Comparative Manufacturing of Hybrid Composites with Waste Graphite Fillers for UAVs

Materials of Unmanned Aerial Vehicles (UAVs) parts require specific techniques and processes to provide high standard quality, sufficiently strong, and lightweight materials. Composite materials with a proper technique have been considered to improve the performance of UAVs. Usually, the hybrid composite is developed by mechanical properties with the addition of the filler component (i.e., particle) in a matrix. This research work aims to develop the effective composite materials with better mechanical properties. Considering the manufacturing of hybrid composite materials, the vacuum process is an affecting factor on mechanical properties. The comparison of the hand lay-up process (HL) and vacuum infusion process (VI) with controlled pressure and temperature are studied in this research. In addition, graphite fillers (i.e., 5 wt%, 7.5 wt%, 10 wt%, and 12.5 wt%) are added to the studied matrix. Obviously, the ply orientation is one of the factors that affects mechanical properties. Moreover, two types of ply orientation (i.e., [0°/90°]4s and [−45°/45°]4s) are comprehensively investigated to improve mechanical properties in the three-point bending test. The experimental results show that the vacuum infusion process of ply orientation [0°/90°]4s with the addition of 10 wt% graphite filler exhibits remarkable flexural strength from 404 MPa (without filler) to 529 MPa (10 wt% filler). Especially, the ply orientation of [0°/90°]4s has higher flexural strength than [−45°/45°]4s in both processes. Considering the failure, the fracture of the specimen propagates along the trajectory of fiber fabric orientation, leading to the breakage. Subsequently, the flexural strength under the vacuum infusion process is more significant than in the hand lay-up process. Effectively, it is found that the hybrid composite in this manufacturing has a higher strength-to-weight ratio to use in the structure of UAV instead of pure aluminum. It should be noted that the proposed hybrid composite strategy used in this study is not only limited to the UAV parts. The contribution can be extended to use in other applications such as automotive, structural building, and so on.


Introduction
Composite materials combine two or more different structures or chemical compositions to improve their properties [1,2]. Generally, composite materials consist of a matrix and fiber, which serve as the content and reinforcement, respectively [3][4][5]. When two or more types of materials are combined (e.g., fiber and metal, fiber and ceramic, fiber and polymer or metal and metal), several effects need to be observed using experimental or numerical analysis [6,7]. Nowadays, composite materials are developed on the model of structural changes in elements (i.e., fiber, matrix, and particle) to improve the properties of composite materials. Therefore, composite materials are used as materials for ships, automobiles, aircrafts, and UAVs [8,9]. Up to this date, the composites are used as components of the UAV to improve the performance of the UAV in achieving various missions controlled by a mathematical model program (i.e., fuzzy control system and PID system) [10].
Considering the manufacturing, it is a considerable factor affecting the mechanical properties of composite materials [38][39][40]. Depending on the application, there are several methods for manufacturing composites (i.e., hand lay-up process, vacuum infusion process, and compression process). In aviation applications, the vacuum infusion process is used to reduce air bubbles in the materials, resulting in high strength [41][42][43]. Kim et al. [44] studied the manufacturing process of glass fiber/epoxy composites between the hand lay-up with vacuum bagging and vacuum infusion. Evidentially, the hand lay-up process represents simplicity, leading to the reduction in resources. It is widely used in the production of composite materials. Nevertheless, the controlled environmental conditions of the manufacturing (i.e., curing vacuum pressure and temperature profile) directly affected the mechanical properties of composite materials. As evidence, the curing temperature at 100 • C for 2 h was applied to improve the mechanical properties of the carbon fiber/epoxy composite. Hoda et al. [45] studied the carbon fiber/epoxy composites via the control pressure and temperature to determine the optimal pressure and temperature, leading to superior mechanical properties. When the vacuum pressure is decreased, the mechanical properties improve as the air gap is reduced in the materials with the optimum curing temperature at 100 • C. This evidence indicates the environment could affect the mechanical properties [46][47][48].
Therefore, this research aims to enhance the mechanical properties of the hybrid composites laminates from wastes of toner cartridges (i.e., carbon fiber/epoxy composites with graphite fillers) and to compare the manufacturing of the vacuum bagging process and vacuum infusion process with controlled pressure and temperature. To investigate the sensitivity of the composites for higher flexural properties, this study validates the alterations of the ply orientation and angle of the carbon fiber fabric. Finally, the hybrid composite material with adding the graphite filler from waste in this study can contribute to the further manufacturing of the preliminary UAV structure (i.e., drone cover, rib, spar, and frame) instead of using aluminum.

Materials
In this section, the 3K carbon fiber fabric made by Toray Carbon Fibers American, Inc. (see Figure 1a), with the 1 × 1 plain weave corresponding to the weight of 200 g/m 2 and density of 1.8 g/cm 3 , is used to prepare for manufacturing the studied hybrid composites. The matrix is made of the epoxy resin ER550 with the resin and hardener ratio of 100:35, respectively. The graphite filler with the average size of 5 µm diameter from wastes of a printer toner cartridge is measured by the Scanning Electron Microscope (SEM) analysis (see Figure 1b). The vacuum processes are compared between vacuum bagging and vacuum infusion to determine the difference in mechanical properties. For decades, the vacuum bagging process has been introduced and widely used in several industries, especially in UAV components. Due to its ability, the vacuum bagging process yields a more consistent, thinner, lighter, and stronger product than aluminum and other metals. However, up to this date, the vacuum infusion process has been widely used in the cuttingedge technology for many industries. Outstandingly, the vacuum infusion process brings all of the environmental advantages of a closed mold process. The resin curing in a closed environment leads to an excellent fiber-to-resin ratio with minimal-to-no voids in the finished laminate. In this paper the vacuum process is not only compared, but also the effect of waste graphite filler is introduced.

Prepared Specimens
Considering the preparation of the carbon fiber fabric, the angles of the fabric are cut into the specific sizes of 25 × 25 cm 2 at −45°, 45°, 0°, and 90° for 40 sheets per direction. The structure of the fabric is displayed in Figure 2. To mix the filler with the epoxy, this study first combines the epoxy resin with the graphite filler at a ratio of 5, 7.5, 10, and 12.5 wt%. Then, the mixed epoxy resin is gently stirred for a few minutes. After a certain time of stirring, the hardener is poured into the mixture and gently stirred in one direction to avoid the air bubble. To conduct the studied hybrid composite materials, the specimen designs are described in Table 1.

Prepared Specimens
Considering the preparation of the carbon fiber fabric, the angles of the fabric are cut into the specific sizes of 25 × 25 cm 2 at −45 • , 45 • , 0 • , and 90 • for 40 sheets per direction. The structure of the fabric is displayed in Figure 2. To mix the filler with the epoxy, this study first combines the epoxy resin with the graphite filler at a ratio of 5, 7.5, 10, and 12.5 wt%. Then, the mixed epoxy resin is gently stirred for a few minutes. After a certain time of stirring, the hardener is poured into the mixture and gently stirred in one direction to avoid the air bubble. To conduct the studied hybrid composite materials, the specimen designs are described in Table 1.

Prepared Specimens
Considering the preparation of the carbon fiber fabric, the angles of the fabric are c into the specific sizes of 25 × 25 cm 2 at −45°, 45°, 0°, and 90° for 40 sheets per direction. T structure of the fabric is displayed in Figure 2. To mix the filler with the epoxy, this stud first combines the epoxy resin with the graphite filler at a ratio of 5, 7.5, 10, and 12.5 wt Then, the mixed epoxy resin is gently stirred for a few minutes. After a certain time stirring, the hardener is poured into the mixture and gently stirred in one direction avoid the air bubble. To conduct the studied hybrid composite materials, the specim designs are described in Table 1.   In addition to the basic selection of the filler percentage based on [34], the behavior of the improved hybrid composite can be accepted around 10-12.5 wt% of graphite filler. Thus, to observe the wide range of hybrid composite behavior, this study selected 5, 7.5, 10, and 12.5 wt% of the filler content. In addition to the homogenous and agglomeration in the mixing process, it could not be confirmed that the mixing would provide perfectly homogeneous and agglomerate. However, in the experiment this study controlled the significant agglomeration factors (i.e., mixing ratio, mixing time, and stirring rate) at the same condition. This could improve the mechanical properties because the filler provides a bridge between fiber and matrix. In addition to the environment of the curing process, this research considers two different processes: (1) hand lay-up with vacuum bagging; and (2) vacuum infusion. Both processes are controlled by the pressure at −0.8 bar and temperature at 100 • C. However, in this study the humidity is not controlled because the composite did not expose to the air. In fact, the composite is still in the vacuum bag film, which can be used at temperatures up to 170 • C all along the curing time in an oven.

Hand Lay-Up Process
In this process, the manufacturing relies on the hand lay-up process. The aluminum plate is used as a mold for manufacturing the carbon fiber/epoxy laminate layer, where eight layers of carbon fiber fabric with the epoxy resin mixture are laid as described in Table 1. After that, the peel ply, release film, and breather are laid to the carbon fiber/epoxy laminate, respectively. Then, the manufacturing of the laminate has been completed. The laminate is placed in the vacuum bagging and removed the air by the vacuum pump, as shown in Figure 3. To control the environment of the manufacturing, the pressure is assigned at −0.8 bar, and the hybrid composite laminates are cured using the OV301 curing oven at 100 • C with a temperature profile as shown in Figure 4.

Vacuum Infusion Process
Vacuum infusion molding uses a glass plate mold to manufacture carbon fiber/epoxy laminates. In this process, eight layers of the carbon fiber fabric are placed, followed by the peel ply, infusion net, and vacuum bagging sealed by a sealant tape. The temperature profile is shown in Figure 4. The epoxy resin with/without graphite is added to the laminates under the vacuum pressure, as shown in Figure 5. The vacuum pressure was controlled at −0.8 bar for 15 min and dried for two days. Then, the hybrid composites are dried and cured by the curing oven (i.e., OV301) at 100 °C.

Testing Specimens
Testing for mechanical properties can be performed by a variety of testing methods. In UAVs, it is obvious that the wing and other structures are practically subjected to a complex combination of forces, including tension, compression, and shear. In addition to aerospace applications, there are many forces acting on structures with significant force; that is, the bending force caused by the lift and friction of the structures. Thus, this study performs the flexural test to understand aspects of the material's behavior under the flexural load. Therefore, this research pays special attention to the bending test to determine flexural strength. This study used the three-point bending test (see Figure 6) with a Universal Testing Machine (UTM) of 100 kN. Testing is performed in accordance with ASTM D790-02 [49,50] with a crosshead speed of testing of 5 mm/min and a span length of 100 mm. The test specimen measured 191 × 20 × 2 mm 3 (see Figure 7). The flexural strength can be calculated by the following equation:

Vacuum Infusion Process
Vacuum infusion molding uses a glass plate mold to manufacture carbon fiber/epoxy laminates. In this process, eight layers of the carbon fiber fabric are placed, followed by the peel ply, infusion net, and vacuum bagging sealed by a sealant tape. The temperature profile is shown in Figure 4. The epoxy resin with/without graphite is added to the laminates under the vacuum pressure, as shown in Figure 5. The vacuum pressure was controlled at −0.8 bar for 15 min and dried for two days. Then, the hybrid composites are dried and cured by the curing oven (i.e., OV301) at 100 °C.

Testing Specimens
Testing for mechanical properties can be performed by a variety of testing methods. In UAVs, it is obvious that the wing and other structures are practically subjected to a complex combination of forces, including tension, compression, and shear. In addition to aerospace applications, there are many forces acting on structures with significant force; that is, the bending force caused by the lift and friction of the structures. Thus, this study performs the flexural test to understand aspects of the material's behavior under the flexural load. Therefore, this research pays special attention to the bending test to determine flexural strength. This study used the three-point bending test (see Figure 6) with a Universal Testing Machine (UTM) of 100 kN. Testing is performed in accordance with ASTM D790-02 [49,50] with a crosshead speed of testing of 5 mm/min and a span length of 100 mm. The test specimen measured 191 × 20 × 2 mm 3 (see Figure 7). The flexural strength can be calculated by the following equation:

Vacuum Infusion Process
Vacuum infusion molding uses a glass plate mold to manufacture carbon fiber/epoxy laminates. In this process, eight layers of the carbon fiber fabric are placed, followed by the peel ply, infusion net, and vacuum bagging sealed by a sealant tape. The temperature profile is shown in Figure 4. The epoxy resin with/without graphite is added to the laminates under the vacuum pressure, as shown in Figure 5. The vacuum pressure was controlled at −0.8 bar for 15 min and dried for two days. Then, the hybrid composites are dried and cured by the curing oven (i.e., OV301) at 100 • C.

Vacuum Infusion Process
Vacuum infusion molding uses a glass plate mold to manufacture carbo laminates. In this process, eight layers of the carbon fiber fabric are placed the peel ply, infusion net, and vacuum bagging sealed by a sealant tape. The profile is shown in Figure 4. The epoxy resin with/without graphite is adde nates under the vacuum pressure, as shown in Figure 5. The vacuum press trolled at −0.8 bar for 15 min and dried for two days. Then, the hybrid co dried and cured by the curing oven (i.e., OV301) at 100 °C.

Testing Specimens
Testing for mechanical properties can be performed by a variety of test In UAVs, it is obvious that the wing and other structures are practically s complex combination of forces, including tension, compression, and shear. aerospace applications, there are many forces acting on structures with sign that is, the bending force caused by the lift and friction of the structures. Th performs the flexural test to understand aspects of the material's behavior u ural load. Therefore, this research pays special attention to the bending test flexural strength. This study used the three-point bending test (see Figure 6 versal Testing Machine (UTM) of 100 kN. Testing is performed in accordanc D790-02 [49,50] with a crosshead speed of testing of 5 mm/min and a span mm. The test specimen measured 191 × 20 × 2 mm 3 (see Figure 7). The flex can be calculated by the following equation:

Testing Specimens
Testing for mechanical properties can be performed by a variety of testing methods. In UAVs, it is obvious that the wing and other structures are practically subjected to a complex combination of forces, including tension, compression, and shear. In addition to aerospace applications, there are many forces acting on structures with significant force; that is, the bending force caused by the lift and friction of the structures. Thus, this study performs the flexural test to understand aspects of the material's behavior under the flexural load. Therefore, this research pays special attention to the bending test to determine flexural strength. This study used the three-point bending test (see Figure 6) with a Universal Testing Machine (UTM) of 100 kN. Testing is performed in accordance with ASTM D790-02 [49,50] with a crosshead speed of testing of 5 mm/min and a span length of 100 mm.
The test specimen measured 191 × 20 × 2 mm 3 (see Figure 7). The flexural strength can be calculated by the following equation: where σ is the flexural strength, F is the maximum load subjected to the specimens of the test, L is the span length between the supports, b is the width of a specimen for testing, and t is the thickness of a specimen for testing.
where σ is the flexural strength, F is the maximum load subjected to the specimens of the test, L is the span length between the supports, b is the width of a specimen for testing, and t is the thickness of a specimen for testing.

Hand Lay-Up Process
The hand lay-up manufacturing is the process to create the laminate, where the air gap is sufficiently reduced, thus improving the strong bonding between the matrices. Subsequently, the maximum load is increased by the coordination between the matrix and reinforcement. In this study, the maximum load applied to the specimen can be obtained by the three-point bending test. Figure 8 shows the maximum load of specimens using the hand lay-up process. As a result, in the orientation of [0°/90°]4s the maximum load applied to the hybrid composites is increased by the addition of the graphite filler. The maximum load at the ply orientation of [0°/90°]4s is increased by 15.7%, 48%, and 55.8% with the addition of the graphite filler at 5, 7.5, and 10 wt%, respectively, compared to the carbon fiber/epoxy without graphite (0 wt%). Evidently, the tendency graph of maximum load at the ply orientation of [0°/90°]4s is similar to the tendency graph of ply orientation [−45°/45°]4s. In the ply orientation of [−45°/45°]4s, it is obvious that the percentages of the graphite at 5, 7.5, and 10 wt% affect the maximum load by 2.4%, 14.9%, and 28.7%, compared to the carbon fiber/epoxy without graphite (0 wt%). Considering the graphite with polymer, the curing heats the graphite to increase the bonding capability between the fiber and matrix. This process can significantly improve the adhesion between the fiber and matrix, resulting in higher mechanical properties. Compared with 12.5wt%, the additional graphite is considered at the ply orientations of [0°/90°]4s and [−45°/45°]4s. Consequently, there is a substantial decrease in the maximum load structure applied to the specimen by 10.9% and 9.6%, respectively. Obviously, the maximum load decreases as the addition of where σ is the flexural strength, F is the maximum load subjected to the specimens of the test, L is the span length between the supports, b is the width of a specimen for testing and t is the thickness of a specimen for testing.

Hand Lay-Up Process
The hand lay-up manufacturing is the process to create the laminate, where the ai gap is sufficiently reduced, thus improving the strong bonding between the matrices. Sub sequently, the maximum load is increased by the coordination between the matrix and reinforcement. In this study, the maximum load applied to the specimen can be obtained by the three-point bending test. Figure 8 shows the maximum load of specimens using the hand lay-up process. As a result, in the orientation of [0°/90°]4s the maximum load applied to the hybrid composites is increased by the addition of the graphite filler. The maximum load at the ply orientation of [0°/90°]4s is increased by 15.7%, 48%, and 55.8% with the addition of the graphite filler at 5, 7.5, and 10 wt%, respectively, compared to the carbon fiber/epoxy without graphite (0 wt%). Evidently, the tendency graph of maximum load a the ply orientation of [0°/90°]4s is similar to the tendency graph of ply orientation [−45°/45°]4s. In the ply orientation of [−45°/45°]4s, it is obvious that the percentages of the graphite at 5, 7.5, and 10 wt% affect the maximum load by 2.4%, 14.9%, and 28.7%, com pared to the carbon fiber/epoxy without graphite (0 wt%). Considering the graphite with polymer, the curing heats the graphite to increase the bonding capability between the fibe and matrix. This process can significantly improve the adhesion between the fiber and matrix, resulting in higher mechanical properties. Compared with 12.5wt%, the additiona graphite is considered at the ply orientations of [0°/90°]4s and [−45°/45°]4s. Consequently there is a substantial decrease in the maximum load structure applied to the specimen by 10.9% and 9.6%, respectively. Obviously, the maximum load decreases as the addition o

Hand Lay-Up Process
The hand lay-up manufacturing is the process to create the laminate, where the air gap is sufficiently reduced, thus improving the strong bonding between the matrices. Subsequently, the maximum load is increased by the coordination between the matrix and reinforcement. In this study, the maximum load applied to the specimen can be obtained by the three-point bending test. Figure 8 shows the maximum load of specimens using the hand lay-up process. As a result, in the orientation of [0 • /90 • ] 4s the maximum load applied to the hybrid composites is increased by the addition of the graphite filler. The maximum load at the ply orientation of [0 • /90 • ] 4s is increased by 15.7%, 48%, and 55.8% with the addition of the graphite filler at 5, 7.5, and 10 wt%, respectively, compared to the carbon fiber/epoxy without graphite (0 wt%). Evidently, the tendency graph of maximum load at the ply orientation of [0 • /90 • ] 4s is similar to the tendency graph of ply orientation [−45 • /45 • ] 4s . In the ply orientation of [−45 • /45 • ] 4s , it is obvious that the percentages of the graphite at 5, 7.5, and 10 wt% affect the maximum load by 2.4%, 14.9%, and 28.7%, compared to the carbon fiber/epoxy without graphite (0 wt%). Considering the graphite with polymer, the curing heats the graphite to increase the bonding capability between the fiber and matrix. This process can significantly improve the adhesion between the fiber and matrix, resulting in higher mechanical properties. Compared with 12.5wt%, the additional graphite is considered at the ply orientations of [0 • /90 • ] 4s and [−45 • /45 • ] 4s . Consequently, there is a substantial decrease in the maximum load structure applied to the specimen by 10.9% and 9.6%, respectively. Obviously, the maximum load decreases as the addition of the increasing graphite filler could affect the agglomerates, causing porosity in the matrix. Therefore, it is found that the adhesion between the matrix and reinforcement represents the weakness to the hybrid composite. the increasing graphite filler could affect the agglomerates, causing porosity in the matrix Therefore, it is found that the adhesion between the matrix and reinforcement represents the weakness to the hybrid composite. The relationship between the flexural strength and percentage of graphite is analyzed in Figure 8. As a result, when the load is applied to the specimen via the three-point bend ing test, the flexural strength can be determined. By analyzing the flexural strength, it can be observed that the ply orientation affects the mechanical properties at the ply orienta tions of [0°/90°]4s, which possess higher flexural strength rather than the orientation o [−45°/45°]4s. By placing the orientation of [0°/90°]4s with/without the graphite filler at 0, 5 7.5, 10, and 12.5 wt%, the results show the augmentation of flexural strength by 138.5% 169.5%, 207.3%, and 188.7%, respectively, compared to the ply orientation of [−45°/45°]4s This is because the ply orientation of [0°/90°]4s has a plain weave between vertical and horizontal. Subsequently, when the force is applied by bending test, the perpendicula force inside such ply orientation can absorb the flexural strength. It can be seen that the angle of [−45°/45°]4s represents the oblique angle of the fabric when the shear is applied to specimens, resulting in the resistance in the shear force. Figure 9 shows the maximum load affected on the specimen via the vacuum infusion process. As a result, when the percentage of graphite fillers is changed to 5, 7.5, 10, and 12.5 wt%, the maximum load increases, similar to the hand lay-up process. In the case o the ply orientation of [0°/90°]4s, when graphite is added by 5, 7.5, 10, and 12.5 wt%, the maximum load can be increased by 13%, 33%, 72.2%, and 28% compared to the case of the carbon fiber/epoxy without graphite (0 wt%). In the case of the ply orientation o [−45°/45°]4s, the increased graphite percentage of 5, 7.5, 10, and 12.5 wt% can affect the maximum load, corresponding to the augment of 14.3%, 36.3%, 41%, and 28% compared to the case of the carbon fiber/epoxy without graphite (0 wt%). Compared to the case o the additional graphite with 10 wt%, the case of 12.5 wt% of the ply orientation of [0°/90°]4 The relationship between the flexural strength and percentage of graphite is analyzed in Figure 8. As a result, when the load is applied to the specimen via the three-point bending test, the flexural strength can be determined. By analyzing the flexural strength, it can be observed that the ply orientation affects the mechanical properties at the ply orientations of [0 • /90 • ] 4s , which possess higher flexural strength rather than the orientation of [−45 • /45 • ] 4s . By placing the orientation of [0 • /90 • ] 4s with/without the graphite filler at 0, 5, 7.5, 10, and 12.5 wt%, the results show the augmentation of flexural strength by 138.5%, 169.5%, 207.3%, and 188.7%, respectively, compared to the ply orientation of [−45 • /45 • ] 4s . This is because the ply orientation of [0 • /90 • ] 4s has a plain weave between vertical and horizontal. Subsequently, when the force is applied by bending test, the perpendicular force inside such ply orientation can absorb the flexural strength. It can be seen that the angle of [−45 • /45 • ] 4s represents the oblique angle of the fabric when the shear is applied to specimens, resulting in the resistance in the shear force. Figure 9 shows the maximum load affected on the specimen via the vacuum infusion process. As a result, when the percentage of graphite fillers is changed to 5, 7.5, 10, and 12.5 wt%, the maximum load increases, similar to the hand lay-up process. In the case of the ply orientation of [0 • /90 • ] 4s , when graphite is added by 5, 7.5, 10, and 12.5 wt%, the maximum load can be increased by 13%, 33%, 72.2%, and 28% compared to the case of the carbon fiber/epoxy without graphite (0 wt%). In the case of the ply orientation of [−45 • /45 • ] 4s , the increased graphite percentage of 5, 7.5, 10, and 12.5 wt% can affect the maximum load, corresponding to the augment of 14.3%, 36.3%, 41%, and 28% compared to the case of the carbon fiber/epoxy without graphite (0 wt%). Compared to the case of the additional graphite with 10 wt%, the case of 12.5 wt% of the ply orientation of The relationship between the flexural strength and percentage of graphite fillers i shown in Figure 9. When the ply orientations are compared, the orientation of [0°/90°] yields higher flexural strength than the orientation of [−45°/45°]4s. It is signified that th ply orientation can directly affect the mechanical properties. The flexural strength is in creased by combining the graphite fillers at ply orientations of [0°/90°]4s, while the graph ite fillers are added by 5, 7.5, and 10 wt%. Subsequently, the flexural strength is increase by 6%, 22%, and 31%, compared with 0 wt% of the graphite. By adding 12.5 wt% of graph ite, the flexural strength is significantly reduced by 25.8% compared to the 10 wt% of th graphite. The tendency of flexural strength on the ply orientation of [−45°/45°]4s is simila to the ply orientation of [0°/90°]4s with graphite fillers of 5, 7.5, and 10 wt%. Consequently the flexural strength increased by 10.6%, 20.4%, and 35.6%, when compared to the cas without graphite filler. When the graphite filler is added greater than 10 wt% (i.e., 12. wt%), the flexural strength can be minimized by comparing the flexural strength at 1 wt% percentage of the graphite filler. Moreover, in the case of ply orientation [−45°/45°] with 10 wt% and 12.5 wt%, the flexural strength is decreased by 23.7%. As a result, it i signified that increasing the amount of graphite fillers did not always augment the flex ural strength. In fact, the highest strength point relies on the optimal content of differen fillers. In this case, the optimal content of the studied filler could be found at around 1 wt% graphite filler.

Vacuum Infusion Process
The relationship between the manufacturing processes and flexural strength show that the hand lay-up process has less strength than the vacuum infusion process in bot aspects of the ply orientation and percentage of the graphite filler, as shown in Figure 10 Considering the ply orientation of [0°/90°]4s at 0, 5, 7.5, 10 and 12.5 wt% of the graphit fillers based on the vacuum infusion process, it is obvious that the flexural strength ca be increased by 43.1%, 31.2%, 18.3%, 20.3%, and 0.2% compared with the hand lay-u process. Together with the ply orientation of [−45°/45°]4s, the flexural strength based o The relationship between the flexural strength and percentage of graphite fillers is shown in Figure 9. When the ply orientations are compared, the orientation of [0 • /90 • ] 4s yields higher flexural strength than the orientation of [−45 • /45 • ] 4s . It is signified that the ply orientation can directly affect the mechanical properties. The flexural strength is increased by combining the graphite fillers at ply orientations of [0 • /90 • ] 4s , while the graphite fillers are added by 5, 7.5, and 10 wt%. Subsequently, the flexural strength is increased by 6%, 22%, and 31%, compared with 0 wt% of the graphite. By adding 12.5 wt% of graphite, the flexural strength is significantly reduced by 25.8% compared to the 10 wt% of the graphite. The tendency of flexural strength on the ply orientation of [−45 • /45 • ] 4s is similar to the ply orientation of [0 • /90 • ] 4s with graphite fillers of 5, 7.5, and 10 wt%. Consequently, the flexural strength increased by 10.6%, 20.4%, and 35.6%, when compared to the case without graphite filler. When the graphite filler is added greater than 10 wt% (i.e., 12.5 wt%), the flexural strength can be minimized by comparing the flexural strength at 10 wt% percentage of the graphite filler. Moreover, in the case of ply orientation [−45 • /45 • ] 4s with 10 wt% and 12.5 wt%, the flexural strength is decreased by 23.7%. As a result, it is signified that increasing the amount of graphite fillers did not always augment the flexural strength. In fact, the highest strength point relies on the optimal content of different fillers. In this case, the optimal content of the studied filler could be found at around 10 wt% graphite filler.
The relationship between the manufacturing processes and flexural strength shows that the hand lay-up process has less strength than the vacuum infusion process in both aspects of the ply orientation and percentage of the graphite filler, as shown in Figure 10.

Comparison with Aluminum
Generally, most of the materials used in the manufacturing process for the U structions are made of aluminum, which is heavy and strong. In this research, the strength of aluminum is comprehensively compared with the hybrid composite m Table 2 shows the flexural strength between the pure aluminum alloy 6061 an composite materials (i.e., 10 wt% of [0°/90°]4s). According to Table 2, the hybrid co materials demonstrate higher flexural strength in the manufacturing process com the pure aluminum alloy 6061. However, the strength is considered in conjunct the design weight. The weight of the specimen material is taken to determine the to-weight ratio. As a result, hybrid composites produce higher strength-to-weig than aluminum. By considering the comparison of the flexural strength regardin and VI manufacturing, it can be seen that the VI process provides the higher strength than the HL process. This is because the VI process represents a pressur uum process in removing the entire air in the laminate and replacing with the ep or filler. Thus, it is confirmed that hybrid composite materials can be used as an al substance for producing that can be used as structural UAVs.

Materials
Flexural Strength (MPa) Weight (g) Strength-to-Mass Ratio (M

Comparison with Aluminum
Generally, most of the materials used in the manufacturing process for the UAV constructions are made of aluminum, which is heavy and strong. In this research, the flexural strength of aluminum is comprehensively compared with the hybrid composite materials. Table 2 shows the flexural strength between the pure aluminum alloy 6061 and hybrid composite materials (i.e., 10 wt% of [0 • /90 • ] 4s ). According to Table 2, the hybrid composite materials demonstrate higher flexural strength in the manufacturing process compared to the pure aluminum alloy 6061. However, the strength is considered in conjunction with the design weight. The weight of the specimen material is taken to determine the strength-to-weight ratio. As a result, hybrid composites produce higher strength-to-weight ratios than aluminum. By considering the comparison of the flexural strength regarding the HL and VI manufacturing, it can be seen that the VI process provides the higher flexural strength than the HL process. This is because the VI process represents a pressurized vacuum process in removing the entire air in the laminate and replacing with the epoxy resin or filler. Thus, it is confirmed that hybrid composite materials can be used as an alternative substance for producing that can be used as structural UAVs.

Fracture Characteristics
Concerning the strength of the material, the fracture of the material is also clarified in this subsection. This analysis can reveal the fractures of hybrid composite materials. The crack caused by force on the specimens is performed by the three-point bending test. By applying the testing force, the crack patterns on specimens are discovered, as shown in Figure 11. The studied specimen is tested by the manufacturing process with coordination between the reinforcement (i.e., carbon fiber fabric) and matrix (i.e., epoxy resin). Considering the failure, the fracture of the specimen cracks following the trajectory of fiber orientation. In the composite laminates, macro-micro images are provided in Figure 12 to observe the crack and failure area. Considering the failure, the fracture of the specimen cracks following the trajectory of fiber orientation. In the composite laminates, macro-micro images are provided in Figure 12 to observe the crack and failure area. As illustrated in Figure 13, the carbon fiber fractures are investigated by the scanning electron microscope (SEM) with the commercial model of JOEL JSM-6010LV. At 10 wt% of the graphite filler (see Figure 13a,b) it can be seen that the matrix formulates as a group, indicating the ductility of the matrix; whereas, the matrix at 12.5 wt% of the graphite filler (see Figure 13c,d) disseminates in small pieces corresponding to the flexural strength or less ductility (see in Figure 10), signifying the degradation of strength. In the case of the vacuum infusion process (see Figure 13a,c), it can be seen that the matrix formulates as a group, indicating the strong bonding between the fiber and matrix when subjected to loads. In addition, it is obvious that the fibers are held together with less gap, while the vacuum bagging has fewer matrix components at the fracture (see Figure 13b,d). Therefore, the vacuum infusion process at 10 wt% of the graphite filler yields better perfor- Considering the failure, the fracture of the specimen cracks following the trajectory of fiber orientation. In the composite laminates, macro-micro images are provided in Figure 12 to observe the crack and failure area. As illustrated in Figure 13, the carbon fiber fractures are investigated by the scanning electron microscope (SEM) with the commercial model of JOEL JSM-6010LV. At 10 wt% of the graphite filler (see Figure 13a,b) it can be seen that the matrix formulates as a group, indicating the ductility of the matrix; whereas, the matrix at 12.5 wt% of the graphite filler (see Figure 13c,d) disseminates in small pieces corresponding to the flexural strength or less ductility (see in Figure 10), signifying the degradation of strength. In the case of the vacuum infusion process (see Figure 13a,c), it can be seen that the matrix formulates as a group, indicating the strong bonding between the fiber and matrix when subjected to loads. In addition, it is obvious that the fibers are held together with less gap, while the vacuum bagging has fewer matrix components at the fracture (see Figure 13b,d). Therefore, the vacuum infusion process at 10 wt% of the graphite filler yields better perfor- As illustrated in Figure 13, the carbon fiber fractures are investigated by the scanning electron microscope (SEM) with the commercial model of JOEL JSM-6010LV. At 10 wt% of the graphite filler (see Figure 13a,b) it can be seen that the matrix formulates as a group, indicating the ductility of the matrix; whereas, the matrix at 12.5 wt% of the graphite filler (see Figure 13c,d) disseminates in small pieces corresponding to the flexural strength or less ductility (see in Figure 10), signifying the degradation of strength. In the case of the vacuum infusion process (see Figure 13a,c), it can be seen that the matrix formulates as a group, indicating the strong bonding between the fiber and matrix when subjected to loads. In addition, it is obvious that the fibers are held together with less gap, while the vacuum bagging has fewer matrix components at the fracture (see Figure 13b,d). Therefore, the vacuum infusion process at 10 wt% of the graphite filler yields better performance in flexural strength than the vacuum bagging process. It should be noted that the energy dispersive X-ray spectroscopy (EDS) represents several physical phenomena corresponding to the electron interactions used for imaging sample surfaces. It is possible to take advantage of these interactions to obtain chemical information, where the EDS detector is a tool for measuring the energy of the emitted photons in the X-ray electromagnetic spectrum. However, this study is not relevant to the investigation of the chemical information or energy amount. In fact, this work pays attention to the hybrid structural analysis with flexural strength. Thus, the EDS analysis is not considered in this study.

Conclusions
In this study, the hybrid composite materials are introduced by the combination of carbon fiber/epoxy with waste graphite fillers. The main objective is to expand/promote It should be noted that the energy dispersive X-ray spectroscopy (EDS) represents several physical phenomena corresponding to the electron interactions used for imaging sample surfaces. It is possible to take advantage of these interactions to obtain chemical information, where the EDS detector is a tool for measuring the energy of the emitted photons in the X-ray electromagnetic spectrum. However, this study is not relevant to the investigation of the chemical information or energy amount. In fact, this work pays attention to the hybrid structural analysis with flexural strength. Thus, the EDS analysis is not considered in this study.

Conclusions
In this study, the hybrid composite materials are introduced by the combination of carbon fiber/epoxy with waste graphite fillers. The main objective is to expand/promote the utilization of hybrid composite materials (e.g., waste graphite fillers) into the UAV application, instead of using the conventional material, i.e., aluminum. Thus, the mechanical properties of hybrid composite materials are comprehensively analyzed based on the three-point bending tests. As a result, it is found that the hybrid composite materials provide higher flexural strength and strength-to-weight ratio than the pure aluminum alloy 6061. Obviously, the additional waste graphite fillers exhibit a significant factor, improving the flexural strength. At 10 wt% of the filler, it reveals the highest flexural strength in the case of ply orientation of [0 • /90 • ] 4s and of [−45 • /45 • ] 4s . Visibly, the waste graphite fillers at 10 wt% compromising in fiber composites with the ply orientation of [0 • /90 • ] 4s provide remarkable properties. Therefore, the study's waste toner (graphite filler) could improve the mechanical properties as the graphite filler successfully reduces the air gap in the lamination. In addition, employing the contaminated graphite fillers from waste toner cartridges can effectively reduce and reuse the large abundance of waste and practically eliminate the waste from commercial and industrial systems. This utilization could also diminish hazardous waste and pollution in both local and global environments. Moreover, this study shows that the ply orientation symbolizes the important factor affecting the flexural strength and load. The ply orientation directly alters the fracture characteristics of specimens from the testing. Nevertheless, the manufacturing process is also one of the factors impacting the flexural strength. The performed experiment indicates the specimens triggered by the vacuum infusion process embody the higher flexural strength. Subsequently, the hybrid composite material has a higher strength-to-weight ratio than the pure aluminum alloy, leading to the incredibly strong and lightweight materials. This study also confirmed that the hybrid composite material from the waste toner could be used to manufacture various parts of industries, including automobile and aviation today and in future, resulting in the structure's strength enhancement with weight reduction. In future works, this application can be used as an alternative material for manufacturing UAV parts, such as ribs, wings, and fuselage. Moreover, different manufacturing processes to conduct specific parts of the UAV body should be analyzed as further work of this study.