Center for the Predictive Modeling and Simulation of High Energy Density Dynamic Response of Materials (PSAAP DOE)

Executive Summary

Caltech proposes the formation of a Multidiscipline Simulation Center (MSC) to develop a multidisciplinary Predictive Science methodology focusing on high-energy-density dynamic response of materials as it arises in hypervelocity impact. We propose to demonstrate Predictive Science by means of a concerted and highly integrated experimental, computational, and analytical effort that focuses on an overarching ASC-class problem: hypervelocity normal and oblique impact of metallic projectiles and targets, at velocities up to 10 km/s. Hypervelocity impact gives rise to pressures in the Mbar range and strain-rates up to 1011s-1, providing a grand challenge problem in Predictive Science that is also well-matched to the direct interests of the NNSA mission. Materials selected for study will include metals such as tantalum and iron that have been extensively studied under the previous ASC/ASAP Center. Depending on impact velocity and material choices, physics that will challenge modeling and simulation can include melting and vaporization, dissociation, ionization, and plasma formation; luminescence and radiative transport, solid-solid phase transitions, high-strain-rate deformation and thermomechanical coupling; fracture, fragmentation, spall and ejecta; deformation instabilities such as shear banding; hydrodynamic instabilities, mixed-phase flows, and-depending on material choices-mixing. The overarching hypervelocity impact application, in conjunction with a rigorous and novel methodology for model-based uncertainty quantification, will provide the intellectual backbone of the Center and its chief organizing principle. In particular, the quantification of margins and uncertainties (QMU) will drive and closely coordinate the experimental, computational, modeling, software development, verification and validation efforts within a Yearly Assessment format. This Yearly Assessment cycle will result in precise quantitative measures that will serve to track progress towards achieving a verified and validated predictive capability. Integrated and component experiments will be conducted by participating faculty at Caltech's Small Particle Hypervelocity Range (SPHR) and High Strain-Rate Testing (HSRT) facilities. A broad array of state-of-the-art active, passive, and time-resolved diagnostic techniques will supply high-quality data in support of validation and uncertainty quantification. The ability to conduct the experimental campaign in-house will allow close coordination with the computational campaign, a requirement for the rigorous quantification of modeling uncertainties. One key outcome of this coordinated experimental/computational effort will be the identification of the major sources of modeling uncertainty, which will be targeted for higher-fidelity modeling with the aid of component experiments.

The previous ASC/ASAP Center has produced a wealth of validated material properties and models, including pressure- and temperature-dependent equations of state and elastic moduli and multiscale models of plasticity and fracture for Ta and Fe. We will build on this extensive foundation and compute material properties relevant to hypervelocity impact phenomena. A key role will be accorded to multiscale mathematics with a view to defining Fast Multiscale Models well suited for uncertainty-quantification analysis. The proposed integrated simulation will leverage the existing Virtual Test Facility (VTF) developed by the previous ASC/ASAP Center. The VTF will provide a scalable parallel framework for Lagrangian and Eulerian approaches to solid, fluid and plasma dynamics that will implement adaptive mesh refinement coupled with finite-element, finite volume and particle-based solvers. The computational science and engineering effort will develop a professional-quality software framework for the integration of the physics modules developed by the disciplinary groups, maintain the framework according to professional standards, ensure the effective utilization of petascale NNSA computing resources and oversee the scheduling and execution of the computational campaign on NNSA platforms as part of the Yearly Assessment cycle. We plan to continue and expand the extensive interactions of the previous ASC/ASAP Center with the NNSA Laboratories through regularly scheduled summer student internships at NNSA Laboratories, through a program of comentoring and co-advising of graduate students by NNSA Laboratory scientists, by fostering collegial scientific interactions with NNSA Laboratory scientists, and by other means.

The Fundamental Chemistry and Physics of Munitions Under Extreme Conditions (ARO)

Task: Top-down Multiscale simulations of the drop weight test of energetic materials

Existing mesoscale/continuum approaches and reactive "ignition and growth" models of the initiation and heterogeneous detonation are based on relatively simple phenomenological models where the constitutive parameters and the kinetics of energy release processes are based on ad hoc empirical forms fitted to the experiment. The data available to calibrate such a fitting form are limited (e.g., there is no data on the reactants Hugoniot near the von Neumann spike or for the products near the CJ state). There may be many constitutive models all in reasonable agreement with available experiment data.
Such engineering approaches have proved to be useful predicting the detonation wave parameters in EM composites. However, they cannot be used for prediction of the sensitivity because the kinetics parameters included in existing phenomenological models is not directly coupled to the mechanical processes at the microscopic level. To capture the key properties that affect the sensitivity and response of EM under shock or thermal loading, it is imperative to provide a seamless coupling at different levels of the nonlinear mechanical response at high-rate strains with molecule-level energy exchange in hot spots and thermochemical kinetics to properly describe the energy localization processes that lead to rapid chemistry and govern shock-to-detonation transition.
A critical issue in multiscale modeling of EM is how the strength properties and failure mechanisms (shear banding, fracture, etc) of the material influence dissipation of the impact mechanical energy. This requires a detailed description of the anisotropic nonlinear response of the material under large deformations to determine preferred slip planes and shear flows in EM micrograins under stresses generated by the interference of reflected and transferred shock waves, Mach stems, rarefaction waves, Taylor waves, and shock focusing or defocusing at the grain boundaries. It is also requires an accurate description of void collapse or material friction at the non-planar grain interfaces. For example, the commonly used von Mises plasticity or Maxwell viscoelastic models assume isotropic constitutive properties under uniaxial flow which necessarily leads to a symmetric collapse or to free slip boundary conditions at the interface.

In order to determine the multiscale coupling of chemical kinetics to the physical processes in heterogeneous EM under shock or impact conditions, we propose to develop a top-down multiscale simulation strategy that:

• starts with continuum description of a multigranular system as at the left in the figure, considering a distribution of 100s of such grains,

• applies a shock to the left side,

• describes the propagation of local strain, stress, and temperature fields in term of microscopic elastic parameters

• uses the stresses acting on each of the grains as in the middle to determine how the strength properties and failure mechanisms of EM (shear banding, fracture, etc) influence the dissipation of the mechanical energy of shock or impact.

• Use continuum approaches incorporating anisotropic nonlinear elasticity to determine how resolved shear stresses along different slip systems build up to affect formation of the shear flows in EM micrograins

• calculate the large deformations that affect the plasticity mechanisms

• Determine how hot spot formation depends on the smoothness of the grains and the packing.

• Use these results eventually to obtain a continuum model for the "drop weight" test including chemical reactions at the impact-generated shear bands, grain-grain sliding, or void collapse.

We would use wherever possible results from detailed reaction kinetics and constitutive parameters obtained from atomistic simulations.

ArMAX: A Lightweight, Hierarchical, MAX Phase Armor (NRL)

The high strain rate properties and the constitutive models of the component materials proposed for the ArMAX armor will be determined using the split Hopkinson pressure bar and quasi-static materials testing system over the strain rate range of 10-4 to 104 s-1 and wide range of temperatures. In addition to standard compression specimen, the recently developed shear compression specimen (SCS) will also be used to determine the shear dominant properties of materials such as polyurea. The constitutive laws, stress as a function strain, strain rate and temperature will be provided for use in the computation simulations of the ArMAX armor system. The recovered specimens (particularly MAX Phase) materials will be subjected to microstructural examination to characterize the mechanisms of deformation at high strain rates including kinking of nanolaminates. Penetration experiments using a model penetrator will be performed on the ArMAX armor package with the goals of (i) characterizing penetration resistance of variously constructed armor systems, (ii) provide highly resolved data for validation of computation tool kit, (iii) screening promising designs for further development and use in genetic algorithm based optimization and (iv) identifying important microstructural and geometrical parameters that influence penetration resistance. The principal objectives of the proposed work in computational model and tool development are: i) The development of validated and integrated 3-D dynamic computational toolkit for simulating the interaction of the proposed ArMAX armor against potential threats. This would be based on the direct numerical simulation of scaled penetration tests performed at Caltech and full scale ballistic testing performed at NSWC, Dahlgren and iii) The optimization of the geometry and layout of the various components including the distribution and geometry of the MAX phases, polymer interfaces and polyurea encasing for best ballistic performance. The ArMAX armor plate consisting of the MAX Phase ceramic tiles, constraint layers, tile isolation layers, impedance mismatch layers and surface blast-mitigation coatings will be treated as a composite armor. The armor will be discretized according to the materials with appropriate interface conditions. The material interfaces between elements will be represented by cohesive elements to simulate fracture and failure. In addition to the depth of penetration, detailed comparisons will be made on the observed failure modes and as well as any time resolved data such as stress gages will be made. At the end of Phase I, it is anticipated that a fully validated toolkit for studying
penetration mechanics of the newly proposed ArMAX armor system. Implementation of the genetic algorithm in the context of ArMAX armor optimization will involve identifying designs that minimize areal density and yet possesses excellent energy dissipation capability. Various topologies will be optimized including the advanced concepts of hierarchical design proposed for Phase II.

Mechanics and Mechanisms of Impulse Loading, Damage and Failure of Marine Structures and Materials (MURI)

This proposal seeks to develop a fundamental understanding of the mechanics and mechanisms of impulse loading and the resulting damage and failure in marine structures and materials of relevance to the Navy. The current state of knowledge provides detailed understanding of individual aspects; however, impulse failure of marine structures is a highly inter-connected and highly coupled phenomenon. Therefore, an integrated multidisciplinary approach based on the comprehensive understanding of fluid/structure interactions, impulse loads arising from explosions and wave slamming and their effects on materials and structures of marine craft, made of composites and sandwich structures, forms the basis of the proposed research. This approach emphasizes the delicate interplay between the source, the fluid, the structure and the material through a collaborative effort that includes theory, large-scale simulation, and controlled and highly instrumented dynamic experimentation.The proposed effort will be pursued in the context of three highly relevant loading conditions:

• Loading due to bubble collapse and cavitation in underwater explosions (UNDEX) and their effect on fiber reinforced composite and sandwich structures;

• Shock focusing phenomena in convergent geometries and their effect on composite materials/structures;

• Wave slamming and the effect of repeated transient loading on sandwich structures and light-weight metals.

High-speed quantitative visualization of the fluid-solid interfacial conditions will be used to probe the basic physics and mechanisms of impulse load creation, transmission, deformation and failure for planar and curved surfaces relevant to marine structures. Appropriate scaling laws will be used (when necessary) to infer the boundary conditions generated by the impulse loads on realistic marine structures. These loads will then be applied to the materials/structures of interest and the resulting dynamic deformation and failure modes will be characterized using a wide range of diagnostic tools based on full field, high-speed optical and infrared thermography.

Comprehensive 3-D fluid dynamical (CFD) and structural/material mechanics (FEM) models will be developed. Multiscale modeling including a realistic description of cavitation, bubble collapse, wave slamming, composite materials and sandwich structures will form an integral part of this computational effort. The highly resolved data from the experiments will be used to validate the computational models and codes. All these results will be incorporated into comprehensive theories that can be used to design impulse loading mitigation technologies.

To pursue these ambitious goals, the proposal gathers a diverse and distinguished team with expertise spanning fluid mechanics, thermal sciences, explosion mechanics, solid and structural mechanics, materials science and computational science and engineering. The team has many existing research connections and a proven record of collaborative research.

The proposed research will develop novel experimental techniques, advanced diagnostics and a new generation of validated computational models. With its unprecedented attention to the solid/fluid interface under impulse conditions, this proposal is also likely to raise new questions and lead to the discovery of new phenomena. All these are expected to have a significant technological impact on the design of next generation light-weight and high-speed marine structures including littoral combat ships, destroyers, cruisers, and advanced sea bases. Finally, this proposal provides unique opportunity for training of students and postdoctoral scholars in an area of importance to the national defense in an interdisciplinary setting.