Pore Collapse Mechanism Research Articles

Continuum models for meso-scale simulations of HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane) guided by molecular dynamics: Pore collapse, shear bands, and hotspot temperature

Meso-scale calculations of energy localization and initiation in energetic material microstructures must capture the deformation and collapse of pores and high-temperature shear bands, which lead to hotspots. Because chemical reaction rates depend sensitively on temperature, predictive continuum models need to get the pore-collapse dynamics and resulting hotspot temperatures right; this imposes stringent demands on the fidelity of thermophysical model forms and parameters and on the numerical methods employed to perform high-resolution meso-scale calculations. Here, continuum material models for β-HMX are examined in the context of shock-induced pore collapse, treating predictions from all-atom molecular dynamics (MD) simulations as ground truth. Using atomistics-consistent material properties, we show that the currently available strength models for HMX fail to correctly capture pore collapse and hotspot temperatures. Insights from MD are then employed to advance a Modified Johnson–Cook (M-JC) strength model, which is shown to capture key aspects of the physics of shock-induced localization in HMX. The study culminates in a MD-guided strength model for β-HMX that produces continuum pore-collapse results in better alignment on several aspects with those predicted by MD, including pore-collapse mechanism and rate, shear-band formation in the collapse zone, and temperature, strain, and stress fields in the hotspot zone and the surrounding material. The resulting MD-informed/MD-determined M-JC model should improve the fidelity of meso-scale simulations to predict the detonation initiation of HMX-based energetic materials in microstructure-aware multi-scale frameworks.

Challenging the paradigm for reactive material's ignition from shear to pressure: Thermomechanical study of Al-PTFE

Structural reactive materials (SRMs) are attracting growing interest in the warheads industry, while the Al-PTFE system is probably the most popular. Due to their final application, high-strain rate (impact) testing is the most realistic method for SRMs. For this purpose, a synchronized Split Hopkinson (Kolsky) Pressure Bar with high-speed infrared and optical cameras is used to characterize the sintered Al-PTFE system, thus enabling the simultaneous investigation of its thermal energy release and mechanical properties under impact. The characteristic specimen heating rates during such experiments are of the order of 3.85 × 108 °C/min, which most likely caused evaporation of the reaction products, that were therefore not identified in a post-mortem analysis of the remaining fragments. Whereas the conventional wisdom has it that shear loading causes SRM ignition, the main result of this work just points to the opposite, showing unambiguously that pressure is the reaction-triggering loading mode, with the potential involvement of a pore collapse mechanism.

Energy localization efficiency in 1,3,5-trinitro-2,4,6-triaminobenzene pore collapse mechanisms

Atomistic and continuum scale modeling efforts have shown that the shock-induced collapse of porosity can occur via a wide range of mechanisms dependent on pore morphology, the shockwave pressure, and material properties. The mechanisms that occur under weaker shocks tend to be more efficient at localizing thermal energy but do not result in high, absolute temperatures or spatially large localizations compared to mechanisms found under strong shock conditions. However, the energetic material 1,3,5-trinitro-2,4,6-triaminobenzene (TATB) undergoes a wide range of collapse mechanisms that are not typical of similar materials, leaving the collapse mechanisms and the resultant energy localization from the collapse, i.e., hotspots, relatively uncharacterized. Therefore, we present the pore collapse simulations of cylindrical pores in TATB for a wide range of pore sizes and shock strengths that trigger viscoplastic collapses that occur almost entirely perpendicular to the shock direction for weak shocks and hydrodynamic-like collapses for strong shocks that do not break the strong hydrogen bonds of the TATB basal planes. The resulting hotspot temperature fields from these mechanisms follow trends that differ considerably from other energetic materials; hence, we compare them under normalized temperature values to assess the relative efficiency of each mechanism to localize energy. The local intra-molecular strain energy of the hotspots is also assessed to better understand the physical mechanisms behind the phenomena that lead to a latent potential energy.

Conkunitzin-C2 from Conus consors induces an incomplete block of the Shaker K+ channel using a unique mode of action

Voltage-gated potassium channels (KVs) are activated in response to changes in the electrical membrane potential by coupling a voltage-sensing module with K+ selective pore. Toxins targeting the channel pore can interfere with ion permeation by physically occluding the pore (‘plug mechanism’), by inducing a collapse of the selectivity filter (‘pore-collapse mechanism’), or by trapping the ions in ectopic sites formed at the toxin-channel interface (‘lid mechanism’) to induce a complete channel block.

Strain-rate dependency and impact dynamics of closed-cell aluminium foams

Strain rate sensitivity and deformation mechanisms of closed-cell aluminium foams under low-velocity impact loadings are investigated in this study. Instrumented drop-weight impact experiments and Finite Element (FE) modelling were conducted to explore the deformation rate dependency of aluminium foams (manufactured by CYMATTM corporation). An X-ray micro-Computed Tomography (XCT) reconstructed foam geometry was used in the FE modelling approach to explore actual deformation mechanisms and strain rate sensitivity of foams. The deformation and pore collapse mechanisms were explored through investigating the stress and plastic strain contours. Our results show that the FE modelling with rate-dependent material properties agreed with the dynamic experimental results. The foam showed significant rate sensitivity within the examined range of strain rates. Our modelling and experimental results collectively indicate that the rate sensitivity of the base material is the primarily responsible for enhancing strength during impacts. Furthermore, the FE modelling with rate-independent material properties and unique foam topology confirms the negligible inertia effect at low velocity impacts.

Theoretical study of the defect evolution for molecular crystal under shock loading

We simulate the shock loading process of β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine by molecular dynamics and calculate the isoentropic curve, Hugoniot curve, temperature field, velocity field, stress field, and density field. Based on the simulation results, we develop a physical model to describe the pore collapse, crack expansion, and hot spot formation mechanisms and calculate a set of key parameters, such as pore collapsing speed, rarefaction wave speed, and crack expansion speed. A microscopic physical picture for defect evolution at the early time of shock loading is obtained.

Mechanical response and dynamic deformation mechanisms of closed-cell aluminium alloy foams under dynamic loading

The dynamic compressive response of closed-cell (CYMAT™) stabilised aluminium alloy foams (SAF) has been investigated using a modified Split Hopkinson Pressure Bar (SHPB) in conjunction with a high-speed camera. Tests have been carried out on 45 mm diameter and 23 mm thick cylindrical specimens. The elastic-plastic pore collapse mechanism has been investigated using Digital Image Correlation (DIC) and micro-computed X-ray tomography. A stress-strain relationship for individual specimens at different impact velocities has been obtained with the combination of an analytical method and SHPB theory. The large deformation (∼80%) has been measured from eight strain gauges’ data using a wave separation algorithm. The test results exhibited a significant increase in elastic and plastic strength during the pulse loading. The X-ray tomography data of pre-impacted and post-impacted SAF specimens have been extensively analysed to elucidate the internal elastic-plastic pore collapse mechanisms.

Macro and micro collapse mechanisms of closed-cell aluminium foams during quasi-static compression

The pore collapse mechanisms of closed-cell aluminium foams during quasi-static compression have been investigated. A suite of experiments and numerical simulations were carried out to elucidate the deformation pathways of individual pores during quasi-static compressive loading. X-ray computed tomography was utilized to generate 3D views of the foams before and after deformation. The tomography based foam geometry was imported into the finite element software ABAQUS/Explicit for simulations. The results showed that the simulations accurately reproduced the experimentally observed yielding and post yielding behaviour of the foams. As expected, pores with thin cell-walls were observed to deform at faster rates both experimentally and in simulations. The simulations aided to reveal the complex deformation evolution of cell-walls and junctions (plateau borders) during compression. While the cell-walls experienced bending, buckling and rotation by forming hinges; the plateau borders experienced considerably less deformation. The thickness/strength of cell-walls and topological foams' heterogeneities are observed as the governing factors for collapse. Significant lateral strain is observed at the cell level, although the bulk lateral strain was negligible. Finally, it was also observed that the micro-pores and cracks present within the cell-walls contribute to their deformation.

Modelling and characterization of cell collapse in aluminium foams during dynamic loading

Plate-impact experiments have been conducted to investigate the elastic–plastic behaviour of shock wave propagation and pore collapse mechanisms of closed-cell aluminium foams. FE modelling using a meso-scale approach has been carried out with the FE software ABAQUS/Explicit. A micro-computed tomography-based foam geometry has been developed and microstructural changes with time have been investigated to explore the effects of wave propagation. Special attention has been given to the pore collapse mechanism. The effect of velocity variations on deformation has been elucidated with three different impact conditions using the plate-impact method. Free surface velocity (ufs) was measured on the rear of the sample to understand the evolution of the compaction. At low impact velocities, the free-surface velocity increased gradually, whereas an abrupt rise of free-surface velocity was found at an impact velocity of 845 m/s with a copper flyer-plate which correlates with the appearance of shock. A good correlation was found between experimental results and FE predictions.

Modeling and simulation of preshock desensitization in heterogeneous explosives using a mesoscopic reaction rate model

To understand and predict the response of explosive materials, numerical models are utilized to simulate various scenarios. Various hazard and vulnerability scenarios for explosives involve multiple shock compression. In the present study, a kind of mesoscopic model for shock ignition of solid heterogeneous explosives is examined in order to demonstrate its availability to account for the desensitization caused by multiple shocks in explosives. Since the mesoscopic model is based on the assumption of the elastic viscoplastic pore collapse mechanism, and on the other hand the desensitization mechanism is also usually described in connection with the closure of pores, the ability of the mesoscopic model to predict the desensitization effects must be analyzed. For this purpose, the mesoscopic model has been implemented in a hydrodynamic code (LS-DYNA) as a user-defined equation of state. For verification, the double shock, reflected shock and detonation-quenching experiments have been modeled and simulated. The data show that the model can reproduce various features of some of the previously reported experiments involving the preshock desensitization of solid explosives.

Pore Collapse and Regrowth in Lithiated Si Anodes Determined By in-Operando Neutron Reflectometry

Structure and composition changes of an 11 nm thick amorphous silicon (a-Si) thin film anode, capped with a 4 nm thick alumina film are measured, in operando, by neutron reflectometry (NR) and electrochemical impedance spectroscopy in a lithium half-cell. NR data are analyzed to quantify the a-Si thickness and composition at various states of charge over six cycles. The a-Si anode expands and contracts upon lithiation and delithiation, respectively, while maintaining its integrity and low interfacial roughness (£1.6 nm) throughout the cycling. The apparently non-linear expansion of the a-Si layer volume versus lithium content agrees with findings from previous thin-film a-Si anode studies. However, a proposed pore collapse and regrowth (PCRG) mechanism explains apparently minimal volume expansion of the film as a whole for low Li content in terms of linear expansion of the solid domains within the initially porous amorphous Si. In the PCRG model, the porosity is first consumed by the expansion of solid domains upon lithiation, after which the film as a whole expands linearly with Li content with a proportionality factor found to be 8.48 cm3/mol-Li, which is similar to that of crystalline Si. Porosity is reversibly reestablished at 5-28% upon delithiation. Data also show that the alumina protective layer on the a-Si film functions as an effective artificial solid electrolyte interphase (SEI), which maintains its structural integrity, low interfacial roughness, and a relatively small transport resistance. No additional spontaneously-formed SEI is observed in this study.

Two-dimensional mesoscale simulations of shock response of dry sand

Simulations were done to gain insight whether the shock response of dry sand at low stresses would vary with porosity and whether the effects of friction between grains under confinement could be extracted from the planar plate impact experimental data. The sand sample was modeled as grains separated by voids representing porosity. The simulation procedure coupled grain deformations with frictional sliding at grain boundaries. The shock response of dry sand varied considerably with porosity. The sample compacted through pore closure followed by inelastic pore collapse mechanisms affecting the inhomogeneous response and shock rise time. The sample attained final compaction in the shock state long after attaining peak longitudinal velocity/stress. The calculated shock Hugoniot for a sample of high (40%) porosity was in agreement with experimental data. The Us-Up slopes for sand of 10% and 20% porosity were found to be negative. The calculated σH-ρH Hugoniot suggested that the two slopes would become positive at higher stresses in order to approach the solid Z-cut quartz Hugoniot at full compaction. High porosity sand may never exhibit negative slopes. It is concluded that the effects of friction between grains can be successfully extracted from a coupled experimental-computational approach. This requires measuring the velocity profile in the back buffer, elastic buffer material, and code capable of simulating frictional sliding between grains. The dispersion effect increased the slope of the velocity profile with propagation distance but did not result in a wave speed reduction or shock attenuation. This may be due to the small grain size and sample thickness as well as the absence of grain fragmentation in the present simulations.

Pore collapse and regrowth in silicon electrodes for rechargeable batteries.

Structure and composition of an 11 nm thick amorphous silicon (a-Si) thin film anode, capped with 4 nm of alumina are measured, in operando, by neutron reflectivity (NR) and electrochemical impedance spectroscopy in a lithium half-cell. NR data are analyzed to quantify the a-Si thickness and composition at various states of charge over six cycles. The a-Si anode expands and contracts upon lithiation and delithiation, respectively, while maintaining its integrity and low interfacial roughness (≤1.6 nm) throughout the cycling. The apparently non-linear expansion of the a-Si layer volume versus lithium content agrees with previous thin-film a-Si anode studies. However, a proposed pore collapse and regrowth (PCRG) mechanism establishes that the solid domains in the porous LixSi film expand linearly with Li content at 8.48 cm(3) mol(-1) Li, similar to crystalline Si. In the PCRG model, porosity is first consumed by expansion of solid domains upon lithiation, after which the film as a whole expands. Porosity is reestablished at 5-28% upon delithiation. Data show that the alumina protective layer on the a-Si film functions as an effective artificial solid electrolyte interphase (SEI), maintaining its structural integrity, low interfacial roughness, and relatively small transport resistance. No additional spontaneously-formed SEI is observed in this study.

Numerical analysis of concrete under a wide range of stress and with different saturation condition

This paper is devoted to the description of elastoplastic damage behavior of concrete under a wide range of confining pressure and different saturation conditions. Based on experimental investigations, two plastic flow mechanisms are identified: plastic shearing coupled with damage at low confining pressure and plastic pore collapse at high confining pressure. The plastic shearing mechanism describes plastic distortion and failure by the formation of shear bands. On the other hand, the pore collapse mechanism is related to the destruction of pore structures. Furthermore, influences of water saturation on the mechanical behavior of concrete are also studied in this paper. The increase of water saturation induces the decrease of capillary pressure leading to an additional plastic deformation. The proposed model is implemented in a fully coupled finite element calculation code. The performance of the model is examined by comparing numerical results with laboratory data issued from triaxial compression tests under a large range of confining pressures.

Elastoplastic damage modeling the mechanical behavior of rock-like materials considering confining pressure dependency

In this paper, the effect of confining pressure on the mechanical behavior of rock-like materials, such as concrete, is studied by using a coupled elastoplastic damage model. Damage mechanism is coupled, in different manners, with two plastic flow mechanisms: plastic shear mechanism developed under low confining pressure and plastic pore collapse mechanism observed under high confinement. The proposed model is applied to study a series of laboratory tests performed under extremely low to very high confining pressure, up to 650MPa. The numerical predictions of proposed model are in good agreement with the experimental results.

Deformation modes in an Icelandic basalt: From brittle failure to localized deformation bands

According to the stress state, deformation mode observed in rocks may be very different. Even in the brittle part of the crust a differential stress can induce shear failure but also localized compacting deformation, such as compaction bands in porous sedimentary rocks. The mode of deformation controls many hydrodynamic factors, such as permeability and porosity. We investigate in this paper two different modes of deformation in an Icelandic basalt by using laboratory seismological tools (elastic waves and acoustic emissions) and microstructural observations. First of all, we show that at low effective confining pressure (Peff=5MPa) an axial loading induces a shear failure in the basalt with an angle of about 30° with respect to the main stress direction. On the contrary, at high effective confining pressure (Peff≥75MPa and more) an increase of the axial stress induces a localization of the deformation in the form of subhorizontal bands again with respect to the main stress direction. In this second regime, focal mechanisms of the acoustic emissions reveal an important number of compression events suggesting pore collapse mechanisms. Microstructural observations confirm this assumption. Similar compaction structures are usually obtained for porous sedimentary rocks (20–25%). However, the investigated basalt has an initial total porosity of only about 10% so that compaction structures were not expected. The pore size and the ratio of pore to grain size are likely to be key factors for the particular observed mechanical behavior.

Characterization of dynamic damage mechanisms in Palmetto wood as biological inspiration for impact resistant polymer composites

Palmetto wood has been previously identified as a potential biological template for inspiring the development of synthetically engineered materials with hierarchical microstructures that exhibit enhanced mechanical behavior. Previously, the multi-scale mechanical behavior has been studied under quasi-static loading in order to understand the relationship between the microstructure of Palmetto wood and its mechanical behavior. In this study, the mechanical behavior of dry Palmetto wood is investigated under dynamic loading using low velocity impact. The experimental results reveal that the macrofiber concentration of the Palmetto wood plays a key role in the dynamic failure mechanisms. Under low velocity impact, the dynamic damage was found to be dominated globally by axial loading induced by bending leading to localized, shear-dominated debonding at the macrofiber-porous cellulose matrix interface, as well as compressive loading induced by indentation under the projectile leading to local crushing of the porous cellulose matrix and shear cracking of the macrofibers and matrix. By increasing the macrofiber concentration, it was found that the dominant failure mechanism could be transformed from the former to the latter by increasing the energy absorbed by indentation in order to increase impact resistance. This explains why the macrofiber concentration gradually decreases radially towards the center of the wood stem, since the outer portion of the wood has a high indentation resistance while the inner portion absorbs more energy through bending. A new model was proposed for to better understand the variation in mechanical behavior with macrofiber concentration and loading rate consistent with the evolution of the observed damage mechanisms. It was found that the greatest effect of increasing loading rate and macrofiber concentration was to increase the elastic modulus by 450–600% and the yield stress associated with the pore collapse mechanism by 125–175%. There is also a coupling between the evolution of plastic strain and damage that depends more strongly on macrofiber concentration than loading rate. These two effects combine to cause a significant increase in the density of energy absorption by 75–133% with increasing macrofiber concentration and strain rate. Therefore, the structure of Palmetto wood can be used as a template to guide the development of more impact resistant polymer composites, such as inserting 12 to 20vol.% pultruded carbon fibers into the foam core of sandwich composite structures.

A micromechanical modeling of ductile behavior of a porous chalk: Formulation, identification, and validation

SUMMARYPorosity strongly affects the overall ductile behavior of cohesive geomaterials undergoing plastic deformation. In the present paper, we proposed an original micromechanical approach that suitably couples Drucker–Prager‐type plasticity, evolving porosity under general triaxial loadings. The resulting model has the advantage to be based on a single macroscopic yield function, which also plays the role of plastic potential. It is shown that this yield function is particularly appropriate to account for the pore collapse and plastic shearing mechanisms that govern the mechanical behavior of the studied Lixhe chalk. Finally, the new model is implemented and validated by comparison to triaxial tests data, covering a wide range of confining pressures. Copyright © 2011 John Wiley & Sons, Ltd.

A two scale model of porous rocks with Drucker–Prager matrix: Application to a sandstone

The present paper is devoted to a micromechanical model of porous rocks and its application to a sandstone. This original model takes advantage of a recent homogenization-based macroscopic yield function which couples Drucker–Prager type plasticity of the solid matrix and evolving porosity. Its formulation and implementation are described. Application to a Vosges sandstone shows that, except for very low confining pressures for which the mechanical behavior is quasi-brittle, the model predicts well the ductile behavior at moderate or high confining pressures (for which the pore collapse mechanism is expected to play a dominant role).

Numerical Simulation of Mechanical Behaviour of Cement Paste under Compressive Pressure

This paper contributes to numerically study the mechanical behavior of cement paste under compressive pressure. First, the main results from an experimental study on the mechanical behavior of cement paste are studied. Then an elastoplastique model with two flow mechanisms is proposed to describe the mechanical behavior of cement paste subjected to confining pressure. A particular emphasis is put on the pore collapse mechanism which is developed at higher confining pressure. Finally, numerical simulations and experimental data are compared in order to verify the capacity of the proposed model to reduce numerically the basic characteristics of cement paste under different levels of confining pressure.