# Analysis of Fatigue Performance of Spot-Welded Steel T-Profiles Under Cyclic Torsional Loading
## 1 Introduction  
Steel T-profiles are essential for a wide range of industries. They are crucial components of structural frameworks, support beams, and trusses used in construction, ensuring the durability and strength of buildings, bridges, and industrial facilities [1]. Due to their rigidity and capacity to support loads, these profiles are vital in the production process for constructing machine frames and equipment supports. In the transportation sector, they provide structural support for train tracks, bridges, and tunnels, particularly in areas experiencing dynamic loads. Additionally, they reinforce chassis construction in automotive applications, enhancing vehicle longevity and safety. These profiles are also widely utilized in renewable energy systems, agricultural, maritime engineering, and infrastructure projects, indicating their versatility and dependability across various industries. Spot welding is one of the techniques used to produce T-profiles, especially when fabricating structural and automotive components [2]. The initial steps in the procedure involve preparing the steel sheets, cutting them to size, and ensuring their cleanliness. T-profiles are then created by spot welding perpendicular sheets together. Steel T-profiles can be subjected to various types of loadings such as torsional, axial, bending, shear, or a combination, depending on the application and environmental conditions [3]. Torsional loading involves twisting forces applied to the T-profile about its longitudinal axis. This kind of loading is frequently seen in situations where the T-profile is twisted or rotated, such as in car chassis during cornering or in structural frameworks subjected to seismic or wind pressures. When exposed to cyclic torsional loadings, steel T-profiles face many challenges, especially in structural frameworks and automotive chassis applications. These challenges must be addressed to prevent fatigue failure resulting fromrepetitive twisting forces. Thus, fatigue resistance is crucial. By uniformly distributing torsional loads throughout the cross-section, a well-designed structure extends the fatigue life of T-profiles and reduces stress concentrations that can cause premature failure. Innovative design approaches, such as refining the cross-sectional form and including reinforcement features, are used to optimize T-profiles for better fatigue resistance. In the literature, a few studies have focused on the cycling performance of spot-welded structures. [PERSON] (2018) predicted the cyclic endurance of spot-welded connections in beams subjected to both bending and torsional forces, analyzing the physical mechanisms associated with stress induced by buckling. A two-dimensional model of buckling was employed to determine the stress distribution. [PERSON] et al. (2018) conducted high-cycle fatigue tests to examine the failure mechanisms of resistance spot-welded DP600 steel. This study revealed that fatigue crack initiation predominantly occurred in the heat-affected zone or near the fusion zone. Subsequently, fatigue crack propagation extended through both the thickness and width directions until final failure. The primary factors affecting the failure mechanism of spot welds under fatigue loading were the stress concentration at the junction of the two bonded sheets, the stress intensity factor, and the base metal's strength. [PERSON] and [PERSON] (2018) conducted a series of fatigue life tests on modified tensile shear (MTS) test specimens joined by spot welding to investigate the impact of electrode force on fatigue life. The experimental results revealed that the number of cycles until failure varied depending on the spot-welding configurations, including electrode force and welding schedules. Their results suggested that through-thickness cracking was the main characteristic associated with fatigue failure. [PERSON] and [PERSON] (2018) analyzed high-cycle fatigue of spot welds using the extended finite element method (XFEM) and finite element method (FEM).