## 3 Results and Discussion
### Fatigue Results  
When designing safety-critical aircraft or aeroengine structures, the fatigue life curve, also referred to as the Wohler or S-N curve, determines the level of stress that a material can sustain for a certain number of cycles before failing. Fracture occurs at a comparatively high number of cycles, referred to as HCF, which is characterised by repetitive cycling at applied loads that primarily cause elastic strains in the material. In contrast, LCF is characterised by a relatively low number of cycles and repetitive cycling at applied stresses that cause plastic deformation in the material. This study conducted the fatigue tests under ambient room temperature conditions, with a constant amplitude stress ratio of R = 0.1. The applied stresses in the fatigue test were (0.5, 0.6, 0.7, 0.8, and 0.9) % of the ultimate tensile strength of the RSW joint for similar and dissimilar welded cases. Figure 13 illustrates the relationship between the stress amplitude and endurance limit (fatigue life) of welded titanium joints across different cases. The fatigue results for the 1-1 mm welded joint (case A) exhibited the greatest fatigue life in comparison to case B (0.5-0.5 mm) and case C (0.5-1 mm). The S-N curve indicates that the fatigue life for case A increased as the applied stress level increased from 188 MPa (0.9% of UTS) to 338.4 MPa (0.5% of UTS), as shown in Figure 13. The endurance limit decreased from its greatest value  
Figure 12: XRD analysis of the dissimilar Ti/ AISI 304 joint (case I). of 1.39 \(\times\) 10\({}^{6}\) cycles to its lowest value of 1.61 \(\times\) 10\({}^{5}\) cycles for this particular case. The findings of the S-N curve for case A showed that the data points were very close to each other when the applied stress was within 0.8-0.9% of the ultimate tensile strength (UTS) of case A. The data distribution, however, was much wider when the stress amplitude was between 0.5% and 0.7% of the UTS in case A. It is worth mentioning that the lowest number of cycles that case A could tolerate before failure was 1.61 \(\times\) 10\({}^{5}\) at an applied stress of 552.6 MPa (0.9% of UTS for case A). The internal defects within the specimen may have an uneven size distribution when subjected to low stress, which might explain this. Figure 13 shows that the fatigue life of case B (0.5-0.5 mm) followed the same pattern as case A. As the applied stress increased from 307 MPa (0.9% of UTS for case B) to 552.6 MPa (0.5% of UTS for case B), the fatigue life decreased from 7.37 \(\times\) 10\({}^{5}\) cycles, which was the highest, to 3.64 \(\times\) 10\({}^{5}\) cycles, which was the lowest value. It can be noted that case A revealed a higher fatigue life than case B despite the tensile strength of case B being higher than that of case A. Therefore, the fatigue strength of a spot-welded joint is not directly proportional to the material's strength. Consistent with previous studies, the findings of this paper show that the spot weld fatigue performance is unaffected by the strength of the material under low load and high cycle conditions [76,87,88]. Both the spot weld's loading characteristics and the stress concentration factor of the weld's surrounding circumferential notch determine the fatigue strength. According to Figure 13, which shows the S-N curve for case C (0.5-1 mm), the highest fatigue life was 7.69 \(\times\) 10\({}^{5}\) cycles at an applied stress of 163 MPa, which is 0.5% of UTS for case C. On the other hand, the lowest fatigue life of 2.63 \(\times\) 10\({}^{4}\) cycles was detected at an applied stress of 293.4 MPa, which is 0.9% of UTS for case C. It should be noted that at a stress level of 0.9% of the joints' ultimate strength, the fatigue life before failure was higher for the equal thickness welded joint in cases A and B than for the unequal thickness joint in case C. This result is caused by a heat imbalance between the uneven titanium sheets [49]. Figure 13: S-N curves of the different cases of titanium resistance spot-welded joints.