Temperature and rate-dependent plastic deformation mechanism of carbon nanotube fiber: Experiments and modeling

https://doi.org/10.1016/j.jmps.2023.105241Get rights and content

Highlights

  • High-temperature dynamic mechanical behaviors were investigated by a modified Hopkinson bar.

  • Microstructure evolution of CNT fibers was observed by the in-situ SEM tensile equipment.

  • The time-temperature equivalency principle of tensile strength of CNT fibers was discussed.

  • The mechanical behaviors of CNT fibers at different temperatures were described by a mathematical shear-lag model.

  • An effective strategy was proposed to improve the thermal-mechanical properties of CNT fibers.

Abstract

High-performance carbon nanotube (CNT) fibers inevitably encounter complicated thermal environments while used in aerospace structures and intelligent actuators. Catastrophic damage is unavoidably induced due to the loss of their mechanical properties. Herein, a modified Hopkinson bar and an in-situ SEM tensile equipment were introduced to study the high-temperature time-dependent mechanical behaviors and microstructure evolution of individual CNT fibers. It was found that the interaction between CNTs was weakened by thermal expansion and defects were introduced into CNTs by thermal oxidation. In addition, the effects of temperature on the viscoplastic strain transition and durability of CNT fibers were studied through a series of relaxation experiments. A shear-lag unit cell and a three-level mathematical model were developed for describing the hierarchical structure evolution of CNT fibers. These models clarified the mechanism of strength loss of CNT fibers at high temperatures and characterized the effect of thermal expansion on their microstructure and mechanical properties. The overlap length and the distance between tubes as a function of temperature were discussed. According to the modeling, an effective strategy was proposed to improve the mechanical properties of CNT fibers at high temperatures by restraining thermal expansion and thermal oxidation. This work would provide crucial theoretical and experimental guidance for the improvement of the thermal dynamic stability of CNT fibers and their composites in extremely high-temperature operating conditions.

Introduction

Advanced carbon-based fibers, such as carbon nanotube (CNT) fiber, and graphene fiber, have received more attention in aircraft and satellite constituents owing to their excellent specific strength, stiffness, fatigue, corrosion resistance, etc. (Iqbal et al., 2021). CNT fibers, as a type of carbon nanotube macro-assemblies, inherited the excellent nanomechanical properties of individual CNT (Chen et al., 2013; Li et al., 2019; Zhilyaeva et al., 2019). High-performance CNT fibers have the advantages of transferring load, heat, and electricity, which are some of the best candidates for lightweight advanced fibers (Bai et al., 2018; Barbalinardo et al., 2021; Behabtu et al., 2013; Cheng et al., 2019; Lee et al., 2022; Yuan et al., 2019; Zhu et al., 2020). However, aeronautical structures often suffer bird strikes, dust erosion, UV exposure, etc. (Chen et al., 2021; Cheng et al., 2021; Pan et al., 2022). Especially, when the aircraft is flying in a dense atmosphere, the air friction will cause temperature increment. Meanwhile, supercoiled CNT fibers possess large torsional and tensile actuation strokes, and provide reversible contractile actuation with high energy densities and power densities (Chu et al., 2021; Deng et al., 2017; Leng et al., 2021; Mu et al., 2019). Accordingly, CNT fibers have attracted great interest in intelligent actuators and artificial muscles. The response of intelligent actuators can be triggered by environmental vibrations, such as humidity, light, electricity, and temperature (Jang et al., 2020; Jayathilaka et al., 2019; Zhang et al., 2022). Among them, thermal driving is a very typical and common driving method (Dong et al., 2021; Kim et al., 2018). Thus, the impact resistance and bearing capability of advanced fiber composites under longstanding loading and transient dynamic loading at high temperatures are of vital importance for application in aerospace and actuators.

It is generally known that the time-dependent behavior of materials is crucial for the design and application of materials in impact protection, load-durability, and so on (Lea and Jardine, 2018; Naraghi et al., 2014; Zhang et al., 2020; Li et al., 2023). Previous studies have been conducted on the mechanical properties of CNT fibers under quasi-static loading and dynamic loading (Bai et al., 2018; Liu et al., 2020a; Zhang et al., 2012; Zhang and Xu, 2022). As a one-dimensional material on a micron scale, the dynamic properties of CNT fibers under a high strain rate have been mainly tested by a miniaturized Hopkinson bar (Lim et al., 2010; Wang et al., 2016; Wu et al., 2012), and the tensile strength of CNT fibers increases and the failure mode gradually transforms from the inter-tube slippage to cascade-like breaking as the strain-rate increases (Hu et al., 2021b; Wang et al., 2018; Xu et al., 2016). The interface between CNTs has been proved to be dynamically strengthened, which contributes to the good impact resistance of CNT fibers (Gao et al., 2022; Hu et al., 2021a; Wang et al., 2018). Furthermore, time-dependent discontinuous loading experiments were conducted to reveal the microstructure evolution behavior of CNT fibers (Liu and Yang, 2015; Sun et al., 2013b; Xue et al., 2021). The CNT fibers exhibit prominent twisting-dependent mechanical behaviors during the stretching and relaxation process, where the twisted structure evolved and the internal CNTs rearranged. The previous research also indicated that the intertube cohesion of twisted fiber restricts the slippage of CNTs, while the misalignment of CNTs enhances the viscous properties and improves the stress relaxation of CNT fibers.

Heretofore, the above research was mainly conducted at room temperature. As is known to all, the temperature has a great influence on the mechanical properties of materials and the loss of mechanical properties at high temperatures is unavoidable (Chen et al., 2022; Liu et al., 2020b; Zhang et al., 2021; Su et al., 2020). Accordingly, the investigation of the thermomechanical property of materials plays an important role in the design and application of materials. A novel in-situ thermomechanical testing stage has been designed to test single crystal Silicon (Elhebeary and Saif, 2018; Kang and Saif, 2013). Zhang et al. measured the quasi-static mechanical properties of CNT fibers at extreme temperatures in anaerobic conditions (Zhang et al., 2019). Because of the temperature dependence of the carbon bond strength and intertube interaction, the tensile strength and Young's modulus of the CNT fibers decreased with the increase in temperatures. Moreover, in addition to the effect of high temperature on the internal microstructure of CNT fibers, thermal oxidation may cause damage and structural changes in the CNTs in the atmosphere (Trakakis et al., 2020), which also affects the mechanical properties of CNT fibers. These researches on the temperature-dependent mechanical behaviors of CNT fibers focused on the continuous quasi-static tensile tests, but the temperature effects on the long-term stability and short-term impact have not been systematically investigated.

The nonlinear stress-strain relationship of CNT fibers is attributed to the multi-scale structures (Galuppi et al., 2019; Zhao et al., 2014). Considering the slipping between carbon nanotubes, the mathematical model of CNT fibers has been established to describe the constitutive relationship of CNT fibers on the macro scale at room temperature (Li et al., 2012). The structural characteristics of CNT fibers were obtained and the effects of multi-scale structural parameters on the strength and toughness of fibers have been characterized (Sun et al., 2013a). At the same time, the details of mechanical behaviors of the CNT fibers during deformation, for example straightening and slippage were explained (Park et al., 2019). The geometric parameters and interface properties of each level in the hierarchical structure material are essential to its mechanical properties (Gao et al., 2003; Ji and Gao, 2010). However, the direct relationship between the mechanical properties and geometric characteristics of each hierarchy at different temperatures and establishing a temperature-dependent mathematical model are essential and need to be further elaborated.

In this work, a series of time-dependent experiments were performed to uncover the high-temperature mechanical performance of CNT fibers. The in-situ scanning electron microscope (SEM) method was conducted to investigate the microstructure evolution of CNT fibers. A modified Hopkinson tension bar was designed to investigate the dynamic mechanical behavior of individual CNT fiber under high temperatures. Mathematical modeling was derived to describe and predict the temperature-dependent mechanical behaviors of CNT fibers. The main interests of the study were focused on revealing the temperature mechanism of the microstructures evolution and the failure behavior of CNT fibers, and thus propose an effective strategy to design the CNT fiber with better thermal-mechanical stability.

Section snippets

Preparation of CNT fibers

CNT fibers (TNF 400) were produced by Time Nano Co. Ltd (Chengdu, China) through a floating catalyst chemical vapor deposition (CVD) method, which were termed as T-CNT fibers. And the other CNT fibers (JCCFB5) were produced by Best Materials Co. Ltd (Chengdu, China) through the CVD method either, which were termed as J-CNT fibers. The T-CNT fibers were ductile, of which the fracture strain was about 10%−20%. While the J-CNT fibers were brittle but stronger. The longer CNT fiber was cut into

Quasi-static stretching behavior

To study the influence of temperature on mechanical performance, the quasi-static (10−3 s  1) stress-strain curves of T-CNT fibers and J-CNT fibers at 298 K, 473 K, and 673 K were obtained, respectively. Before the stretching, samples and the device were kept at the target temperature for 20 min. The typical tensile results were presented in Fig. 3a and Fig. 3b. All curves include the following three stages: linear elastic-like stage, hardening stage, and failure stage. For strain-hardening

Temperature effects on strength of CNT fibers

CNT fibers have a complex hierarchical microstructure. As shown in Fig. 5a, the hierarchical organizations of structural components consisted of the CNT level, the bundle level, and the fiber level from the atomic scale to the macroscale (Gao et al., 2018). At the primary level, assume that the tensile strength of a perfect CNT was σ0, and the average strength of that with defects was σI. In the secondary level, the tensile strength of a CNT bundle that consisted of CNTs was σII. These CNTs

Hierarchical fiber structure

The plastic deformation mechanism of CNT fiber under different temperatures and strain rates was clearly illustrated in previous analyses and discussions. However, how to model the mechanical behavior of this hierarchical fiber under different temperatures is still a big challenge. As shown in Fig. 5a, a three-level hierarchical structure of CNT fiber was proposed to understand the mechanical behaviors of CNT fibers. The length of the nanotube and bundle inside the fiber is much shorter than

Thermal-mechanical stability

Previous mathematical models have shown that the distance h between CNTs was closely related to the mechanical properties of CNT fibers. It was reasonably estimated that densification could be an effective way to optimize the performance of CNT fibers. Meantime, the thermal oxidation and thermal expansion resulted in a significant reduction in mechanical properties of CNT fibers, which deteriorated the properties of the fiber. Therefore, limiting the thermal expansion and weakening the thermal

Conclusions

In this work, we systematically investigated the long-term and short-term mechanical behaviors of CNT fibers by designing experimental tests and mathematic modeling under different temperatures and loading rates. The experimental results of the tensile loading and stress relaxation showed that high temperatures not only weakened the tensile strength of the fiber, but also reduced the load transfer efficiency, strain-rate enhancement effect, and long-term bearing durability of the fiber.

CRediT authorship contribution statement

Deya Wang: Conceptualization, Investigation, Methodology, Visualization, Software, Writing – original draft. Pengfei Wang: Conceptualization, Investigation, Supervision, Validation, Writing – review & editing. Yangfan Wu: Software, Formal analysis, Validation. Lehu Bu: Software, Formal analysis. Jie Tian: Resources. Mao Liu: Resources. Gengzhi Sun: Supervision, Writing – review & editing. Lin Mei: Methodology. Songlin Xu: Conceptualization, Supervision, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No.: 11872361), and China Fundamental Research Funds for the Central Universities (Grant No.: WK 2480000008).

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