# Torsional Fatigue Performance of a Spot-Welded Structure: An XFEM Analysis
## 1 Introduction
Larger diameters and thicknesses correlated with longer fatigue lives within certain limits, but beyond a threshold, increased thickness did not improve fatigue strength. Notably, using a greater number of small spot welds was more effective in enhancing fatigue strength compared to larger spots with equivalent cross-sectional areas. [PERSON] et al. [5] performed fatigue tests on specimens welded with friction spot welding using a stress ratio of R = 0.10. The two-parameter Weibull distribution was used to statistically examine the fatigue life of the linked overlapped sheets. The findings demonstrate that an infinite life of the service component can be attained at a comparatively low load, equivalent to 10% of the maximum supported by the joint. It was observed that circumferential cracking propagated around the spot-welded zone for high loads. On the other hand, for lesser loads, the crack spreads circumferentially around the weld nugget; however, because insufficient solicitation occurred to boost plastic deformation in the weld nugget, the fracture tended to spread in the direction of width. Similar research was done by [PERSON] et al. in experimental research [6], which looked at the fatigue behavior of a single friction spot welded of Ti6 Al4V and AA5754 dissimilar structures. In the specimens, two fatigue failure modes were noted. The upper aluminum sheet failed under high cycle loads because of increased stress concentration at the notch tip, which caused a fracture to initiate and propagate through the thickness towards the joint surface. Under low cyclic load, a \"through weld\" failure was noted, as the crack rapidly spread over the interfacial surface until reaching a critical point where the remaining cross-section could not withstand the shear overload, resulting in failure. The impact of varying nugget diameters on the fatigue lifetimes of distinct welded joints of Dual-Phase (DP) 800 and Twinning-Induced Plasticity (TWIP) 1000 steel was investigated by [PERSON] et al. [7]. [PERSON] and [PERSON] [8] compared and validated the numerical modeling of fatigue failure of spot-welded joint using the finite element method (FEM) and extended FEM (XFEM) for high cycle fatigue. In [PERSON] et al. 's investigation [9], the fatigue service life of an interface crack between two elastic layers was evaluated using the Paris law and the linear elastic fracture mechanics concept. [PERSON] and [PERSON] [10] analyzed the fatigue failure of the spot-welded structure subjected to tensile loading using XFEM for different values of the cyclic load, the load ratio, and nugget size. In aerospace applications, due to various factors and operational conditions, torsional loadings frequently occur. For instance, during flight maneuvers such as rolls and turns, different parts of an aircraft experience torsional loads, such as the wings, which are subjected to twisting forces as the aircraft changes its orientation. Also, changes in airflow and aerodynamic forces can induce torsional loads on aircraft components, where the wings, control surfaces, and other protruding elements are exposed to varying wind conditions during flight, leading to torsional stresses. Structural asymmetry in an aircraft, whether intentional or due to design constraints, can lead to uneven distribution of loads, including torsional forces. Atmospheric turbulence, sudden changes in wind direction, and ground-related forces during landing and taxiing are other factors that may lead to the presence of torsional loadings. The performance of spot-welded structures subjected to torsional loading used in aerospace applications needs to be addressed during its design considerations. In the literature, only one study was investigated. Three different spot weld modeling approaches--rigid, solid element, and umbrella spot welds--that use static and multiaxial low cycle torsional fatigue analysis were the focus of [PERSON] et al. 's study [11].