# Experimental Wear Characterization and Durability Enhancement of an Aeronautic Braking System
## 3 Results and Discussion  
### Endurance Test Results and Wear Characterization of the Base Configuration  
The endurance test of the base configuration was performed on the test bench by monitoring the different working parameters as a percentage of the reference results obtained during the preliminary ATP. The results are represented in Figures 3-6. Figure 3 reports the trend of the release threshold voltage, which increases up to a value of about 200% of the maximum value obtained during the preliminary ATP. The start curve represents the values of the release threshold voltage when the anchor starts to be released, and the end curve reports the values of the voltage when the anchor's release stops. The start and end curves should be coincident if the anchor is uniformly released. Given that these two curves are not coincident after 2500 actuation cycles, the anchor's release is not uniform, and the air gap progressively increased during the test while the anchor was engaged. The increase in the release threshold voltage suggests that particles of worn material from the disk pack are located between the solenoid and the anchor, and the difference between the start and end curves suggests that the worn material is not uniformly distributed at the interface, but a greater number of wear particles are located in a portion of the interface, which is also justified by the test rig slope. Figures 4 and 5 show the trend of the engagement threshold voltage and that of the inductance, respectively. These results suggest that the air gap increases due to the wear progress. In fact, the air gap increase implies both that the inductance decreased during the test and that the engagement threshold voltage increased because a greater power supply is necessary for the solenoid to generate a sufficient magnetic field to engage the anchor. Figure 4: Engagement threshold voltage in the base configuration. Figure 5: Inductance in the base configuration. Figure 3: Release threshold voltage in the base configuration. Figure 6 shows the trend of the braking torque, which is higher than the minimum value resulting from the ATP and reaches 300% of the reference maximum value. This result demonstrates that despite the short durability, the braking system maintains a sufficient braking capacity. It is worth evaluating the changes in the friction coefficient during the test by calculating  
\[f=\frac{4T_{b}}{NF(D+d)}\, \tag{1}\]  
where \(N\) is the number of contact mating surfaces, \(F\) is the normal force on the disks, \(f\) is the friction coefficient, and \(D\) and \(d\) are the maximum and minimum diameters of the contact surface, respectively. The results are reported in Figure 7, where it is shown that the friction coefficient slightly fluctuates around an average value of 0.3. From the overall analysis of the experimental results obtained on the test rig, one can conclude that the braking system in its base configuration stops working after 3000 actuation cycles, where 1 actuation cycle is considered the sequence of a retraction and an extension. This number of cycles is considerably lower than the minimum number of cycles prescribed for the TBO. At the end of this endurance test, a visual analysis of the sample braking components was conducted using a digital microscope. When the brake was dismounted from the test rig and opened, a large number of wear particles were evident, and a visual inspection highlighted that the stator disks were more worn than the rotor disks, as can be observed in Figure 8.  
The trend of the release threshold voltage (Figure 3) is strongly related to this evidence. In fact, as the stator disks are made of ferromagnetic materials, they release ferromagnetic particles, which are attracted by the magnetic field generated by the solenoid, and they crowd around the interface between the solenoid and anchor, causing the nonuniform release of the anchor, as also demonstrated by the visual inspection of the solenoid. Figure 6: Braking torque in the base configuration.