# Simulation, Fabrication and Testing of UAV Composite Landing Gear
## 3 Results and Discussion
### Failure Mode Analysis of the Tested Models  
The distribution and orientation of the fibers in the polymer matrix, the presence of voids and deposition defects, or the appearance of microcracks in the material microstructure are some of the main defects specific to the FFF process for composite filaments reinforced with short carbon fiber. In order to highlight these main defects, representative samples were taken from the landing gears to be prepared for structural and morphological analysis. In this regard, the samples have been cross-sectioned and longitudinally sectioned, embedded into acrylic resin and polished.  
In the case of the landing gear made of PLA GF, it was found that interlayer voids (Figure 8a) appeared between the layers of extruded material but also typical voids that appear in the case of filaments with short fibers such as rectangular voids (Figure 8b). In the microscopic images of the PP GF30 landing gear, the following can be observed: the gray lines (Figure 9a) or gray dots (Figure 9b) represent the glass fibers, and the darker areas represent voids from the FFF manufacturing process of the PP GF30 filament. It was observed that this PP GF30 (glass fiber reinforced) filament showed much fewer defects compared to carbon-fiber-reinforced filaments analyzed in other studies [43; 44; 45].  
Figure 8: Microscopic analysis of the landing gear made of PLA GF: (**a**) longitudinal section (100\(\times\)); (**b**) cross-section (100\(\times\)).  
Figures 10 and 11 show that the distribution of carbon fibers in the polymer matrix is approximately uniform, having an orientation along the deposition direction similar to those published in other studies [43; 44; 45; 46]. After analyzing the microstructure, the appearance of defects, such as voids and interlayer voids, was observed. Following the microscopic analyses of the landing gear models fabricated by filaments reinforced with carbon fiber, the following can be observed: the white lines or the white dots represent the carbon fibers, and the darker areas represent voids or interlayer voids that come from the FFF process of landing gears.  
The conclusions that can be drawn after completing the microscopic analysis of the investigated materials are:  
* Typical defects of short fiber-reinforced composites were found, similar to those published in other studies [43; 44; 45; 46; 47; 48];
* The formation of elongated interlayer voids or voids of any shape found in the matrix or filament [49];
* Regarding the orientation of the carbon fibers, in most materials, the orientation of the carbon fibers along the deposition direction can be observed [48].  
Figure 10: Microscopic analysis of the landing gear fabricated of PET CF15: (**a**) longitudinal section (100\(\times\)); (**b**) cross-section (100\(\times\)).  
Figure 9: Microscopic analysis of the landing gear made of PP GF30: (**a**) longitudinal section (100\(\times\)); (**b**) cross-section (100\(\times\)).  
From the microscopic analysis of the CFRP landing gear in the longitudinal section and in the cross-section were observed the following: interlayer region, carbon fiber alignment, and the orientation of the carbon fibers and the carbon fiber are distributed uniformly in the resin. Of course, even in this case, a defect (void) can be observed that occurred in the manufacturing process of the CFRP landing gear (Figure 12).