Publications

Over the past few years, the CTH multiphysics hydrocode has overhauled its software quality and testing processes, implementing current best practices in software quality and building a robust V&V test suite comprised of traditional hydrocode verification problems, including ASC Tri-Lab Test Suite and Enhanced Tri-Lab Test Suite problems, as well as validation problems for some of CTH’s most frequently used equations of state, materials models, and other key capabilities. Substantial progress towards building this new test suite was made in FY19 and FY20. In FY21, the test suite has been expanded to include verification and validation tests of the Steinberg-Guinan-Lund (ST) viscoplastic model and the Johnson Cook (JFRAC) fracture model. Additionally, two new verification tests were added, covering hydrodynamics and high explosive (HE) modeling capabilities: the Kidder Gaussian density problem and the Escape of HE Products (EHEP) problem from the Tri-Lab Test Suite. This report discusses each of these test problems in detail. Verification test results are compared to analytic solutions. Validation test results are compared to experimental data. Wherever possible, convergence or mesh refinement studies are included. Additionally, while implementing the Kidder verification problem, a bug was identified that affects the use of tables to initialize pressure or density in 1D or 2D calculations. A brief discussion of the bug and its fix is included. CTH demonstrates good performance overall on the new test suite problems. Simulation results showed good agreement with analytic solutions for the Kidder problem, with convergence rates ranging between 1.8 and sub-linear, and relatively good agreement for the EHEP problem, though convergence rates for pressure and density were nearly 0. The ST and JFRAC strain rate loading verification tests show good agreement with analytic solutions. Likewise, CTH simulation results show good agreement with experimental validation data, including Taylor rod impact testing, for the materials tested. Future V&V work will focus on adding 2D and 3D versions of existing verification tests as well as adding validation tests of other frequently used capabilities such as other fracture models.

Composite materials are used as alternatives to conventional metallics in a multitude of applications including military ground vehicles, aircraft, space launch and re-entry vehicles and even personnel protection where weight savings are critical. In application, these materials are susceptible to high velocity impacts from various threats and it is essential that the response of these materials, under relevant conditions, be understood in order to provide optimized effective designs. This work details an on-going effort to validate the anisotropic multiple constituent model (MCM) within the CTH hydrocode. Within the CTH framework, the anisotropic MCM model is coupled with an equation of state (EOS) and provides continuum averaged stress and strain fields for each constituents (fiber and resin) of a composite microstructure from which progressive damage evaluations can be performed. In this paper we focus on recent validation efforts where woven S2/SC15 (glass/epoxy) composite panels were impacted with steel spheres at various impact velocities and angles of obliquity. The experimental testing was performed at the Shock Thermodynamics Applied Research (STAR) Facility at Sandia National Laboratories to provide data for further validation of the MCM model under oblique impact conditions. Oblique impacts result in stress fields which exercise the anisotropy of the strength model and the EOS coupling of the MCM model more robustly. Results are presented for both the CTH MCM model predictions and the experimental testing. The primary comparison metrics evaluated are the predicted and observed damage extent, overall damage pattern, and residual velocity of the projectile.

The CTH multiphysics hydrocode is used in a wide variety of important calculations. An essential part of ensuring hydrocode accuracy and credibility is thorough code verification and validation (V&V). In the past, CTH V&V work (particularly verification) has not been consistently well documented. In FY19, we have made substantial progress towards addressing this need. In this report, we present a new CTH V&V test suite composed of traditional hydrocode verification problems used by similar ASC codes as well as validation problems for some of the most frequently used materials models and capabilities in CTH. For the verification problems, we present not only results and computed errors, but also convergence rates. Validation problems include mesh refinement studies, providing evidence that results are converging. CTH performs well overall on the new V&V test suite, though the test suite itself still only covers a fraction of CTH capabilities. Accuracy and convergence rates for traditional verification problems such as Sedov, Noh, and the 1D Riemann problems are comparable to similar ASC codes. Likewise, CTH simulation results show good agreement with experimental validation data for the AVAVE, EPPVM, and Johnson-Cook strength models as well as for the Mie-Griineisen EOS for the materials tested. Future V&V work will add testing for other frequently used material models and code capabilities, helping to provide confidence in simulation results for critical analyses. ACKNOWLEDGEMENTS The authors would like to thank Brian Carnes, Jim Cox, and Kevin Dowding for helpful discussions. We would also like to acknowledge the valuable resources published by Los Alamos National Laboratory, such as the code verification tool ExactPack 1 , which was used in this work to calculate and plot analytic solutions, and the Standardized Definitions for Code Verification Problems 2 , which was helpful in implementing the ASC Tri-Lab Test Suite problems. Finally, we wish to thank Kim Mish, Angel Urbina, and Walter Witkowski for supporting this work. 1 R. Singleton Jr, D. M. Israel, S. W. Doebling, C. N. Woods, A. Kaul, J. W. Walter Jr, and M. L. Rogers, Exactpack documentation, tech. rep., Los Alamos National Lab. (LANL), Los Alamos, NM (United States), 2016. 2 J. Kamm, S Doebling, D. Israel, and R. Singleton, Standardized definitions for code verification test problems, tech. rep., LA-UR-14-20418 (Rev. 2), 2014.

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The use of S2 glass/SC15 epoxy woven fabric composite materials for blast and ballistic protection has been an area of on-going research over the past decade. In order to accurately model this material system within potential applications under extreme loading conditions, a well characterized and understood anisotropic equation of state (EOS) is needed. This work details both an experimental program and associated analytical modelling efforts which aim to provide better physical understanding of the anisotropic EOS behavior of this material. Experimental testing focused on planar shock impact tests loading the composite to peak pressures of 15 GPa in both the transverse and longitudinal orientations. Test results highlighted the anisotropic response of the material and provided a basis by which the associated numeric micromechanical investigation was compared. Results of the combined experimental and numerical modeling investigation provided insights into not only the constituent material influence on the composite response but also the importance of the plain weave microstructure geometry and the significance of the microstructural configuration.

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This article details the implementation and application of the glass-specific computational constitutive model by Holmquist and Johnson (J Appl Mech 78:051003, 2011) to simulate the dynamic response of soda-lime glass under high rate and high pressure shock conditions. The predictive capabilities of this model are assessed through comparison of experimental data with numerical results from computations using the CTH shock physics code. The formulation of this glass model is reviewed in the context of its implementation within CTH. Using a variety of experimental data compiled from the open literature, a complete parameterization of the model describing the observed behavior of soda-lime glass is developed. Simulation results using the calibrated soda-lime glass model are compared to flyer plate and Taylor rod impact experimental data covering a range of impact and failure conditions spanning an order of magnitude in velocity and pressure. The complex behavior observed in the experimental testing is captured well in the computations, demonstrating the capability of the glass model within CTH.

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This study details and demonstrates a strain-based criterion for the prediction of polymer matrix composite material damage and failure under shock loading conditions. Shock loading conditions are characterized by high-speed impacts or explosive events that result in very high pressures in the materials involved. These material pressures can reach hundreds of kbar and often exceed the material strengths by several orders of magnitude. Researchers have shown that under these high pressures, composites exhibit significant increases in stiffness and strength. In this work we summarize modifications to a previous stress based interactive failure criterion based on the model initially proposed by Hashin, to include strain dependence. The failure criterion is combined with the multi-constituent composite constitutive model (MCM) within a shock physics hydrocode. The constitutive model allows for decomposition of the composite stress and strain fields into the individual phase averaged constituent level stress and strain fields, which are then applied to the failure criterion. Numerical simulations of a metallic sphere impacting carbon/epoxy composite plates at velocities up to 1000 m/s are performed using both the stress and strain based criterion. These simulation results are compared to experimental tests to illustrate the advantages of a strain-based criterion in the shock environment.

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