main metric for RSW quality [79]. The tensile test was carried out using 10 welded cases of varying thicknesses to meet this criterion. The samples were then measured for their tensile shear strength, which would be utilised in the fatigue test that followed. For titanium grade 2 alloy, the shear strength samples for 1-1 mm (case A), 0.5-0.5 mm (case B), and 0.5-1 mm (case C) cases were manufactured using the optimal RSW parameters of the alloy as noted in [44]. The shear strength findings of cases A, B, and C in Figure 4 showed that the maximum shear strength of 613 MPa was observed for 0.5-0.5 mm (case B), while the lowest shear strength of 333.5 MPa was observed for 1-0.5 mm (case C), however, case A (1-1 mm) exhibited a shear strength of 374.7 MPa which is situated between the shear strength of cases B and C. It is important to note that welding cases A and B of equal thickness sheets showed higher strain before fracture than welding case C of unequal thickness. The greater bulk resistance of the thicker sheet compared to the thinner sheet is responsible for the reduced strain experienced by unequal-thickness RSW joints before failure. Because of this, the resulting weld nugget is asymmetrical, meaning that it penetrates the thicker sheet more deeply than the thinner one [80]. Another possible cause of the experienced strain decline is a change in the final solidification line. In RSW joints \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Description** & **Equation** \\ \hline Sa (\(\mu\)m) & Arithmetic mean surface height & \(Sa=\frac{1}{m\times n}\int\limits_{0}^{m}\int\limits_{0}^{n}[Z\ (x,\ y)|dxdy\) \\ Sq (\(\mu\)m) & Mean square surface height & \(Sq=\sqrt{\frac{1}{m\times n}\int\limits_{0}^{m}\int\limits_{0}^{n}z^{2}\ (x,\ y)dxdy}\) \\ \multirow{2}{*}{Sz (\(\mu\)m)} & Maximum height between the & \multirow{2}{*}{\(Sz=Sp-Sv\)} \\  & highest peak and the deepest valley & & \\ Ssk (-) & Surface skewness & \(Ssk=\frac{1}{m\times n\times Sq}\int\limits_{0}^{m}\int\limits_{0}^{n}Z\ (x,y)^{3}dxdy\) \\ \multirow{2}{*}{Sku (-)} & Surface kurtosis & \multirow{2}{*}{\(Sku=\frac{1}{m\times n\times Sq}\int\limits_{0}^{m}\int\limits_{0}^{n}Z\ (x,y)^{4}dxdy\)} \\ \multirow{2}{*}{Sdq (-)} & The surface gradient & \multirow{2}{*}{\(Sdq=\sqrt{\frac{1}{m\times n}\int\limits_{0}^{m}\int\limits_{0}^{n}\int \limits_{0}^{n}\left[\left(\frac{\pi(x,y)}{\delta x}\right)^{2}+\left(\frac{ \pi(x,y)}{\delta y}\right)^{2}\right]dxdy}\)} \\ \multirow{2}{*}{Vv(p) (\(\mu\)m)} & Valley void volume at the material & \multirow{2}{*}{\(V_{\varphi}(p)=\frac{1}{\lambda}\int\limits_{p}^{100}\left(z_{p}-z\right)f(p)dp\)} \\ \hline \end{tabular}
\end{table} 
Table 5: Selected parameters of 3D fatigue-fractured surface topography according to ISO 25178-2 [75]. with unequal thickness, the final solidification line is moved from the sheet/sheet interface to the geometric centre of the total combined thickness [81]. Shear strength samples for AISI 304 stainless steel were welded under ideal RSW conditions as reported in [6] for the 1-1 mm (case D), 0.5-0.5 mm (case G), and 0.5-1 mm (case H) cases. The shear strength of AISI 304 stainless steel revealed that the 1-1 mm case (D) had the maximum shear strength at 526.8 MPa as shown in Figure 5, in contrast to 515.2 MPa for the 0.5-0.5 case (G) and 369.5 MPa for the 0.5-1 mm case (H).