Size Effect Of Mechanical Strength Of Carbon Nanotubes

Carbon nanotubes have a wide range of potential applications in the fields of civil engineering due to that they can substantially improve the mechanical strength and heat preservation properties of building materials.These applications all benefit from their excellent mechanical properties.Fully understanding the physical mechanism of these excellent mechanical properties could play an important role for the practical application of carbon nanotubes.Since the experimental study of nano-scale materials are very difficult to carry out,the molecular dynamics method,which can describe the mechanical properties of carbon nanotubes accurately and can better control the uncertain factors,and take into account the influences of the simulate a variety of factors such as temperature,defect concentration,diameter and loading condition on the mechanical properties of carbon nanotubes,is now considered as an ideal tool to study the nanomaterials.Based on the molecular dynamics simulation,we studied the size effect of mechanical strength of carbon nanotubes under the axial loading.The effects of temperature,defect and diameter on the size effect are also investigated comprehensively.The stress-strain curves of pristine carbon nanotubes under the axial tension were analyzed by using molecular dynamics simulation.The influence of temperature on the fracture strain of carbon nanotubes was systematically studied.The results show that the higher the temperature is,the smaller fracture strain of the carbon nanotubes is,and the more easily it is broken.The statistics of the fracture strain of pristine carbon nanotubes with fixed tube length were analyzed and the distribution was fitted very well by the Weibull statistical strength theory.Later,the relationship between the fracture strain and the length of the carbon nanotubes was obtained by the molecular dynamics simulations and the size effect of the pristine carbon nanotubes at finite temperatures was discovered.It is thought that the size effect of mechanical strength of material generally comes from defects existing in the materials.However,the size effect that found in the pristine carbon nanotubes is a result of the random thermal motion of the atoms.Therefore,it is a "dynamic disorder" mechanism that leads to the new strength size effect.The size effect of tensile strength of defective carbon nanotubes has been studied systematically in this paper.By using molecular dynamics method,the stress-strain curves of the carbon atoms with random defects were obtained by tensile-fracture simulation.It was found that the fracture strain of the defect carbon tube was significantly smaller than that of the pristine carbon tube.This paper then performed the numerical simulation of the fracture strain of the carbon nanotubes with fixed length,and the statistical distribution of the fracture strain is consistent with the prediction of the Weibull statistic theory.This paper further simulated the fracture strain of defective carbon nanotube with different lengths and the length-dependence of the carbon nanotube strength on the tube length is also fitted very well by Weibull theory,showing that the Weibull theory is applicable to discuss the mechanical strength of carbon nanotube.This paper also simulated the effect of defect concentration and temperature on the size effect,and found the overall effect is very weak.The simulation of tensile strength of defect carbon nanotubes with different diameters found that the decrease of the fracture strain became weaker with the increase of the diameter.The result of the present paper deepens our understanding of the size effect of the mechanical properties of carbon nanotubes.It proves that the Weibull statistical size effect theory applies also to the carbon nanotube as it applies to macro-scale system.A new size effect mechanism of "dynamic disorder" of nanomaterials is discovered.The fully understanding of the underlying physical mechanism of these size effect of the carbon nanotubes has important effect on their application in the field of civil engineering,and the dynamical disorder mechanism of size effect may be very helpful to understand the stability properties of those civil engineering structures under dynamical loading.