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LDRD final report : mesoscale modeling of dynamic loading of heterogeneous materials

Robbins, Joshua R.; Dingreville, Remi P.; Voth, Thomas E.; Furnish, Michael D.

Material response to dynamic loading is often dominated by microstructure (grain structure, porosity, inclusions, defects). An example critically important to Sandia's mission is dynamic strength of polycrystalline metals where heterogeneities lead to localization of deformation and loss of shear strength. Microstructural effects are of broad importance to the scientific community and several institutions within DoD and DOE; however, current models rely on inaccurate assumptions about mechanisms at the sub-continuum or mesoscale. Consequently, there is a critical need for accurate and robust methods for modeling heterogeneous material response at this lower length scale. This report summarizes work performed as part of an LDRD effort (FY11 to FY13; project number 151364) to meet these needs.

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TYPE SAND Report YEAR 2013
OSTIDOI

LDRD Final Report: Topology Optimization for Nonlinear Transient Applications Using a Minimally Invasive Approach

Robbins, Joshua R.

The purpose of this project was to devise, implement, and demonstrate a method that can use Sandias existing analysis codes (e.g., Sierra, Alegra, the CTH hydro code) with minimal modification to generate objective function gradients for optimization-based design in tran- sient, non-linear, coupled-physics applications. The approach uses a Moving Least Squares representation of the geometry to substantially reduce the number of geometric degrees of freedom. A Multiple-Program Multiple-Data computing model is then used to compute ob- jective gradients via finite differencing. Details of the formulation and implementation are provided, and example applications are presented that show effectiveness and scalability of the approach.

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TYPE SAND Report YEAR 2018
OSTIDOI

Microstructural modeling of ferroic switching and phase transitions in PZT

Proceedings of SPIE - The International Society for Optical Engineering

Robbins, Joshua R.; Khraishi, Tariq A.; Chaplya, Pavel

Niobium doped Lead Zirconate Titanate (PZT) with a Zr/Ti ratio of 95/5 (i.e., PZT 95/5-2Nb) is a ferroelectric with a rhombohedral structure at room temperature. A crystal (or a subdomain within a crystal) exhibits a spontaneous polarization in any one of eight crystallographically equivalent directions. Such a material becomes polarized when subjected to a large electric field. When the electric field is removed, a remanent polarization remains and a bound charge is stored. A displacive phase transition from a rhombohedral ferroelectric phase to an orthorhombic anti-ferroelectric phase can be induced with the application of a mechanical load. When this occurs, the material becomes depoled and the bound charge is released. The polycrystalline character of PZT 95/5-2Nb leads to highly non-uniform fields at the grain scale. These local fields lead to very complex material behavior during mechanical depoling that has important implications to device design and performance. This paper presents a microstructurally based numerical model that describes the 3D non-linear behavior of ferroelectric ceramics. The model resolves the structure of polycrystals directly in the topology of the problem domain and uses the extended finite element method (X-FEM) to solve the governing equations of electromechanics. The material response is computed from anisotropic single crystal constants and the volume fractions of the various polarization variants (i.e., three variants for rhombohedral anti-ferroelectric and eight for rhomobohedral ferroelectric ceramic). Evolution of the variant volume fractions is governed by the minimization of internally stored energy and accounts for ferroelectric and ferroelastic domain switching and phase transitions in response to the applied loads. The developed model is used to examine hydrostatic depoling in PZT 95/5-2Nb.

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TYPE Conference YEAR 2007
ScopusOSTI

Modeling dislocations in a polycrystal using the generalized finite element method

Robbins, Joshua R.

Modeling the interaction of dislocations with internal boundaries and free surfaces is essential to understanding the effect of material microstructure on dislocation motion. However, discrete dislocation dynamics methods rely on infinite domain solutions of dislocation fields which makes modeling of heterogeneous materials difficult. A finite domain dislocation dynamics capability is under development that resolves both the dislocation array and polycrystalline structure in a compatible manner so that free surfaces and material interfaces are easily treated. In this approach the polycrystalline structure is accommodated using the GFEM, and the displacement due to the dislocation array is added to the displacement approximation. Shown in figure 1 are representative results from simulations of randomly placed and oriented dislocation sources in a cubic nickel polycrystal. Each grain has a randomly assigned (unique) material basis, and available glide planes are chosen accordingly. The change in basis between neighboring grains has an important effect on the motion of dislocations since the resolved shear on available glide planes can change dramatically. Dislocation transmission through high angle grain boundaries is assumed to occur by absorption into the boundary and subsequent nucleation in the neighboring grain. Such behavior is illustrated in figure 1d. Nucleation from the vertically oriented source in the bottom right grain is due to local stresses from dislocation pile-up in the neighboring grain. In this talk, the method and implementation is presented as well as some representative results from large scale (i.e., massively parallel) simulations of dislocation motion in cubic nano-domain nickel alloy. Particular attention will be paid to the effect of grain size on polycrystalline strength.

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TYPE Conference YEAR 2010
OSTI

Multimode Metastructures: Novel Hybrid 3D Lattice Topologies

Boyce, Brad B.; Garland, Anthony G.; White, Benjamin C.; Jared, Bradley H.; Conway, Kaitlynn C.; Adstedt, Katerina A.; Dingreville, Remi P.; Robbins, Joshua R.; Walsh, Timothy W.; Alvis, Timothy A.; Branch, Brittany A.; Kaehr, Bryan J.; Kunka, Cody; Leathe, Nicholas L.

With the rapid proliferation of additive manufacturing and 3D printing technologies, architected cellular solids including truss-like 3D lattice topologies offer the opportunity to program the effective material response through topological design at the mesoscale. The present report summarizes several of the key findings from a 3-year Laboratory Directed Research and Development Program. The program set out to explore novel lattice topologies that can be designed to control, redirect, or dissipate energy from one or multiple insult environments relevant to Sandia missions, including crush, shock/impact, vibration, thermal, etc. In the first 4 sections, we document four novel lattice topologies stemming from this study: coulombic lattices, multi-morphology lattices, interpenetrating lattices, and pore-modified gyroid cellular solids, each with unique properties that had not been achieved by existing cellular/lattice metamaterials. The fifth section explores how unintentional lattice imperfections stemming from the manufacturing process, primarily sur face roughness in the case of laser powder bed fusion, serve to cause stochastic response but that in some cases such as elastic response the stochastic behavior is homogenized through the adoption of lattices. In the sixth section we explore a novel neural network screening process that allows such stocastic variability to be predicted. In the last three sections, we explore considerations of computational design of lattices. Specifically, in section 7 using a novel generative optimization scheme to design novel pareto-optimal lattices for multi-objective environments. In section 8, we use computational design to optimize a metallic lattice structure to absorb impact energy for a 1000 ft/s impact. And in section 9, we develop a modified micromorphic continuum model to solve wave propagation problems in lattices efficiently.

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TYPE SAND Report YEAR 2021
OSTIDOI
Results 26–50 of 69
Results 26–50 of 69