What is so special about carbon fiber materials in lightweighting cars?

Carbon fiber reinforced polymer (CFRP) is composed of carbon fiber as a reinforcing material and resin as a matrix material. Early carbon fiber composites were mainly used in the military field.

With the improvement of material performance, molding process and the decline of price cost, carbon fiber composites are increasingly used in general industry and sports and leisure fields. Driven by the global trend of energy conservation and environmental protection, the fuel consumption of automobiles has attracted more and more attention. Lightweight design has become an effective way to save energy and reduce emissions for automobiles, and the material performance and development trend of carbon fiber composites are in line with the development needs of the automobile industry.

Carbon fiber composites are 50% lighter than steel and 30% lighter than aluminum. The weight reduction effect is obvious. Therefore, many automobile manufacturers have begun to try to use a large number of carbon fiber composites in the process of automobile manufacturing and modification in pursuit of extreme lightweight. With the development of new energy vehicles, carbon fiber composites will be more and more widely used in automobiles.

Characteristics of carbon fiber composite parts

1. Lightweight and high strength

The density of carbon fiber reinforced resin-based composite materials commonly used in automobiles is 1.5-2.0g/cm3, which is only 1/4-1/5 of ordinary carbon steel and about 1/3 lighter than aluminum alloy. In addition, the mechanical properties of carbon fiber composite materials are better than those of metal materials. Its tensile strength is 3-4 times higher than that of steel, its stiffness is 2-3 times higher than that of steel, its fatigue resistance is 2 times higher than that of steel, its weight is 3-4 times lighter than that of steel, and its thermal expansion coefficient is 4-5 times smaller.

If calculated by specific strength, carbon fiber composite materials greatly exceed carbon steel, and can exceed some special alloy steels, so they have higher specific strength. The use of carbon fiber materials can reduce the weight of the car body while reducing the power demand, and then use a smaller drive engine and suspension device, and reduce the impact risk by reducing kinetic energy. This spiral result will further reduce the weight of the car body. Therefore, replacing the original steel parts with carbon fiber parts has a significant lightweight effect.

2. Good designability

Carbon fiber composite materials have very strong designability, and can be flexibly designed according to different application requirements. According to the stress of the product structure, the structure and arrangement of the fiber can be adjusted to make anisotropic and different thickness products, and the sandwich structure can be applied to improve the overall rigidity of the component to achieve the best lightweight design.

Arranging the carbon fiber according to the direction of force can give full play to the anisotropy of the composite material strength, thereby saving materials and reducing weight. Due to its isotropy, metal materials will have the problem of excessive strength in the other direction after meeting the technical requirements of the maximum force direction.

For products with corrosion resistance requirements, matrix resins and reinforcement materials with good corrosion resistance can be selected during design, while other performance requirements, such as dielectric properties and heat resistance, can be met by selecting appropriate raw materials. In addition, in order to make the product cost acceptable, low-cost materials can be appropriately selected for replacement, such as mixed plying of different fibers, which can save material costs while meeting the performance indicators of the components.

3. Component integration

From the perspective of energy saving, the car should be designed with the smallest air resistance while taking into account its aesthetics. During stamping, the shape and structure of steel sheets are often limited due to process reasons. However, composite materials can be used to make various curved surfaces by utilizing their fluidity during molding, so as to achieve an integrated molding effect and meet the requirements of aerodynamic design and aesthetic needs.

Modularization and integration are also a development trend of automobile structure. Carbon fiber composite materials can form all parts, protrusions, ribs, edges, etc. of different thicknesses in one piece through reasonable mold design. Therefore, composite materials are suitable for manufacturing automobile parts that are difficult to manufacture with steel sheets, have low production efficiency, and are difficult to ensure precision.

For example, Lotus sports cars use carbon fiber materials as the goal of the whole vehicle, and make the body parts lightweight and integrated, which not only reduces the weight of the body, but also greatly increases the stiffness and strength of the parts, improving the performance of the whole vehicle.

4. impact resistance

Carbon fiber composite materials also have good impact resistance. Carbon fiber composite materials based on polymers have certain viscoelastic mechanical properties and can absorb certain impact energy.

In addition, there are microcracks and local debonding on the interface between the matrix material and the fiber, and there is a small local relative movement between the carbon fiber and the matrix, and there is friction.

Due to the effects of viscoelasticity and interface friction, the vibration attenuation coefficient is large, so when the vehicle is impacted, it can absorb a large amount of impact energy, which is beneficial to improve personal safety.

5. Good corrosion resistance

Many parts on the car are subject to corrosion from chemical agents such as engine oil, gasoline, and automobile transmission fluid, as well as harsh environments such as high temperature, severe cold, and salt spray. It is difficult for traditional metal materials to ensure quality consistency and service life under different environments.

However, carbon fiber composite products generally do not have rust and corrosion problems. Polymer-based composite materials have excellent acid resistance, seawater resistance, and can also resist alkali, salt and organic solvents. Therefore, it is an excellent corrosion-resistant material, and automotive parts made of it have a long service life and extremely low maintenance costs.

6. Carbon fiber/thermoplastic composites

The matrix resins used in carbon fiber reinforced resin composites are mainly divided into two categories, one is thermosetting resin and the other is thermoplastic resin. Thermosetting resins are composed of reactive low molecular weight prepolymers or high molecular weight polymers with active groups; during the molding process, they are cross-linked and polycondensed under the action of curing agents or heat to form an infusible and insoluble cross-linked structure. Commonly used in composite materials are epoxy resins, bismaleimide resins, polyimide resins and phenolic resins.

Thermoplastic resins are composed of linear high molecular weight polymers, which melt and dissolve under certain conditions and only undergo physical changes. Commonly used ones are polyethylene, nylon, polytetrafluoroethylene and polyetheretherketone. In carbon fiber reinforced resin-based composites, carbon fibers play a reinforcing role, while the resin matrix shapes the composite material into a whole that bears external forces and transfers the load to the carbon fibers through the interface. Therefore, it has a direct impact on the technical performance, molding process and product price of carbon fiber composites, and the composite method of carbon fibers will also affect the performance of composite materials.

7. Carbon fiber reinforced nylon 6 composites

Short-cut carbon fiber reinforced thermoplastic resin composites (SCFRTP) are an important component of engineering materials. This type of composite material usually has good mechanical properties, good economic benefits and can be manufactured into various shapes of products by hot pressing, extrusion or injection molding. Nylon 6 is a thermoplastic resin material with a wide range of applications, and filler reinforcement modification is a common and effective modification method to improve the performance of nylon 6 composites.

The research on carbon fiber reinforced nylon 6 composites (PA6/CF) has received a lot of attention in recent years. The performance of PA6/CF composites is affected by many factors: processing methods and processes, the content and length of carbon fibers in the composite material, the dispersion and orientation of carbon fibers, the interface structure between carbon fibers and PA6, and the characteristics of nylon 6 and carbon fibers themselves.

However, the mechanical properties of thermoplastic composites mainly depend on the microstructure of the polymer matrix and the interface properties between the fiber and the matrix, so it is particularly important to compare and analyze the microstructure and macroscopic properties of composite materials.

In addition, PA6 is a semi-crystalline polymer composite material, and the mechanical properties of PA6/CF depend on the crystal structure and morphology of the PA6 matrix, which in turn depends on the processing technology of the composite material. There are few reports on the systematic connection between the micromorphology and structure of PA6/CF composite materials and macroscopic performance.

8. Carbon fiber reinforced polypropylene composite materials

Carbon fiber can improve the elastic modulus of polypropylene, but reduce the tensile strength, impact strength and elongation at break of the material. Carbon fiber has a certain heterogeneous nucleation effect on polypropylene, which improves the non-isothermal crystallization peak temperature and crystallization degree of the polypropylene matrix, and promotes the formation of β crystal in the PP matrix. During the isothermal crystallization process, polypropylene is easy to nucleate and crystallize at the end of the carbon fiber section.

EPDM-g-MAH and SEBS-g-MAH have good dispersibility in the PP matrix, which significantly improves the impact strength and elongation at break of PP. The PP/CF composite materials with EPDM-g-MAH and SEBS-g-MAH have higher impact strength and elastic modulus.

The addition of toughening agent in PP/CF composite material leads to the decrease of nucleation ability and slowing down of crystal growth rate of matrix polypropylene. Toughening agent does not change the crystal form of PP/CF composite material. The nucleation effect of carbon fiber in PP/CF composite material with toughening agent becomes obvious, so that the spherulites in the composite material mainly grow near the carbon fiber.