# Optimization and Experimental Verification of Lower
## 3 Experimental Verification of Lower Articulation
### Fatigue Test  
#### 3.2.1 Fatigue Test Method  
To evaluate the fatigue performance of the lower articulation, fatigue test conditions were established based on the vehicle manufacturer specifications, advanced cases, and standards EN 12663-1 and EN 13749 [21,22]. In addition, one unit of the lower articulation was produced and a fatigue test was conducted to verify its experimental validation. The fatigue testing procedure for the lower articulation was carried out according to the steps presented in Figure 13. The total number of fatigue test cycles was set to 1.0 \(\times\) 10\({}^{7}\), and the test was conducted in three steps. In Step 1, a total of 6.0 \(\times\) 10\({}^{6}\) cycles of repetitive loading were applied, followed by a liquid penetrant inspection to check for crack formation. If no cracks were detected in Step 1, the test continued with the same static load while increasing the dynamic load to 120% for Step 2, which was performed for 2.0 \(\times\) 10\({}^{6}\) cycles. After completing Step 2, the dynamic load was increased to 140% for Stage 3, which was conducted for 2.0 \(\times\) 10\({}^{6}\) cycles. After the tests, a final visual inspection was performed to check for crack formation.  
Figure 12: Stress-time curve of structural test: (**a**) tension; (**b**) compression; (**c**) right; (**d**) left; (**e**) vertical up; (**f**) vertical down.  
The method for applying fatigue loads in each direction on the lower articulation is shown in Figure 14. As illustrated, initially, a static tensile load was applied continuously in the left direction, followed by 5000 cycles of repetitive loading under this static load. Next, after releasing the static tensile load, a dynamic vertical load was applied for 500 cycles. Once the dynamic vertical load application was completed, a static compressive load was applied, and 5000 cycles of dynamic compressive loading were performed. Finally, the static compressive load was released, and the vertical dynamic load was reapplied for 500 cycles.  
Table 6 lists the equipment, standard loads, and speed conditions used for the fatigue testing of the lower articulation in each direction. The testing equipment for the lower articulation fatigue tests includes actuators for applying loads, load cells for measuring loads, strain gauges for measuring stress, and data acquisition systems (DAQs) for data collection and analyses. Figure 15 shows the fatigue testing setup for the lower articulation. Through these fatigue tests, the fatigue performance of the lower articulation was evaluated, and the safety was verified by checking for crack formation in each test step.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Direction of Fatigue Testing** & **Test Equipment** & \begin{tabular}{c} **Fatigue Load** \\ **Conditions** \\ \end{tabular} & **Testing Speed** \\ \hline \hline \multirow{2}{*}{Tension and Compression Right and Left} & \begin{tabular}{c} MTS Hydraulic \\ Actuator \\ MTS Hydraulic \\ Actuator \\ \end{tabular} & \(\pm 40\pm 15\) kN & \(3\) mm/min \\  & \
\begin{tabular}{c} MTS Hydraulic \\ Actuator \\ \end{tabular} & \(\pm 25\) kN & \(3\) mm/min \\  &
\begin{tabular}{c} MTS Hydraulic \\ Actuator \\ \end{tabular} & \(-90\pm 10\) kN & \(3\) mm/min \\ \hline \hline \end{tabular}
\end{table}
Table 6: Fatigue test conditions.  
Figure 14: Load histories of components imposed on lower articulation.  
Figure 13: Definition of fatigue test steps.