There are many kinds of composite materials, mainly metal-based fiber reinforced composite materials, which have high specific strength and obvious lightweight effect, but are more difficult to process and weld than aluminum alloys. Foamed aluminum is a functional and structural integrated material, which has the characteristics of low density (about 10% of aluminum), high strength, good vibration damping, good sound insulation and heat insulation, etc. The structural foam aluminum material is used to manufacture the roof cover of the light car.
1. Body skin material selection
At present, the car body is mostly made of skeleton structure and skin, and the research group has done research on the selection of skin materials.
(1) Requirements for body skin materials. Lightweight, safety, and low cost are factors that must be considered in the selection of body skin materials. Table 1 summarizes the mechanical properties, specific gravity and unit cost of the five alternative body skin materials. It can be seen from the table that the titanium alloy plate has high specific strength (strength/specific gravity), and the lightweight effect is good, but the price is expensive; although the 304 stainless steel plate has high specific gravity, it has high strength and good comprehensive mechanical properties, good weldability, and the price is not too expensive. , so it is the preferred material for lightweight cars in the initial stage; the specific strength of aluminum alloy is worse than that of stainless steel, and the price is lower than that of stainless steel. Although the ductility and weldability are not as good as stainless steel, due to the rapid development of design and welding technology, it soon The material of choice for lightweight cars; ABS engineering plastics have low specific strength, excellent low water absorption, corrosion resistance, non-toxic and tasteless, excellent chemical properties and electrical insulation properties, and can be heat-resistant and not deformed at low temperatures. It also has high impact toughness under certain conditions, and the price is the lowest, and it is also used more; carbon fiber plate has the highest specific strength but large direction difference, poor ductility, best lightweight effect, and the price is also very expensive, but lower than titanium alloy plate.
| performance | 304 stainless steel | 5052 aluminum plywood | TC4 titanium plate | ABS board | CCF300 carbon fiber board |
| Tensile strength σb/MPa | 520 | 230 | 902 | 43 | 3950 |
| Yield strength σ0.2/MPa | 205 | 195 | 824 | 50 | – |
| Specific gravity/(g/cm3) | 7.93 | 2.68 | 4.54 | 1.05 | 1.76 |
| Unit price/(yuan/m2) | 356 | 145 | 2225 | 9 | 700 |
(2) Material selection. From the analysis of the above data alone, it is difficult to obtain the best body skin material. Therefore, the Analytic Hierarchy Process is introduced here to quantitatively compare the weighted consideration of the above five alternative body skin materials. The analysis model is shown in Figure 1. The analysis adopts the analytic hierarchy process proposed by T.L.Saaty in the 20th century. By comparing the assignments according to the subjective feeling of the judges on the judgment factors, a simple matrix is constructed, and then accurate weight values are obtained through calculation to help decision-making. Through calculation, analysis and quantitative comparison, it is concluded that among the five alternative body skin materials, after comprehensively considering the three aspects of safety (mechanical properties), light weight (material proportion), and low cost (unit cost), CCF300 carbon fiber sheet It is the optimal body skin material.
2. Properties and applications of carbon fiber materials
(1) Carbon fiber material. Carbon fiber (CF) is a new type of fiber material with carbon content of more than 95%, high strength and high modulus, which is made of flake graphite microcrystals and other organic fibers stacked along the fiber axial direction. Carbon fiber resin composite The properties of the material are ① the specific gravity is less than 1/4 of the steel, the tensile strength is 7~9 times that of the steel (generally greater than 3500MPa), and the specific strength is high. ② The tensile modulus of elasticity is also higher than that of steel (generally 23000~43000MPa), and the rigidity is good. However, carbon fiber materials only show high strength along the fiber axis, weak shear strength, poor impact resistance, and easy damage. Therefore, when manufacturing into structural components, they often take advantage of their light tensile strength to avoid the Do the part that takes the side impact. The disadvantages of carbon fiber are: 1. The stress calculation is complicated, and complex stress calculation (longitudinal rigidity, transverse rigidity) is required during processing, and the carbon fiber sheets are overlapped and formed according to the calculation. ②Because it is formed after the mold is made, it is difficult to change the size, and it is difficult to change the style. ③It is easy to turn white and aging in the sun. ④The price is higher, the output is limited, the process is complicated, the cost of opening the mold is higher, and the processing is difficult.
(2) Examples of carbon fiber materials. Carbon fiber material products are available in cloth, sheet, sheet and tube forms, as shown in Figure 2. Cloths and sheets can be bonded to existing objects as reinforcing materials, or can be made by self-made molds; plates and pipes are made into structures, and it is difficult to guarantee the connection strength. For mass production, it is best to directly customize components after design.
(3) Examples of carbon fiber products. Carbon fiber has been used in many products, not only for skin materials, but also into various parts as shown in Figure 3. Carbon fiber composite materials can reduce weight by 40% to 60% when used in car bodies and chassis. The brake pads used as carbon fiber brake discs can withstand high temperatures of 2500C and have excellent braking stability. The outer ring of the hub is made of carbon fiber, and the inner hub is made of light alloy. With stainless steel screws, the weight of the hub is about 40% lower than that of the general hub of the same size. For example, the weight of the hub at 20 can be reduced from 18kg to 6kg. The carbon fiber driveshaft not only reduces the mass by 60%, but also has better fatigue resistance and durability. Carbon fiber is also used in transmission shafts, leaf springs, frames and other components. At present, steel materials account for about 3/4 of the car body mass. If all the steel parts of the car are replaced by carbon fiber composite materials, the car body mass can be reduced by 300kg, the fuel efficiency can be increased by 36%, and the carbon dioxide emissions can be reduced by 17%. In addition, carbon fiber is also used in pressure vessels and wind turbine blades, and it also has broad application prospects in new energy fields such as wind energy, nuclear energy and solar energy.
