Ever since the exfoliation of free standing two-dimensional (2D) material, these materials have been under the microscope. The main reason for this is because they exhibit some unique and attractive properties which are useful for applications, in composites, sensors, energy storage devices and electronics. Research on two dimensional (2D) materials, such as Graphene and Molybdenum disulfide (MoS2), now involves researchers worldwide, implementing cutting edge technology to study them. However, when considering using 2D materials in such promising applications, one of the major concerns is the mechanical failure, which remains heavily underexplored.
In this work, we demonstrate the use of sequential and concurrent multiscale modeling to study the effect of various fracture modes on Graphene and Molybdenum disulfide (MoS2). Some of the fracture modes explored are pure tensile loading, shear loading, a combination of tensile and shear loading and cyclic loading to investigate fatigue. It is an established fact that multiscale modeling is an effective way to study materials over a realistic length and time scale. Atomistic simulation methodologies based on interatomic potential is used to obtain the mechanical and thermal properties of nanoscale materials. Based on the properties such as Young’s modulus, Poison’s ratio, heat conductivity and heat capacity at atomic scale the Finite Element equations of thermoelasticity are formulated. Then we present the concurrent multiscale theory. The challenge that arises in this phase is the formulation of interfacial conditions between the atomic region and the continuum region. In our method we divide the entire region into two sub-regions. For regions where critical physical phenomena, such as crack initiation and propagation occur, we adopt Molecular Dynamics (MD) with small time step to simulate the material behavior with relative high resolution. For non-critical regions we adopt finite element (FE) method with large time steps to reduce computational effort.
This approach enables one to study the crack extension, propagation, and branching under different loading conditions, without using any fracture criteria. We have explored three different loading modes including mode I tensile loading, mixed mode loading (combination of mode I and mode II shear loading) and cyclic loading. We were able verify the brittle nature of Graphene and analyze the partial ductile nature of MoS2. Some of the applicable fracture mechanics concepts are also introduced for comparison purpose. We were also able to successfully show the adverse effect fatigue loading has and further compare the nature of fracture in these materials. We believe that the results obtained will be very useful when considering 2D materials for large scale applications. This multiscale model is not restricted to Graphene and MoS2 and is a powerful tool to be used for studying 2D materials.
|Advisor:||Lee, James D|
|Commitee:||Keidar, Michael, Leng, Yongsheng, Solares, Santiago, Wang, Leyu|
|School:||The George Washington University|
|Department:||Mechanical & Aerospace Engineering|
|School Location:||United States -- District of Columbia|
|Source:||DAI-B 81/10(E), Dissertation Abstracts International|
|Subjects:||Materials science, Nanoscience, Molecular physics|
|Keywords:||2D material, Concurrent multiscale Modeling, Continuum mechanics, Fracture analysis, Molecular dynamics, Sequential multiscale modeling|
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