Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel.
Sci Adv. 2016 Feb 5;2(2):e1500969. doi: 10.1126/sciadv.1500969. eCollection 2016 Feb.
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 m(0.5), typical of moderately brittle materials and applicable also to graphene.
晶体结构的缺陷通常被认为会降低碳纳米管的理想强度。然而，这种缺陷所引发的断裂机制，以及固体力学理论在纳米尺度上的有效性，仍存在争议。我们表明，单壁纳米管（SWNTs）的断裂韧性符合经典的断裂力学理论，即使对于最小的可能空位缺陷（约 2 Å）也是如此。通过模拟含有常见类型缺陷的 SWNTs 的拉伸，我们展示了缺陷边界处的应力集中如何导致在相对较低的应变下脆性（不稳定）断裂，从而使 SWNTs 的理想强度降低多达 60%。我们发现，由于 SWNT 的桁架状结构，这种尺度上的缺陷不是尖锐的，而且应力集中是有限的且较低的。此外，作为宏观尺度上的几何性质的应力集中与 SWNT 的断裂韧性（一种材料性质）有关。由此得到的 SWNT 断裂韧性为 2.7 MPa m(0.5)，这是典型的中等脆性材料，也适用于石墨烯。
