Michael W. Burkett

Failure phenomenology of confined ceramic targets and impacting rods

Abstract The mechanism by which a long rod penetrates a steel-encased ceramic block was sought by performing impact experiments at a range of velocities and examining the fracture and deformation in the recovered targets and impactors. The key processes are the crushing of a small volume of ceramic adjacent to the leading surface of the advancing penetrator, and the subsequent flow of the fine fragments lateral to and then opposite the direction of attack. The results suggest that nonconventional material properties such as the dynamic compressive failure energy and the friction, flow and abrasive properties of the finely fragmented material govern the penetration resistance of confined ceramics. This understanding of penetration mechnism can be used to guide development of specialized tests and failure models to measure pertinent material properties and to predict penetration behavior, respectively.

Influence of Shock Prestraining and Grain Size on the Dynamic‐Tensile‐Extrusion Response of Copper: Experiments and Simulation

The mechanical behavior of, and damage evolution in high‐purity Cu is influenced by strain rate, temperature, stress state, grain size, and shock prestraining. The effects of grain size on the tensile mechanical response of high‐purity Cu have been probed and are correlated with the evolution of the substructure. The dynamic extrusion response of shock prestrained Cu demonstrates the significant influence of grain size on the large‐strain dynamic tensile ductility of high‐purity copper. Eulerian hydrocode simulations utilizing the Mechanical Threshold Stress constitutive model were performed to provide insight into the dynamic extrusion process. Quantitative comparisons between the predicted and measured deformation topologies and extrusion rates are presented.

Sensitivity of hypervelocity impact simulations to equation of State

Abstract We investigate the sensitivity of hypervelocity impact simulations to differences in equations of state (EOSs), comparing our hydrocode calculations to recent experiments on lead at G. M. Delco ( Pomykal, 1986 ). The results suggest that the hydrocode does not correctly predict all the details of debris cloud structure, even when these are significant EOS differences. We used three different equations of state, one of which is new. It was generated just for sensitivity studies using the most recent experimental EOS data.

Models of high velocity impact phenomena

Abstract Models of craters formed by impacts at velocities of up to 24.5 km/sec have been computed using the Smooth Particle Hydrodynamics, MESA, EPIC and CALE codes. These modeling efforts are compared to data obtained from the Hypervelocity Microparticle Impact project at Los Alamos using the van de Graaff accelerator. A factor of 5 increase in yield strength was needed to account for high strain rate effects and to match the data. Structure in the data is addressed by using crater volume instead of crater diameter cubed. Detailed code comparisons were made between the four codes with good agreement found.

Burn front and reflected shock wave visualization in an inertially confined detonation of high explosive

Proton radiography was used to investigate the spatiotemporal evolution of the burn front and associated reflected shocks on a PBX-9502 charge confined between an outer cylindrical steel liner and an inner elliptical tin liner. The charge was initiated with a PBX-9501 booster and a line wave generator at 30° from the major axis of the ellipse. This configuration provides a large region where the high explosive (HE) is not within the line of sight of the detonation line and thus offers a suitable experimental platform to test various burn models and EOS formulations. In addition, the offaxis initiation allows for the burn fronts to travel around the charge through different confining paths. Simulations with the hydrocode PAGOSA were performed to assess the accuracy of several HE burn methodologies.

Stress fields generated by kinetic energy projectile interaction with ceramic targets

Two-dimensional axisymmetric calculations were performed with the Eulerian hydrocode MESA2D and the Lagrangian structural analysis code PRONTO2D. The calculated stress distributions were compared shortly after impact and found to be similar in magnitude and profile. For certain geometric configurations, the interaction of the kinetic energy penetrators with the ceramic targets produce high compressive principal stresses as well as, significant tensile principal stresses ahead of the projectile/ceramic interface. The principal tensile stresses fracture the ceramic ahead of the penetrator. The crack trajectories measured from a recovered ceramic target were compared with crack trajectory estimates based upon MESA2D principal stress states within the tile. The fracture process degrades the ceramic and allows the projectile to penetrate a fractured ceramic media. 12 refs., 3 figs.

Hydrodynamic simulations of gaseous Argon shock compression experiments

The lack of published Ar gas shock data motivated an evaluation of the Ar Equation of State (EOS) in gas phase initial density regimes. In particular, these regimes include initial pressures in the range of 13.8 – 34.5 bar (0.025 – 0.056 g/ cm3) and initial shock velocities around 0.2 cm/μs. The objective of the numerical evaluation was to develop a physical understanding of the EOS behavior of shocked and subsequently multiply re-shocked Ar gas through Pagosa numerical simulations utilizing the SESAME equation of state. Pagosa is a Los Alamos National Laboratory 2-D and 3-D Eulerian continuum dynamics code capable of modeling high velocity compressible flow with multiple materials. The approach involved the use of gas gun experiments to evaluate the shock and multiple re-shock behavior of pressurized Ar gas to validate Pagosa simulations and the SESAME EOS. Additionally, the diagnostic capability within the experiments allowed for the EOS to be fully constrained with measured shock velocity, particle vel...

Coupled Plasticity and Damage Modeling and Their Applications in a Three‐Dimensional Eulerian Hydrocode

Previously developed constitutive models and solution algorithms for continuum‐level anisotropic elastoplastic material strength and an isotropic damage model TEPLA have been implemented in the three‐dimensional Eulerian hydrodynamics code known as CONEJO. The anisotropic constitutive modeling is posed in an unrotated material frame of reference using the theorem of polar decomposition to compute rigid‐body rotation. TEPLA is based upon the Gurson flow surface (a potential function used in conjunction with the associated flow law). The original TEPLA equation set has been extended to include anisotropic elastoplasticity and has been recast into a new implicit solution algorithm based upon an eigenvalue scheme to accommodate the anisotropy. This algorithm solves a two‐by‐two system of nonlinear equations using a Newton‐Raphson iteration scheme. Simulations of a shaped‐charge jet formation, a Taylor cylinder impact, and an explosively loaded hemishell were selected to demonstrate the utility of this modelin...

Modeling Anisotropic Plasticity: 3D Eulerian Hydrocode Simulations of High Strain Rate Deformation Processes

Previously developed constitutive models and solution algorithms for anisotropic elastoplastic material strength have been implemented in the three‐dimensional Conejo hydrodynamics code. The anisotropic constitutive modeling is posed in an unrotated material frame of reference using the theorem of polar decomposition to obtain rigid body rotation. Continuous quadratic yield functions fitted from polycrystal simulations for a metallic hexagonal‐close‐packed structure were utilized. Simple rectangular shear problems, R‐Value problems, and Taylor cylinder impact data were used to verify and validate the implementation of the anisotropic model. A stretching rod problem (involving large strain and high strain‐rate deformation) was selected to investigate the effects of material anisotropy. Conejo simulations of rod topology were compared for two anisotropic cases.