Basic Introduction Of Carbon Fiber Composite Materials for Automobiles

 

With the advent of the era of lightweight automobiles, more and more lightweight materials continue to appear in the automotive industry, from the widely used PP, PC, PA, ABS, etc. to the hot carbon fiber composite materials in recent years. The world’s exploration of lightweight automobiles has never stopped, so today the editor has compiled an article about the knowledge of carbon fiber composite materials for automobiles, let us learn about it together.

 

 

1. Concept Of Carbon Fiber Composite Materials

Carbon fiber reinforced composite materials refer to composite materials with carbon fiber, fabric, fiber felt, etc. as reinforcement and resin, metal, ceramic, etc. as matrix. Common ones are resin-based carbon fiber composite materials (CFRP), ceramic-based carbon fiber composite materials (CMC) and metal-based carbon fiber composite materials (MMC). Carbon fiber composite materials for automobiles are mainly resin-based composite materials.

 

Excellent Performance Of Carbon Fiber Composite Materials

 

1. Mechanical Properties

Carbon fiber composite materials have high tensile strength, large modulus, low density, and high specific strength and specific modulus. Compared with traditional metal materials, carbon fiber composite materials are also lighter in weight on the basis of high strength and high modulus, and have obvious density advantages. Compared with aluminum alloy materials, the same components can reduce weight by 50% using carbon fiber composite materials; compared with steel, the weight reduction can reach 70%. The following table compares the mechanical properties of carbon fiber composite materials and common metal materials:

 

Table Comparison Of Mechanical Properties Of Carbon Fiber Composite Materials and Common Metal Materials

 

Material Types		Density/（g/cm^3）	Tensile Strength/Mpa	Elastic Modulus/Mpa	Specific Strength/m	Specific Modulus/km
High-Strength Steel		7.8	1000	214000	1.3	0.27
Aluminum Alloy		2.8	420	71000	1.5	0.25
Magnesium Alloy		1.79	280	45000	1.6	0.25
Titanium Alloy		4.5	942	112000	2.1	0.25
Carbon Fiber Composite Materials		2.0	1100	40000	5.5	0.2
Carbon Fiber Composite Materials	High Strength Type	1.5	1400	130000	9.3	0.87
	High Modulus Type	1.6	1100	190000	6.2	1.2

 

2. Thermodynamic Properties

Carbon fiber composite materials have good resistance to high and low temperatures. When isolated from air (under inert gas protection), they still have strength at 2000℃ and do not break brittlely under liquid nitrogen. At the same time, their thermal expansion coefficient is small, their specific heat capacity is high, they can store a large amount of heat energy, and they have excellent resistance to thermal shock and thermal friction.

 

3. Corrosion Resistance

Carbon fiber composite materials have high corrosion resistance. In addition to being oxidized by strong oxidants such as concentrated nitric acid, hypochlorous acid and dichromate, general acids and alkalis have little effect on them. Compared with steel and alloy materials, carbon fiber composite materials are safer and more durable as automotive materials. The following is a comparison of the toughness, high temperature resistance and corrosion resistance of carbon fiber composite materials and common metal materials:

 

Table Comparison of the toughness, high temperature resistance and corrosion resistance of carbon fiber composite materials and common metal materials

 

Material Properties	High Strength Steel	Aluminum Alloy	Magnesium Alloy	Titanium Alloy	Carbon Fiber
Toughness	Strong	Poor	Poor	Genera	Very Strong
High Temperature Resistance	200℃	200℃	300℃	500℃	2000℃
Corrosion Resistance	General	Strong	Poor	Strong	Very Strong

 

2. Common Matrix and Reinforcement Materials

In composite materials, carbon fiber reinforcement is the main body that bears external forces; the resin matrix transmits stress loads to the carbon fiber body in the form of shear stress through the interface layer. At the same time, the resin matrix also protects the carbon fiber reinforcement from external damage.

 

1. Resin Matrix

When selecting the resin matrix of the composite material, its strength, stiffness, fatigue resistance, operating temperature, and resistance to moisture and heat aging should be considered. Commonly used resin matrices are divided into the following two categories:

 

1) Thermosetting Resin

Such as: epoxy resin, phenolic resin, polyimide resin, etc. After thermosetting resin is processed and formed, an insoluble and infusible three-dimensional network structure is formed in the polymer molecules. Thermosetting resin-based materials have high strength and are currently the most commonly used type of resin in carbon fiber composite materials. However, it does not melt at high temperatures, is difficult to process, and is not easy to recycle.

 

 

Figure Epoxy Resin Particles

 

2) Thermoplastic Resins

Such as: vinyl resin, nylon, polytetrafluoroethylene resin, etc. Thermoplastic resins can be melted at a certain temperature and plasticized after cooling. They have good toughness and processability, and the materials can be recycled and have low cost. Thermoplastic carbon fiber composites are highly favored by the automotive materials industry. New high-performance thermoplastic resins are gradually replacing traditional thermosetting resins and becoming the matrix of automotive carbon fiber composites.

 

 

Figure PTFE Resin Particles

2. Reinforcement

Reinforcement material is another important component of carbon fiber composites. Composite materials made of different forms of reinforcement also have different properties. When selecting reinforcement materials, factors such as the mechanical performance requirements of the composite material, the interface compatibility between the resin matrix and the reinforcement, and the production process requirements should be considered. In theory, there are three types of reinforcements for carbon fiber composites: carbon fiber, carbon fiber fabric, and carbon fiber felt. In the application of automotive materials, they are mostly in the form of fiber reinforcement. Fiber reinforcement can be divided into three categories: chopped carbon fiber, continuous carbon fiber, and mixed fiber.

 

1) Chopped Carbon Fiber

Generally refers to carbon fiber reinforcement below 20mm. When chopped carbon fiber is used as a reinforcement material, it has good infiltration with the resin matrix, strong bonding, and high tear strength of the composite material. However, the uniformity of chopped carbon fiber during the laying process is difficult to control, which can easily lead to large anisotropy of the mechanical properties of the composite material.

2) Continuous Carbon Fiber

Generally refers to carbon fiber filament reinforcement above 40mm. Continuous carbon fiber as a reinforcement material can effectively improve the tensile fracture strength and toughness of the composite material, but its processing process is more difficult than chopped carbon fiber.

 

3) Hybrid Fibers

Hybrid fiber reinforced composite materials refer to composite materials with two or more fiber reinforcements in the same matrix. Composite materials with different properties can be obtained by changing the components, content and structure of each fiber.

 

Figure Carbon Fiber Material

 

III. Molding Method

With the widespread use of carbon fiber composite materials, its processing methods are constantly being innovated. From the traditional labor-intensive hand lay-up molding to the current mechanized spray molding and injection molding, the molding methods of carbon fiber composites are developing towards high quality, high efficiency and low cost. Common molding methods and characteristics:

 

1. Hand Lay-up Molding

Hand-made paste has simple process, low investment and wide application. However, the product has loose texture, low density, low strength, and mainly relies on manual labor, with unstable quality and low production efficiency.

 

2. Winding Molding

The fiber can remain continuous and complete, and is generally suitable for continuous carbon fiber reinforcement. Mechanized production, short production cycle, and low labor intensity. The product does not require mechanical processing, but the equipment is complex, the technical difficulty is high, and the process quality is difficult to control.

 

3. Injection Molding

It is divided into reaction injection molding (RIM) and reinforced reaction injection molding (RRIM). It has the characteristics of high production efficiency and low energy consumption, and can be used as a composite of chopped carbon fiber or sheet reinforcement materials.

 

4. Resin Transfer Molding (RTM)

Short production cycle, no pollution, low labor cost, product size can be precisely controlled, and can be used to manufacture large-scale high-quality products with relatively complex processes.

 

 

 

Figure Carbon Fiber Rack in Production Workshop

1. Difficulties In The Application Of Carbon Fiber Composites
2. High Cost

From the production of carbon fiber to the preparation of its composite materials, high cost investment is involved. Compared with metal materials such as steel and aluminum, the cost of carbon fiber composite materials is 20% higher.

 

2. Long Production Cycle, Low Production Efficiency, And Inability To Mass Produce

Traditional metal materials can be formed in just a few tens of seconds, while the resin molding of carbon fiber composite materials takes several hours. The long production cycle leads to low production efficiency and inability to achieve mass production.

 

3. Difficulty In Product Design And Development, Lack Of Mature Database

The production technology of carbon fiber composite materials is not mature enough. From the selection of resin matrix and reinforcement to the selection of connection points, mechanical distribution of connection parts and composite processes, there is a lack of mature data support, and the development of new products is difficult.

 

4. Difficulties In Connection And Recycling Technology

Punching holes on the surface of carbon fiber composites will cause its strength loss, so it is not suitable for traditional riveting and screwing when connected. The commonly used method is to use adhesives for bonding, but this method has poor durability and is prone to aging.

 

1. Application Of Innovative Technologies
2. High-Pressure Resin Transfer Molding Technology (HP-RTM) to Shorten The Production Cycle

Use high-pressure technology to achieve high-pressure injection, high-pressure impregnation and high-pressure curing of the resin matrix. That is, the resin matrix quickly fills the mold cavity under high pressure, accelerates the infiltration between the resin and the reinforcement, and accelerates the resin reaction system to shorten the curing cycle. BMW and SGL have jointly developed epoxy resins with curing times of 2 min and 5 min, and adopted HP-RTM technology to achieve a “one-minute cycle”.

 

2. Use Thermoplastic Matrix Prepreg To Shorten The Production Cycle, And The Matrix Can Be Hot-Melted And Reused To Reduce Costs

Due to the good thermal stability of thermosetting resin materials, the resin matrix of carbon fiber composites currently uses thermosetting resins, and thermoplastic resins are used less. However, the molding process time of thermoplastic resins is short, and they are reprocessable and recyclable, which can shorten the production cycle of composite materials and reduce production costs. At present, General Motors and Teijin are jointly developing composite molding technology for polypropylene and polyamide resins and carbon fibers.

 

3. Carbon Fiber/Glass Fiber Hybrid Processing And Molding Technology

In view of the shortcomings of carbon fiber composite materials, which have poor impact resistance and fracture toughness, carbon fiber and glass fiber are mixed and used together as reinforcement materials to achieve complementary performance. Daimler and Chrysler use overlapping feeding and mixed carbon fiber and glass fiber to reduce the number of parts, reduce weight, and increase stiffness by 22%.

 

4. New Reinforcement Structure-Three-Dimensional Fabric

Three-dimensional fabric has excellent three-dimensional space and can achieve good impregnation effect when compounded with resin. This excellent structure also fundamentally solves the problem of low shear strength and easy delamination between composite layers of traditional composite materials.

 

1. Summary

In recent years, with the continuous efforts of experts and scholars at home and abroad, carbon fiber has been increasingly applied to the automotive field due to its excellent performance. It is believed that in the near future, carbon fiber composite materials will appear more in low-end and mid-end models, and will no longer be exclusive to high-end models.