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The control mechanisms of the strength and toughness of carbon nanotube (CNT) fibers are revealed by analyzing the load-bearing and deformation characteristics of multi-scale structures in the fiber under tensile loading. A theoretical model is established to investigate the effect of the multi-scale structures on the strength and toughness of CNT fibers. Based on our previous experimental results on tension with in situ micro-Raman monitoring [Li et al., Nanotechnology 22, 2011], the macro- and micro-mechanical behaviors of the fiber are analyzed. The tensile behaviors of the fiber are correlated with the load-bearing and deformation processes involved in the multi-scale structures in the fiber, such as the nanotube bundle and the thread in microscopic scale, and the CNT in nanoscale. The CNT fiber exhibits high strength and toughness simultaneously depending on the multi-scale structure of the material, the differences in the properties between bundles and threads, and the unique interfaces formed by the tabular geometric configuration of double-walled CNTs. A constitutive relationship for CNT fiber materials is developed to provide information on the role of multi-scale structures on the strength and toughness of fibers. Both strength and toughness of CNT fibers can be enhanced by increasing the volume ratio of bundles to threads, the interfacial shear strength, and the interface slippage friction resistive force among the CNTs. |
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