Temperature and Strain-Rate Dependent Plastic Deformation of Carbon Nanotube
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TEMPERATURE AND STRAIN-RATE DEPENDENT PLASTIC DEFORMATION OF CARBON NANOTUBE Chengyu Wei, Kyeongjae Cho, Department of Mechanical Engineering, Stanford University, California; Deepak Srivastava, NASA Ames Research Center, MST27A-1, Moffett Field, California ABSTRACT In this work we use classical molecular dynamics to study strain rate and temperature dependent plasticity of carbon nanotube (CNT) under compressive strain. We focus on two types of defects: sp3 bond formation and bond rotation. Our simulation shows that thermal fluctuations help the strained CNT to overcome the local energy barrier to obtain plastic deformation. The yielding strain of a compressed CNT found to be strain-rate and temperature dependent, and low strain rate limit of the yielding strain is estimated to be less that 6%. INTRODUCTION Carbon nanotubes (CNTs) are formed by wrapping a graphite plane around certain directions. Experimental and theoretical studies find that CNTs have unusual strength [1,2] because of the strong sp2 bonds leading to Young’s modulus of 1TPa or higher. There are many studies on possible ways to use CNTs in nano-scale devices or as nanofibers in composite materials [3-6]. For the applications it is important to understand how CNT respond to external mechanical loads and the mechanisms of formation of possible defects leading to plastic deformations. It is thus essential to understand the intrinsic mechanical properties of CNT at atomistic level. Previous atomistic simulations on this subject were generally conducted at zero temperature or without considering strain rate effect, while under real experimental conditions these factors can play important role. The subject we are interested in this work is the plastic deformation of CNTs at finite temperatures, and specifically the strain-rate and temperature dependant behavior of yielding strain of CNTs. SIMULATIONS All the simulations in this work are conducted with Tersoff-Brenner interaction potential [7,8] for carbon atoms and with 0.5 fs time step in MD simulation. 1. Plastic deformation of CNT under compressive strain at finite temperatures Previous simulations using classical molecular dynamics (MD) [9] and tight-binding (TB) [10] simulations have confirmed the high strength of CNTs. MD simulations found CNTs to be super-elastic up to strain as large as 15%, while in TB simulations CNTs collapse at 12 % strain with sp3 diamond-like bond formed at some sections of the nanotube. Both studies were conducted at zero temperature. In this work, we study how temperature dependent fluctuations can help CNTs to overcome possible energy barriers AA6.5.1
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