## 3 Simulation Modeling and the Validation of the Rudder Mechanism
### Experimental Verification  
The fatigue life test is a test meant to evaluate the durability of a material or a structure during repeated loading by cyclically applying loads at certain stress levels. As the rudder mechanism is the main force-transmitting component of the rudder system, consid  
Figure 4: Simulation Modeling Flowchart. ering that the polar low-temperature environment mainly involves sea ice and waves, the low-temperature fatigue life of the moving components will change with the continuous interaction of sea ice and waves with the rudder mechanism. The traditional service condition simulation life test, which is time-consuming, inefficient, and costly, is accelerated by applying stresses and environmental conditions that are greater than those in the traditional service conditions without changing the failure mechanism [25]. Therefore, corresponding accelerated fatigue test research must be carried out on the rudder bar structural design test rig under an ice-water mixing environment. The overall design scheme of the test bench is as follows: take the servo motor as the power source, drive the speed reducer through the torque-type method of control, turn the rotational force into thrust through the screw, and then realize the reciprocating motion of the underwater vehicle rudder bar mechanism by PLC programming. The motor and reducer form a whole system, which generates a corresponding change in rotational speed and forms a certain torque. It can deliver power to the drag object to obtain the desired torque and speed. The physical object processed by the test bench is shown in Figure 5, and this scene is that of the object being placed in a polar environment simulation laboratory during the experiment. The rudder stock specimens used for the experiment are shown in Figure 6. The experiment was conducted at four temperatures: room temperature (20 \({}^{\circ}\)C), 0 \({}^{\circ}\)C, \(-\) 20 \({}^{\circ}\)C, and \(-\)40 \({}^{\circ}\)C. It simulates the operating frequency of the rudder in reality, with an applied frequency of 15 Hz and a duration from installation to fracture, and calculates its fatigue loading frequency. Based on the finite element simulation results, the parts of the rudder system of most concern are determined, and then an accelerated life test is used to pres  
Figure 5: Physical diagram of the low-temperature fatigue testing machine for an underwater vehicle rudder mechanism. Figure 6: Guide rod test pieces. the parts of concern and apply resistance to the rudder shaft to increase the rudder drive torque. Finally, in the simulation laboratory, the temperature is lowered to the required temperature for the test and maintained for half an hour before applying the loads for the test. During the experiment, it was found that the rudder stock was partially covered with ice. To simulate the icing situation of the rudder stock mechanism during underwater missions in the Arctic region, the rudder stock specimens were quickly moved from 0 \({}^{\circ}\)C to a low-temperature air environment before each experiment. In addition, the relative humidity in the polar environment simulation laboratory was high, and the wind generated by the air compressor increased the amount of icing. Therefore, local icing was added as a defect of the rudder stock to investigate its impact on the operation of the mechanism [26]. The bottom left corner of Figure 5 shows a partially enlarged image of the \(-40\)\({}^{\circ}\)C icing scenario during the experiment. The ice thickness on the surface of the rudder stock head was measured at different temperatures. The ice thickness was 0 at room temperature and 0 \({}^{\circ}\)C, and 0.6 mm and 1.6 mm at \(-20\)\({}^{\circ}\)C and \(-40\)\({}^{\circ}\)C, respectively. The lower the temperature is, the thicker the ice. The tester outside the lab judged whether the test was properly proceeding by monitoring the real-time torque feedback to match the given torque.