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We describe models for the behavior of hot-pressed boron carbide that is subjected to extreme dynamic environments such as ballistic impact. We first identify the deformation and failure mechanisms that are observed in boron carbide under such conditions, and then review physics-based models for each of these mechanisms and the integration of these models into a single physics-based continuum model for the material. Atomistic modeling relates the composition and stoichiometry to the amorphization threshold, while mesoscale modeling relates the processing-induced defect distribution to the fracture threshold. The models demonstrate that the relative importance of amorphization and fracture are strongly dependent on the geometry and impact conditions, with the volume fraction of amorphized material being unlikely to be significant until very high velocities (~3 km/s) are reached for geometries such as ball impact on plates. These connections to the physics thus provide guidelines for the design of improved boron carbide materials for impact applications.

This work develops RVE-based finite element (FE) models to understand how the microstructure of Ultra-High-Molecular-Weight Polyethylene (UHMWPE) composites affects the overall mechanical behaviour of the laminate. The models represent a [0/90] configuration with a random fibre packing sequence through the thickness of each ply, as well as a variation in the cross-sectional shape of the fibres, both obtained from laminate cross section micrograph images. The uncertainty of interface properties and its effects on the overall mechanical response is also investigated. The response of the fibre is assumed to be viscoelastic-plastic and transversely isotropic and the three-dimensional constitutive behaviour is implemented through a user-defined subroutine in the LS-DYNA explicit FE code. Constituent properties are calibrated using experimental results on UHMWPE single fibres and a generic thermoplastic polyurethane resin material. The numerical results generated by the RVE models are validated against experimental results found in the open literature. Special focus was given to the in-plane shear and out-of-plane compression response of UHMWPE laminates. Our results can be used as inputs in a homogenised continuum level model, to express the effect of uncertainties which propagate from the microstructure to the macro-scale response.

Computational models of the brain and its biomechanical response to skull accelerations are important tools for understanding and predicting traumatic brain injuries (TBIs). However, most models have been developed using experimental data collected on animal models and cadaveric specimens, both of which differ from the living human brain. Here we describe efforts to noninvasively measure the biomechanical response of the human brain with MRI—at non-injurious strain levels—and generate data that can be used to develop, calibrate, and evaluate computational brain biomechanics models. Specifically, this paper reports on a project supported by the National Institute of Neurological Disorders and Stroke to comprehensively image brain anatomy and geometry, mechanical properties, and brain deformations that arise from impulsive and harmonic skull loadings. The outcome of this work will be a publicly available dataset ( http://www.nitrc.org/projects/bbir ) that includes measurements on both males and females across an age range from adolescence to older adulthood. This article describes the rationale and approach for this study, the data available, and how these data may be used to develop new computational models and augment existing approaches; it will serve as a reference to researchers interested in using these data.

In this work, AgGaS2 thin films are investigated and their applications in photovoltaic cells is attempted. Less porosity and fine grains formed by pulse plating technique compared to dc plating technique. The film thickness measured by surface profilometer and its ranging from 500 to 950 nm with increase in duty cycle. The single-phase polycrystalline nature noticed in X-ray diffraction pattern. Composition of films estimated by Energy dispersive X-ray analysis (EDAX). In XPS studies, peaks in the Ag 3d spectrum at 367.34 eV and 373.42 eV could be the binding energies of Ag3d5/2 and Ag3d3/2, respectively. At 1146 eV and 1118 eV, respectively, the peaks of Ga 2p1/2 and Ga 2p3/2 were noted. At 162.1 eV and 153.1 eV, the sulphur doublet occurred. AFM studies observed that the grain size and surface roughness decreases as the duty cycle increases. The transport properties at room temperature measured, the film resistivity and mobility increased with increase of duty cycle. The carrier density decreases with duty cycle increases. Studies of photo electrochemical cells showed an efficiency of 5.85%. Mott-Schottky plots yield a flat band potential ranging from 0.8 V to 1.00 V in the range of (SCE). The film showed p-type conductivity.

We examine the statistical distribution of critical nucleation pressures necessary to dynamically grow voids during the spall failure of an AZ31B magnesium alloy. The approach uses laser-driven micro-flyers to generate spall over times of the order of tens of nanoseconds, allowing us to focus on void nucleation processes rather than void coalescence processes. Our methodology combines quantitative postmortem characterization of void mediated failure with time-resolved interferometry of the failure event, and reveals the dynamics of the failure process. We infer the distribution of the underlying nucleation pressures and explore the associated strain rate dependence of spall strength in these alloys.

We have investigated the deformation and failure mechanisms of a rolled AZ31B magnesium alloy under both quasi-static ( $$10^{-3}\,{\text {to}}\,10^{-2}\,{\text{s}}^{-1}$$ ) and dynamic ( $$10^{3}\,{\text{s}}^{-1}$$ ) compressive loading. The observed anisotropy in the plastic response originates from the different in-plane and out-of-plane deformation mechanisms activated when the loading orientation changes. For plasticity driven by extension twins, extension twin–twin boundaries were found to dominate in the late stage of deformation. Severe localization was observed after deformation mediated by pyramidal slip and contraction twinning compared to the deformation mainly caused by extension twinning. The fracture process was established as a function of both loading rate and loading orientation. Moreover, the intermetallic inclusions in the material, which appeared to be hard and brittle, might lead to macroscopic fracture under compressive loading.

Computational models of the human head are promising tools for the study and prediction of traumatic brain injuries (TBIs). Most available head models are developed using inputs (i.e., head geometry, material properties, and boundary conditions) derived from ex-vivo experiments on cadavers or animals and employ linear viscoelasticity (LVE)-based constitutive models, which leads to high uncertainty and poor accuracy in capturing the nonlinear response of brain tissue under impulsive loading conditions. To resolve these issues, a framework for the development of fully subject-specific 3D human head models is proposed, in which model inputs are derived from the same living human subject using a comprehensive in-vivo brain imaging protocol, and the viscous dissipation-based visco-hyperelastic constitutive modeling framework is employed. Specifically, brain tissue material properties are derived from in-vivo magnetic resonance elastography (MRE), and full-field strain-response of brain under rapid rotational acceleration is obtained from tagged MRI, which is used for model validation. The constitutive model comprises the Ogden hyperelastic strain energy density and the Upadhyay-Subhash-Spearot viscous dissipation potential. The simulated strain-response is compared with experimental data and with predictions from subject-specific models employing two commonly used LVE-based constitutive models, using a rigorous validation procedure that evaluates agreement in spatial strain distribution, temporal strain evolution, and differences in maximum values of peak and average strain. Results show that the head model developed in this work reasonably captures 3D brain dynamics, and when compared to LVE-based models, provides improvements in the prediction of peak strains and temporal strain evolution.

Computational models of the human head are promising tools for estimating the impact-induced response of brain, and thus play an important role in the prediction of traumatic brain injury. Modern biofidelic head model simulations are associated with very high computational cost, and high-dimensional inputs and outputs, which limits the applicability of traditional uncertainty quantification (UQ) methods on these systems. In this study, a two-stage, data-driven manifold learning-based framework is proposed for UQ of computational head models. This framework is demonstrated on a 2D subject-specific head model, where the goal is to quantify uncertainty in the simulated strain fields (i.e., output), given variability in the material properties of different brain substructures (i.e., input). In the first stage, a data-driven method based on multi-dimensional Gaussian kernel-density estimation and diffusion maps is used to generate realizations of the input random vector directly from the available data. Computational simulations of a small number of realizations provide input-output pairs for training data-driven surrogate models in the second stage. The surrogate models employ nonlinear dimensionality reduction using Grassmannian diffusion maps, Gaussian process regression to create a low-cost mapping between the input random vector and the reduced solution space, and geometric harmonics models for mapping between the reduced space and the Grassmann manifold. It is demonstrated that the surrogate models provide highly accurate approximations of the computational model while significantly reducing the computational cost. Monte Carlo simulations of the surrogate models are used for uncertainty propagation. UQ of strain fields highlight significant spatial variation in model uncertainty, and reveal key differences in uncertainty among commonly used strain-based brain injury predictor variables.

Description, Twin boundary migration transitions from atomic shear to atomic shuffle at high stresses and strain rates., Twinning is a prominent deformation mode that accommodates plasticity in many materials. This study elucidates the role of deformation rate on the atomic-scale mechanisms that govern twin boundary migration. Examination of Mg single crystals deformed under quasi-static compression was compared with crystals deformed via plate impact. Evidence of two mechanisms was uncovered. Atomic-level observations using high-resolution transmission electron microscopy revealed that twin boundaries in the -axis quasi-statically compressed single crystals are relatively smooth. At these modest stresses and rates, the twin boundaries were found to migrate predominantly via shear (i.e., disconnection nucleation and propagation). By contrast, in the plate-impacted crystals, which are subjected to higher stresses and rates, twin boundary migration was facilitated by local atomic shuffling and rearrangement, resulting in rumpled twin boundaries. This rate dependency also leads to marked variations in twin variant, size, and number density in Mg. Analogous effects are anticipated in other hexagonal closed-packed crystals.

Boron carbide is an excellent protection material but it is also very brittle, and granular flow, triggered by progressive fracture during impact, greatly influences its ballistic performance. Transmission electron microscopy observations of samples recovered from controlled pressure-shear plate impact experiments have elucidated the extent to which local failure mechanisms accompany dynamic granular flow. These observations point to the interplay of quasiplasticity, the formation of nanoscale amorphous bands, cleavage and intragranular cracking. Association of these mechanisms with granular flow and identification of a critical velocity for the onset of amorphization provides a foundation for modeling and predicting armor performance.

This is the first part of a 2-paper series describing a generalized multi-mechanism constitutive model for the dynamic failure of quasi-brittle materials (e.g., ceramics and geomaterials), and focus on the poorly understood mechanism of amorphization. Amorphization has long been recognized as a significant deformation mechanism in a variety of quasi-brittle materials. In this part, we develop an amorphization model based on observations in both experiments and atomistic simulations. We consider the onset of amorphization bands and the deformation of amorphous phases inside the bands. The sliding along amorphization bands introduces damage to the materials, which eventually results in material failure, and the response of the failed material is described as granular flow. Using boron carbide (BC) as a representative material, we determine the material parameters and validate the model using plate impact experiments under different shock conditions. Finally, we use the model to predict the response of BC under dynamic Vickers indentation, and compare the simulation results with experiments from the literature to demonstrate the capability of the proposed model.

Plastic flow is studied as a function of strain rate for uniaxial compression along the principal directions of an AZ31B magnesium alloy processed by equal channel angular extrusion across 8 decades of strain rate. The effect of strain rate on flow stress is found to be a function of loading orientation. Work hardening rates are also found to vary with strain rate and orientation. In situ high-speed imaging reveals the prominence of heterogeneous deformation at small macroscopic strains. Postmortem electron microscopy further shows significant texture reorientation under multiple orientations of loading—some aided by twins and others by dislocation slip. It is observed that twin activity is more favorable at high strain rates. While very little twinning and significant grain growth were observed from complementary quasi-static experiments by Krywopusk et al. (unpublished), we observe significant twinning with negligible grain growth at high strain rates.

We examine the influence of shear anisotropy of brain tissue on the potential for mild traumatic brain injury. First we develop a new constitutive description for the white matter in the brain that can capture the anisotropic behavior of the white matter in both tension and shear. The material parameters for the models are determined using a set of three experiments already published in the literature. The calibrated and parameterized model is then implemented in a computational (finite element) model of the head. This computational model is two-dimensional and is used to simulate a previously published injury-causing event in the National Hockey League, using axonal strain as criterion to assess the level of diffuse axonal injury. It is demonstrated that the inclusion of shear anisotropy affects both the nature and the extent of predicted injury. Further, the locations of the predicted injury are more consistent with observations in the literature.

We describe a laser-launched micro-flyer apparatus designed for spall strength measurement. The launcher uses a single pulse from a pulsed laser that is stretched in time to nominally 20 nanoseconds using an optical ring cavity, while inexpensive multi-lens arrays are used to spatially homogenize the beam. The velocimetry technique that we developed for the experiment provides the required sub-nanosecond time resolution. We demonstrate the capability of the apparatus to interrogate the spall strength of AZ31B Mg alloy thin foils, a material system with potential applications as a lightweight protection material. Numerical simulations and fractography are very useful to determine the quality of the experimental data and help to interpret our results. The simulations and fractography analyses of the experiments suggest that the short shock pulse duration in the experiment causes incipient spallation. The short pulse also likely introduces stochasticity to the measured spall strength through limited activation of failure mechanisms within the samples. The shocked AZ31B Mg alloy has spall strengths that are greater than previously reported figures for fine grained Mg alloys, likely because the laser based system achieves higher strain rates than in prior work on this material.

We present a hybrid approach for simulating hypervelocity impacts onto asteroids. The overall system response is separated into two stages based on their different characteristic timescales. First, the short-timescale fragmentation phase is simulated using a modified version of the Tonge–Ramesh material model implemented in a Material Point Method framework. Then, a consistent hand-off to an N -body gravity code is formulated to execute the long-timescale gravitational reaccumulation calculation. We demonstrate this hybrid approach by considering the 5 km/s head-on impact of a 1.21 km diameter basalt impactor on a 25 km diameter target asteroid. The impact event resulted in the fragmentation, but not complete disruption, of the entire target. A granular core is observed at the end of the fragmentation simulations, which acts as a gravity well over which reaccumulation occurs in the N -body simulations. Our results suggest that disruption thresholds for rocky asteroids are higher when energy-dissipating mechanisms such as granular flow and pore collapse are included.

An understanding of the dynamic failure of damaged ceramics is important in protection applications, where the interaction of the projectile with cracked material is a contributing factor in the overall system performance. In this paper, we investigate the effects of pre-existing internal cracks on the quasi-static and dynamic compressive behavior of an advanced ceramic. We present experiments on a hot-pressed boron carbide in which internal cracks are generated through thermal shocking after which the initial material damage is quantified. Damage characterization was performed via Resonant Ultrasound Spectroscopy (RUS) and high-resolution Computed Tomography (CT). A computational procedure is developed to determine the three-dimensional structure of the internal crack network in the initially damaged material from a series of CT images. The failure and strength of the material is then evaluated experimentally. The uniaxial compressive strength of the predamaged boron carbide samples is determined under both quasistatic and dynamic loading scenarios and this is correlated with the pre-existing crack structure as determined by CT. Damaged samples were found to have average compressive strength of 1.14 GPa in quasistatic loading and 0.68 GPa in dynamic loading compared to 2.98 ± 0.6 GPa and 3.70 ± 0.3 GPa for pristine material, respectively. High speed photography employed during dynamic testing indicates that pre-existing cracks may lead to different failure mechanisms from what is normally seen in pristine material. Ultimately, these insights can be used to design improved materials that are more resistant to dynamic failure.

Suhas Eswarappa Prameela, K. T. Ramesh and Tim Weihs highlight how students and postdocs can develop their competencies and skills during research collaboration.

Advances in brain imaging and computational methods have facilitated the creation of subject-specific computational brain models that aid researchers in investigating brain trauma using simulated impacts. The emergence of magnetic resonance elastography (MRE) as a non-invasive mechanical neuroimaging tool has enabled in vivo estimation of material properties at low-strain, harmonic loading. An open question in the field has been how this data can be integrated into computational models. The goals of this study were to use a novel MRI dataset acquired in human volunteers to generate models with subject-specific anatomy and material properties, and then to compare simulated brain deformations to subject-specific brain deformation data under non-injurious loading. Models of five subjects were simulated with linear viscoelastic (LVE) material properties estimated directly from MRE data. Model predictions were compared to experimental brain deformation acquired in the same subjects using tagged MRI. Outcomes from the models matched the spatial distribution and magnitude of the measured peak strain components as well as the 95th percentile in-plane peak strains within 0.005 mm/mm and maximum principal strain within 0.012 mm/mm. Sensitivity to material heterogeneity was also investigated. Simulated brain deformations from a model with homogenous brain properties and a model with brain properties discretized with up to ten regions were very similar (a mean absolute difference less than 0.0015 mm/mm in peak strains). Incorporating material properties directly from MRE into a biofidelic subject-specific model is an important step toward future investigations of higher-order model features and simulations under more severe loading conditions. Statement of Significance: The study presents a method to calibrate and evaluate subject-specific finite element brain models using a combination of advanced magnetic resonance imaging (MRI) data. The imaging data is acquired in human volunteers and includes anatomical MRI, magnetic resonance elastography (MRE), and tagged MRI to generate subject-specific geometry, calibrate subject-specific material properties, and evaluate simulation response using subject-specific brain deformation. This dataset of MRE and tagged MRI allows for a unique evaluation of whether material properties from MRE can be used to create biofidelic computational models of the human brain. The study develops a calibration procedure to readily calculate linear viscoelastic material parameters from MRE data and then provides a sensitivity study of the effect of mechanical heterogeneity of the brain on simulation response. The calibrated computational models are used to simulate each subject's tagged MRI experiment; the results show good agreement between the simulated and experimental strain fields. The presented study and results will be informative in guiding the calibration of subject-specific computational brain model from experimental MRE data. The processed MRI, MRE, and tagged MRI data are publicly available at https://www.nitrc.org/projects/bbir/.

Both engineered and natural materials may be subjected to extremes of elevated pressure, temperature, and strain rate due to shock loading; examples include ballistic impact of projectiles on armor, planetary impacts, and high-speed machining operations. Experimental techniques for ascertaining the macroscopic response of materials to shock loading are well established, but insight into fundamental mechanisms of deformation requires the ability to characterize the evolution of microstructure in real time. Experiments in which specimens are recovered and characterized after shock loading have been widely used to understand structure-property relationships. But these shock recovery experiments only reveal information at the end state of the material, from which the evolution of the structure during unloading must be inferred. There is always the possibility that the structure of the material continues to evolve during unloading, leading to a potential misunderstanding of the structural evolution during shock compression and release. In this paper we describe the results of shock recovery and time-resolved in situ x-ray diffraction studies of deformation twinning in an extruded fine-grained AMX602 magnesium alloy. The samples were shock compressed along the plate normal and extrusion directions, then released back to ambient conditions. Analysis of the microstructure before and after shock loading indicates a substantial change in crystallographic texture reflecting substantial deformation twinning. Texture evolution from in situ synchrotron x-ray diffraction measurements show significant twinning during shock compression followed by detwinning during stress release. These results not only provide insight into the complex twinning-detwinning behavior of this particular alloy, but also illustrate the utility of in situ characterization for bridging the knowledge gap between shock recovery experiments and the transient behavior of materials during shock loading more generally.

An increase in arterial pressure within the cerebral vasculature appears to coincide with ischemia and dysfunction of the neurovascular unit in some cases of traumatic brain injury. In this study, we examine a new mechanism of brain tissue damage that results from excessive cerebral arteriole pressurization. We begin by considering the morphological and material properties of normotensive and hypertensive arterioles and present a computational model that captures the interaction of neighboring pressurized arterioles and the surrounding brain tissue. Assuming an axonal strain-induced injury criterion, we find that the injury depends on vessel spacing, proximity to an unconfined free surface, and the relative difference in stiffness between the arterioles and the surrounding tissue. We find that a steeper heterogeneity (stiffer vessels surrounded by softer brain tissue) causes larger axial strains to develop at some distance from the arteriole wall, within the brain parenchyma. For a more gradual heterogeneity (softer vessels), we observe more larger strain fields close to the arteriole walls. Both deformation patterns are comparable to damage seen in previous pathology studies on postmortem TBI patients. Finally, we use an analytical model to approximate the interplay between internal pressure, arteriole thickness, and the variation in mechanical properties of arterioles.

The mechanical response of single crystal silicon carbide (SiC) of two hexagonal polytypes (six layer, 6H- and four layer, 4H) was investigated using nanoindentation. Indentations were performed on two specific crystallographic orientations of single crystals i.e., normal to the basal, ( 0001 ) and prismatic, ( 10 1 ¯ 0 ) planes, in the load range between 25 mN and 500 mN. A significant anisotropy in the hardness is observed with the basal orientations showing a higher hardness compared to prismatic orientations. In both orientations, the 6H-SiC polytype exhibits higher hardness than the 4H-SiC polytype. It is also observed that the hardness decreases with increasing indentation load, suggesting that SiC crystals exhibit indentation size effect. However, unlike hardness, elastic modulus is independent of indentation load and the elastic anisotropy is insignificant. Severe cracking, particularly at higher indentation loads is noticed near the edges of the indentation imprints. The indentation fracture toughness, K I C i computed from the imprints shows slightly higher values for 6H-SiC compared to the 4H-SiC. However, for both the polytypes, a slightly higher K I C i is observed for basal indentations compared to the prismatic ones.

A physics-based finite strain crystal plasticity constitutive model for body-centered-cubic (BCC) single crystals is developed to capture the strong temperature, rate, and orientation dependence of mechanical behavior. The key features of the model include twinning-anti-twinning asymmetry of shearing resistance, a yield criterion that incorporates atomistics-informed non-Schmid effects, and a flow rule formulated based on the theory of thermally activated motion of screw dislocations via nucleation of double kinks. The implementation of the constitutive model in a finite-element program is briefly discussed. The material constants in the model are determined by calibrating the model against literature-based experimental data on single-crystal Molybdenum subjected to uniaxial compression and uniaxial tension. Experiments of uniaxial compression on a single crystal specimen with a hole were performed for validation of the calibrated model for BCC Molybdenum. Measurements of deformations in the vicinity of the hole were used to assess the ability of the model in predicting localized deformation patterns around the hole. The model is able to effectively describe the anisotropic and temperature-dependent stress-strain response of a molybdenum crystal up to a homologous temperature of 0.3.

In-situ ultra high speed optical imaging with 200 ns temporal resolution is used to gain insight into the evolution of twinning in single crystal magnesium under dynamic loading. Under compression along the a-axis, nucleation of twins is observed to occur on two conjugate { 10 1 ¯ 2 } twin planes. Twin nucleation appears to be stress-driven with the first twins nucleating at resolved stresses of 5–7 MPa. These first twins propagate across the specimen at very high speeds of the order 1 km/s. After the first twins stop growing, twin boundary growth is observed to be very small and relatively slow. The nucleation of additional twins from the boundaries of pre-existing twins is found to be preferred over twin boundary growth at these rates of loading. As a result, twin nucleation is found to have a dominant contribution to twin volume fraction evolution at later times. Lastly, twinning is found to have a dominant contribution to the net plastic strain at these strain rates.

Pressure-shear plate impact experiments generate normal and transverse particle velocities during high strain rate deformations. Traditionally, freespace lenscoupled tabletop laser interferometry techniques are used together with diffraction gratings to interrogate the evolving velocity vector at the back face of the target plate. Recently, fiberoptic velocimetry (photon Doppler velocimetry or PDV) has become commonplace for measuring normal particle velocities above 200m/sec. In this work, we demonstrate transverse velocity detection using a modified PDV system where we subtract the measured normal velocity history from a concurrent velocity history measured at a canted angle to the target surface to obtain the transverse velocity component. This modified system is time-multiplexed to reduce the number of components, and uses an erbium doped fiber amplifier (EDFA) to boost the angled signal intensity while maintaining low noise. The system operates as a heterodyne interferometer, but features a frequency upshifted reference leg to improve data analysis at the particle velocities expected in the experiment. We demonstrate by direct comparison that this inexpensive and simple approach is as effective as traditional grating methods.

We demonstrate the use of X-ray phase contrast imaging with sub-microsecond temporal resolution to obtain quantitative visualization of dynamic fracture processes in brittle solids. We examine an amorphous solid (fused silica), a ceramic single crystal (single-crystal quartz), and a polycrystalline ceramic (boron carbide), in the form of single-edge notched specimens loaded using a three-point apparatus at nominal strain rates up to $\sim $ 800 s−1. We observe that the crack tip speed for boron carbide is relatively independent of mode I stress intensity factor rate ( $\dot {K}_{\mathrm {I}}$ ) for these rates of loading, while that of fused silica and single-crystal quartz increases with $\dot {K}_{\mathrm {I}}$ . Further, for the amorphous and single crystal cases, we observe the development of a crack tip instability in the form of crack branching as the crack tip speed approaches 45% of the Rayleigh wave speed. This suggests that strain-rate-dependent mechanisms contribute to crack branching. Such mechanisms may, in turn, affect the macroscopic fracture properties of these materials.

We report in situ time-resolved measurements of the dynamic evolution of the volume fraction of extension twins in polycrystalline pure magnesium and in the AZ31B magnesium alloy, using synchrotron x-ray diffraction during compressive loading at high strain rates. The dynamic evolution of the twinning volume fraction leads to a dynamic evolution of the texture. Although both the pure metal and the alloy had similar initial textures, we observe that the evolution of texture is slower in the alloy. We also measured the evolution of the lattice strains in each material during deformation which, together with the twin volume fractions, allows us to place some constraints on the relative contributions of dislocation-based slip and deformation twinning to the overall plastic deformation during the dynamic deformations.

The distributions of size and chemical composition in the regolith on airless bodies provides clues to the evolution of the solar system. Recently, the regolith on asteroid (25143) Itokawa, visited by the JAXA Hayabusa spacecraft, was observed to contain millimeter to centimeter sized particles. Itokawa boulders commonly display well-rounded profiles and surface textures that appear inconsistent with mechanical fragmentation during meteorite impact; the rounded profiles have been hypothesized to arise from rolling and movement on the surface as a consequence of seismic shaking. We provide a possible explanation of these observations by exploring the primary crack propagation mechanisms during thermal fatigue of a chondrite. We present the in situ evolution of the full-field strains on the surface as a function of temperature and microstructure, and observe and quantify the crack growth during thermal cycling. We observe that the primary fatigue crack path preferentially follows the interfaces between monominerals, leaving them intact after fragmentation. These observations are explained through a microstructure-based finite element model that is quantitatively compared with our experimental results. These results on the interactions of thermal fatigue cracking with the microstructure may ultimately allow us to distinguish between thermally induced fragments and impact products., 23 pages, 7 figures

Magnesium (Mg) and its alloys have been the subject of intensive scientific research and development in the communities of materials science and engineering, mechanical engineering and manufacturing. Considering their light weight and high specific strength, current and potential applications include the aerospace industry, automobiles, and vehicle and personnel armors. This range of applications demands a good understanding of the behavior under extreme conditions such as impact or high strain rate loading. The past two decades have witnessed a surge of studies of the mechanical responses of Mg and its alloys under impact loading, both experimentally and using simulations and modeling at different spatial and temporal scales. Experimental examinations at strain rates up to 1 0 7 s − 1 (shock wave loading) have been published. In terms of simulations and modeling efforts, multi-physics, multi-scale investigations, from first principles calculations (density functional theory, DFT), molecular dynamics (MD), discrete dislocation dynamics (DDD), crystal plasticity (CP) and continuum mechanics have all been explored. To address the challenges in design, manufacturing and application of Mg alloys, the US Army Research Laboratory (US-ARL) created the Materials in Extreme Dynamic Environments (MEDE) Collaborative Research Alliance (CRA) in 2012. The goal of the Metals Program within the MEDE CRA has been to observe, understand, and design the mechanisms active within Mg and Mg alloys in these extreme conditions. In this paper, fundamental aspects of plastic deformation of Mg and Mg-alloys and the history of the research efforts in experiments, modeling, and simulations available in the literature are critically reviewed. Key findings and contributions from the Materials in Extreme Dynamic Environments (MEDE) Metals Collaborative Materials Research Group (CMRG) are presented, followed by summary and future perspectives.

We present a simplified constitutive model, based on the dominant deformation mechanisms, to capture the mechanical behavior of magnesium. This approach takes into account (1) the accumulation and annihilation of 〈 a 〉 and 〈 c + a 〉 dislocations, (2) the volume fraction of extension twins, and (3) two material domains — the parent region and the twinned region. We first show that the model successfully captures the material responses of single crystal magnesium under different loading conditions. We then extend the model for application to a polycrystalline magnesium alloy. The results provide a simple and straightforward approximate model for magnesium and its alloys under various loading conditions.

Based on transmission electron microscopy results from pure Mg single crystal examined in the current work, and Mg alloys and other hexagonal metals in literature, a characteristic dislocation substructure inside 10 1 ¯ 2 twins is identified. Abundant non-basal [c] and ⟨c + a⟩ perfect dislocations, as well as basal I1 stacking faults with widths on the order of 100 nm distributed preferentially in the vicinity of a twin boundary, with a low density zone in the middle of the twin. Considering the ubiquity of 10 1 ¯ 2 twins, this characteristic dislocation substructure should be considered in modeling of hexagonal metal alloy deformation.

A finite deformation mechanism-based thermodynamically consistent constitutive framework is presented for describing the dynamic behaviors of brittle materials under impact loading. The framework is developed based upon a multiplicative decomposition of the deformation gradient in terms of multiple mechanisms, including recoverable elasticity, crack-induced damage, and other inelastic mechanisms such as subgrain and granular plasticity. The finite deformation kinematics that captures the multiple mechanisms is structured within a thermodynamically consistent framework, and the consequent coupling of the various mechanisms is articulated. Specific constitutive equations are formulated for a Mie–Gruneisen equation of state, micromechanics-based dynamic-fracture-induced damage growth, subgrain or lattice plasticity for slip and other deformation modes, and granular plasticity for granular flow and pore collapse post-fragmentation. Using hot-pressed silicon carbide (SiC-N) as the model material, this integrative model is calibrated using available experiments that interrogate specific mechanisms. The effects of loading rate, the influence of confinement, and the path-dependent constitutive behaviors of the material predicted by the model are demonstrated. The model performance at the application scale is then evaluated by simulating previously performed sphere-on-cylinder impact experiments.

Electrical response from dynamic compression tests on x-cut single crystal α-quartz specimens at strain rates of 103 s − 1 reveal nonsimilar stress-charge behavior during damage evolution. Specifically, when quartz is undergoing extensive and irreversible dynamic brittle fracture under a compressive stress impulse of up to 1.8 GPa, the effective piezoelectric stress coefficient (d) exhibits approximately a 24% increase from loading to unloading. The experimental results are examined in the framework of the theory of linear piezoelectricity in order to understand the role of increasing crack density and crystal orientation on electroelastic properties.

We examine the strengthening mechanisms within a high-strength aluminum alloy with the objective of providing guidelines for increased strength. First, we measure the mechanical behavior of the age-hardenable Al–Cu–Mg–Ag alloy known as Al 2139 in the T8 condition, and observe strengths of 500 MPa at quasistatic strain rates and average strengths of up to 600 MPa at high strain rates. Next, we explore the reasons for the high strength of this alloy by considering the contributions of various strengthening mechanisms to the total strength of the material. Finally, we develop an analytical approach to estimating the strengthening developed through the mechanism of dislocation cutting of closely spaced plate-like semi-coherent precipitates. Our results suggest that dislocation cutting of the $$\Omega$$ phase is the primary strengthening mechanism in this alloy.

Normal plate impact experiments were conducted to study the spall behavior of AMX602 magnesium alloy fabricated via Spinning Water Atomization Process (SWAP), which was followed by cold compaction and hot extrusion. Incipient spall damage in the specimens was measured at different shock conditions using 51 mm and 105 mm diameter bore gas guns. The Hugoniot Elastic Limit (HEL) was measured to be approximately 187 ± 11 MPa. The spall strengths extracted from the free surface velocity profiles of the shocked specimens was found to increase by 8% for shock stresses ranging from 1.61 to 4.53 GPa. The experimental data also revealed some inherent scatter in the spall strength data due to microstructural heterogeneity. Post-test fractographic analysis of the shock-recovered specimens suggests that the initiation of spall was most likely associated with intermetallic Al 2 Ca-bearing precipitates forming during the hot extrusion process. The subsequent spall failure propagated via cavitation events that involved the nucleation of nano- and micrometer-sized voids with very limited plastic growth.

Boron carbide has a wide range of solubility, but the effects of stoichiometry on its microstructure and mechanical response are not well understood. In this study, detailed microstructural characterization was carried out on three hot-pressed B-rich boron carbide samples. Lattice parameter measurements from XRD identified the compositions to be B 4.2 C, B 5.6 C and B 7.6 C. Local substitutional disorder was observed by Raman spectroscopy, particularly for more B-rich samples. Electron energy loss spectroscopy observations suggest that excess boron preferentially substitutes for carbon atoms in the B 11 C icosahedra; after which additional boron modifies the CBC chains. Moreover, the boron content has salient effects on boron carbide densification and microstructure. Improved densification was observed in the more B-rich samples (B 5.6 C and B 7.6 C), and there is a transition from few or no intragranular planar defects (B 4.2 C), to numerous stacking faults (B 5.6 C), to copious twins (B 7.6 C). Nanoindentation experiments revealed that the highest value for B 4.2 C is statistically larger than that for B 5.6 C or B 7.6 C, suggesting that the hardness of boron carbide is reduced by boron substitution.

Mechanics lies at the heart of many of the underpinnings of modern technological civilization: materials, infrastructure, transportation, health and security. The mechanics of dynamic failure processes also has a major bearing on the potential catastrophes that threaten civilization, including airbursts and major asteroid impacts. Recent events (such as the Chelyabinsk meteoroid) have demonstrated the need to understand major impact and fragmentation events. Many of the fundamental problems of current interest in national security also involve impact and fragmentation, typically studied through large-scale computational simulations. In the Murray lecture, these issues were addressed by describing fundamental high-strain-rate experiments, high-speed visualization, theoretical and computational modeling of failure processes, and simulations of asteroid damage and disruption. This paper focuses on experimental results on meteorites and a basalt.

Diffuse axonal injury (DAI) is a debilitating consequence of traumatic brain injury (TBI) attributed to abnormal stretching of axons caused by blunt head trauma or acceleration of the head. We developed an anatomically accurate, subject-specific, three-dimensional (3D) computational model of the human brain, and used it to study the dynamic deformations in the substructures of the brain when the head is subjected to rotational accelerations. The computational head models use anatomy and morphology of the white matter fibers obtained using MRI. Subject-specific full-field shearing motions in live human brains obtained through a recently developed tagged MRI imaging technique are then used to validate the models by comparing the measured and predicted heterogeneous dynamic mechanical response of the brain. These results are used to elucidate the dynamics of local shearing deformations in the brain substructures caused by rotational acceleration of the head. Our work demonstrates that the rotational dynamics of the brain has a timescale of ∼100 ms as determined by the shearing wave speeds, and thus the injuries associated with rotational accelerations likely occur over these time scales. After subject-specific validation using the live human subject data, a representative subject-specific head model is used to simulate a real life scenario that resulted in a concussive injury. Results suggest that regions of the brain, in the form of a toroid, encompassing the white matter, the cortical gray matter, and outer parts of the limbic system have a higher susceptibility to injury under axial rotations of the head.

Single crystals of pure Mg were deformed along the crystallographic 〈 a 〉 axis under normal plate impact loading of microsecond duration. Post-test electron microscopy analyses of the recovered specimens revealed the presence of extension twins. Twins were observed to grow to significant sizes and an estimate of the twin tip velocity was obtained. Secondary extension twinning was also observed.