Darby J. Luscher

Evaluating the effects of loading parameters on single-crystal slip in tantalum using molecular mechanics

This study is aimed at developing a physics-based crystal plasticity finite element model for body-centred cubic (BCC) metals, through the introduction of atomic-level deformation information from molecular dynamics (MD) investigations of dislocation motion at the onset of plastic flow. In this study, three critical variables governing crystal plasticity mediated by dislocation motion are considered. MD simulations are first performed across a range of finite temperatures up to 600K to quantify the temperature dependence of critical stress required for slip initiation. An important feature of slip in BCC metals is that it is not solely dependent on the Schmid law measure of resolved shear stress, commonly employed in crystal plasticity models. The configuration of a screw dislocation and its subsequent motion is studied under different load orientations to quantify these non-Schmid effects. Finally, the influence of strain rates on thermal activation is studied by inducing higher stresses during activation at higher applied strain rates. Functional dependence of the critical resolved shear stress on temperature, loading orientation and strain rate is determined from the MD simulation results. The functional forms are derived from the thermal activation mechanisms that govern the plastic behaviour and quantification of relevant deformation variables. The resulting physics-based rate-dependent crystal plasticity model is implemented in a crystal plasticity finite element code. Uniaxial simulations reveal orientation-dependent tension–compression asymmetry of yield that more accurately represents single-crystal experimental results than standard models.

Self-consistent modeling of the influence of texture on thermal expansion in polycrystalline TATB

This paper presents a modeling approach for simulating the anisotropic thermal expansion of polycrystalline (1,3,5-triamino-2,4,6-trinitrobenzene) TATB-based explosives which utilizes microstructural information including the porosity, crystal aspect ratio and processing-induced texture. A self-consistent homogenization procedure is used to relate the macroscopic thermoelastic response to the constitutive behavior of single-crystal TATB. The model includes a representation of the grain aspect ratio, porosity and, crystallographic texture attributed to the consolidation process. A quantitative model is proposed for describing the evolution of the preferred orientation of basal planes in TATB during consolidation and an algorithm constructed for developing a discrete representation of the associated orientation distribution function. Analytical and numerical solutions using this model are shown to produce textures consistent with previous measurements and characterization for isostatically and uniaxially ‘die-pressed’ specimens.Predicted thermal strain versus temperature results for textured specimens are shown to be in agreement with corresponding experimental measurements. Results from these simulations are used to identify qualitative trends. Key conclusions from this work include the following. Both porosity and grain aspect ratio have an influence on the thermal expansion of polycrystal TATB, considering realistic material variability. The preferred orientation of the single-crystal TATB [0 0 1] poles within a polycrystal gives rise to pronounced anisotropy of the macroscopic thermal expansion. The extent of this preferred orientation depends on the magnitude of the deformation and, consequently, is expected to vary spatially throughout manufactured components much like the porosity. The modeling approach presented here has utility toward bringing spatially variable microstructural features into macroscale system engineering models.

Equations of state for the α and γ polymorphs of cyclotrimethylene trinitramine

Equations of state for the α and γpolymorphs of the energetic molecular crystal cyclotrimethylene trinitramine (RDX) have been developed from their Helmholtz free energies. The ion motion contribution to the Helmholtz free energy is represented by Debye models with density-dependent Debye temperatures that are parameterized to vibrational densities of states computed from dispersion-corrected density functional theory. By separating the vibrational density of states into low frequency modes of mainly lattice phonon character and high frequency modes of intramolecular character we were able to significantly improve the description of the heat capacity at low temperatures and the thermal contribution to the pressure. The ion motion contribution to the Helmholtz free energy of the high pressureγpolymorph was constructed from that of the αpolymorph to reproduce the temperature-independent transformation pressure seen experimentally. The static lattice energies for both polymorphs were constructed to reproduce published isothermal compression data. The equations of state have been applied to the prediction of the path of the principal Hugoniot in the equilibrium phase diagram.

A single-crystal model for the high-strain rate deformation of cyclotrimethylene trinitramine including phase transformations and plastic slip

A continuum model for the high-rate, thermo-mechanical deformation of single-crystal cyclotrimethylene trinitramine (RDX) is developed. The model includes the effects of anisotropy, large deformations, nonlinear thermo-elasticity, phase transformations, and plastic slip. A multiplicative decomposition of the deformation gradient is used. The volumetric elastic component of the deformation is accounted for through a free-energy based equation of state for the low- (α) and high-pressure (γ) polymorphs of RDX. Crystal plasticity is addressed using a phenomenological thermal activation model. The deformation gradient for the phase transformation is based on an approach that has been applied to martensitic transformations. Simulations were conducted and compared to high-rate, impact loading of oriented RDX single crystals. The simulations considered multiple orientations of the crystal relative to the direction of shock loading and multiple sample thicknesses. Thirteen slip systems, which were inferred from in...

Taylor impact tests and simulations of plastic bonded explosives

Taylor impact tests were conducted on plastic bonded explosives PBX 9501 and PBXN-9 for impact velocities between 80 and 214 m/s. High-speed photography was used to image the impact event at a rate of one frame for every 25 μs. For early times, PBXN-9 showed large-deformation mushrooming of the explosive cylinders, followed by fragmentation by an amount proportional to the impact speed, was observed at all velocities. PBX 9501 appeared to be more brittle than PBXN-9, the latter demonstrated a more viscoelastic response. The post-shot fragments were collected and particle size distributions were obtained. The constitutive model ViscoSCRAM was then used to model the Taylor experiments using the finite element code ABAQUS. Prior to the Taylor simulations, ViscoSCRAM was parameterized for the two explosives using uniaxial stress-strain data. Simulating Taylor impact tests validates the model in situations undergoing extreme damage and fragmentation.

Anomalous plasticity of body-centered-cubic crystals with non-Schmid effect

Abstract Plastic deformations in body-centered-cubic (bcc) crystals have been of critical importance in diverse engineering and manufacturing contexts across length scales. Numerous experiments and atomistic simulations on bcc crystals reveal that classical crystal plasticity models with the Schmid law are not adequate to account for abnormal plastic deformations often found in these crystals. In this paper, we address a continuum mechanical treatment of anomalous plasticity in bcc crystals exhibiting non-Schmid effects, inspired from atomistic simulations recently reported. Specifically, anomalous features of plastic flows are addressed in conjunction with a crystal plasticity model involving two non-Schmid projection tensors widely accepted for representing non-glide components of an applied stress tensor. Further, modeling results on a representative bcc tantalum are presented and compared to experimental data at a range of low temperatures to provide physical insight into deformation mechanisms in these crystals with non-Schmid effects.

State Estimate of Wind Turbine Blades Using Geometrically Exact Beam Theory

As wind turbine blades fatigue, the blade’s dynamic response to loading may be expected to change. The kinematic quantities that exhibit significant changes are important for wind turbine blade operation from the perspective of measurement, estimation, and performance or even life cycle prediction. A state estimate providing accurate information on these features would lead to better estimates of remaining fatigue life and provide valuable information to the turbine control systems for the purpose of maximizing total energy output of a wind turbine system. In this work, we implement an observer for state estimation of nonlinear systems using the system Jacobian to correct the system output by updating the force input to a reference model in an iterative Newton-Raphson scheme. We apply this method to a surrogate wind turbine blade modeled using a geometrically exact beam theory to estimate its state given available measurements. LA-UR-12-25441.