# Experimental Wear Characterization and Durability Enhancement of an Aeronautic Braking System
## 3 Results and Discussion
### Endurance Test of the First Optimized Configuration with Eight Springs  
The results of the endurance test performed on the eight-spring configuration are presented in Figures 10-13, where they are compared with the results obtained for the base configuration.  
Figure 11 compares the release threshold voltage for the 8-spring and base configurations (10 springs), also showing that, in the 8-spring configuration, the anchor starts undergoing partial and nonuniform engagement after around 2500 actuation cycles. The release threshold voltage trend again suggests that particles of worn material from the disk pack are located between the solenoid and the anchor and that the worn material is not uniformly distributed at the interface.  
Figure 12: Inductance in the 8-spring and 10-spring (base) configurations.  
Figure 10: Release threshold voltage in the 8-spring and 10-spring (base) configurations.  
Figures 11 and 12 compare the engagement threshold voltage and inductance in the eight-spring and base configurations, respectively, suggesting that wear progress also occurs in the eight-spring configuration. Nevertheless, the engagement threshold voltage always remains less than the reference maximum ATP value.  
Figure 13 reports the braking torque comparison for the eight-spring and base configurations. The results show that in the eight-spring configuration, the initial braking torque is less than the minimum value established with the ATP, but it rapidly reaches acceptable values, although less than in the base configuration, as was expected.  
Figure 14 shows the trend of the friction coefficient during the test, which varies from a relatively low value of around 0.1 to a high value close to 0.5.  
The overall analysis of the results for the eight-spring configuration demonstrates that, although this configuration has a better wear resistance than the base configuration, as it reaches the failure point after around 4500 actuation cycles, this improvement is not sufficient, as the failure occurs before the TBO.  
Again, in this case, the visual analysis of the sample braking components at the end of the endurance test demonstrated the presence of a large number of ferromagnetic particles, as can be observed in Figure 15.  
Therefore, the failure of the braking system is due to the accumulation of ferromagnetic particles between the anchor and solenoid, mainly originating from wear progress on the stator disks. These results lead to the conclusion that possible further optimization in terms of the wear resistance and durability of the system lies in a reduction in the ferromagnetic wear material and thus in a change in the friction pair materials.  
The carbon fiber used as friction material for the rotor disks could be identified as possibly responsible for the severe wear phenomenon on the stainless-steel stator. This could be due to different factors, such as the low operating temperature of the brake and the different wear behaviors of the matrix and reinforcement of carbon fiber. For this reason, a different friction material was analyzed.  
Figure 14: Friction coefficient in the 8-spring configuration.  
Figure 13: Braking torque in the 8-spring and 10-spring (base) configurations.