Projects

Compared to their non-porous counterparts, metallic foams are known to exhibit improved functionality (e.g., damping capacity) when subjected to high-speed impact loading. Here, we report the results of a molecular dynamics study of bicontinuous nanoporous gold (NPG) subjected to impact loading. Additionally, we investigate the effects of a heterogeneous impact zone on the fate of a witness specimen initially protected by an NPG target. A cube-shaped flyer object (Lo = 408.6 Å) composed of full-density f.c.c. gold (FDG) strikes with impact speed Uflyer= 1.0 km/s an initially stationary equal-sized NPG target, which subsequently transmits a dispersed shock wave into a protected FDG witness specimen located at the downstream end of the target. The NPG target is stochastic, with porosity of 0.5 and mean ligament diameter L =64±6 Å. A corresponding simulation for an FDG target serves as a baseline case. As anticipated, the sharp, planar shock imparted by the FDG flyer rapidly becomes highly curved and broadened along the shock direction in the NPG target. Intense plastic and convective flows (ejecta and jetting) lead to spatially heterogeneous mass flow and energy localization, which persist across the entire length of the NPG target studied. Whereas the shock transmitted by the FDG target leaves the witness specimen intact and essentially undamaged, the heterogeneous stress and flow fields imparted by the NPG target results in destruction of the witness. Independent simulations for the same cube-shaped NPG target shocked on the three statistically equivalent {1 0 0} target faces reveal modest run-to-run variability in the target and witness responses. The difference in the effects of the NPG and FDG targets on the witness response motivates additional study of the failure evolution inside the witness. It appears from the preliminary results that the porous-nonporous interface might induce a failure pattern that, in some aspects, resembles failure waves observed in brittle solids. 

Constitutive description of the plastic flow in metallic foams has been rarely explored in the literature. Even though the material is of great interest to researchers, its plasticity remains a topic that has a much room for exploration. With the help of the rich literature that explored the material deformation mechanism, it is possible to introduce a connection between the results of the atomistic simulations and the well-established continuum constitutive models that were developed for various loading scenarios. In this work, we perform large-scale atomistic simulations of metallic gold foams of two different sizes at a wide range of strain rates under uniaxial compression. By utilizing the results of those simulations, as well as the results we reported in our previous works, a physical atomistic-continuum dislocations-based constitutive modeling connection is proposed to capture the compressive plastic flow in gold foams for a wide range of sizes, strain rates, temperatures, and porosities. The results reported in this work present curated datasets that can be of extreme usefulness for the data-driven AI design of metallic foams with tunable nanoscale properties. Eventually, we aim to produce an optimal physical description to improve integrated physics-based and AI-enabled design, manufacture, and validation of hierarchical architected metallic foams that deliver tailored mechanical responses and precision failure patterns at different scales. 

Nanoporous gold (NP-Au) is of great interest to researchers due to its high surface area; and accordingly, the wide range of applications that the material can be utilized for especially those where high temperature is involved. Therefore, the effect of temperature on NP-Au is studied by performing Molecular Dynamics (MD) simulations at temperatures between 300 K and 700 K. Moreover, an Arrhenius type formulation is proposed to modify existing scaling laws to capture the temperature effect. Also, a series of temperature dependent modifications to an existing dislocation based constitutive model are proposed. The simulation results show that while the elastic modulus and yield stress are temperature dependent, their tension–compression asymmetries are not. Under both compression and tension, material strength is controlled by surface stresses and dislocation mobility. However, the dislocation density required to plastically deform the material is found to be completely temperature independent under tension, and becomes temperature dependent under compression once there is sufficient amount of ligaments merging and collapse. 

While several studies assessed the behavior of nanoporous gold (NP-Au) under different loading conditions for various material characteristics and loading scenarios, very limited attention was given to the effect of strain rate on material response. In this study, the effect of strain rate is investigated by performing novel atomistic simulations on NP-Au under uniaxial loading up to large compressive and tensile strains (60% strain) for strain rates in the range of 10^6/s and 10^9/s. This paper explores the material response under uniaxial loading and proposes a size, relative density, and strain rate dependent dislocation based constitutive model that describes the plastic flow in NP-Au. In addition, modified Gibson and Ashby (G-A) scaling relations that capture the effect of strain rate are proposed to predict the elastic modulus, yield stress and ultimate stress. The simulation results show that the elastic modulus is strain rate independent similar to that of bulk materials. Additionally, the yield stress and its compression-tension asymmetry are strain rate dependent. Under compression, strain hardening is found to be strain rate dependent, and it is controlled by the amount of dislocation density for strain rates below 10^8/s; whereas, it is controlled by the coupling effect of dislocation density and dislocation mobility for higher strain rates. Under tension, the material shows higher ductility and softening with increasing strain rate. Also, the material deformation and failing mechanisms change at strain rates exceeding 10^8/s due to the transition in dislocation activity within the ligaments. 

Significant attention in the literature is directed toward the development of scaling relations that relate the properties of nanoporous metals to bulk materials in order to help in their design. Although nanoporous gold has been under extensive study to develop the proper scaling relations, the literature still lacks a specific model that predicts its properties based on a combination of surface parameters, ligament size, and relative density. This work is part of the ongoing trials to introduce such scaling relations. Therefore, utilizing literature-reported results, the authors are proposing scaling relations that account for the coupling effect of surface area to solid volume ratio, ligament size, and relative density to predict the elastic modulus, yield stress, and ultimate stress under uniaxial loading. Moreover, a comparison between the proposed model and existing scaling laws in the literature is presented. 

Nanoporous (NP) metals with ligament sizes up to a few tens of nm show exceptional mechanical properties such as high strength and stiffness per weight. While their elasticity and yield strength have been the subject of numerous studies, less is known about the plastic deformability of these materials under large compressive and tensile strains. In this study, the effect of ligament size is investigated using large-scale atomistic simulations to probe the elastic response, plastic response, and deformation mechanisms of nanoporous gold under uniaxial compression and tension to strains in excess of 60 percent. This paper explores the full range of the material response under uniaxial loading, focusing on the modifications to strain hardening under compression and ductility and delayed failure under tension. It was found that the elastic modulus experiences a compression-tension asymmetry that decreases with ligament size increase. Under compression, strain hardening is found to be ligament size dependent. This size dependency can be explained by the coupling effect of the change in surface area to solid volume ratio evolution and defects accumulation. Under tension, the material shows higher ductility with ligament size increase causing a delay in failure. This is attributed to differences in dislocation density. The results reported in this work will help in evaluating the effect of ligament size on the material response, and eventually enhance the design of novel nanoporous foams with tailored mechanical response.