Composite Sheet Usage in Electric Vehicles

Composite materials are a type of material manufactured by combining two or more materials macroscopically. Composite materials are used in many sectors today due to their properties. Polymer composite sheets can be used not only in sandwich structures produced by using various core materials, but also in laminated structures consisting solely of fiber and matrix material composition. In this article, the design and manufacture of wheel hub covers produced using the vacuum-assisted resin infusion (VARTM) method for the development of the aerodynamic structure of Demobil 09, an electric vehicle of the Solaris Solar Car Team, and the effect of these components on the aerodynamic structure were examined through both experimental and finite element method analyses. In this study, the surface quality, strength structure, fiber-matrix ratio, adhesive capability at the fiber-matrix interface, and contribution to aerodynamic structure of products manufactured using vacuum-assisted resin infusion and hand lay-up methods in composite sheet production were examined.

2. Vacuum-Assisted Resin Infusion Casting Method and Demobil '09 Vehicle

The vacuum infusion method is a method that works on the principle of resin forming the matrix in composite structure advancing in a vacuumed environment. The primary purpose of using this method is to obtain the product without manual handling after production preparations are completed. The use of the vacuum infusion method in composite manufacturing has certain advantages compared to other production methods. These advantages can be listed as follows: • In the hand lay-up method the resin/fiber mix ratio, in the infusion method is reversed, resulting in the use of less resin with more solid material structure and enabling long-term use of the material. • Inconsistent material consumption dependent on personal factors is prevented. It ensures homogeneity in the appearance and structure of the material. For example, during resin use, the vacuum infusion method allows resin to distribute homogeneously to every point of the product without human contact. • Through the vacuum bag, resin is added to the fiber layers that are covered using spiral tubes, thereby preventing resin from contaminating the environment. At the same time, the spread of resin and gases resulting from the reaction is prevented. Demobil '09 is the ninth electric vehicle designed and manufactured by the Solaris Team, a four-wheeled, two-seater chassis and body made of carbon-fiber fabric-reinforced polymer composite structure, manufactured by the team in 2017 (See Figure 1). Demobil '09, driven from the rear left wheel by an in-wheel electric motor, has a LiIon battery pack with 5 [kWh] capacity.

(a) A view from CFD analysis of the vehicle,

(a) A view from CFD analysis of the vehicle, Figure 1. Demobil '09 Vehicle

2.1 Preparation Process

The preparation process before starting production with the vacuum-assisted resin infusion casting method is an important factor for the product structure. The stages of the preparation process can be listed as follows: • Selection of mold material and manufacturing process: The appropriate mold material was selected based on how many times products would be taken from the mold and considering the production cost of the mold. Styrofoam with a density of 25 [kg/m³] was selected for hub cap production. The reason for selecting styrofoam with a density of 25 [kg/m³] is that it allows production with the desired surface roughness while having low production and material costs. The mold on which the production process will be performed is machined to the desired shape and precision using CNC or lathe machines. • Mold surface improvement: In composite production using the vacuum-assisted resin infusion casting method, the mold must be completely clean and as smooth as possible. For this reason, two coats of gelcoat can be applied to achieve the desired quality of the mold surface. • Mold Release Film: To be able to remove the product from the mold without damage, a mold release film with appropriate dimensions must be placed on the mold. • Carbon Fiber Fabric Placement: The carbon fiber fabric with properties specified in Table 1 is cut to appropriate dimensions and placed on the mold surface. In accordance with our reference loads, the use of two layers of carbon fiber fabric was deemed appropriate. As seen in Figure 2, a displacement of 0.0002507 [mm] was detected as a result of the analysis.

Table 1. Technical properties of Dowaksa 200 [gr/m²] 3K-Plain carbon fiber fabric used

 

Figure 2. Displacement amount and density resulting from the use of two layers of carbon fiber

  • Release Fabric: A release fabric that enables easy removal of the net is placed on the carbon fiber fabric. • Resin flow pattern design: The flow pattern is designed based on the shape and complexity of the part to be produced. In order for the resin flow to distribute homogeneously to all points of the product, the flow pattern was designed as shown in Figure 3. In accordance with this pattern, spiral tubes and T connectors were positioned as shown in Figure 5.

Figure 3. Flow pattern

• Vacuum bag placement: Double-sided tapes are placed to secure the vacuum bag to the mold boundaries without air leakage. The vacuum bag is cut according to the void calculated in the inner surface of the mold and glued to the double-sided tape without air leakage. • Resin: Connected to the vacuum pump from the two T connectors on the mold. Resin transfer to the mold is provided from one side of the connections, while the other T connector is connected to the vacuum tank. This production apparatus is shown in Figure 3. The properties of the resin used are shown in Table 2. The use of a vacuum tank is not only to balance the vacuum or divide it into parallel paths, but also to prevent excess resin used in the mold from reaching the vacuum pump.

Figure 4. Vacuum infusion production apparatus

Table 2. Corepox MEL-2203/MEH-2970-3 Epoxy Resin Technical Properties

• The basic arrangement of materials used in the vacuum-assisted resin infusion casting apparatus is shown in Figure 5

Figure 5. Material arrangement in the vacuum infusion production method

2.2 Production and Analyses

In this article, differences between vacuum infusion and hand lay-up techniques were observed and the results in Table 3 were obtained.

Table 3. Differences resulting from vacuum infusion and hand lay-up production types

Figure 6. Surface qualities of hub caps manufactured using vacuum infusion and hand lay-up methods

Aerodynamic design is an important design element especially for electric vehicles. As a result of the analyses performed, the calculation of the aerodynamic drag force acting on a moving vehicle is as follows. In the equation, ρ represents air density, A represents the front cross-sectional area of the vehicle, V represents velocity, and C_drag represents the drag coefficient of the vehicle. Upon examination of the equation, it was observed that the aerodynamic drag force acting on the vehicle increases with the square of velocity. The calculation of power expended by the vehicle to overcome the aerodynamic drag force is as follows. Besides the aerodynamic structure of a vehicle, surface quality of the body is also an important factor. The surface qualities of hub caps manufactured using vacuum-assisted resin infusion casting and hand lay-up methods are shown in Figure 6. In the analyses of the electric vehicle Demobil 09 of the Solaris Solar Car Team with and without the manufactured hub caps, the results in Table 4 were obtained. The analysis was performed while the electric vehicle (Demobil 09) was traveling at 120 [km/h].

Table 4. Power consumption obtained as a result of the analyses performed

At the end of the production process, production stages, quantities of materials used in production methods, and static analysis results of products were evaluated. In the hub cap manufactured using the vacuum-assisted resin infusion casting technique, 63 [gr] of resin mixture and two carbon fiber fabrics with an area of 0.15 [m²] were used. At the end of production, the total weight of the product was measured as 123 [gr]. As a result of static analysis, maximum displacement was determined as 0.0002507 [mm]. In the hub cap manufactured using the hand lay-up technique, 121 [gr] of resin mixture and two carbon fibers with an area of 0.15 [m²] were used. At the end of production, the total weight of the product was measured as 181 [gr]. As a result of static analysis, maximum displacement was determined as 0.003104 [mm]. When the quantities of materials used, their costs, and the static analyses of the products were evaluated, it was found that the product manufactured using the vacuum-assisted resin infusion casting method is more durable in terms of strength and service life, and more economical in terms of the cost of materials used during production. It was observed that the surface quality of the product manufactured using the vacuum-assisted resin infusion casting technique is of higher quality than the product manufactured using the hand lay-up technique. Regarding the importance of surface quality in terms of aerodynamic structure, it was observed that products manufactured using the vacuum-assisted resin infusion casting technique are more efficient in terms of aerodynamic structure. Talha Batuhan Korkut / Department of Mechanical Engineering - Dokuz Eylül Üniversitesi Ahmet Özkan / Department of Metallurgy and Materials Engineering - Dokuz Eylül Üniversitesi Asst. Prof. Dr. Aytaç Gören / Department of Mechanical Engineering - Dokuz Eylül Üniversitesi

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