Residency Abstracts

Shock experiments were conducted on additively manufactured 316L stainless steel samples with 500-micron pores to investigate the collapse of the large pore and its effects on the shock wave propagation.

The project conducted at Los Alamos National Laboratory (LANL) will entail resonant ultrasound spectroscopy (RUS) experiments of additively manufactured (AM) 316 stainless steel (SS) hybrid-AM samples and conventional AM samples. Hybrid-AM manufacturing involves the use of secondary manufacturing processes and energy sources to alter specified layers within a build. These supplemental processes and energy sources act synergistically with the traditional as build parameters and alter microstructure, reduce the local average grain size, increase dislocation density, and impart or relieve residual stresses. Typically, the changes in mechanical properties are not confined within a single layer but rather have a compounding effect on the preceding layers. The ability to control material properties in localized regions of a build has significant part performance advantages, but the introduced changes present unique challenges in nondestructive evaluation (NDE). The project conducted at LANL is an attempt to resolve and understand the nondestructive inspection of these hybrid-AM components using RUS.

The project involved examining the propagation of shock waves through various structures. Hopkinson bar testing was performed to achieve (relatively) mid-range strain rates in lattices, some bistable and some with varied geometry. High speed video with digital image correlation data was taken of these tests, allowing the examination of the propagation of the shock through the lattices. At the same time, modeling and simulation was performed of previously run high strain rate shock propagation in 3D printed structures.

When materials are shocked and tear apart due to spall failure, the individual pieces from the original part have unique material characteristics due to the high stresses experienced during spall failure and may cause more or less damage then their unshocked counterparts. Therefore, developing an accurate model of the damage from an exploded material requires a deep understanding of the fragments' material characteristics as well as the material characteristics of the original part. I investigated the changes in material characteristics of martensitic steel that was shocked and experienced catastrophic failure to support the development of a more accurate computer model of martensitic steel's failure and range of effect.

My first residency was through Los Alamos National Laboratories. During this residency, I learned to apply the LANL ASC Multiphysics FLAG code, an arbitrary Lagrangian-Eulerian (ALE) code with MHD and advanced material models, to my research on electrothermal instability physics. This multi-dimensional, multi-material, multi-temperature code has been used to study fundamental ETI physics, and continues to evolve to address more topics in the HED regime. Direct comparisons of FLAG calculations to A/lambda and ED target designs are well-suited to address the role of dominant, localized Joule heating and ETI in plasma formation in high-current conductors. I studied magneto Rayleigh-Taylor (MRT) evolution and relevant physics using the FLAG code, and was able to both qualitatively and quantitatively compare these simulations to experimental and theoretical data. These comparisons showed excellent agreement.

For residency II, I was able to construct an operational magneto-PL setup and subsequently rebuilt the magneto-THz setup in the lab. Currently, very few groups in the world have simultaneous magneto-PL and magneto-THz setups in the same lab. Upon the successful construction of the two setups, I hope to utilize residency III to continue spectroscopic studies on nanomaterials.

My residency project dealt with shock propagation in structures comprised of interesting microstructural geometries. Over the course of the project, I worked with my mentors to conduct analysis of phase contrast imaging (PCI) experimental data taken at the Dynamic Compression Sector (DCS). The work consisted of data from several experiments, including an examination of spall in various types of magnesium, shock propagation in foams of varying composition and density, and shock propagation in patterned additively manufactured microstructures. Work was also begun on simulating shocks in the additively manufactured geometries. In addition to the computational work, I was able to observe shock data collection in gas gun experiments, and also took part in preparations and sample building for further experiments to be run at DCS in the near future. The project allowed me the opportunity to learn a great deal about shock physics, gain familiarity with shock physics experiments, learn and compare several different data analysis techniques, and prepare to take part in further experimental work.

A systematic study was undertaken employing phase field dislocation dynamics (PFDD), a powerful physics-informed meso-scale model, to investigate the influences of material properties and void obstacle geometries on nanovoid strengthening in fcc metals. Of particular interest was the impact of dislocation dissociation, a common occurrence in fcc metals, on the strengthening trends as well as the mechanisms active for dislocations to overcome nanovoid obstacles.

ANPs have emerged as potentially hyperresponsive nanomaterials for environmental sensing. Due to the extreme optical nonlinearities and reliance on sensitive cross-relaxation pathways within the nanoparticle, small perturbations in the nanoparticle’s environment manifest into large changes in luminescence intensities of the ANP’s optical emission. In the realm of nanoscale sensing, ANPs have previously demonstrated notable changes in luminescence intensities when changes in pressure, temperature, and material substrates were applied to the nanoparticles. In this residency, we further explored ANP sensing in areas of high fields, such as high electric and magnetic fields.

There were two primary goals to this residency project. The first goal was for me to learn the theory of molecular dynamics and application through LAMMPS. This was accomplished by me reading literature in parallel with scripting progressively more advanced simulations. The second goal was for me to test LAVA with the perspective of a newcomer to LAMMPS before its release. LAVA is a Python code named with the “La” in LAMMPS and the “VA” in VASP and can perform molecular dynamics and atomistic in a semi-automated fashion with each of these software, respectively. The intent is to provide a method for those inexperienced with such simulations to still produce results, and to streamline the production pipeline for more experienced users with automation tools.

Current literature for impact of Stacking Fault Energy (SFE) on critical stress for a dislocation to shear a void is limited to atomistic models, which have well known spatial and temporal limitations. Further, the limited availability of robust interatomic potentials for various fcc transition metals means that complete, systematic studies focusing on the importance of SFE for void shearing are lacking. Adding SFE to dissociated dislocations in a meso-scale model such as Phase Field Dislocation Dynamics (PFDD) will allow for more accurate predictions of void hardening effects in fcc metals, as well as further the PFDD model. This added physics is a crucial step towards the goal of accurately predicting dislocation mediated pore growth in a meso-scale PFDD model. For this residency, the PFDD model developed by the Beyerlein group at UCSB was used as the meso-scale model within which SFE physics was included. Five fcc transition metals, Copper, Silver, Nickel, Platinum, and Rhodium, were used and edge dislocations were allowed to dissociate into Shockley partials. Working together with my LANL advisor, a project framework was devised to systematically investigate the effect of SFE on the obstacle strength of nanovoids. The edge dislocations were allowed to shear an array of voids under stress, with the void sizes and spacings varied for each metal. Critical stress values were calculated for each void size and spacing, and results were compared and contrasted with current literature.

In this project, we determined the principle of photon energy down conversion, where high-energy X-ray photon energies get attenuated down to