## 1 Introduction  
With the continuous development of commercial vehicles and the ongoing expansion of the transport market, semi-trailers (as shown in Figure 1) have become increasingly dominant in transporting goods. Compared to ordinary trucks, semi-trailers offer a significant improvement: Their transport efficiency can be increased by up to 30-50%, while reducing both transportation costs (by 30-40%) and fuel consumption (by 20-30%). The results of relevant experiments indicate that for every 100 kg reduction in overall vehicle weight, the fuel consumption per 100 km decreases by approximately 0.3 to 0.6 liters [1]. As a critical component of semi-trailers, the fifth wheel plays an essential role in connecting the tractor and the trailer, which is vital for ensuring the vehicle's safety, stability, and reliability. To enhance these aspects further, optimizing the lightweight design of the fifth wheel represents an important area of research [2]. The fatigue performances of the fifth wheels of tractors have been studied. The mounting position of the fifth wheel on a semi-trailer largely determines the dynamics of the cab and durability of the components [3]. As the forward offset of the fifth wheel increases, the axle loads on the first, fourth, fifth, and sixth wheels increase, whereas the axle loads on the second and third wheels decrease [4]. The tractor-semi-trailer with the Split fifth wheel coupling (SFWC) model has less deviation tracking and better wear resistance than the conventional tractor-semi-trailer [5]. By building a finite element model of the semi-trailer's fifth wheel, suspension, mechanical spring, pneumatic spring, and press components and performing fatigue analysis, it was determined that the location of the saddle where the fatigue damage occurred was in the connection area of the second beam and the force-transferring components [6; 7]. According to the corresponding fatigue damage areas, combined with the layout characteristics of the semi-trailer traction seat, it is possible to perform reasonable maintenance of the fifth wheel to ensure its service life [8]. For the lightweight design and structural optimization of the fifth wheel, it is necessary to determine the corresponding conditions to verify its strength and safety performance. According to the actual condition of the fifth wheel, the stress-strain distribution is obtained, in which the maximum equivalent stress of the fifth wheel is located in the contact area of the rear reinforcement, and the maximum deformation is located at the rear top of the saddle body, which provides a basis and reference for the structural improvement and optimization of the fifth wheel [9; 10]. According to the dynamic test methods specified in the relevant standards, a fatigue life simulation test of the fifth wheel was conducted, and the validity of the simulation test results was verified through a bench test to ensure that the fatigue life of the model's fifth wheel was higher than the standard requirements [11]. The load spectrum of the upper surface of the fifth wheel was obtained by simulating the time-domain excitation signal of the C-grade road, and the limit fatigue life was estimated with the help of finite element software, according to the load change of the fifth wheel in the actual vehicle transport process, and the fatigue-prone key parts were identified [12]. For the lightweight design of the fifth wheel, in the optimization with mass as a single objective, a model is established by constraining the first-order modal frequency and equivalent stress of the structure, and the lightweight design of the traction seat is investigated using the optimization method of the dimensional parameters to achieve the effect of lightweight design [13; 14]. For multi-objective optimization, the mass and first-order intrinsic frequency are usually taken as the objective functions for model building, and the static-dynamic performance and fatigue reliability are verified by the optimal solution to achieve the unity of lightweight and safety [15].