Molecular Dynamic Study On The Crack Propagation In Graphene Under Tear Loading

Graphene has broad potential application in various fields because of its excellent physical properties.With the development of graphene fabrication and processing technology,its application becomes feasible.However,in an actual production process,the graphene sheet cannot avoid the defects that affect its mechanical properties.The defects may develop into cracks under the load and further affect the performance of micro-/nano-devices.Understanding the crack propagation path under a load is an effective way to avoid the fatal failure of graphene.On the other hand,it is necessary to guarantee the accuracy of graphene sheet size when processing the graphene-based devices with specific functions.In preparing a graphene ribbon,the mechanical tearing approach is a fast and simple method.Therefore,it is necessary to study the fracture process of a graphene sheet under a tearing load.To deal with this problem,several models of graphene sheets with initial cracks were established,and molecular dynamics method was used to simulate the crack propagation in a graphene sheet.In the simulation,a fixed region and a moving region for loading were set on both sides of the initial crack.A constant displacement load is provided on the moving region for tearing the sheet.The main results of the study are as follows:First,a model of graphene sheet with an initial crack was constructed,and the crack propagation under an in-plane tearing load was studied.The simulation results show that the loading speed and the length ratio of the fixed region to the moving region have no influence on the crack propagating direction,mainly along the zigzag direction;but have a small influence on the total displacement of the moving region when the tearing is completed;However,when the loading speed is too high,the moving region may break and escape from the graphene sheet.The total displacement as completing tearing is lower when the length ratio increases.Considering the effect of the length ratio,the total displacement of the model with the crack at the armchair edge(the armchair model)reduces up to 53%,and the total displacement of the model with the initial crack at the zigzag edge(the zigzag model)reduces up to 66%;The length ratio influences the crack propagation path in the armchair model.In general,the crack propagates at the side near the shorter one of the fixed region and the moving region in an armchair model,while the crack propagation path in a zigzag model always propagates along the straight line from the initial crack.The temperature influences obviously the crack propagation path in an armchair model but slightly in a zigzag model.Tearing an armchair model at high temperature will generate carbon chains,and the crack boundary is not smooth.Second,when the moving direction of the moving region is perpendicular to the graphene sheet(out-of-plane tearing),the crack propagation was studied with consideration of the essential factors,such as temperature,loading speed and the length ratio.Results show that the temperature has no effect on the crack propagation path of graphene under the out-of-plane tearing.The total displacement of the moving region in an armchair model decreases up to 10% when increasing the length ratio,or up to 4% for a zigzag model.The effect of loading speed on the out-of-plane tearing is the same as that of the in-plane tearing.Note that when the loading speed is greater than 0.8 ?/ps(80 m/s),the moving region in a zigzag model breaks from the graphene sheet before finishing tearing.In addition,it is found that the graphene sheet has a critical crack length,over which the tearing mode is independent of the crack growth rate.Finally,results show that the crack propagation direction is always along the zigzag direction under either an in-plane or an out-of-plane tearing.It is also independent of the initial crack direction and the loading speed.However,the total displacement of the moving region in a graphene sheet under an in-plane tearing is smaller than that under an out-of-plane tearing.Therefore,the tearing efficiency is higher under an in-plane tearing,but the crack boundaries are smooth under an out-of-plane tearing.The results of this thesis clarify the crack propagation path and the tearing efficiency of a graphene sheet under the different tearing modes.It is significant for fabricating a graphene ribbon by the mechanical tearing method.