Additively manufactured lattice metamaterials offer design versatility in strength and energy absorption and provide an additional degree of freedom through the selection of the lattice topology. Under quasistatic loading, the unit cell structure can strongly affect the stiffness, yield, and post-yield behavior, but whether and to what degree the effect of lattice topology persists into dynamic loading scenarios, up to the compaction shock regime, has not been established. LLNL's ALE3D hydrocode was used to perform a computational investigation of dynamic loading in multiple lattice types, including the gyroid, octet, Schwarz D, and rhombic dodecahedron, under impact velocities from 0.25 to 2.25 km/s. Shock Hugoniots for each lattice topology are generated and compared, suggesting that above a critical velocity, distinctions between architectures may not persevere and compacted lattices behave similarly. To investigate the transition between topology-dependent quasistatic compression and the topology-independent regime above the critical velocity, a onedimensional elastic-linear hardening plasticity-densified solid (E-LHP-DS) shock model for lattice materials was developed that relies upon confined compression to link the quasistatic and shock mechanics. Unlike similar works, the model does not assume rigid behavior prior to yield or locking behavior at densification, allowing a richer exploration of lattice mechanics. With only six parameters, the analytical model simultaneously fit quasistatic confined compression simulations for relative densities 0.1 <= rho <= 0.9 and predicted dynamic compaction behavior to traverse several distinct shock modes, each defined by a critical impact speed (equivalently, critical stresses). Comparing the numerical results to the one-dimensional E-LHP-DS shock model predictions suggests that the topology-independence under strong shocks is linked to the onset of densification, which can be predicted based on quasistatic confined compression results.

Owing to their ability to provide tunable mechanical responses, lattice materials are frequently studied to elucidate their response to static and dynamic loads. However, these roles are typically in opposition: static loads must be supported sufficiently far away from the onset of buckling or yielding, whereas dynamic loads are typically ameliorated by crushing of the lattice, which provides excellent energy-absorption due to the large plastic deformation accompanying densification. In contrast, this work considers the octet truss as an exemplar topology, in a structural role where it must simultaneously support static loads while enduring high-amplitude impulsive loads. This study focuses on the ability to withstand impulsive loads without yielding, an essential prerequisite to enduring dual loading. Computational studies using the ALE3D hydrocode were performed to examine the response of the octet truss under a short temporal width impulse shape associated with laser-driven shocks. A key finding was that covering the lattice with a solid face sheet and treating this face sheet thickness as a design variable allows the Taylor-like pulse to be attenuated prior to entering the weaker lattice, at the cost of added mass up front. Experimental validation was accomplished by laser-driven shock testing, using octet trusses printed out of Ti-5Al-5V-5Mo-3Cr. The results show that for a given quantity of mass, the attenuation is maximized when as much mass as possible is moved into the face sheet, leaving a more slender lattice structure. The effect of placing mass in the face sheet rather than lattice beams dominates the effect of relative density, to the point where a low-mass structure with most of the mass concentrated in the face sheet can outperform a high-mass structure with most of the mass in the lattice. By further understanding the propagation of short pulse width waves within under-dense structures, this study expand the domain of applicability of such structures, including lattice materials, to challenging dual-loading regimes spanning decades of strain rates.

The ability to differentiate between atmospheric radionuclide signatures from underground nuclear explosions (UNEs) and signals from other sources, such as medical isotope-production facilities and nuclear reactors, can be critical to the detection and monitoring of unannounced, low-yield nuclear events. Signatures having anomalously high amplitudes, compared to background levels, remain the best indicator in screening for a UNE. However, isotopic composition can further validate a suspected UNE signature, but separation from any atmospheric background composition is first necessary. To date, evaluating the challenges of performing this separation has typically involved comparing an observed background with a highly idealized deterministic model of radioxenon signature production by a UNE that does not consider the influence of post-detonation chemical/physical processes in the detonation cavity or the subsequent gas transport mechanisms that can also affect the isotopic composition of the detected gas signature. In addition, purely deterministic models, as previously employed, overlook the uncertainty inherent in estimating critical parameters characterizing the UNE and its detonation environment. In this paper, we create detailed, multi-parameter models of radionuclide evolution using the widely accepted England and Rider post-detonation radionuclide decay-chain network coupled to detailed models simulating physical production and transport processes affecting the gas signature. Because these models are governed by uncertain parameters including barometric fluctuations, realistic ranges of variation for each of the parameters influencing isotopic composition are then defined. A Latin-Hypercube sampling approach is used to obtain a random distribution of isotopic production and gas transport results associated with a given value of each parameter. We apply these results to background histories of two stations, one providing 4-isotope background measurements and the other providing two-isotope measurements associated with the 2013 DPRK announced UNE.

On December 5, 2022, an indirect drive fusion implosion on the National Ignition Facility (NIF) achieved a target gain G_{target} of 1.5. This is the first laboratory demonstration of exceeding "scientific breakeven" (or G_{target}>1) where 2.05 MJ of 351 nm laser light produced 3.1 MJ of total fusion yield, a result which significantly exceeds the Lawson criterion for fusion ignition as reported in a previous NIF implosion [H. Abu-Shawareb et al. (Indirect Drive ICF Collaboration), Phys. Rev. Lett. 129, 075001 (2022)PRLTAO0031-900710.1103/PhysRevLett.129.075001]. This achievement is the culmination of more than five decades of research and gives proof that laboratory fusion, based on fundamental physics principles, is possible. This Letter reports on the target, laser, design, and experimental advancements that led to this result.

Granular material is showing very often in geotechnical engineering, petroleum engineering, material science and physics. The packings of the granular material play a very important role in their mechanical behaviors, such as stress-strain response, stability, permeability and so on. Although packing is such an important research topic that its generation has been attracted lots of attentions for a long time in theoretical, experimental, and numerical aspects, packing of granular material is still a difficult and active research topic, especially the generation of random packing of non-spherical particles. To this end, we will generate packings of same particles with same shapes, numbers, and same size distribution using geometry method and dynamic method, separately. Specifically, we will extend one of Monte Carlo models for spheres to ellipsoids and poly-ellipsoids.

The Material Point Method (MPM) has appealing attributes for simulations involving large deformation of materials with history-dependent constitutive laws. It avoids the mesh tangling errors of Lagrangian finite element methods, as well as the advection errors typical of Eulerian or ALE methods. Recent developments in the MPM have led to a better understanding of the error and dissipation that arise in the particle-to-grid and grid-to-particle mapping, but significant error and high-frequency noise can still occur in simulations involving strong shock waves. This error appears to be associated with the kinematics on the background computational grid common to all MPM implementations. An investigation of forward- and reverse-ballistic impact simulations has revealed that sub grid-scale variations in the material state arise as the shock front enters a grid cell, and that this non-equilibrium state can persist unless the background grid has some means to relax the resultant particle-scale noise. Relaxation can occur when (i) the background grid shape functions have non-constant gradient, (ii) the shocked material has a sufficiently high velocity relative to the background grid, or (iii) the velocity field is enriched with additional degrees of freedom for treating weak discontinuities between particles.

Micromechanics theories such as Mori–Tanaka's approximation and Herve-Zaoui's layered-inclusion approximation have been used extensively to predict homogenized stiffness, inclusion stresses, and matrix strains in various composites. While these theories accurately predict homogenized properties, the accuracy of their predictions of stresses and strains within individual phases of cementitious composites has not been assessed with experimental measurements or used to infer phase properties. Here, we therefore use in-situ X-ray tomography, 3D X-ray diffraction, and digital volume correlation to evaluate homogenized stiffness, inclusion stresses, and matrix strains in two cementitious composites. We compare measurements with predictions of Mori–Tanaka's mean-field approximation and Herve-Zaoui's layered-inclusion approximation. We provide some of the first direct support that these micromechanics theories can accurately predict both homogenized sample stiffness and individual phase responses. We also show that combining in-situ X-ray measurements with these theories provides a novel route for inferring the properties of specific phases.

Investigations of shock compression of heterogeneous materials often focus on the shock front width and overall profile. The number of experiments required to fully characterize the dynamic response of a material often belie the structure–property relationships governing these aspects of a shock wave. Recent observations measured a pronounced shock-front width on the order of 10 s of ns in particulate composites. Here, we focus on particulate composites with disparate densities and investigate whether the mechanical interactions between the phases are adequate to describe this emergent behavior. The analysis proceeds with a general Mie–Grüneisen equation of state for the matrix material, a general drag force law with general power-law scaling for the particle-matrix coupling of the phases, and a volume fraction-dependent viscosity. Lie group analysis is applied to one-dimensional hydrodynamic flow equations for the self-consistent interaction of particles embedded in a matrix material. The particle phase is characterized by a particle size and volume fraction. The Lie group analysis results in self-similar solutions reflecting the symmetries of the flow. The symmetries lead to well-defined scaling laws, which may be used to characterize the propagation of shock waves in particle composites. An example of the derived scaling laws for shock attenuation and rise time is shown for experimental data on shock-driven tungsten-loaded polymers. A key result of the Lie analysis is that there is a relationship between the exponents characterizing the form of the drag force and the exponent characterizing the shock velocity and its attenuation in a particulate composite. Comparison to recent experiments results in a single exponent that corresponds to a conventional drag force.

This article presents a novel approach to modeling beam elements that accommodate large displacements/rotations in the context of the material point method (MPM). The MPM is a hybrid Lagrangian/Eulerian approach for solving solid mechanics problems involving extremely large deformations and rotations. A solid body is discretized into a set of Lagrangian material points, called particles in the MPM. The equations of motion are solved on a fixed Eulerian background grid. The beam particle developed herein consists of two end nodes, each possessing three translational and three rotational degrees of freedom (DOF) in 3D. The end nodes are tracked and define the evolving beam particle domain. As in the conventional MPM, the linear momentum is conserved on the background grid, but here we also use this grid to enforce C1 continuity between beam particles by mapping angular velocities and accelerations for the beam rotational DOF to/from the background grid. The contact interactions between the beam particles are treated using multiple velocity fields on the background grid. Spatial nodes representing the spatial extent of the beam particle are introduced, which allows for collision detection and associated frictional contact. The effectiveness of the proposed approach is demonstrated through a series of numerical examples. Beam structures subjected to large displacements and rotations are compared with analytical/numerical solutions, in which good agreement is obtained.

Quantifying the ways in which local particle rearrangements contribute to macroscopic plasticity is one of the fundamental pursuits of granular mechanics and soft matter physics. Here we examine local rearrangements that occur naturally during the deformation of three samples of 3D granular materials subjected to distinct boundary conditions by employing in situ x-ray measurements of particle-resolved structure and stress. We focus on five distinct rearrangement measures, their statistics, interrelationships, contributions to macroscopic deformation, repeatability, and dependence on local structure and stress. Our most significant findings are that local rearrangements (1) are correlated on a scale of three to four particle diameters, (2) exhibit volumetric strain-shear strain and nonaffine displacement-rotation coupling, (3) exhibit correlations that suggest either rearrangement repeatability or that rearrangements span multiple steps of incremental sample strain, and (4) show little dependence on local stress but correlate with quantities describing local structure, such as porosity. Our results are presented in the context of relevant plasticity theories and are consistent with recent findings suggesting that local structure may play at least as important of a role as local stress in determining the nature of local rearrangements.

Continuum modeling of granular media is made possible by the existence of a length scale at and above which grain-resolved properties can be meaningfully homogenized. Progress has been made in identifying such length scales relevant to local structural properties such as porosity. However, a systematic analysis of scales above which different mechanical properties can be homogenized has yet to emerge. Here, X-ray tomography and 3D X-ray diffraction data are examined to identify such length scales. The data was obtained in-situ in compressed granular materials with rigid and flexible confinement. The experimental data are supplemented with validated discrete element simulations which examine different system sizes and different boundary conditions. Our study reveals a hierarchy in the length scales of granular solids, with lengths governing structural variables being the shortest, lengths of stress variables being intermediate, and lengths of energy dissipation being the longest. All structural and mechanical length scales obey a power law based on the theory of Geostatistics, implying that the length scales can be found by analyzing samples significantly smaller than the length scales themselves. The length scales are also found to be sensitive to boundary conditions, implying that they are extrinsic features of granular media.

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. As recently reported, a burning plasma state, where the alpha-heating in the plasma is the primary source of heating, was achieved in laboratory experiments. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin ``burn propagation'' into surrounding cold fuel, enabling the possibility of high energy gain. While ``scientific breakeven'' (i.e. unity target gain) has not yet been achieved, this talk reports the first controlled fusion experiment on the National Ignition Facility to produce capsule gain greater than unity (here 5.8) and reach ignition by many different formulations of the Lawson criterion. In the talk, we will discuss some key basic physics inertial confinement fusion (ICF) principles behind the burning plasma and ignition results as well as discuss future challenges.

Predicting the mechanical performance of concrete from composition and processing remains a grand challenge. Our work focuses on predicting damage evolution in ultrahigh-performance concrete (UHPC), where analysis is challenging due to a lack of separation in the length scales of heterogeneity and a limited availability of data to describe the strength and failure of individual phases. Cement-hydration modeling informs segmentation of CT images from mechanical tests, providing a highly resolved 3-D representation of microstructure and damage evolution. Mesoscale simulations of damage in UHPC reveal how the relative strength of cement phases affects patterns of intra- and inter-granular fracture. We find that the nano-indentation hardness of individual phases is not necessarily a predictor of the relative shear strength needed to match experimentally observed fracture patterns. The mesoscale model predicts complicated material responses as emergent phenomena from relatively simple models of individual phases - a path towards forward modeling UHPC constitutive response.

The existence of a deep borehole in the Earth's crust disturbs the local stresses and creates a stress concentration that may result in breakout and damage to the borehole. Maintaining wellbore integrity mitigates environmental impacts such as groundwater contamination, gas leakage to the atmosphere, and fluid spills and seepage at the surface. In this paper, the stability of deep boreholes (5 km) is examined by laboratory experiments and numerical models in the context of nuclear waste disposal in Israel. Two rock types in southern Israel are considered: the crystalline basement (granite) and the Zenifim Formation (arkose). A series of room-temperature triaxial rock deformation experiments were conducted at different confining pressures. This mechanical characterization was then used to parameterize the elastic properties and damage behavior of the rocks. This facilitated modeling the stability of the deep boreholes by two different formulations of damage rheology: a dynamic-oriented formulation used to model deformation immediately after the creation of the open hole and a quasi-static formulation used to model longer stress corrosion regime. The calibrated modeling results indicate greater stability with Zenifim arkose than the crystalline granite for deep borehole conditions despite the granite having a greater triaxial compressive strength. Dissipation associated with dilation and porous compaction in the arkose during deformation plays a significant stabilizing role in the borehole compared to crystalline rocks. These results suggest that common strength-based borehole stability assessment may lead to inaccurate predictions. Three-dimensional modeling of bottom-hole stress conditions and the effects of transient borehole geometry show conventional two-dimensional analysis may not be conservative when predicting borehole damage.

The objective of this paper is to develop an analytical expression for temperature in a thermodynamically consistent model with a Mie-Gruneisen equation for pressure. This expression for the temperature is obtained for both constant and variable specific heat models and can be used in common models for the material yield strength that include thermal softening due to increase in temperature. Also, the material parameters in the model can easily be calibrated to match known experimental data and the model has been used to determine the value of the shock pressure at the onset of melting during shock loading.

Accurate models for the strength of granular materials during compaction are essential for modeling of wellbore completion, ballistic penetration, and asteroid impact, and in order to infer equation of state or compaction relationships (porosity vs. pressure) from a Hugoniot measurement of 1-D shock stress. Whereas solid materials may exhibit a clear Hugoniot elastic limit in shock velocimetry measurements, dynamic shear strength measurements for granular materials are non-trivial. It is commonly assumed that the shear strength of a granular material approaches that of the solid phase as the porosity goes to zero, with a possible reduction of strength due to thermal softening, phase change, or damage. Using mesoscale simulation we investigate the evolution of shear stress during compaction for granular SiO2, to assess the validity of this assumption. The results show a significant drop in continuum shear stress due to two purely mechanical effects. The first is a local shear relaxation due to the increase in confining stress that occurs from contact stresses at internal pore surfaces during pore collapse. The second, and more significant effect, is due to spatial variance in the direction of the deviatoric stress tensors, whereby the magnitude of the spatially averaged deviatoric stress tensor can be much less than the spatially averaged magnitude of deviatoric stress, i.e., the continuum shear stress (apparent strength) for the material may drop towards zero during dynamic compaction, even if locally there are high stresses and material at yield. This stress relaxation occurs even in the absence of thermal softening, phase change, damage, or melt. Importantly, unlike those mechanisms, this shear relaxation effect is reversible, and shear stress can recover under reshock or sustained shear loading.

Dataset containing supporting materials for article titled "The influence of packing structure and interparticle forces on ultrasound transmission in granular media". An enclosed PDF file describes the data.