Innovative Application of Carbon Fiber Composites in Rail Transportation: Multi-Axial Warp Knitted Fabrics and Pultrusion Process

 

01 Introduction

 

The current carbon fiber composite manufacturing technology in the field of rail transportation mainly follows the traditional molding methods, including the use of autoclaves, manual laying, and centrifugal casting (OOA) and other technologies. Autoclaves are widely used in the aviation industry due to their excellent performance. The manual laying method is difficult to meet the large-scale production requirements of complex components and has a greater impact on the environment. Due to technical limitations, the OOA method encounters difficulties in manufacturing parts with high fiber content, which limits its scope of application. Pultrusion technology, as a molding method that can be continuously produced, is valued for its advantages such as high fiber content, high raw material utilization efficiency and high production efficiency. According to research data, carbon fiber materials reinforced with multi-axial braiding show excellent performance and can meet different mechanical performance requirements through precise hierarchical design. Applying such multi-axial braids to the pultrusion process can effectively supplement the deficiencies of pultruded products in non-axial performance, thereby broadening the application field of composite materials.

 

02 Multiaxial Warp Knitted Fabric

 

Multiaxial warp knitted fabric (abbreviated as MWK) is composed of warp yarn (0°), weft yarn (90°) and axial yarn (θ), where the angle of the lining yarn varies from -20° to +20°. During the weaving process, the weaving yarn will pass through the entire fabric and tightly bind all the pre-laid load-bearing yarns in the thickness direction.

 

These load-bearing yarns are usually made of fibers with high mechanical properties, such as glass fiber, carbon fiber, Kevlar fiber and ultra-high molecular weight polyethylene fiber, while the weaving yarns are mostly made of low-cost high-strength polyester yarns.

 

The use of weaving yarns enhances the interlayer shear strength and multi-directional dimensional stability of the fabric, reduces the risk of layer cracking, and makes the multiaxial warp knitted fabric have good formability and resin permeability. In contrast, woven fabrics are woven by interlacing weft and warp yarns. For example, plain weave fabrics form a stable but poorly formable structure by alternating weft yarns with each warp yarn up and down; twill fabrics have good formability and permeability but slightly inferior stability by alternating weft yarns through warp yarns at a certain regularity; satin fabrics are formed by weft yarns passing through multiple warp yarns. Although they have good formability and permeability, they have weak stability, and asymmetry may cause stress concentration in multi-layer fabrics.

 

Due to the spatial obstruction of woven fabrics, their mechanical properties are relatively weak. In contrast, multi-axial warp knitted fabrics have significant advantages in providing superior mechanical properties. Their layers (in the thickness direction) are tightly combined by braiding yarns, allowing flexible design according to needs, and are currently ideal reinforcement materials.

 

The manufacturing process of this fabric ensures that the yarn can be fully straightened, thereby maximizing the performance of the yarn and maintaining the stability of the fabric size when stretched in the longitudinal and transverse directions. Due to the presence of braiding yarns, multi-axial warp knitted fabrics allow resin to penetrate faster when used as a composite material substrate, which helps to improve the performance of composite products.

 

As a composite reinforcement, multi-axial warp knitted fabrics show excellent performance in tensile, bending, shear and impact tests.

 

Figure Carbon Fiber Multi-Axial Warp Knitted Fabric

 

Carbon fiber multi-axial warp knitted fabric uses braided wire to bind multiple layers of carbon fibers at different angles together. Multi-axial fabric can reduce the laying time of fiber/fabric during composite material manufacturing. Due to the presence of braided wire, when using liquid molding processes such as RTM and VARI, the resin can quickly infiltrate the fiber and improve the curing efficiency.

 

Carbon fiber warp knitted fabric, with its unique weaving method, keeps the carbon fiber straight and meets the requirements of the direction, proportion and sequence of the ply. At the same time, the braid has good integrity and high ply efficiency. The warp knitting method makes the fabric lay well, making it an ideal composite material reinforcement material. The main features of multi-axial fabric are as follows:

 

1. High tensile strength and good mechanical properties. Multi-axial warp knitting technology enables the yarn layer of the fabric to be straightened and oriented in a specific direction, so that the mechanical properties of the reinforcing yarn are fully utilized.

 

2. Good dimensional stability and shear performance. Due to the introduction of yarns in the ±45° direction in the multi-axial warp knitted fabric, the shear deformation of the fabric is suppressed.
3. Good laying and pre-setting effects. Multi-axial warp knitted fabrics have high laying efficiency. Since the fabric has “packaged” the fibers at various angles, they can be directly cut and laid, and can also be pre-set, which significantly improves the process efficiency.
4. Good resin impregnation. Multi-axial warp knitted fabrics can easily form channels in the fabric fibers, which reduces the impregnation and penetration pressure, has good permeability and fast penetration speed, and is conducive to the uniform distribution of resin.

 

03 Pultrusion Process

 

Pultrusion technology is a continuous, efficient and cost-effective process for producing high-performance fiber composite materials, which is particularly suitable for large-scale production. This technology optimizes the reinforcing function of the fiber. In the classic pultrusion process, the fiber is fully stretched to fully utilize its axial strength.

 

In addition, pultrusion technology can produce products with more stable quality due to its high level of automation, fewer production steps and lower technical and environmental impact, which is a significant advantage compared to other manufacturing processes.

 

According to international development trends, the production of large-scale products with complex cross-sections and thick walls is becoming the direction of pultrusion technology development, and it plays an increasingly important role in many fields.

 

Figure Continuous Fiber Pultrusion Process Flow

 

Traditional impregnation pultrusion technology is a fast and continuous production method. In this process, as shown in the steps, reinforcing fibers (commonly glass fibers) are first spread and introduced through a creel. Next, these fibers enter an impregnation tank, where they are impregnated with resin. The impregnated fibers are then introduced into a mold with a heating function for heating and curing.

 

After the part is cured, it is pulled out of the mold by a pull-out device and cut to a specific size as required. The key links in the pultrusion process include the impregnation, curing and pulling of the fibers. For products with complex cross-sections, the fiber bundles must be preformed by a preforming device before entering the mold.

 

There are two main ways to impregnate the fibers: one is to impregnate through an open impregnation tank, and the fibers are fully impregnated when passing through the tank containing resin; the other is to inject the resin directly into the mold through an injection device to impregnate the reinforcing material, which is called closed mold injection.

 

The advantage of the pultrusion process is that it can achieve efficient continuous production. Compared with the autoclave or molding process, it has higher production efficiency and lower cost.

 

04 Application of Multi-Axial Warp Knitted Fabrics Combined with Pultrusion Process in Rail Transit

 

In recent years, my country’s rail transit industry has developed rapidly, not only in domestic high-speed EMUs, but also exported my country’s high-speed railway and transportation technology solutions and overall vehicles to many countries.

 

With the further development of my country’s high-speed rail technology, the requirements for vehicle structure and lightweight are increasing year by year. Compared with metal materials such as aluminum alloys, carbon fiber composite materials have obvious advantages in specific strength and specific stiffness, and are ideal materials for lightweight rail transit vehicle bodies. At present, the application of carbon fiber composite materials in rail transit vehicles has begun to extend and expand from non-load-bearing parts such as interior decoration and in-vehicle equipment to load-bearing parts such as car bodies and equipment compartments, and from small parts such as skirts and fairings to large components and structures such as top covers, driver’s cabs, and vehicle bodies. The proportion of carbon fiber composite materials is increasing year by year.

 

CRRC Qingdao Sifang Co., Ltd. released the CETROVO new generation carbon fiber subway (80km/h) vehicle in 2018. Its body is made of all-carbon fiber composite materials. The pultruded parts include the body shoulder beam and the chassis side beam. As shown in the figure, the reinforcing materials of the two pultruded parts include carbon fiber yarn, unidirectional warp knitted fabric and fiber felt.

 

 

Figure Schematic diagram of the main structure of the CFRP car body and the longitudinal beam of the carbon fiber pultrusion and the cross section of the actual figure

 

Continuous unidirectional fiber pultrusion has high production efficiency and low cost. The length of the parts is only limited by the production space, but the space occupied by the yarn pultrusion unwinding frame for complex and thick parts is large. In addition, the mechanical properties of continuous fiber pultrusion parts are highly directional. As long as they are suitable for products that mainly bear axial loads, if the products bear loads in other directions, the continuous fiber pultrusion products will be unable to cope with them.

 

The preparation of carbon fiber yarn pultrusion samples will adopt a similar pultrusion process. The difference is that the impregnation tank is changed from an open impregnation tank to a closed mold injection process, and the rest of the processes are the same.

 

Figure Schematic Diagram of Multi-Axial Warp Knitted Fabric Pultrusion Equipment

 

Sample Test

Carbon fiber fabric pultrusion flat plate and carbon fiber yarn pultrusion flat plate test specimens are prepared and tested according to ISO or ASTM standards. The specimens are loaded from 0 N increments to the ultimate load at a given loading rate. The figure shows some photos of the pultrusion flat plate specimens before and after the test. The specimens were intact before the test, and different forms of damage appeared after the test. Photos of carbon fiber fabric pultrusion flat plate specimens before and after the test, and photos after the 0° tensile test show that the middle area of ​​5 samples has been damaged.

 

The failure mode of the 0° tensile test specimen is fiber delamination in the middle area and partial fiber breakage. In the 0° compression test, the failure mode of the specimen is buckling in the middle, and partial fiber breakage occurs. In the 90° tensile and compression test, the failure mode after the tensile and compression test is similar to that of 0°, that is, the specimens eventually have fiber breakage, accompanied by delamination. In the V-type notch test, delamination mainly occurs in the 45° direction, and its failure form is mainly delamination and a small number of fiber breakages.

 

Figure Carbon Fiber Multiaxial Fabric Pultrusion Sample Before and After Test

 

The test results show that the mechanical properties of HFC20-25K yarn deformation performance in the 0° direction are excellent, which is 1956Mpa, and the performance strength and modulus in the 90° direction are very low, with a strength of only 37.1Mpa, which is much lower than the 90° strength and modulus of LPTN800 fabric. This is mainly because there is no continuous fiber to carry the unidirectional yarn pultrusion in the 90° direction, and it is mainly carried by resin, so the unidirectional fiber pultrusion performance in the 90° direction is low.

 

The performance of the LPTN800 pultrusion sample in the 0° direction is lower than that of the yarn pultrusion sample in the 0° direction, mainly because the proportion of carbon fiber in the four-axial warp knitted fabric in the 0° direction is greatly reduced, which makes the performance in the 0° direction decrease, and the tensile and compressive properties in the 90° direction are significantly increased, mainly because the fibers in the four-axial warp knitted fabric in the 90° direction and ±45° direction significantly improve the performance in the 90° direction.

 

05 Summary

 

With the advancement of technology and the enhancement of environmental protection awareness, the application of carbon fiber composite materials in the field of rail transit will be further expanded. In the future, we can foresee the development of lighter, more efficient and environmentally friendly carbon fiber materials, which will greatly improve the energy efficiency and performance of rail transit vehicles.

 

Further optimization and innovation of multi-axial warp knitting and pultrusion technology will make the production of carbon fiber composite materials more efficient, reduce costs, and make their application in rail transit more extensive.

 

In addition, with the in-depth research on material recycling and reuse technology, the sustainability of carbon fiber composite materials will be significantly improved, making an important contribution to the sustainable development of the rail transit industry.