Edge Crack Sensitivity In A Stretched H-BN Strip

Graphene,hexagonal boron nitride(h-BN)and other two-dimensional(2D)crystals possess excellent mechanical properties,which have attracted the attention of scholars from various countries.Defects are inevitably introduced in 2D crystals during preparation,and the fracture behavior has a great influence on the application of 2D crystals.Therefore,it is important to predict the failure of h-BN by modeling and analyzing crack propagation in h-BN.Stress field near the crack tip dominates the crack behavior,and atomic defects,grain boundaries or dislocations near the crack tips strongly affect the stress distribution.MD method is used in this work with the modified Tersoff potential.First,we establish a verification model of h-BN,and calculate the mechanical parameters such as Young's modulus by LAMMPS(an open-source software).This method is verified to be reliable as our simulated results are comparable with the experimental ones.Then,a series of strips models of h-BN monolayer with an initial edge crack are established by changing the crack size.Tensile loads are applied to the upper and lower sides of the strip model.The critical crack lengths for crack initiation are obtained under different conditions.Furthermore,the remote stress?_remote at crack initiation for various original crack length is examined,which show that Griffith criterion overestimates?_remote for short cracks<2.5nm while underestimates that for longer cracks.Thus we propose a modified exponent factor 0.28 for Griffith criterion rather than 0.5.The asymptotic stress field around nano-crack is calculated by MD atomic virial stress theory,comparing with continuum theory of stress intensity factor K and stress concentration factor,which indicts that elastic continuum overestimates local stresses around the crack front as the bond breaking stress remains?54.4GPa at the tip.The critical energy release rate G_c at initiation for small crack<1.5nm are also much higher than theoretical toughness G_Ic.Our results provide a clear understanding of the nano-crack fracture in a 2D strip for the continuum limitation in discrete hexagonal lattices,and provides us a clear understanding of the nano edge crack insensitivity in a stretched h-BN strip at the atomic scale further for the Griffith criterion limitation.