DEVELOPMENT OF A SELF-CONSISTENT COUPLED ATOMISTIC-CONTINUUM MODEL TO STUDY THE BRITTLE AND DUCTILE FRACTURE IN METALLIC MATERIALS
Johns Hopkins University
Modeling fracture and failure of material is a complex phenomenon that needs atomic-scale understanding of the kinetics and energetics of different deformation mechanisms. Several efforts has been made over the years to model the fracture at continuum scale e.g, cohesive zone model, phase-field model. The success of these continuum scale fracture models rely on the appropriate incorporation of the interaction between the crack and the different deformation mechanisms within the material such as interatomic decohesion, dislocation nucleation, mobility of the dislocations, dislocation reaction, twining etc. Hence, there is a need to develop a systematic framework to quantify these interactions and develop physics-based constitutive laws that can be used in continuum scale fracture models. This dissertation develops a concurrent coupled atomistic-continuum model to capture the interaction between different deformation mechanisms on the propagation of crack. The atomistic region is modeled using time-accelerated Molecular Dynamics(MD) and for the continuum region, the density-based Crystal Plasticity Finite Element(CPFE) model is used. Hyperdynamics is used for the time acceleration of the MD. The atomistic-continuum coupling is achieved by enforcing geometric compatibility and force equilibrium in the interface region. A sequence of steps is performed to characterize and quantify the dislocations at the interface and then transfer those dislocations from the atomistic to the continuum region in the density form. The propagation of the dislocations in the density form is modeled by solving the transport equation of a conserved quantity, also known as the advection equation. The mesh-less Reduced Kernel Particle Method(RKPM) is used to solve the advection equation over the continuum domain. The developed concurrent coupled atomistic-continuum model is used to study the brittle and ductile propagation of a crack in a nickel single crystal. A parametrized form of crack propagation law and the evolution of dislocation density is extracted from the model. The concurrent model has also been used to construct the free energy functional of the phase-field model where the evolution of different energy contributions during the fracture process is obtained. These evolution laws can be employed in full continuum scale models to study the fracture process at a larger spatial scale.
Molecular Dynamics, Crystal Plasticity, Concurrent Model, Phase Field, Hyperdynamics, Accelerated Molecular Dynamics