Strain-Sensing Characteristics of Carbon Nanotube Yarns Embedded in Three-Dimensional Braided Composites under Cyclic Loading

Carbon nanotube yarns are embedded in three-dimensional (3D) braided composites with five-axis yarns, which are used as strain sensors to monitor the damage of 3D braided composites. In the cyclic mechanical loading experiment, the strain-sensing characteristics of 3D braided composites were studied by in situ measuring the resistance change of the embedded carbon nanotube yarn.-e 3D five-directional braided composite prefabricated part based on carbon nanotube yarns was developed, and the progressive damage accumulation experiments were carried out on carbon nanotube yarns and specimens embedded in carbon nanotube yarns. -e research results show that there is a good correlation between the change of relative resistance of the carbon nanotube yarn and the strain of the composite specimen during cyclic loading and unloading. When the tensile degree of the specimen increases beyond a certain range, the carbon nanotube yarn sensor embedded in the specimen shows resistance hysteresis and produces residual resistance. -erefore, the fiber can better monitor the progressive damage accumulation of 3D five-direction braided composites.


Introduction
As an advanced structural load-bearing material, three-dimensional (3D) braided composite materials, due to their excellent properties such as nonlayered structure, good overall performance, one-time braiding, and flexible design of special-shaped parts, have a growing share in engineering applications that pay great attention to strength and light characteristics, especially in the field of aerospace [1]. Although the strength of composite materials is superior, invisible damage such as fatigue and even impact may accumulate and cannot be identified due to external load during use. e monitoring ability of composite materials has become the biggest weakness restricting their development. erefore, it is very important to monitor the local damage and development status of the composite structure in real time for its safe application [2,3]. So, in situ structural health monitoring (SHM) of 3D braided composites is a very important research topic.
In recent years, researchers have proposed a variety of composite monitoring and sensing technologies. However, some of these technologies, such as ultrasonic technology, cannot be applied in the mechanical operation of composite materials, while other technologies, such as active piezoelectric sensors, optical fiber sensors, acoustic emission sensors, and other embedded sensing technologies, can monitor the composite structure in real time [4][5][6]. For the manufacturing methods, identifiability and damage types of composite materials, each different monitoring technology, have their own advantages and disadvantages.
Literature research [7,8] provides us with a simple monitoring technology based on resistance change, that is, the monitoring method based on the piezoresistive effect. In these studies, piezoresistive effect monitoring technology was successfully applied to conductive carbon fiber-reinforced polymer. e conductivity of the carbon fiber can be used to monitor the damage indexes in the polymer, such as fiber fracture, delamination, and matrix cracking, which may be related to the damage mechanism in composites. Muto et al. [9] studied the application of the piezoresistive effect monitoring method in nonconductive composites. e research showed that the mechanical load exerted on the composite can be monitored by inserting carbon fiber into glass fiber-reinforced polymer during the manufacturing process and by the electrical response of the carbon fiber. However, due to different elastic moduli between the two materials and the limitation of carbon fiber conductivity, in fact, the carbon fiber sensor does not monitor the progressive cumulative damage of the composite, but only predicts the final fracture of the composite. erefore, it is necessary to find a kind of conductive fiber with strong conductive ability, which can monitor the damage of low tensile axial strain through the change of resistance, so as to realize the damage monitoring of the conductive fiber for nonconductive composites.
Carbon nanotube (CNT) has a unique seamless tube structure. In 1991, Iijima [10] discovered CNT for the first time, which caused extensive research. CNT is regarded as the best new material for real-time structural health monitoring because of its lightweight, good mechanical properties, high conductivity, and easy to build the sensor network [11][12][13]. Compared with traditional strain sensors, CNT sensors can overcome many limitations of existing conventional sensors, such as limited monitoring space and small weight. erefore, the additional impact of CNTs in the monitoring process is relatively small, which has the potential to embed in situ monitoring in the composite structure. CNT fiber is composed of single CNT, and then CNT fiber tape is twisted together to make carbon nanotube yarns. Carbon nanotube yarns have an orderly fiber stable structure, so they have good repeatability and stable resistance strain behavior [14]. Previous studies have also shown that [15][16][17] the embedded CNT fiber sensor can be used to monitor the generation and expansion of damage in composite structures in real time. erefore, the composite material samples of the carbon nanotube yarn sensor are prepared by a 3D four-step braiding process.
rough cyclic loading and unloading experiments, the strain-sensing performance of the carbon nanotube yarn sensor for composite materials was analyzed. By observing the change of the resistance of the embedded carbon nanotube yarns with the tensile loading of the composites, the occurrence and progressive damage accumulation of the composites are monitored, which provides the basis for the application of 3D braided composite in situ sensors based on carbon nanotube yarns.

Research on the Embedding Method of the Carbon Nanotube Yarn Sensor
Carbon nanotube yarns were embedded into 3D braided composites by the 3D five-direction four-step braiding process. e 3D braiding steps are shown in Figure 1 [18].
Taking 3D four-step 1 × 1 square braiding as an example, the main yarn carriers are arranged around the main body with the number of rows m and columns n, expressed as ab.
e yarn carrier moves through four steps to realize one braiding cycle, which is called four-step method.
Step 1: the yarn carrier moves along the adjacent row direction for one braiding motion; that is, the center distance between two adjacent yarn carriers is listed; Step 2: the yarn carrier moves along the adjacent column direction for one braiding motion, that is, the center distance between two adjacent yarn carriers; Step 3: the motion direction of the yarn carrier in Step 1 is changed to the opposite direction; Step 4: the motion direction of the yarn carrier in Step 2 is changed to the opposite direction. After a cycle of braiding, after the above four steps, the arrangement of yarn carriers returns to the original state, as shown in Figure 2.
Carbon nanotube yarn is braided with the carbon fiber in the form of an axial yarn. When the prefabricated part is braided, a carbon nanotube yarn sensor is braided into every 5 carbon fibers. e spatial structure of the composite prefabricated part is shown in Figure 3.

Design of the Strain-Sensing Experimental
System for the Carbon Nanotube Yarn Sensor

Strain-Sensing Mechanism of the Carbon Nanotube Yarn.
When analyzing the piezoresistive effect of the carbon nanotube yarn sensor, firstly, based on Ohm's law, In the formula, R 0 is the original resistance of the yarn; ρ is the resistivity of the yarn; L is the length of the yarn; A is the cross-sectional area of the yarn. e change rate of resistance is In the formula, ΔR is the change of resistance of the yarn; Δρ is the change of yarn resistivity; ΔL is the change of yarn length; ΔA is the change of yarn cross-sectional area. e change in yarn length can be expressed as In the formula, ε 11 is the strain coefficient. e latter part of formula (3) can be neglected when the strain is small. e sensor cross section can be expressed as In the formula, ϑ 12 is Poisson's ratio of carbon nanotube yarns; therefore, It can be seen from equation (6) that the resistance change of the carbon nanotube yarn sensor is caused by the change of strain and resistivity. Because of the symmetry between the resistance change rate and the load, the resistance change rate is linear in the load range [19].

Strain-Sensing Experimental System of the Carbon Nanotube Yarn.
e experimental system consists of an AG-250KNE Shimadzu universal material experimental machine (produced by Shimadzu company of Japan), 50 mm extensometer, resistance strain gauge, 10 V DC power supply, analog-to-digital converter, computer, and other relevant connecting equipment. It is used to measure tension and displacement data, longitudinal strain data, in situ resistance of the specimen during mechanical loading and data acquisition and processing. e strain-sensing experimental system based on the carbon nanotube yarn sensor is shown in Figure 4. e tensile speed is 2 mm/min. e experiments in this paper are carried out at room temperature.

Verification of the Strain-Sensing Mechanism of the Carbon Nanotube Yarn.
e relationship between strain and resistance change rate of the carbon nanotube yarn under tension is analyzed to verify the feasibility of the above strain-sensing technology theory.
Under monotonic tension, the relationship between the strain of the carbon nanotube yarn and the change rate of relative resistance is shown in Figure 5. It can be seen from the figure that there is a good linear relationship between strain and resistance change rate in the elastic range, and the experimental results are consistent with the theory of strain sensor technology. Detailed analysis is carried out in the results and discussion.  carbon nanotube array dry spinning method, belonging to the multiwall carbon nanotube fiber) developed by Suzhou Hengqiu Graphene Technology Co., Ltd. e main technical parameters of carbon nanotube yarns are shown in Table 1. e experimental specimens were prepared by a 180 × 120 rectangular braiding machine (developed by Tiangong University) in the Institute of Composite Materials, Tiangong University. e 3D five-direction braiding process and vacuum-assisted resin transfer molding curing process (VARTM) were used. e specification of the 3D braided composite specimen is 250 mm × 25 mm. Wipe both ends of the carbon nanotube yarns in the specimen and apply silver glue. e two ends of the carbon nanotube yarns in the specimen are connected with copper wires, as shown in Figure 6. e schematic diagram of the tensile test of the experimental specimen is shown in Figure 7; that is, longitudinal tensile stress is applied at both ends of the specimen and connected to the strain-sensing experimental system to test its strain-sensing characteristics.

Strain-Sensing Characteristics of the Carbon Nanotube Yarn under Monotonic Tension.
e monotonic tensile test of the carbon nanotube yarn was carried out to analyze the relationship between stress and strain, strain, and resistance change rate of the carbon nanotube yarn.
Under monotonic tension, the typical stress-strain curve of the carbon nanotube yarn under monotonic tension is shown in Figure 8. It can be seen from the figure that the stress and strain of the carbon nanotube yarn have a good linear correlation in the strain range of 1%. When the strain exceeds about 1.9%, the linear relationship between stress and strain becomes smaller and smaller; that is, the slope decreases obviously. Under monotonic tension, the stress and strain of the carbon nanotube yarn show obvious twostage behavior. In the first stage, the strain increases linearly with the stress, which is mainly due to the tensile deformation, untwisting, and diameter reduction of the carbon nanotube yarn under certain tensile stress. In the second stage, the strain decreases obviously with the change of stress, mainly because when the tensile stress continues to increase, the further torsion of the carbon nanotube yarn leads to the separation or splitting in the carbon nanotube layer in the yarn, until the possible local damage or fracture.
is yield behavior can also be attributed to the weak van der Waals force interaction and the slip phenomenon of carbon nanotubes, which is consistent with the existing research results [20,21].
Under monotonic tension, the typical curve of strain and relative resistance change rate of the carbon nanotube yarn is shown in Figure 9. It can be seen from the figure that the strain of the carbon nanotube yarn has a good linear relationship with the resistance change rate. Contrary to the mechanical behavior, even if the second stage yield behavior leads to a significant reduction of strain with stress, the linear correlation between resistance change rate and strain is still good; that is, the resistance change rate and strain of the selected carbon nanotube yarn have a highly linear behavior under large strain. e linear relationship between carbon nanotube yarn resistance and strain can be attributed to the relative slip deformation of the carbon nanotube in the tensile state.
is relative slip reduces the contact between carbon nanotube bundles, increases the strain, and increases the yarn resistance until the carbon nanotube bundles are completely separated from each other.
In conclusion, it is feasible to apply the theory of strainsensing technology to carbon nanotube yarn strain sensing. At the same time, the resistance change rate of the carbon nanotube yarn maintains a linear correlation with the strain under large strain, so it can be embedded in 3D braided composites to monitor the progressive damage and accumulation of fabrics.

Strain-Sensing Characteristics of the Carbon Nanotube
Yarn under Cyclic Loading. Progressive damage accumulation experiment was carried out on carbon nanotube yarns; that is, 10 progressive tensile cycles were loaded until fracture. e typical real-time resistance and mechanical loading curves of the carbon nanotube yarn sensor are shown in Figure 10.
It can be seen from Figure 10 that, with the gradual increase of the loading stress of the progressive tensile cycle, the initial resistance of the carbon nanotube yarn sensor also increases; that is, the resistance lag occurs with the continuous loading time. At the same time, it is also found that the resistance of the carbon nanotube yarn sensor always shows a consistent correlation with the bearing stress, which indicates that it has good repeatability, stable resistance strain behavior, and reliable strain-sensing performance. In addition, at the end of each progressive tensile cycle, the initial resistance and length of the yarn were measured under free loading, and the relationship between the resistance change of the carbon nanotube yarn sensor and the yarn length was plotted. e results are shown in Figure 11.
It can be seen from Figure 11 that there is a good linear correlation between the initial resistance of the carbon nanotube yarn sensor, that is, the change of relative resistance and the instantaneous length of the yarn. e reason is that, with the gradual increase of stress and strain, the permanent deformation of the yarn is caused, resulting in the linear growth behavior of the two. e delay of resistance change during the first load may be related to the accuracy of the measuring instrument or the degree of pretensile, and the sensitivity of the sensor needs to be further studied. On the contrary, the mechanical and electrical properties of the yarn always show linear resistance strain behavior before breaking. e resistance variation of carbon nanotube yarns and the instantaneous length of the yarn show a linear relationship independent of the load level, so the strain can be sensed by the resistance variation.      Discrete Dynamics in Nature and Society In conclusion, the strain-sensing characteristics of the carbon nanotube yarn show good consistency under monotonic tension and cyclic loading, especially when the mechanical behavior shows a certain softening stage; that is, the slope of the stress-strain curve decreases obviously. On the contrary, the resistance change rate and strain of the carbon nanotube yarn can maintain a highly linear behavior under large strain. erefore, the carbon nanotube yarn can be used to monitor the damage of its embedded braid based on the mechanism of strain-sensing technology.

Strain-Sensing Characteristics of Composite Specimens under Cyclic
Loading. Progressive damage accumulation experiments were carried out on 3D five-direction braided composite specimens, that is, 10 progressive cyclic loading and unloading until fracture. e electromechanical properties of composites under a wide range of mechanical loading are analyzed.
Under cyclic loading and unloading, the stress-strain curve of the 3D five-direction braided composite specimen is shown in Figure 12. It can be seen from the figure that the axial mechanical strain and stress of the 3D braided composite specimen show good monotonic consistency. at is, they increase monotonically with the increase of mechanical loading and decrease monotonically with unloading. At the same time, it is also found that when the tensile stress increases to a certain extent, the specimen does not return to absolute zero strain under zero mechanical load. at is, the residual strain is produced, which indicates that the progressive cyclic loading and unloading cause the residual damage of the specimen. As shown in the figure, the strain of the specimen seems to recover to about 0.1% after the fourth cycle and about 0.15% after the ninth cycle. Residual damage is caused by matrix cracks or fiber separation in the fabric due to the continuous increase of tensile stress.
Under cyclic loading and unloading, the curve between resistance change rate and specimen strain of the embedded 3D five-direction braided composite carbon nanotube yarn sensor is shown in Figure 13. It can be seen from the figure that the resistance change rate of the carbon nanotube yarn sensor shows good monotonic consistency with the strain. At the same time, it is also found that, with the continuous increase of cyclic loading and unloading stress, the electrical properties of the specimen show characteristics similar to the mechanical properties. For example, after the strain exceeds about 1.0% in the fourth cycle (about 35% of the fracture stress), the residual resistance is observed after unloading, and the change value of resistance will not return to zero. With the continuous increase of bearing stress, the residual resistance of the carbon nanotube yarn sensor also increases accordingly. At the same time, when the strain exceeds about 1%, the residual resistance is generated after the specific point. With the increase of stress in progressive cyclic loading and unloading, the resistance gradually increases in an approximately linear manner. In the experiment, all embedded sensors show similar characteristics.
is specific point represents the strain threshold of the material and is actually the damage monitoring threshold of the specimen. Previous studies have shown that when the 3D braided composites exceed this strain threshold, matrix cracks or fiber separation may occur [22].
In conclusion, when the internal damage of the 3D braided composite specimen occurs, the resistance of the Discrete Dynamics in Nature and Society embedded carbon nanotube yarn sensor will change, but with the continuous increase of load until fracture, the resistance change rate and strain of the carbon nanotube yarn sensor still have a good linear relationship. At the same time, the residual resistance can well detect the occurrence and accumulation of induced damage in composites. erefore, carbon nanotube yarn can be used to monitor the progressive cumulative damage of 3D braided composites.

Conclusion
e experimental results of progressive damage accumulation of the carbon nanotube yarn show that the resistance and stress of the carbon nanotube yarn show good monotonic consistency and show a linear increase behavior with the change of yarn length. In contrast to the mechanical properties, the resistance change rate of the carbon nanotube yarn is linearly proportional to the instantaneous length of the yarn and independent of the load. erefore, based on the piezoresistive effect, the structural strain can be monitored by the resistance change of the carbon nanotube yarn. e experimental results of progressive damage accumulation of 3D braided composites show that when the mechanical load of progressive tensile cycle exceeds about 35% of the fracture stress of the composites, the carbon nanotube yarn sensor shows a large resistance measurement value and forms a hysteresis loop, so it can better monitor the strain of 3D braided composites and realize better monitoring of progressive damage accumulation in 3D braided composites. erefore, the results of this study provide useful guidance for the application of carbon nanotube yarns embedded in 3D braided composite in situ sensors.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.   Discrete Dynamics in Nature and Society