Defects in crystalline structure are commonly believed to degrade the ideal strength of carbon nanotubes. However, the fracture mechanisms induced by such defects, as well as the validity of solid mechanics theories at the nanoscale, are still under debate. We show that the fracture toughness of single-walled nanotubes (SWNTs) conforms to the classic theory of fracture mechanics, even for the smallest possible vacancy defect (~2 Å). By simulating tension of SWNTs containing common types of defects, we demonstrate how stress concentration at the defect boundary leads to brittle (unstable) fracturing at a relatively low strain, degrading the ideal strength of SWNTs by up to 60%. We find that, owing to the SWNT’s truss-like structure, defects at this scale are not sharp and stress concentrations are finite and low. Moreover, stress concentration, a geometric property at the macroscale, is interrelated with the SWNT fracture toughness, a material property. The resulting SWNT fracture toughness is 2.7 MPa m0.5, typical of moderately brittle materials and applicable also to graphene.
展开▼
机译:通常认为晶体结构的缺陷会降低碳纳米管的理想强度。然而,由此类缺陷引起的断裂机制以及固体力学理论在纳米尺度上的有效性仍在争论中。我们表明,即使对于最小的空位缺陷(〜2Å),单壁纳米管(SWNT)的断裂韧性也符合经典的断裂力学理论。通过模拟包含常见缺陷类型的单壁碳纳米管的张力,我们证明了缺陷边界处的应力集中如何在相对较低的应变下导致脆性(不稳定)断裂,从而使单壁碳纳米管的理想强度降低了60%。我们发现,由于SWNT的桁架状结构,这种规模的缺陷并不明显,应力集中有限且较低。此外,应力集中是宏观尺度上的几何特性,它与材料特性SWNT断裂韧性相关。所得的SWNT断裂韧性为2.7 MPa m 0.5 sup>,是中等脆性材料的典型特征,也适用于石墨烯。
展开▼