Search NASA⌕ Search

Engineering topics

Tonks, Michael R

Publications and source records attributed to Tonks, Michael R.

9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Development of a Grain Boundary Pinning Model that Considers Particle Size Distribution Using the Phase Field Method

Grain boundary (GB) migration significantly impacts material behavior. However, GB migration is slowed or even halted by resistive pressure applied by pores or particles. Zener’s original investigation of particle pinning, and subsequent modifications by other researchers, describe the resistive pressure for various spatial distributions of particles with respect to GBs. In this work, we develop a pinning model that considers the impact of the particle size distribution and we verify it by comparing to mesoscale phase field and Monte Carlo simulations. Resistive pressure expressions are developed that are functions of the percentage of GB area covered by particles and of the particle volume fraction for any spatial distribution of particles. In both expressions, the mean value of the resistive pressure decreases with increasing standard deviation of the particle radius.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2014

Molecular dynamics simulations of intergranular fracture in UO2 with nine empirical interatomic potentials

The intergranular fracture behavior of UO2 was studied using molecular dynamics simulations with a bicrystal model. The anisotropic fracture behavior due to the different grain boundary characters was investigated with the View the MathML source symmetrical tilt S5 and the View the MathML source symmetrical tilt S3 ({1 1 1} twin) grain boundaries. Nine interatomic potentials, seven rigid-ion plus two core–shell ones, were utilized to elucidate possible potential dependence. Initiating from a notch, crack propagation along grain boundaries was observed for most potentials. The S3 boundary was found to be more prone to fracture than the S5 one, indicated by a lower energy release rate associated with the former. However, some potential dependence was identified on the existence of transient plastic deformation at crack tips, and the results were discussed regarding the relevant material properties including the excess energies of metastable phases and the critical energy release rate for intergranular fracture. In general, local plasticity at crack tips was observed in fracture simulations with potentials that predict low excess energies for metastable phases and high critical energy release rates for intergranular fracture.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2014

NEAMS FPL M2 Milestone Report: Development of a UO₂ Grain Size Model using Multicale Modeling and Simulation

This report summarizes development work funded by the Nuclear Energy Advanced Modeling Simulation program's Fuels Product Line (FPL) to develop a mechanistic model for the average grain size in UO₂ fuel. The model is developed using a multiscale modeling and simulation approach involving atomistic simulations, as well as mesoscale simulations using INL's MARMOT code.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2014

Demonstrating the Temperature Gradient Impact on Grain Growth in UO2 Using the Phase Field Method

Grain boundaries (GBs) are driven to migrate up a temperature gradient. In this work, we use a phase field (PF) model to investigate the impact of temperature gradients on normal grain growth. GB motion in 2D UO2 polycrystals is predicted under increasing temperature gradients. We find that the temperature gradient does not significantly impact the average grain growth behavior, because the curvature driving force is dominant. However, it does cause significant local migration of the individual grains. In addition, the change in the GB mobility due to the temperature gradient results in larger grains in the hot portion of the polycrystal.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2013

Intergranular fracture in UO2: derivation of traction-separation law from atomistic simulations

In this study, the intergranular fracture behavior of UO2 was studied by molecular dynamics simulations using the Basak potential. In addition, the constitutive traction-separation law was derived from atomistic data using the cohesive-zone model. In the simulations a bicrystal model with the (100) symmetric tilt E5 grain boundaries was utilized. Uniaxial tension along the grain boundary normal was applied to simulate Mode-I fracture. The fracture was observed to propagate along the grain boundary by micro-pore nucleation and coalescence, giving an overall intergranular fracture behavior. Phase transformations from the Fluorite to the Rutile and Scrutinyite phases were identified at the propagating crack tips. These new phases are metastable and they transformed back to the Fluorite phase at the wake of crack tips as the local stress concentration was relieved by complete cracking. Such transient behavior observed at atomistic scale was found to substantially increase the energy release rate for fracture. Insertion of Xe gas into the initial notch showed minor effect on the overall fracture behavior.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2013

A Quantitative Comparison between C0 and C1 elements for Solving the Cahn-Hilliard Equation

The Cahn–Hilliard (CH) equation is a time-dependent fourth-order partial differential equation (PDE). When solving the CH equation via the finite element method (FEM), the domain is discretized by C1-continuous basis functions or the equation is split into a pair of second-order PDEs, and discretized via C0-continuous basis functions. In the current work, a quantitative comparison between C1 Hermite and C0 Lagrange elements is carried out using a continuous Galerkin FEM formulation. The different discretizations are evaluated using the method of manufactured solutions solved with Newton’s method and Jacobian-Free Newton Krylov. It is found that the use of linear Lagrange elements provides the fastest computation time for a given number of elements, while the use of cubic Hermite elements provides the lowest error. The results offer a set of benchmarks to consider when choosing basis functions to solve the CH equation. In addition, an example of microstructure evolution demonstrates the different types of elements for a traditional phase-field model.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2013

Guidance to Design Grain Boundary Mobility Experiments with Molecular Dynamics and Phase-Field Modeling

Quantitative phase-field modeling can play an important role in designing experiments to measure the grain boundary (GB) mobility. In this work, molecular dynamics (MD) simulation is employed to determine the GB mobility using Cu bicrystals. Two grain configurations are considered: a shrinking circular grain and a half loop grain. The results obtained from the half loop configuration approaches asymptotically to that obtained from the circular configuration with increasing half loop width. We then verify the phase- field model by directly comparing to the MD simulation results, obtaining excellent agreement. Next, this phase-field model is used to predict the behavior in a common experimental setup that utilizes a half loop grain configuration in a bicrystal to measure the GB mobility. With a 3D simulation, we identify the two critical times within the experiments to reach an accurate value of the GB mobility. We use a series of 2D simulations to investigate the impact of the notch angle on these two critical times and we identify an angle of 60? as an optimal value. We also show that if the notch does not have a sharp tip, it may immobilize the GB migration indefinitely.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗