Research And Application Progress Of Stitching Technology For Composite Materials

Composite stitching technology is a composite material prefabrication technology that uses stitches to make multi-layered fabrics into quasi three dimensional fabrics or to separate the separated fabrics into integral structures.

The technology originated in the middle and late twentieth Century. It has attracted much attention due to its ability to improve the damage tolerance of composite materials and greatly improve the impact resistance of composite materials, and has been widely applied in recent years.

In this paper, the characteristics, main sewing methods and process parameters of composite stitching technology and their optimum application scope are systematically introduced. The main research achievements on the important mechanical properties such as tension, compression, bending, laminar shear and impact compression of composite materials are summarized. Finally, the major research and application progress of composite stitching technology at home and abroad are described and prospected.

1. Characteristics of stitching technology

Compared with the traditional composite textile, weaving and stacking technology, suture technology mainly has the following characteristics:

(1) the design is strong, the direction of stitching the prefabricated body, the laying distance and the fiber structure can be optimized, and the stitching method and the suture area can also be adjusted as required.

(2) suture has little effect on the original fiber distribution, and a reasonable degree of uniform stress state can be achieved by reasonable setting of suture parameters.

(3) sutures can bear most loads, and reduce the stress concentration of surrounding resins, which can significantly improve the interlaminar properties of composites.

It is highly automated and has developed a highly automated suture equipment for improving suture consistency and stitching efficiency.

5. The assembly technology is excellent. Suture is a kind of connection technology. Compared with other joining technologies such as bonding and riveting, the stitched composite material has strong integrity and is difficult to cause local stress concentration.

Two. Main suture methods and process parameters

In the application of structure, there are mainly 3 sutures: improved suture stitching, chain stitching and tufting (Tufting) suture (see Figure 1).

The locking suture belongs to double side suture. In the improved lock suture, the suture is carried by the stitch from the side of the preform. The bottom line is then carried out by the sewing needle to carry out the next cycle. The junction between the line and the bottom line is located on the surface of the preform, which reduces the upward stitching and the bending and stress concentration effect of the prefabricated body to a maximum extent, as shown in Figure 1 (a).

Lock stitching generally requires that the preform has smaller curvature changes. It is widely used in the edge stitching of large size panels and the connection and stitching of stiffeners and skins, and the stitching thickness can reach 20mm.

Chain stitching is unilateral suture. The crescent shaped stitch is located on the same side as the cycloidal crochet. As the needle moves along the suture, the needle passes through the preform repeatedly and connects it around the sleeve, as shown in Figure 1 (b).

Chain stitching is usually suitable for suture with larger curvature and thinner preform, and the stitching thickness is generally not more than 10mm.

Tufting suture is also one side stitching. The suture follows the stitches from the prefabricated side to the other side. When the stitches exit, the stitches are left in the prefabricated body to complete suture, as shown in Figure 1 (c).

Tufting suture can stitch a thick preform. However, because Tufting suture is the only way to retain sutures through the friction between the stitches and the fibers in the prefabricated part, it is usually necessary to add other positioning methods to ensure that the stitches remain inside the preform and improve the quality of stitching.

The main suture parameters include stitch type, stitch diameter, suture density and suture direction. These parameters can directly affect the performance of preformed body after curing.

When choosing suture, the strength, wear resistance and temperature resistance of the suture should be considered, and the matching with the corresponding resin system. The common types of stitches include carbon fiber, glass fiber, Kevlar fiber and polyester fiber.

High temperature resistant Kevlar fibers, such as Kevlar29, are of light weight, high wear resistance and high toughness. They are widely used in aviation.

The large diameter suture can better improve the interlaminar damage tolerance of composite materials, but at the same time, it will also increase the fiber bending and damage inside the precast body and the resin accumulation in the internal stitches of the composites, resulting in the reduction of the tensile and compressive strength of the components. Therefore, the diameter of the stitches should be selected according to the prefabricated structure.

The stitching density mainly includes 2 parameters: Stitch spacing and row spacing. The greater the stitching density, the more serious the fiber damage and fiber buckling phenomenon is. The more fat area inside the prefabricated body is, the greater the impact on the in-plane performance of the workpiece; conversely, the lower the stitching density, the smaller the interlayer performance is.

Therefore, it is necessary to design suture density reasonably to improve the overall performance of composite parts.

Zhao Long and other studies of China Aviation Composite Materials Co., Ltd. show that when the stitching density is 5~6 needles /cm2, the comprehensive properties of materials are the best.

The stitching direction of the preform is 0 degree, 45 degree and 90 degree. The tensile strength of the composite is greatly influenced by the direction of stitching. The strength of the 0 degree suture is the least, while the 45 degree and 90 degree suture are the same.

Three. The effect of stitching on the mechanical properties of composites.

Suture will cause the buckling and damage of the fibers in the prefabricated part, and easy to form the fat rich area at the seams, thus forming the stress concentration point, which is the main reason for the decrease of the in-plane properties of the material. However, the stitching will greatly enhance the interlaminar damage capacitance of the composite material, and the existence of the thread will also prevent the expansion of the crack. Therefore, the effect of stitching on the mechanical properties of the composite material is somewhat complex.

A large number of studies have shown that suture will cause the decrease of tensile strength of materials, and due to the characteristics of stitching, the failure mode of materials is different from that of traditional composite laminates, and the tensile strength will gradually decrease with the increase of stitching density and stitch diameter.

However, Wei Yuqing et al. And Wu Gang have found that when the stitching density is less than 5~6 /cm2, the tensile failure mode of the material is mainly fiber fracture, and the tensile strength loss rate of stitched composite material is not large.

The effect of stitching on compressive strength of materials is not a simple increase or decrease relationship. Compression strength of stitched composite laminates is sometimes increased sometimes due to laminates design and stitching parameters.

Cheng Xiaoquan and other studies found that the compressive properties of stitched 0 degree unidirectional laminates were reduced by about 24%, but it had little effect on the compressive properties of 90 degree laminate.

Wu Gang and so on studied the compressive properties of laminates with [45/0/ to 45/90]4S and [90/45/90/ to 45/0/ to 45/90/45/90]2S stitches. It was found that the stitching had little reduction in the compression performance, and the change of the stitching parameters had the trend of increasing the compression performance. The stitching in the 0 degree direction had the least influence on the compression performance of the laminate.

Many scholars have found that although the bending properties of composite laminates have been reduced by stitching, the degree of descent is generally not more than 20%, and the density of stitching has little effect on the bending properties of composite laminates.

However, in his research, Liu Li found that proper optimization of stitching density can improve the bending properties of materials, such as stitching density of 4 pin /cm2, and the flexural strength of the material is 27.8% higher than that before stitching.

Sun Qiyong also systematically studied the flexural properties of stitched 3D braided composites. The results showed that the braided angle was 20 degrees, the lap length was 70mm, and the bending performance of three dimensional braided composite specimens with medium density stitching was excellent.

The shear strength of composite laminates increases first and then decreases with the increase of stitching density. This is due to the excessive fiber density and the stress concentration at the stitches, which makes the shear strength of the laminates decrease.

The focus of stitching density optimum depends on the stacking sequence and stitching parameters of laminates.

Suture can significantly increase the GIC value of laminates, increase stitching density, suture strength, reduce the young's modulus of stitches, increase the thickness and axial stiffness of specimens, and improve the GIIC value of specimens.

The suture can obviously reduce the impact damage of composite laminates and increase the compressive strength (CAI) of laminates after impact. A number of experiments show that the reasonable design of suture parameters can increase the CAI of laminates by 40% or even up to 400%.

Four. Application status of stitching technology

The stitching technology has been applied for nearly 30 years. It can enhance the thickness direction of composite structures, mainly for improving the damage tolerance of composite structures.

At present, the stitching equipment has developed from the first generation of manual controlled industrial sewing machine and the second generation computer controlled plane stitching equipment to third generation of computer controlled multi needle stitching equipment, which can realize two dimensional and three-dimensional stitching of various structures.

In recent years, the rapid development of liquid forming technology has laid a good foundation for the wide application of stitching technology.

Composite stitching technology is applied to solid rocket motor nozzle throat liner, expansion section, extension cone, brake disc, screw, aircraft wing and so on.

The ACT plan of the US National Aeronautics and Space Administration (NASA) has developed a 13.5m * 2.7m suture /RFI half wingspan wing panel, as shown in Figure 2, and has successfully carried out 200 semi wingspan ground tests.

At the same time, Boeing also developed the third generation of stitching equipment for large complex structural parts such as fuselage.

In addition, the United States Air Force Wright laboratory and the US Navy Air Force general headquarters also jointly formulated the ALAFS plan. The plan identified 7 key technologies: wing body design, wing structural layout, internal piping design, fuselage tank design, beam layout, internal rib layout and continuous design of the upper and lower girders. The RTM and RFI molding technology for stitching composite materials will be the main technical plan for the implementation of the plan.

At present, China, especially CIC composite materials Co., has successfully applied stitching /RTM, suture / RFI, suture / VARI technology to various composite structural parts, greatly improving the interlaminar strength, impact impedance and integrity of the composite structure, and reducing the assembly cost of the structural parts.

Figure 3-6 shows some typical /LCM stitched structures developed in China.

Five, conclusion

Composite stitching technology solves the problem of low interlaminar performance and small impact damage tolerance of traditional composite materials.

At present, the technology of liquid forming in China has become increasingly mature. With the further optimization of the third generation of sewing equipment and the reduction of manufacturing cost of composite materials, composite stitching technology can not only be taken seriously in the field of aviation and aerospace, but also will be widely promoted in the fields of ships and automobiles, making great contributions to the lightweight of various structural and functional parts.