Session Y1: ME.3 Inelastic Deformation, Fracture, and Spall XII

Modeling Small-Scale Damage Experiments with TEPla

Small-scale experiment simulations provide both focused model validation and parameter value development. Material response to loading is a complex mixture of simultaneously occurring processes such as hardening, melting and failure. The work presented here concentrates on the TEPla model of ductile failure development and evolution. Simulation results for two small-scale experiments are presented. A biaxial loading experiment is described in ``Plastic Deformation and Fracture of Steels Under Dynamic Biaxial Loading'' (C.K. Syn, et al., UCRL-CONF-205148). A gas-gun driven flyer plate impacts a buffer plate. The generated non-planar shock is transmitted through the buffer into a target plate, which is very thin in comparison to its diameter. The result is a biaxial tensile load which causes the target to stretch and fracture and provides a non-uniaxial test of TEPla. The RDamage experimental series studies damage initiation and fracture followed by spall layer recollection (A.M. Kaul, et al., Proc. APS-SCCM-2009). An electromagnetically-driven cylindrical shell impacts a cylindrical target shell, producing a failure surface and released spall layer. An extended EM drive allows recollection of this layer. Simulation tests parameter values for development and crush-out of porosity.   

Deformation behavior of a Ce-Al bulk metallic glass

The mechanisms of stress relaxation in metallic glasses under high strain rates are an area of active study. The lack of extended structure forces strain accommodation through alternative modes to slip. For example, amorphous Ce$_{3}$Al has been shown to undergo a phase transition to the crystalline FCC Ce$_{3}$Al at 25 GPa under quasistatic loading. Whether this mechanism extends to high strain rates has yet to be determined. We present results of an initial study into the ultrafast deformation characteristics of a Ce-Al bulk metallic glass. Using the Janus laser at the Jupiter Laser Facility (LLNL), thin targets $\sim$30 $\mu $m in thickness were shocked over a range of pressures up to 50 GPa. The velocity of the target rear surface was measured using a line-imaging VISAR to reveal features in the wave profile attributed to stress relaxation. In addition, experiments were performed on crystalline forms of Ce-Al prepared through heat treatment of the amorphous material. Preliminary results reveal a distinct precursor wave in the amorphous material below 20 GPa, which gives way to a complex multiwave structure above 30 GPa. Results of analyses in terms of the contribution of elastic energy to Gibbs' free energy of the initial phase are also presented.   

Shock Response of Bi/W Composites

This work investigates the shock response of composite pellets, whose constituents have a widely disparate shock melting response; a low melting phase, Bi, and a high melting phase, W. Samples were mixed using low-energy ball milling, followed by uni-axial pressing with and without heating to yield a range of compositions, densities, and microstructures. Laser-driven shocks were generated in the samples, and the shocked samples were collected for post-mortem analysis. On the laser drive side, we observe craters up to several hundred micrometers deep, which presumably form as Bi is shock-melted, and material is unloaded as tensile stresses develop from the release wave interactions. We find that the depth of the crater (i.e. the melting depth) is primarily governed by the composition and sample porosity. On the spall surface, we observe various behaviors, ranging from no damage to large spall regions, depending on the composition of the sample. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Coupling Instrumented Experiments with Microstructure-based Simulations of Reactant Configuration Effects on Shock-Initiation of Reactions in Intermetallic-Forming Powder Mixtures

The shock-initiation of reactions in intermetallic-forming powder mixtures is dominated by the configuration of reactants, which is influenced by the intrinsic and extrinsic properties of constituents. Instrumented experiments coupled with microstructure-based simulations can be used to understand the meso-scale processes and effects of reactant configuration on the onset conditions for reaction initiation. Uniaxial-strain impact experiments are performed to monitor the input and propagated stress-wave profiles and to determine changes in compressibility and wave-velocity associated with powder densification and possible reaction, as a function of impact velocity and different reactant configurations such as size, shape, and distribution of constituent powders. Meso-scale computational simulations through discretely represented constituents with real and synthetically generated microstructures of reactants, imported into CTH simulations, are also used to qualitatively and quantitatively probe the local configuration changes and particle-level processes, following validation of macroscopic properties by correlations with experiments. The simulations reveal the dependence of the starting configuration of reactants on the heterogeneous nature of localized deformation and mixing with processes such as forced or turbulent flow, vortex formation, and dispersion of reactants, influencing the onset conditions for reaction initiation. Understanding of these processes as a function of the effects of starting reactant configuration, and correlating those with synthetically-generated microstructural constructs allows reverse design of reactive powder mixture systems for desired macro-scale performance. This presentation will present an overview of our experimental and modeling approach in understanding the mechanistic aspects of impact-initiation of reactions for design of reactive materials systems.