# Complex Structure of Braid in Fiber Art Creation with Multisensor IoT Technology

Authors: Jingyu Wang; Pengpeng Li; Mengyao Wang
Journal: Journal of Sensors
 (2022)

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## Abstract

Fiber-fabric composites have the characteristics of lighter texture, higher strength, higher damage resistance, better toughness, and flexibility than laminated composites, but their complex structures have not been well studied. This paper is aimed at study the complex structure of woven fabrics in fiber art creation based on multisensor Internet of Things technology and at studying the impact of its composite material mechanical properties. In this paper, it is proposed to use glass fibers and carbon fibers to weave the required structural preforms in a two-dimensional braiding machine and then use the obtained preforms and epoxy resin to prepare three-dimensional two-dimensional braided composite materials with different structures through a molding process. The composites were tested in tension, bending, and compression to obtain data and their failure modes, and the data were compared to obtain correlations. The experimental results in this paper showed that through the tensile, bending, and compression tests of the three kinds of hybrid structure braided composites, the tensile properties of the glass fiber braided composites decreased by about 77%; the influence of angle on the bending properties of carbon fiber braided composites has the largest downward trend of 63%. Two-dimensional biaxial and two-dimensional triaxial preforms are made by weaving glass fiber and carbon fiber, and the angle deviation is basically kept within 2.2%.


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## Body

## 1. Introduction

The development process of carbon fiber research and production in China is relatively slow. From the late 1990s to the present, it has gone through a long process in recent years. However, China has not yet achieved large-scale industrial production, and the informatization degree of carbon fiber production is relatively low. Therefore, improving the informatization degree of carbon fiber production and applying the Internet of Things technology to the carbon fiber spinning production process are of great significance for improving the output and quality of carbon fiber in China.With the rapid development of Internet of Things technology, the combination of Internet of Things technology and carbon fiber production can effectively promote the development of China’s carbon fiber industry. The monitoring system of carbon fiber production process based on the Internet of Things is a complex system engineering. Based on the emerging Internet of Things technology, the system covers multiple links of carbon fiber production and realizes the monitoring of the carbon fiber production process. It improves the efficiency of carbon fiber production and the informatization degree of the carbon fiber industry and promotes the development of carbon fiber production to automation and intelligence.The innovations of this paper are as follows: (1) biaxial glass fiber preforms with different angles, triaxial preforms with different angles, and triaxial carbon fiber preforms with different angles obtained by 64-spindle two-dimensional braiding machine. (2) Two-dimensional braided plate-like composites were prepared by VARI process with three kinds of mixed-braided structural prefabricated parts and epoxy resin, and the fiber content of the composites under different structures and different angles was calculated. (3) Tensile, compressive, and bending tests are carried out on the two-dimensional braided composite materials with different angles and different number of axes and different fibers; the influence of the two-dimensional braided structure on the mechanical properties of the composites was obtained.


## 2. Related Work

The mechanical behavior of the nonwoven structure is considered to be represented by the change in fiber orientation according to the orientation angle and the degree of deformation. Stolyarov and Ershov’s study found that the fiber orientation in the internal fabric structure corresponds to the applied load direction, and the initial stage of deformation with the maximum material stiffness is not accompanied by a significant change in fiber orientation [1]. Although the fiber deformation mechanism in the internal structure of the nonwoven is explained, the deformation stiffness value is low. Krogh et al. investigated different concepts related to photo frame testing for shear properties of woven prepreg fabrics. Although the sample arm modification was found to have a significant effect on the measured shear load, the uniformity of the shear strain field in the sample was not significantly improved [2]. The effect of the plain weave structure on the cutting mechanism and the mechanism of defect occurrence is rarely studied in detail. Zhang and Li established a three-dimensional finite element model of plain weave carbon fiber cloth, studied the occurrence and expansion of delamination, and found that the plain weave structure can prevent stress transfer and crack growth [3]. There are limitations of further research on the applicability of the optimal design of Kevlar fiber-reinforced polymer (KFRP) composites. Priyanka et al. studied the use of different modeling methods, such as homogeneous isotropic and orthotropic material models, and the effects of different parameters [4]. However, the research on fabric weave pattern, matrix material and composite material manufacturing technology, and working and loading conditions is insufficient.It becomes particularly important to design a cooperative computing offloading mechanism. Guo et al.’s research shows how to fully utilize the centralized cloud and distributed MEC resources [5]. However, it faces new challenges such as single networking method, time extension, poor reliability, high congestion, and high energy consumption. The features offered by mobile technology are very important when considering the continued aging of the population and the ensuing increase in frailty. Domingues et al. proposed the development of a noninvasive fiber optic sensor (OFS) architecture suitable for shoe soles for remote monitoring of plantar pressure [6]. Although the research on multisensor IoT technology is relatively deep, there is almost no research on the combination of multisensor IoT technology and fiber art weaving technology.Optical fiber sensors have both communication and sensing functions and have become a bridge connecting people and the world and an important part of accelerating the development of the Internet of Things. Yin et al.’s research showed the basis of fiber optic chemical sensing or biosensing, including the sensing mechanism of various fiber optic sensors, sensing materials, and new technologies for sensing material deposition [7]. Market forecasts and trends in the use of fiber optic sensors confirm that the demand for them will continue to increase. Senkans et al. focus on fiber Bragg grating (FBG) sensor networks and their applications in IoT and structural health monitoring (SHM), especially their coexistence with existing fiber optic communication system infrastructure [8]. There is emerging infrastructure of the Internet of Things. Given a certain fiber length, there is a trade-off between sensing bandwidth, sensitivity, and spatial resolution. Wang et al. constructed a fully linear system for complete theoretical processing, so that a significant enhancement of the signal-to-noise ratio becomes feasible and the sensing bandwidth remains equal to the quartic average case [9]. Although detailed experiments have been carried out on the fiber art weaving process, the research on its complex structure is still insufficient.


## 3. Combination Method of Multisensor IoT Technology and Fiber Art Weaving

### 3.1. Application of Fiber Materials in Fiber Art Creation

In the process of development, fiber art is good at absorbing nutrients from other art types in terms of expression and craftsmanship, which is because fiber materials have inherent advantages, such as linearity, softness, and affinity in their characteristics. The material itself is superior to other materials and has strong plasticity [10].The weaving method is one of the existing modeling methods of fiber materials in ancient times; that is, various types of fiber materials are used as the basic warp and weft elements, and the interweaving treatment method of mutual floating and sinking is organized according to certain rules. Weaving is the basic means of shaping linear fibers. Linear fibers are the original form of fiber materials and have the most essential characteristics of fibers. The art of fiber weaving is shown in Figure1.Figure 1
The art of fiber weaving.As shown in Figure1, the weaving method is one of the traditional fiber material forming techniques. From traditional art forms such as silk, kesi, and tapestries to the changes of modern weaving forms, a weaving method has always been a particularly important process technique. The modern weaving technique breaks the traditional process of copying and translation and turns the artist’s creative master to the works and the materials used and then pays attention to the matching and changes of the texture and texture of the materials [11]. This makes weaving break free from traditional concepts and becomes an important turning point in fiber art in the modern sense, and it is also one of the important expressions of fiber materials [12].


### 3.2. IoT-Based Carbon Fiber Production

The Internet of Things technology is the further development of information technology and an emerging field in the information technology industry. The Internet of Things technology based on wireless sensor network can monitor all aspects of carbon fiber production in real time, help carbon fiber production enterprises to grasp the production status of the enterprise in real time, and improve the monitoring accuracy and production efficiency [13]. In the short term, carbon fiber manufacturers can reduce operating costs, various resource overheads, and unnecessary losses by applying this technology; in the medium and long term, enterprises can use the Internet of Things technology based on wireless sensor network combined with intelligent optimization control system to improve the intelligence and real-time optimization control ability of carbon fiber production, improve the quality of carbon fiber products, and thus improve the core competitiveness of carbon fiber production enterprises, in order to promote the development of China’s carbon fiber industry [14]. In order to ensure the accuracy and effectiveness of information monitoring in all aspects of carbon fiber production, realize intelligent monitoring of carbon fiber production process, and improve carbon fiber production efficiency, this paper designs the following application framework of carbon fiber production process monitoring system based on Internet of Things technology. The overall framework of the carbon fiber production process monitoring system is shown in Figure 2.Figure 2
The overall framework of the carbon fiber production process monitoring system.As shown in Figure2, the front-end sensor node includes temperature, pressure and other types of sensors, preprocessing module, A/D conversion module, and embedded microprocessor. It is mainly used to collect on-site ambient temperature, pressure, and other information and convert analog signals into digital signals [15].


### 3.3. Sensor Fiber Art Weaving Data Fusion

In actual detection, the detection range and reliability of each sensor node are limited. In order to enhance the robustness of the entire network and the accuracy of the collected data, to avoid node failure and interference caused by external factors, multiple identical sensors are often used to measure the same measurement point [16]. The weighting factors of each sensor are W1,W2,⋯,Wn; the relationship between the fusion value and the weighting factor should satisfy the following two formulas:
(1)X^=∑P=1nWpXp,(2)∑P=1nWp=1.SinceX1,X2,⋯,Xn are independent of each other and are unbiased estimates of x, the total mean squared n error is
(3)σ2=E∑P=1nWpX−Xp2=∑P=1nWP2σP2.It can be obtained from the above formula that the total mean square errorσ2 is a multivariate quadratic function about the weighting factor, so its minimum value must exist. The weighting factor corresponding to the minimum total mean square error is
(4)Wp=1σP2∑i=1n1/σi2,p=1,2,⋯,n.Therefore, the weighting coefficient of each sensor is only determined by the measurement variance [17]. The mean square error can be minimized as long as the weights of each sensor conform to formula (3), and at this time,
(5)X¯=∑i=1nXiσi2∑i=1n1σi2−1.The true value of the measured value is an objective constant, which can be estimated according to the arithmetic mean of the existing measured data. Assume that(6)Xi¯k=1k∑p=1kXp,i=1,2,⋯,n.The estimated value at this time is(7)X^=∑i=1nWiXi¯k.The total mean squared error is(8)σ¯2=1k∑i=1kWi2σi2.Using formulas (4), (6), and (7), the estimated value after data fusion can be calculated.


### 3.4. Improved Adaptive Weighting Algorithm

Assuming that there are two groups of sensors for the same measurement point, the two groups of sensors first use the adaptive weighting method to perform data fusion on their respective measured values. The fused values areX1¯,X2¯, respectively, and the corresponding mean square errors are σ1,σ2 [18, 19].Using the batch estimation algorithm, the measurement equation can be rewritten as(9)X=X1¯X2¯=μTXz+V1V2T,(10)R=EVVT=σ1200σ22.At the same time, consider the above two sets of measurement resultsX1¯ and X2¯, which are two measurement data at the same monitoring point at the same time, and the influence of the previous measurement of the sensor on this measurement is negligible [20]. So based on estimated data fusion measurements,
(11)X+=P+HTR−1X=σ22X1¯+σ12X1¯σ12σ22.


### 3.5. Simplified Anisotropic Hyperelastic Model

Fiber-reinforced braided composite materials are regularly arranged, and this composite material has strong directionality[21, 22]. Therefore, from a macroscopic point of view, fiber-reinforced composites are anisotropic materials. And usually, this anisotropy is very pronounced, and its mechanical properties are highly dependent on the orientation of the fibers. During the large deformation forming process of fiber reinforced woven composite materials, the deformation of the material is mainly realized by the large angle change between the yarns, while the in-plane tensile deformation along the yarn direction of the fiber bundle is relatively small. The schematic diagram of the fabric structure and its deformation before and after is shown in Figure 3.Figure 3
Schematic diagram of the fabric structure and its deformation before and after.
(a)
Before deformation(b)
After deformationFigure3 is a schematic diagram of the deformation of the plain woven fabric containing two reinforcing fibers. There are many ways to express the mechanical properties of the woven material, so the strain energy function can be expressed as
(12)W=WC,a0,b0.Similarly, the strain energy function can also be expressed as a function of the invariantIi, so the strain energy function can be expressed by the following formula:
(13)WC,a0,b0=WIi.Carbon fiber woven cloth is called dry carbon fiber woven cloth without matrix material, which is composed of two bundles of reinforcing fibers intertwined[23, 24], and the strain energy can be decomposed into two parts: fiber tensile strain energy and fiber shear strain energy in the plane.
(14)W=WC,a0,b0=WF+WFF,where WF is the energy contributed by fiber stretching and WFF is the shear energy between the interwoven fibers. The tensile energy WF of the fiber can be simplified to the expression of the fiber draw ratio, namely,
(15)WF=WFC,a0,b0=WaFI4a+WbFI4b,where WaF and WbF represent the tensile energy of the warp and weft fibers, respectively, and I4a and I4b can be defined by the following formula:
(16)I4a=a0·C·a0=λa2,I4b=b0·C·b0=λb2.a0 and b0 represent the draw ratio of the warp and weft fibers, respectively. Introduce I10ab to represent the shear angle between the two bundles of fibers
(17)I10ab=Δφ=φ−φ0≈I4aI4b−1/2a0·C·b0−a0·b0.The shear strain energy between AFF fiber bundles can be quantified by ground, which is(18)WFF=WabFFI10ab.To obtain the corresponding parameters in the material model, it is necessary to perform uniaxial tensile experiments on the material. The tensile strain energy function of the braided material can be obtained by fitting the material curve using the test results of the material uniaxial tensile test. By changing, the tensile strain energy function of the fiber can be obtained by the following formula:(19)WaFI4a=k1I4a−12+k2I4a−13+k3I4a−12.


## 3.1. Application of Fiber Materials in Fiber Art Creation

In the process of development, fiber art is good at absorbing nutrients from other art types in terms of expression and craftsmanship, which is because fiber materials have inherent advantages, such as linearity, softness, and affinity in their characteristics. The material itself is superior to other materials and has strong plasticity [10].The weaving method is one of the existing modeling methods of fiber materials in ancient times; that is, various types of fiber materials are used as the basic warp and weft elements, and the interweaving treatment method of mutual floating and sinking is organized according to certain rules. Weaving is the basic means of shaping linear fibers. Linear fibers are the original form of fiber materials and have the most essential characteristics of fibers. The art of fiber weaving is shown in Figure1.Figure 1
The art of fiber weaving.As shown in Figure1, the weaving method is one of the traditional fiber material forming techniques. From traditional art forms such as silk, kesi, and tapestries to the changes of modern weaving forms, a weaving method has always been a particularly important process technique. The modern weaving technique breaks the traditional process of copying and translation and turns the artist’s creative master to the works and the materials used and then pays attention to the matching and changes of the texture and texture of the materials [11]. This makes weaving break free from traditional concepts and becomes an important turning point in fiber art in the modern sense, and it is also one of the important expressions of fiber materials [12].


## 3.2. IoT-Based Carbon Fiber Production

The Internet of Things technology is the further development of information technology and an emerging field in the information technology industry. The Internet of Things technology based on wireless sensor network can monitor all aspects of carbon fiber production in real time, help carbon fiber production enterprises to grasp the production status of the enterprise in real time, and improve the monitoring accuracy and production efficiency [13]. In the short term, carbon fiber manufacturers can reduce operating costs, various resource overheads, and unnecessary losses by applying this technology; in the medium and long term, enterprises can use the Internet of Things technology based on wireless sensor network combined with intelligent optimization control system to improve the intelligence and real-time optimization control ability of carbon fiber production, improve the quality of carbon fiber products, and thus improve the core competitiveness of carbon fiber production enterprises, in order to promote the development of China’s carbon fiber industry [14]. In order to ensure the accuracy and effectiveness of information monitoring in all aspects of carbon fiber production, realize intelligent monitoring of carbon fiber production process, and improve carbon fiber production efficiency, this paper designs the following application framework of carbon fiber production process monitoring system based on Internet of Things technology. The overall framework of the carbon fiber production process monitoring system is shown in Figure 2.Figure 2
The overall framework of the carbon fiber production process monitoring system.As shown in Figure2, the front-end sensor node includes temperature, pressure and other types of sensors, preprocessing module, A/D conversion module, and embedded microprocessor. It is mainly used to collect on-site ambient temperature, pressure, and other information and convert analog signals into digital signals [15].


## 3.3. Sensor Fiber Art Weaving Data Fusion

In actual detection, the detection range and reliability of each sensor node are limited. In order to enhance the robustness of the entire network and the accuracy of the collected data, to avoid node failure and interference caused by external factors, multiple identical sensors are often used to measure the same measurement point [16]. The weighting factors of each sensor are W1,W2,⋯,Wn; the relationship between the fusion value and the weighting factor should satisfy the following two formulas:
(1)X^=∑P=1nWpXp,(2)∑P=1nWp=1.SinceX1,X2,⋯,Xn are independent of each other and are unbiased estimates of x, the total mean squared n error is
(3)σ2=E∑P=1nWpX−Xp2=∑P=1nWP2σP2.It can be obtained from the above formula that the total mean square errorσ2 is a multivariate quadratic function about the weighting factor, so its minimum value must exist. The weighting factor corresponding to the minimum total mean square error is
(4)Wp=1σP2∑i=1n1/σi2,p=1,2,⋯,n.Therefore, the weighting coefficient of each sensor is only determined by the measurement variance [17]. The mean square error can be minimized as long as the weights of each sensor conform to formula (3), and at this time,
(5)X¯=∑i=1nXiσi2∑i=1n1σi2−1.The true value of the measured value is an objective constant, which can be estimated according to the arithmetic mean of the existing measured data. Assume that(6)Xi¯k=1k∑p=1kXp,i=1,2,⋯,n.The estimated value at this time is(7)X^=∑i=1nWiXi¯k.The total mean squared error is(8)σ¯2=1k∑i=1kWi2σi2.Using formulas (4), (6), and (7), the estimated value after data fusion can be calculated.


## 3.4. Improved Adaptive Weighting Algorithm

Assuming that there are two groups of sensors for the same measurement point, the two groups of sensors first use the adaptive weighting method to perform data fusion on their respective measured values. The fused values areX1¯,X2¯, respectively, and the corresponding mean square errors are σ1,σ2 [18, 19].Using the batch estimation algorithm, the measurement equation can be rewritten as(9)X=X1¯X2¯=μTXz+V1V2T,(10)R=EVVT=σ1200σ22.At the same time, consider the above two sets of measurement resultsX1¯ and X2¯, which are two measurement data at the same monitoring point at the same time, and the influence of the previous measurement of the sensor on this measurement is negligible [20]. So based on estimated data fusion measurements,
(11)X+=P+HTR−1X=σ22X1¯+σ12X1¯σ12σ22.


## 3.5. Simplified Anisotropic Hyperelastic Model

Fiber-reinforced braided composite materials are regularly arranged, and this composite material has strong directionality[21, 22]. Therefore, from a macroscopic point of view, fiber-reinforced composites are anisotropic materials. And usually, this anisotropy is very pronounced, and its mechanical properties are highly dependent on the orientation of the fibers. During the large deformation forming process of fiber reinforced woven composite materials, the deformation of the material is mainly realized by the large angle change between the yarns, while the in-plane tensile deformation along the yarn direction of the fiber bundle is relatively small. The schematic diagram of the fabric structure and its deformation before and after is shown in Figure 3.Figure 3
Schematic diagram of the fabric structure and its deformation before and after.
(a)
Before deformation(b)
After deformationFigure3 is a schematic diagram of the deformation of the plain woven fabric containing two reinforcing fibers. There are many ways to express the mechanical properties of the woven material, so the strain energy function can be expressed as
(12)W=WC,a0,b0.Similarly, the strain energy function can also be expressed as a function of the invariantIi, so the strain energy function can be expressed by the following formula:
(13)WC,a0,b0=WIi.Carbon fiber woven cloth is called dry carbon fiber woven cloth without matrix material, which is composed of two bundles of reinforcing fibers intertwined[23, 24], and the strain energy can be decomposed into two parts: fiber tensile strain energy and fiber shear strain energy in the plane.
(14)W=WC,a0,b0=WF+WFF,where WF is the energy contributed by fiber stretching and WFF is the shear energy between the interwoven fibers. The tensile energy WF of the fiber can be simplified to the expression of the fiber draw ratio, namely,
(15)WF=WFC,a0,b0=WaFI4a+WbFI4b,where WaF and WbF represent the tensile energy of the warp and weft fibers, respectively, and I4a and I4b can be defined by the following formula:
(16)I4a=a0·C·a0=λa2,I4b=b0·C·b0=λb2.a0 and b0 represent the draw ratio of the warp and weft fibers, respectively. Introduce I10ab to represent the shear angle between the two bundles of fibers
(17)I10ab=Δφ=φ−φ0≈I4aI4b−1/2a0·C·b0−a0·b0.The shear strain energy between AFF fiber bundles can be quantified by ground, which is(18)WFF=WabFFI10ab.To obtain the corresponding parameters in the material model, it is necessary to perform uniaxial tensile experiments on the material. The tensile strain energy function of the braided material can be obtained by fitting the material curve using the test results of the material uniaxial tensile test. By changing, the tensile strain energy function of the fiber can be obtained by the following formula:(19)WaFI4a=k1I4a−12+k2I4a−13+k3I4a−12.


## 4. Fabrication Experiment of Two-Dimensional Braided Composite Structure

The preparation of two-dimensional braided composites includes two aspects: the preparation of two-dimensional braided preforms and the preparation of two-dimensional braided composites. The prefabricated parts are made of glass fiber produced by Jushi Group and carbon fiber produced by Zhongfu Shenying Company. The two-dimensional braided composite material is obtained by compounding the two-dimensional braided preform with the epoxy resin produced by Dongqi Company through the VARI molding process, and the mechanical properties of the prepared two-dimensional braided composite material are tested.


### 4.1. Preparation of 2D Braided Preforms

Because of the weaving characteristics of two-dimensional weaving, a mandrel needs to be used in the weaving process, so it is necessary to calculate the diameter of the mandrel used for the design of the mandrel according to the width of the yarn. After that, parameters such as weaving pitch and traction speed are determined, and the prefabricated parts are finally woven. The design process is shown in Figure4.Figure 4
Design weaving flowchart.As shown in Figure4, preforms with different parameters can be woven by the braiding machine, and the parameters in the preforms include braiding angle, braiding pitch, coverage factor, and the like.In the experiment, E6D17-1200-380 direct roving produced by Jushi Group Co., Ltd. was used as the braiding yarn, and the linear density was 1200 tex. The width of the yarn is wide, and the yarn width is 2.70 mm when standing still. The yarn of this width is woven on the surface of the mandrel to ensure uniform weaving and a high coverage factor. The shaft yarn is made of A-grade 12K SYT49S carbon fiber produced by Zhongfu Shenying Carbon Fiber Co., Ltd., and the fiber width is 2.00 mm when standing. The equipment adopts the high-speed horizontal 64-spindle two-dimensional weaving machine produced by Jiangsu Xuzhou Qixing Ribbon Machinery Co., Ltd. The instrument is simple to operate, has high weaving efficiency and a high degree of automation, and can weave biaxial and triaxial fabrics of2×2,1×1,and2×1 specifications. The weave of the samples woven in this subject is 2×2.


### 4.2. Core Mold Diameter Design

In the case of weaving the same angle with the same width of yarn, the diameter of the core mold is directly related to the coverage factor of the preform. On the basis that the yarn of the same thickness can be woven, the coverage factor of the preform obtained by using a core mold with a larger diameter is smaller, and vice versa. The size of the coverage factor has a great influence on the mechanical properties of the two-dimensional braided composites. The larger the coverage factor, the tighter the yarn and the higher the strength, and vice versa. The core mold diameters corresponding to different coverage factors are shown in Table1.Table 1
Corresponding mandrel diameters for different coverage factors.
Number of spindles (piece)Weaving yarn width (mm)Braid angle (°)Mandrel diameter (mm) when the coverage factor is 0.9When the coverage factor is 1, the diameter of the core mold (mm)642.70608055642.70556847642.70506343642.70455739642.70405336642.70304732As shown in Table1, the number of yarns in this test is 64, so n=64. In the case where the width of the knitting yarn is not compressed, the width of the knitting yarn is 2.70 mm, so br=2.70. To avoid low coverage factor, the mechanical properties of composite materials are greatly affected, so the coverage factor of prefabricated parts should reach more than 90% (no obvious voids on the surface). When the braided yarn width is 2.70 mm, the coverage factor is 0.9, the number of yarns is 64, the braiding angle is 60°, and the mandrel diameter is 80 mm. When the coverage factor is 1, the mandrel diameter is 55 mm.In the experiment, the core molds are all made of PVC round pipes, which are lighter and have stronger hardness, which is convenient for the installation and disassembly of the core molds. Before the test, sandpaper must be used to polish the surface of the tube and the head and tail to prevent the friction between the tube and the yarn from being too large, causing serious fiber fluff. The PVC round pipe needs to add a fixing clamp at one end to facilitate the mandrel to be fixed on the pulling device. This method helps to give a certain tensile force to the shaft yarn during the weaving process, to ensure that the shaft yarn is parallel to the axial direction, and to prevent the shaft yarn from buckling during the demolding process, which affects the final test result. The parameter table of the two-dimensional braided preform is shown in Table2.Table 2
Parameter table of 2D braided preforms.
NumberingTypes of weaving yarnsNumber of axesMandrel diameter (mm)Braiding pitch/mmAngle (°)Traction speed (mm)Coverage factor (%)1-1Glass fiberDual axis6063114.33.47498.41-2Glass fiberDual axis5563138.64.21394.31-3Glass fiberDual axis5050131.84.00797.92-1Glass fiberThree-axis6063114.33.47499.92-2Glass fiberThree-axis5563138.64.21399.92-3Glass fiberThree-axis5050131.84.00799.93-1Carbon fiberThree-axis6063114.33.47499.93-2Carbon fiberThree-axis5563138.64.21399.93-3Carbon fiberThree-axis5050131.84.00799.9As shown in Table2, all organizational structures in the table are 2×2. The knitting yarn width is 2.70 mm, and the shaft yarn width is 2.00 mm when the coverage factor is calculated, and the knitting speed is 0.191 rad/s. If the weaving speed is too slow, the woven preform will not be formed smoothly; if the weaving speed is too fast, it is easy to cause the yarn to break. After many trials, the weaving speed was chosen to be 0.191 rad/s.


### 4.3. Prefab Parameters

It takes pictures of the prefab head, middle, and tail. The picture was imported into PS for angle measurement, and the three measured angles were averaged. The fabric is flattened according to half of the corresponding diameter, and the thickness is measured at five points of the preform at random with a thickness measuring instrument, and the average value is obtained. A summary of the parameters of the preform obtained by measurement is shown in Table3.Table 3
The actual angle and thickness of each prefab.
NumberingTypes of weaving yarnsNumber of axesMandrel diameter (mm)Braiding pitch (mm)1-16058.80.31.31-25555.81.51.31-35048.92.21.42-16061.01.72.02-25555.20.41.62-35049.60.81.83-16060.20.31.43-25555.61.11.43-35049.80.41.5As shown in Table3, the liquid forming process obtains the composite material by laying the fiber fabric in the mold and pouring the resin. The process is simple to form and has high forming efficiency, but it needs to design corresponding molds. The autoclave forming process uses the prepreg to prepare the composite material through high temperature and high pressure, and the cost of this process is relatively high.In this experiment, the VARTM molding process in the liquid molding process is used; VARTM molding process is one of the liquid molding processes, which has the advantages of low cost, low pollution, and high efficiency, which is widely used in large and complex structures, such as ship hulls, blades, and aviation components. VARTM lays the fiber fabric on a flat plate, then injects resin in a vacuum state, and finally solidifies into a composite material. The VARTM molding process is shown in Figure5.Figure 5
VARI schematic.As shown in Figure5, fabrics, guide nets, release cloths, and other materials are laid in the order shown, and a vacuum effect is formed in the gap between the vacuum bag and the glass plate by a vacuum pump. The vacuum pressure makes the fabric and the release cloth guide mesh closely fit together, which facilitates the subsequent penetration of epoxy resin.


## 4.1. Preparation of 2D Braided Preforms

Because of the weaving characteristics of two-dimensional weaving, a mandrel needs to be used in the weaving process, so it is necessary to calculate the diameter of the mandrel used for the design of the mandrel according to the width of the yarn. After that, parameters such as weaving pitch and traction speed are determined, and the prefabricated parts are finally woven. The design process is shown in Figure4.Figure 4
Design weaving flowchart.As shown in Figure4, preforms with different parameters can be woven by the braiding machine, and the parameters in the preforms include braiding angle, braiding pitch, coverage factor, and the like.In the experiment, E6D17-1200-380 direct roving produced by Jushi Group Co., Ltd. was used as the braiding yarn, and the linear density was 1200 tex. The width of the yarn is wide, and the yarn width is 2.70 mm when standing still. The yarn of this width is woven on the surface of the mandrel to ensure uniform weaving and a high coverage factor. The shaft yarn is made of A-grade 12K SYT49S carbon fiber produced by Zhongfu Shenying Carbon Fiber Co., Ltd., and the fiber width is 2.00 mm when standing. The equipment adopts the high-speed horizontal 64-spindle two-dimensional weaving machine produced by Jiangsu Xuzhou Qixing Ribbon Machinery Co., Ltd. The instrument is simple to operate, has high weaving efficiency and a high degree of automation, and can weave biaxial and triaxial fabrics of2×2,1×1,and2×1 specifications. The weave of the samples woven in this subject is 2×2.


## 4.2. Core Mold Diameter Design

In the case of weaving the same angle with the same width of yarn, the diameter of the core mold is directly related to the coverage factor of the preform. On the basis that the yarn of the same thickness can be woven, the coverage factor of the preform obtained by using a core mold with a larger diameter is smaller, and vice versa. The size of the coverage factor has a great influence on the mechanical properties of the two-dimensional braided composites. The larger the coverage factor, the tighter the yarn and the higher the strength, and vice versa. The core mold diameters corresponding to different coverage factors are shown in Table1.Table 1
Corresponding mandrel diameters for different coverage factors.
Number of spindles (piece)Weaving yarn width (mm)Braid angle (°)Mandrel diameter (mm) when the coverage factor is 0.9When the coverage factor is 1, the diameter of the core mold (mm)642.70608055642.70556847642.70506343642.70455739642.70405336642.70304732As shown in Table1, the number of yarns in this test is 64, so n=64. In the case where the width of the knitting yarn is not compressed, the width of the knitting yarn is 2.70 mm, so br=2.70. To avoid low coverage factor, the mechanical properties of composite materials are greatly affected, so the coverage factor of prefabricated parts should reach more than 90% (no obvious voids on the surface). When the braided yarn width is 2.70 mm, the coverage factor is 0.9, the number of yarns is 64, the braiding angle is 60°, and the mandrel diameter is 80 mm. When the coverage factor is 1, the mandrel diameter is 55 mm.In the experiment, the core molds are all made of PVC round pipes, which are lighter and have stronger hardness, which is convenient for the installation and disassembly of the core molds. Before the test, sandpaper must be used to polish the surface of the tube and the head and tail to prevent the friction between the tube and the yarn from being too large, causing serious fiber fluff. The PVC round pipe needs to add a fixing clamp at one end to facilitate the mandrel to be fixed on the pulling device. This method helps to give a certain tensile force to the shaft yarn during the weaving process, to ensure that the shaft yarn is parallel to the axial direction, and to prevent the shaft yarn from buckling during the demolding process, which affects the final test result. The parameter table of the two-dimensional braided preform is shown in Table2.Table 2
Parameter table of 2D braided preforms.
NumberingTypes of weaving yarnsNumber of axesMandrel diameter (mm)Braiding pitch/mmAngle (°)Traction speed (mm)Coverage factor (%)1-1Glass fiberDual axis6063114.33.47498.41-2Glass fiberDual axis5563138.64.21394.31-3Glass fiberDual axis5050131.84.00797.92-1Glass fiberThree-axis6063114.33.47499.92-2Glass fiberThree-axis5563138.64.21399.92-3Glass fiberThree-axis5050131.84.00799.93-1Carbon fiberThree-axis6063114.33.47499.93-2Carbon fiberThree-axis5563138.64.21399.93-3Carbon fiberThree-axis5050131.84.00799.9As shown in Table2, all organizational structures in the table are 2×2. The knitting yarn width is 2.70 mm, and the shaft yarn width is 2.00 mm when the coverage factor is calculated, and the knitting speed is 0.191 rad/s. If the weaving speed is too slow, the woven preform will not be formed smoothly; if the weaving speed is too fast, it is easy to cause the yarn to break. After many trials, the weaving speed was chosen to be 0.191 rad/s.


## 4.3. Prefab Parameters

It takes pictures of the prefab head, middle, and tail. The picture was imported into PS for angle measurement, and the three measured angles were averaged. The fabric is flattened according to half of the corresponding diameter, and the thickness is measured at five points of the preform at random with a thickness measuring instrument, and the average value is obtained. A summary of the parameters of the preform obtained by measurement is shown in Table3.Table 3
The actual angle and thickness of each prefab.
NumberingTypes of weaving yarnsNumber of axesMandrel diameter (mm)Braiding pitch (mm)1-16058.80.31.31-25555.81.51.31-35048.92.21.42-16061.01.72.02-25555.20.41.62-35049.60.81.83-16060.20.31.43-25555.61.11.43-35049.80.41.5As shown in Table3, the liquid forming process obtains the composite material by laying the fiber fabric in the mold and pouring the resin. The process is simple to form and has high forming efficiency, but it needs to design corresponding molds. The autoclave forming process uses the prepreg to prepare the composite material through high temperature and high pressure, and the cost of this process is relatively high.In this experiment, the VARTM molding process in the liquid molding process is used; VARTM molding process is one of the liquid molding processes, which has the advantages of low cost, low pollution, and high efficiency, which is widely used in large and complex structures, such as ship hulls, blades, and aviation components. VARTM lays the fiber fabric on a flat plate, then injects resin in a vacuum state, and finally solidifies into a composite material. The VARTM molding process is shown in Figure5.Figure 5
VARI schematic.As shown in Figure5, fabrics, guide nets, release cloths, and other materials are laid in the order shown, and a vacuum effect is formed in the gap between the vacuum bag and the glass plate by a vacuum pump. The vacuum pressure makes the fabric and the release cloth guide mesh closely fit together, which facilitates the subsequent penetration of epoxy resin.


## 5. Two-Dimensional Braided Composite Material Performance Test

With the development of industrialization, the application of two-dimensional braided composite materials is gradually widespread. Considering the subsequent use requirements of the material, it is necessary to judge whether the two-dimensional braided composite material in this topic can meet the use requirements, so it is necessary to carry out a mechanical test on the material. The tensile, bending, and compressive properties of two-dimensional braided composites were mainly tested by different angles, different number of axes, and different fiber blends. The influence of angle, number of axes and different fiber blending on the tensile, bending, and compressive properties of composites can be analyzed and compared.


### 5.1. Influence of Shaft Yarn on Tensile Properties

Two-dimensional triaxial braided composite material compared with the two-dimensional biaxial braided composite material, the two-dimensional triaxial preform is more than the two-dimensional biaxial preform plus a set of shaft yarns composed of carbon fibers. The glassy carbon fiber braided composite material and the carbon fiber braided composite material preform in this paper are all two-dimensional and three-axis, and the three axes are all carbon fibers, and the number of axial yarns is 32 (half of the braided yarns). The addition of carbon fiber axial yarn has two main effects on the tensile strength and elongation at break of the braided composites. The tensile strength of the three kinds of mixed structural composites is shown in Table4.Table 4
Tensile strength of three hybrid structural composites.
NumberingAverage tensile strength (MPa)Dispersion coefficient (%)Average tensile modulus (GPa)Dispersion coefficient (%)Average elongation at break (%)1-1315.86.272.9745.782.731-2166.38.772.4227.5416.751-3159.47.982.3046.9915.092-1431.410.134.7189.446.722-2390.98.855.5827.646.052-3368.99.655.9288.366.613-1576.68.33.7.0587.406.883-2542.39.507.8499.667.013-3526.86.636.8987.746.54It can be seen from Table4 that the tensile strengths of 2-1 to 2-3 and 3-1 to 3-3 are larger than those of glass fibers (1-1 to 1-3) at the same angle. Compared with glass fiber braided composites, glass carbon fiber braided composites add carbon fiber shaft yarns to their preforms. At 30°, the strength of the glassy carbon fiber braided composite increases by 36%; at 40°, it increases by 135%; at 45°, it increases by 131%; at 50°, it increases by 161%; at 55°, it increases by 237%; and it increased by 306% at 60°. The data show that basically with the increase of the braiding angle, the reinforcing effect of carbon fiber axial yarn on the two-dimensional braided composite is more obvious. Due to the presence of carbon fiber shaft yarns, the tensile modulus increases and increases with increasing angle. The tensile displacement-load curves of the three hybrid structures at different angles are shown in Figure 6.Figure 6
Tensile displacement-load curves of three hybrid structures at different angles.
(a)
Glass fiber biaxial(b)
Glass fiber carbon fiber triaxial(c)
Carbon fiber triaxialAs can be seen from Figure6, the carbon fiber shaft yarn has a relatively obvious effect on the tensile elongation at break of the composite material, which will lead to a decrease in the elongation at break. The displacement of the glass fiber woven composite material during tensile fracture is large, and the elongation at break is large. The displacement of the glassy carbon fiber braided composite material during tensile fracture is small, and the elongation at break is small. It can be seen from the data that the presence of carbon fiber shaft yarn reduces the tensile elongation at break of the composite material. According to the weaving characteristics of two-dimensional biaxial preforms and two-dimensional triaxial preforms, the axial yarn improves the stability of the preforms to a certain extent. Different from the 2D biaxial preform, the 2D triaxial preform has better overall stability after demoulding from the core mold and does not have the characteristics of good elasticity after demolding of the 2D biaxial preform, which makes the stability of the composite material formed by the two-dimensional triaxial preform after being compounded with the resin also better. During the stretching process, it is necessary to overcome not only the axial force of the braiding yarn but also the breaking force of all the axial yarns and the stabilizing force of the axial yarn and the braiding yarn after curing, so it finally showed an increase in tensile strength and a decrease in elongation at break.During the whole process from the beginning of the stretching to the tensile fracture, the stress-strain of the glassy carbon fiber woven composite with axial yarn and the carbon fiber woven composite at different angles is basically linear, and it is more inclined to the elastic material. Except for 30°, the displacement load curve of the braided composite material corresponding to other braiding angles is divided into two parts, the first part of the displacement load is basically linear, and the slope is large. The second part is similar to linear with a smaller slope, indicating that the presence of carbon fiber shaft yarns makes the braided composite more elastic in the tensile fracture range. When the tensile test was completed, the displacement-load curve showed a very obvious dip and the load no longer increased, indicating that the carbon fiber shaft yarn can make the tensile fracture of the braided composite material instantaneous.


### 5.2. Influence of Braid Angle on Bending Performance

The flexural properties of three kinds of composites braided with different angles of hybrid structure were tested. The test shows that the bending strength is different at different angles and the bending strength is also different at different structures. Table5 shows the calculation of the bending strength obtained by the bending test of the three kinds of mixed-braided structural braided composites.Table 5
Flexural strength of three hybrid structural composites.
NumberingAverage yield load (N)Average flexural strength (MPa)Dispersion coefficient (%)Average flexural modulus (GPa)Dispersion coefficient (%)1-1555444.010.3213.4999.211-2485388.012.5111.49310.021-3477381.611.9812.3279.982-1423338.49.8811.4118.322-2339271.29.487.7317.692-3258206.411.328.28710.983-1634507.79.3234.8949.583-2538430.97.2 I30.1188.653-3490392.58.6521.2008.39As shown in Table5, when the braiding angle of the glass fiber braided composite material increases from 30° to 60°, the bending strength decreases from 444.0 MPa to 194.4 MPa, with a decrease of about 56%. When the braiding angle of the glassy carbon fiber braided composite material increases from 30° to 60°, the bending strength decreases from 338.4 MPa to 146.4 MPa, with a decrease of about 56%. When the weaving angle of carbon fiber braided composites increased from 30° to 60°, the bending strength decreased from 507.7MPai to 184.5 MPa, with a decrease of about 63%. In the degree of decline, the carbon fiber woven composite material is larger than the glassy carbon fiber woven composite material and the glass fiber woven composite material. In the bending test of the glass fiber woven composite at different angles, the bending displacement load curve of the glass fiber woven composite is shown in Figure 7.Figure 7
Bending displacement load curve of glass fiber braided composites.It can be seen from Figure7 that as the braiding angle increases, the displacement corresponding to the maximum bending load is larger. The displacement corresponding to the maximum bending load at 30°, 40°, and 45° is less than 4 mm; the displacement corresponding to the maximum bending load at 50°, 55°, and 60° is greater than 4 mm.The existence of carbon fiber shaft yarn makes the displacement-load curve in the bending test of the braided composite more linear. The bending displacement-load curves of the three hybrid structures at different angles are shown in Figure8.Figure 8
Bending displacement-load curves of three hybrid structures at different angles.
(a)
Glass fiber composite(b)
Glass fiber carbon fiber composite material(c)
Carbon fiber compositeAs shown in Figure8, the displacement-load curves of the glassy carbon fiber braided composite and the carbon fiber braided composite are more linear than the glass fiber biaxial braided composite due to the presence of carbon fiber axial yarns in their preforms. This is because the addition of carbon fiber shaft yarn makes the braided preform more stable, stabilizing the braiding angle, width, length, and other parameters. Braided composites made from 2D triaxial braided preforms also have better stability. Due to the high tensile strength and modulus of carbon fiber itself, it is determined that the triaxial preform braided composite material is more brittle. The maximum bending strength of the glass fiber woven composite material should be relatively large. The smallest displacement is about 4 mm, and the corresponding displacement when the bending strength of the glassy carbon fiber woven composite material is the largest does not exceed 4 mm at different angles. The displacement corresponding to the maximum bending strength of the carbon fiber woven composite material does not exceed 3 mm at different angles. It shows that the carbon fiber shaft yarn can make the woven composite material with smaller bending elongation at break, make the material more elastic, and strengthen the stability of the woven composite material. At the same time, the carbon fiber shaft yarn makes the load of the braided composite material drop in a cliff-like manner when bending and breaking, and the recovery degree of the samples is different after the test is completed. The glass fiber carbon fiber composite material and the carbon fiber composite material are slightly bent, and the glass fiber composite material is more severely bent, indicating that the carbon fiber shaft yarn affects its elastic properties.


### 5.3. Influence of Shaft Yarn on Compression Properties

The shaft yarn used in this project is 12K SYT49S carbon fiber produced by Zhongfu Shenying Company. Adding a set of axial yarns to the two-dimensional biaxial preform makes the braided preform more stable and also improves the compressive strength of the braided composite material to a certain extent. At the same time, the addition of carbon fiber shaft yarn makes the material more elastic, and as the angle increases, the compressive strength changes less and less. The displacement load curves of the three hybrid structures at different angles are shown in Figure9.Figure 9
Displacement load curves of three hybrid structures at different angles.
(a)
Glass fiber(b)
Glass fiber carbon fiber(c)
Carbon fiberIt can be seen from Figure9 that the displacement line of the position load of the glass-carbon fiber woven composite material with carbon fiber added to the preform is more linear during the compression process, while the glass fiber woven composite material is 30° apart. The load-displacement curve of the braided composite material corresponding to other angles is basically divided into two parts: two parts with high slope and low slope. However, the load-displacement curve of the glassy carbon fiber woven composite material shows the same slope in the whole process, indicating that the existence of carbon fiber shaft yarn makes the material have better elasticity in compression. Due to the presence of carbon fiber, the compressive strength of the braided composite material decreases gradually with the increase of the angle, indicating that the carbon fiber axial yarn bears most of the compressive force. The compressive force borne by the braided yarn is smaller than that of the shaft yarn. Therefore, the compressive strength of glassy carbon fiber woven composites and carbon fiber woven composites decreases with the increase of the braiding angle.


## 5.1. Influence of Shaft Yarn on Tensile Properties

Two-dimensional triaxial braided composite material compared with the two-dimensional biaxial braided composite material, the two-dimensional triaxial preform is more than the two-dimensional biaxial preform plus a set of shaft yarns composed of carbon fibers. The glassy carbon fiber braided composite material and the carbon fiber braided composite material preform in this paper are all two-dimensional and three-axis, and the three axes are all carbon fibers, and the number of axial yarns is 32 (half of the braided yarns). The addition of carbon fiber axial yarn has two main effects on the tensile strength and elongation at break of the braided composites. The tensile strength of the three kinds of mixed structural composites is shown in Table4.Table 4
Tensile strength of three hybrid structural composites.
NumberingAverage tensile strength (MPa)Dispersion coefficient (%)Average tensile modulus (GPa)Dispersion coefficient (%)Average elongation at break (%)1-1315.86.272.9745.782.731-2166.38.772.4227.5416.751-3159.47.982.3046.9915.092-1431.410.134.7189.446.722-2390.98.855.5827.646.052-3368.99.655.9288.366.613-1576.68.33.7.0587.406.883-2542.39.507.8499.667.013-3526.86.636.8987.746.54It can be seen from Table4 that the tensile strengths of 2-1 to 2-3 and 3-1 to 3-3 are larger than those of glass fibers (1-1 to 1-3) at the same angle. Compared with glass fiber braided composites, glass carbon fiber braided composites add carbon fiber shaft yarns to their preforms. At 30°, the strength of the glassy carbon fiber braided composite increases by 36%; at 40°, it increases by 135%; at 45°, it increases by 131%; at 50°, it increases by 161%; at 55°, it increases by 237%; and it increased by 306% at 60°. The data show that basically with the increase of the braiding angle, the reinforcing effect of carbon fiber axial yarn on the two-dimensional braided composite is more obvious. Due to the presence of carbon fiber shaft yarns, the tensile modulus increases and increases with increasing angle. The tensile displacement-load curves of the three hybrid structures at different angles are shown in Figure 6.Figure 6
Tensile displacement-load curves of three hybrid structures at different angles.
(a)
Glass fiber biaxial(b)
Glass fiber carbon fiber triaxial(c)
Carbon fiber triaxialAs can be seen from Figure6, the carbon fiber shaft yarn has a relatively obvious effect on the tensile elongation at break of the composite material, which will lead to a decrease in the elongation at break. The displacement of the glass fiber woven composite material during tensile fracture is large, and the elongation at break is large. The displacement of the glassy carbon fiber braided composite material during tensile fracture is small, and the elongation at break is small. It can be seen from the data that the presence of carbon fiber shaft yarn reduces the tensile elongation at break of the composite material. According to the weaving characteristics of two-dimensional biaxial preforms and two-dimensional triaxial preforms, the axial yarn improves the stability of the preforms to a certain extent. Different from the 2D biaxial preform, the 2D triaxial preform has better overall stability after demoulding from the core mold and does not have the characteristics of good elasticity after demolding of the 2D biaxial preform, which makes the stability of the composite material formed by the two-dimensional triaxial preform after being compounded with the resin also better. During the stretching process, it is necessary to overcome not only the axial force of the braiding yarn but also the breaking force of all the axial yarns and the stabilizing force of the axial yarn and the braiding yarn after curing, so it finally showed an increase in tensile strength and a decrease in elongation at break.During the whole process from the beginning of the stretching to the tensile fracture, the stress-strain of the glassy carbon fiber woven composite with axial yarn and the carbon fiber woven composite at different angles is basically linear, and it is more inclined to the elastic material. Except for 30°, the displacement load curve of the braided composite material corresponding to other braiding angles is divided into two parts, the first part of the displacement load is basically linear, and the slope is large. The second part is similar to linear with a smaller slope, indicating that the presence of carbon fiber shaft yarns makes the braided composite more elastic in the tensile fracture range. When the tensile test was completed, the displacement-load curve showed a very obvious dip and the load no longer increased, indicating that the carbon fiber shaft yarn can make the tensile fracture of the braided composite material instantaneous.


## 5.2. Influence of Braid Angle on Bending Performance

The flexural properties of three kinds of composites braided with different angles of hybrid structure were tested. The test shows that the bending strength is different at different angles and the bending strength is also different at different structures. Table5 shows the calculation of the bending strength obtained by the bending test of the three kinds of mixed-braided structural braided composites.Table 5
Flexural strength of three hybrid structural composites.
NumberingAverage yield load (N)Average flexural strength (MPa)Dispersion coefficient (%)Average flexural modulus (GPa)Dispersion coefficient (%)1-1555444.010.3213.4999.211-2485388.012.5111.49310.021-3477381.611.9812.3279.982-1423338.49.8811.4118.322-2339271.29.487.7317.692-3258206.411.328.28710.983-1634507.79.3234.8949.583-2538430.97.2 I30.1188.653-3490392.58.6521.2008.39As shown in Table5, when the braiding angle of the glass fiber braided composite material increases from 30° to 60°, the bending strength decreases from 444.0 MPa to 194.4 MPa, with a decrease of about 56%. When the braiding angle of the glassy carbon fiber braided composite material increases from 30° to 60°, the bending strength decreases from 338.4 MPa to 146.4 MPa, with a decrease of about 56%. When the weaving angle of carbon fiber braided composites increased from 30° to 60°, the bending strength decreased from 507.7MPai to 184.5 MPa, with a decrease of about 63%. In the degree of decline, the carbon fiber woven composite material is larger than the glassy carbon fiber woven composite material and the glass fiber woven composite material. In the bending test of the glass fiber woven composite at different angles, the bending displacement load curve of the glass fiber woven composite is shown in Figure 7.Figure 7
Bending displacement load curve of glass fiber braided composites.It can be seen from Figure7 that as the braiding angle increases, the displacement corresponding to the maximum bending load is larger. The displacement corresponding to the maximum bending load at 30°, 40°, and 45° is less than 4 mm; the displacement corresponding to the maximum bending load at 50°, 55°, and 60° is greater than 4 mm.The existence of carbon fiber shaft yarn makes the displacement-load curve in the bending test of the braided composite more linear. The bending displacement-load curves of the three hybrid structures at different angles are shown in Figure8.Figure 8
Bending displacement-load curves of three hybrid structures at different angles.
(a)
Glass fiber composite(b)
Glass fiber carbon fiber composite material(c)
Carbon fiber compositeAs shown in Figure8, the displacement-load curves of the glassy carbon fiber braided composite and the carbon fiber braided composite are more linear than the glass fiber biaxial braided composite due to the presence of carbon fiber axial yarns in their preforms. This is because the addition of carbon fiber shaft yarn makes the braided preform more stable, stabilizing the braiding angle, width, length, and other parameters. Braided composites made from 2D triaxial braided preforms also have better stability. Due to the high tensile strength and modulus of carbon fiber itself, it is determined that the triaxial preform braided composite material is more brittle. The maximum bending strength of the glass fiber woven composite material should be relatively large. The smallest displacement is about 4 mm, and the corresponding displacement when the bending strength of the glassy carbon fiber woven composite material is the largest does not exceed 4 mm at different angles. The displacement corresponding to the maximum bending strength of the carbon fiber woven composite material does not exceed 3 mm at different angles. It shows that the carbon fiber shaft yarn can make the woven composite material with smaller bending elongation at break, make the material more elastic, and strengthen the stability of the woven composite material. At the same time, the carbon fiber shaft yarn makes the load of the braided composite material drop in a cliff-like manner when bending and breaking, and the recovery degree of the samples is different after the test is completed. The glass fiber carbon fiber composite material and the carbon fiber composite material are slightly bent, and the glass fiber composite material is more severely bent, indicating that the carbon fiber shaft yarn affects its elastic properties.


## 5.3. Influence of Shaft Yarn on Compression Properties

The shaft yarn used in this project is 12K SYT49S carbon fiber produced by Zhongfu Shenying Company. Adding a set of axial yarns to the two-dimensional biaxial preform makes the braided preform more stable and also improves the compressive strength of the braided composite material to a certain extent. At the same time, the addition of carbon fiber shaft yarn makes the material more elastic, and as the angle increases, the compressive strength changes less and less. The displacement load curves of the three hybrid structures at different angles are shown in Figure9.Figure 9
Displacement load curves of three hybrid structures at different angles.
(a)
Glass fiber(b)
Glass fiber carbon fiber(c)
Carbon fiberIt can be seen from Figure9 that the displacement line of the position load of the glass-carbon fiber woven composite material with carbon fiber added to the preform is more linear during the compression process, while the glass fiber woven composite material is 30° apart. The load-displacement curve of the braided composite material corresponding to other angles is basically divided into two parts: two parts with high slope and low slope. However, the load-displacement curve of the glassy carbon fiber woven composite material shows the same slope in the whole process, indicating that the existence of carbon fiber shaft yarn makes the material have better elasticity in compression. Due to the presence of carbon fiber, the compressive strength of the braided composite material decreases gradually with the increase of the angle, indicating that the carbon fiber axial yarn bears most of the compressive force. The compressive force borne by the braided yarn is smaller than that of the shaft yarn. Therefore, the compressive strength of glassy carbon fiber woven composites and carbon fiber woven composites decreases with the increase of the braiding angle.


## 6. Conclusion

In this paper, two-dimensional weaving design weaving and mechanical property testing of composite materials are the main research objects, and three kinds of prefabricated hybrid structures are woven by glass fiber and carbon fiber, respectively: glass fiber two-dimensional biaxial preform, glass-carbon fiber braided two-dimensional triaxial prefab (glass fiber is braided yarn, carbon fiber is shaft yarn), and carbon fiber braided preform, by measuring the width of glass fiber and carbon fiber, starting from the design of the core mold, and then determining the parameters required for weaving the preform, so as to weave the two-dimensional biaxial and two-dimensional triaxial preforms. The composite material was prepared by VARI process of preform and epoxy resin, and the composite material was tested in tension, bending, and compression. The tensile, flexural, and compressive properties of braided composites with three different structures were tested. It provides some theoretical basis for studying the factors affecting the mechanical properties of two-dimensional braided composites and has a certain theoretical value for simulation research. Due to the limitation of time and cost, the mechanical properties tested in this subject are far from enough, and there is a lack of research on the influence of multiple factors on the braided composite materials, which needs to be further studied in the future.



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*Source: 6358247-2022-07-04.xml*