# Torsional Fatigue Performance of a Spot-Welded Structure: An XFEM Analysis
## 1 Introduction  
Spot welding is a crucial technique in the field of aerospace engineering, greatly aiding in the building and assembly of aircraft components. The lightweight and very durable materials, including titanium and aluminum alloys, that are frequently utilized in aerospace applications are one of the main causes of its significance. Spot welding makes it possible to precisely fuse these materials, forming robust and long-lasting connections that are crucial for the structural integrity of aircraft parts. The method ensures that the overall weight of the aircraft is reduced without sacrificing strength, and it is especially useful when fabricating fuselage panels, wing structures, and other crucial components. In addition to its role in material compatibility, spot welding provides benefits in terms of efficiency and speed during the manufacturing process. To achieve the high safety and performance requirements of aerospace engineering, assembly must be done precisely and consistently. Spot welding offers a quick and automated way to attach components securely while cutting expenses and manufacturing time. In order to preserve the dimensional precision of aerospace components, the method also permits targeted heating, reducing the possibility of thermal distortion in the surrounding areas. Overall, spot welding has become a preferred process in aircraft manufacturing due to its speed and precision, which helps the industry satisfy stringent quality criteria and timelables [1]. On the other hand, the operation and safety of airplanes depend heavily on sensors, thermal management systems, and electrical components. The total dependability of these systems is increased by spot welding, which guarantees strong connections between conductive materials. For electrical components to function efficiently and to help minimize any failures that could jeopardize the performance of crucial aeronautical systems, the technique's ability to build consistent, low-resistance junctions is vital [2]. In operational conditions, mechanical components of the structures, including aerospace ones, typically undergo cyclic loading, making the prevention of fatigue failure a primary design consideration. Given that spot welds establish localized connections, forming inherentcircumferential notches, they result in stress concentrations, thereby increasing the likelihood of structural failure due to fatigue. For example, in automotive components, fatigue failures often arise in the region of a spot weld around 80% of the time, at a low-quality cut edge 15% of the time, and fatigue cracks in sheet metal just 5% of the time [3]. Thus, it can be concluded that spot-veld joints' fatigue strength has a major impact on the overall structural durability under typical service conditions. Therefore, a deeper understanding of spot-veld fatigue behavior and the application of reliable fatigue assessment models have to be considered necessary conditions for guaranteeing trustworthy designs. In the literature, there exist a few studies focused on the cycling performance of spot-welded structures. For instance, [PERSON] et al. [3] investigated the fatigue failure of modified tensile-shear specimens using experimental and numerical methods. It focused on auto-mated and manual spot-weld joints, applying various load ranges during fatigue tests. Results showed that manually spot-welded specimens had 30-45% fewer cycles to failure than automated joints. Microscopic examination revealed gaps in manual spot-weld nuggets, but fatigue-induced cracks consistently formed on their peripheries, similar to automated joints. This difference was attributed to stress concentrations from unsymmetrical weld geometry. In another study, using the Coffin-Manson model, [PERSON] et al. [4] investigated fatigue life in spot-weld joints, taking into account varying spot-weld diameters and sheet thicknesses. The study revealed that fatigue life depended on joint type, load amplitude, sheet thickness, and spot-weld diameter.