Dynamic Strengthening of Carbon Nanotube Fibers under Extreme Mechanical Impulses Wanting Xie, †,‡ Runyang Zhang, § Robert J. Headrick, ∥,⊥ Lauren W. Taylor, ⊥ Steven Kooi, # Matteo Pasquali, ∥,⊥ Sinan Mü ftü , § and Jae-Hwang Lee* ,† † Department of Mechanical and Industrial Engineering and ‡ Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003, United States § Department of Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts 02139, United States ∥ Department of Chemistry and ⊥ Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, United States # Institute for Soldier Nanotechnologies, MIT, Cambridge, Massachusetts 02139, United States * S Supporting Information ABSTRACT: A monofilament fiber spun from individual carbon nanotubes is an arbitrarily long ensemble of weakly interacting, aligned, discrete nanoparticles. Despite the structural resemblance of carbon nanotube monofilament fibers to crystalline polymeric fibers, very little is known about their dynamic collective mechanics, which arise from van der Waals interactions among the individual carbon nanotubes. Using ultrafast stroboscopic microscopy, we study the collective dynamics of carbon nanotube fibers and compare them directly with nylon, Kevlar, and aluminum monofilament fibers under the same supersonic impact conditions. The in situ dynamics and kinetic parameters of the fibers show that the kinetic energy absorption characteristics of the carbon nanotube fibers surpass all other fibers. This study provides insight into the strain-rate-dependent strengthening mechanics of an ensemble of nanomaterials for the development of high-performance fibers used in body armor and other protective nanomaterials possessing exceptional stability in various harsh environments. KEYWORDS: Nanomaterial armor, microballistics, high-strain-rate hardening, collective friction, impact delocalization N atural fibers 1−3 are the oldest example of material technology; over the past century, synthetic fibers 4 have accounted for most of the development in high-performance materials (e.g., aramids and carbon fibers), where polymeric fibers have been implemented in a multitude of applications. 4,5 In the meantime, because of their combination of high molecular strength, elastic modulus, and low density, 6−8 carbon nanotubes (CNTs) have been considered the most promising building blocks for the next-generation of high- performance fibers that could enable applications as extreme as space elevators. 9 Progress has been made toward translating CNT properties to the macroscale via several continuous fabrication methods, including the solution spinning 10 and direct spinning 11 of continuous CNT fibers (CNTF). CNTF mechanical properties are currently limited by the length of their constituent CNTs; when macroscopic fibers fail, it is due to the applied stress exceeding the frictional coupling of the individual CNTs rather than the intrinsic breaking strength of individual CNTs. 12,13 In macroscopic CNTFs, quasi-static mechanical characterization reveals an elastic modulus (70− 350 GPa) and tensile strength (0.23−9.0 GPa) well below the modulus (∼1 TPa) and strength (∼50 GPa) of individual CNTs. 14,15 Strain-rate dependence of mechanical properties of CNTFs were reported under low-speed tensile loading conditions, 16,17 where the applied loading speeds were negligible to the mechanical wave propagation speeds of CNTFs. However, under extremely fast deformation, combined with the radial collapse of CNTs enhancing interfacial friction 18,19 and the intertwined morphology of CNT assemblies within CNTF, interfacial interactions among CNTs can be substantially amplified. Thus, the ultrahigh-strain-rate (USR) mechanical performance of CNTFs may be substantially different from the characteristics predicted by the quasi-static characteristics of CNTFs. For the real-time USR characterization of CNTF, it requires microscopic mechanical excitation and precise quantification of the ultrafast deformation of specimens. As seen in the limitation of typical USR characterizations, 20−22 the Received: January 24, 2019 Revised: April 30, 2019 Published: May 14, 2019 Letter pubs.acs.org/NanoLett Cite This: Nano Lett. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.nanolett.9b00350 Nano Lett. XXXX, XXX, XXX−XXX Nano Lett. Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 05/18/19. For personal use only.