Void Aspect Ratio Research Articles

Decoding ceramic fracture: Atomic defects studies in multiscale simulations

Microstructural atomic defects, including voids, cleavage, and inclusions, are commonly observed in alumina materials, and their impact on mechanical properties, such as fracture stress and toughness, is significant. In this paper, we introduce novel alumina models that incorporate experimentally observed void features. An atomic model is established to study the effects of micro-structural void features on fracture properties and atomic structure changes using molecular dynamics simulations. The electron backscatter diffraction and scanning electronic microscopy analysis of experimental samples are used to evaluate microstructural features that are used as inputs to the simulations (e.g., void aspect ratio, void angle). We apply an innovative Atomistic-to-Continuum (ATC) method based on Riemann sums to bridge atomic and continuum mechanics theories, evaluating the resistance of materials with atomic defects to crack propagation. The results show the greatest effects of pore angles on weakening mechanical properties such as peak strength and fracture energy density. The accuracy and efficiency of the ATC method in evaluating stress intensity factors are used to calculate the mechanical responses. Additionally, we establish a multiple layer perceptron neural network to evaluate the complex relationship between void features (aspect ratio, pore angle, relative distance) and typical fracture properties (fracture stress, critical stress intensity factor). A meta-analysis of these results from both machine learning methods and molecular dynamics simulations reveals the significant impact of each void feature on the sensitivity of typical fracture properties (peak strength, critical stress intensity factor at peak strength) and highlights the critical role of aspect ratio on fracture properties.

Void nucleation and growth behavior of TIG welded AA2219 deformed at cryogenic temperatures

This work investigates the failure behaviour of tungsten inert gas (TIG) welded 2219 aluminum alloy butt welds deformed over a range of cryogenic temperatures relevant to liquid hydrogen/oxygen propellent storage tanks. A novel in situ tensile testing machine, able to deform samples at temperatures down to 83 K, was designed to be accommodated on synchrotron computed tomography beamlines. In this way we were able to observe the accumulation of damage from void nucleation through growth to coalescence and ultimate failure by time-lapse X-ray microtomography during tensile straining at 293 K and 123 K. The results reveal that micro-void nucleation is triggered by θ (Al2Cu) precipitates and α-Al + θ (Al2Cu) eutectics in the form of particle fracture and interface decohesion. After nucleation, these freshly nucleated voids, along with pre-existing welding defects, extend with straining along the loading direction. This leads to void linkage, a very short void coalescence stage and subsequent catastrophic failure. The probability distribution function that describes the rate at which voids nucleate by interface decohesion considering the particle orientation was determined through the Eshelby inclusion theory. The evolution of void aspect ratio and void growth were modelled using the Gurson-Tvergaard-Needleman model and Le Roy model, respectively. This suggests that pre-existing defects grow faster, but that the numerous closely spaced nucleated voids play a significant role in failure.

Experimental and theoretical study on elliptical rubber concrete filled double-skin GFRP short tubes under axial compression

This study proposed a double-skin section consisted of an elliptical glass fiber reinforced polymer (GFRP) outer tube, a circular steel inner tube and rubber concrete filled between them, which had the features of aesthetic appeal, structural efficiency and waste recycle. Axial compressive test was conducted on the elliptical rubber concrete filled double-skin GFRP tubular (CFDST) short columns with various void ratios and rubber contents. Load-axial strain-hoop strain data was obtained from the experiments and the contribution of each component (i.e., concrete, GFRP tube, steel tube) to resist the applied load was clarified. Experimental results indicated that the void ratio and rubber content had the most significant effects on the dilation property (i.e., hoop and axial strains) and confinement effect of the rubber concrete. Nevertheless, effects of steel tube thickness and rubber particle size were insignificant. Based on the experimental results and confining concrete theories, a theoretical model, with proper considerations on the effects of void ratio, rubber content and aspect ratio, was proposed to estimate the ultimate load-carrying capacity and ultimate strain of the stub columns under axial compression. Finally, reasonability and accuracy of the proposed model were verified by the experimental results.

Investigation of the porosity of L/LL4 ordinary chondrite Bjurböle using synchrotron radiation microtomography and scanning electron microscopy: Implications for parent body evolution

Porosity is an essential property of chondritic meteorites and is closely related to the genesis, thermal evolution, metamorphism, and thermal properties of the meteorite parent bodies. We study porosity, its texture, and shapes at sub-micron resolution in 3D and 2D within a 0.35 cm3 sample of the L/LL4 ordinary chondrite (OC) Bjurböle using two techniques, synchrotron radiation microtomography (SRμCT) and scanning electron microscopy (SEM). We employ automated segmentation tools that can be applied to both SRμCT and SEM data. Successful segmentation results can be achieved by combining visual qualitative examination and machine learning algorithms.We report novel measurement results of three-dimensional porosity properties of Bjurböle, such as aspect ratio and connectivity of void spaces, and compare the results of 2D and 3D porosity analysis. The Bjurböle sample in this study is a complex, highly porous, and friable medium and the dominant type of porosity is intergranular, continuous porosity, which contains almost all porosity volume. The shapes of the void volumes have an important effect on the connectivity of the porosity and thermal transport properties. Positive correlations between void diameter and aspect ratio as well as void volume and connectivity are present in Bjurböle, which indicate that smaller voids have lower aspect ratios and lower connectivity. In Bjurböle, small, near-spherical voids with few connections have the highest relative frequency, whereas larger void spaces with higher aspect ratios and connectivity are significantly fewer. Completely isolated pores, i.e., voids surrounded by solid material, also have a high relative frequency, and they exist within the chondrules and the matrix. However, the volume percentage of these pores is negligible compared to that of the continuous porosity.Our results support the previously measured high porosities of Bjurböle. The volume percentage of intergranular void spaces, in particular in the matrix, and the measured high porosities are not in line with the results of thermal evolution and sintering models of chondritic parent bodies regarding petrologic type 4, which implies that Bjurböle originates from a parent body with an initial onion shell structure that fragmented during or after its metamorphic peak and quickly reaccreted into a rubble pile.

Yield surface for void growth and coalescence of porous anisotropic materials under axisymmetric loading

Ductile fracture in metallic alloys occurs by growth and coalescence of cavities. Growth, also referred to as homogeneous yielding, refers to rather diffuse plasticity around cavities, while coalescence, also termed as inhomogenous yielding, corresponds to the localization of plasticity along some planes or directions. Coalescence can develop in various patterns; three coalescence modes have been observed experimentally: internal necking, coalescence in columns and void sheeting. Plastic anisotropy of the material is known to have a significant effect on both homogeneous and inhomogeneous yielding. Therefore, in the present study, yield criteria accounting for the transition from homogeneous yielding to inhomogeneous yielding modes in anisotropic porous materials are obtained using kinematic limit analysis on a cylindrical unit-cell with a coaxial cylindrical cavity. Two types of plastic anisotropy are considered: Hill (1948) plasticity and crystal plasticity. The proposed analytical yield criteria are compared to numerical limit analysis computations and are found to qualitatively agree with simulations. In particular, plastic anisotropy, void shape effects and their coupling are well captured, especially regarding yield stresses and deformation modes. Finally, an homogenized model for Hill porous materials is obtained by supplementing evolution laws for microstructural parameters (void aspect ratio and ligament size ratios) derived from sequential limit analysis. Proposed evolution laws are then discussed in the light of numerical results and experimental evidence.

An improved upper bound analysis for study of the void closure behavior in the plane strain extrusion

In this paper, a new mathematical model has been developed to predict the void closure behavior in metal billets during the plane strain extrusion process. For this purpose, an improved upper bound analysis has been conducted by applying the power balance between the internal work rate, energy release rate along the void, and the external power. Considering the limiting condition of the derived equations on the verge of the void closure, a mechanical criterion has been obtained for finding the minimum extrusion ratio required for the closure of the elliptical voids. The formulation has been developed for both frictionless and frictional conditions on the die-billet interface. The process has also been modeled by the finite element method (FEM) and good agreement has been observed between the results of the derived analytical criterion and the FEM modeling for the minimum required extrusion ratio. It has been found that the minimum extrusion ratio required for the void closure is only a function of the extrusion die angle, the void aspect ratio, and the frictional condition on the die-billet interface. Furthermore, extruding the material under the frictional condition, and in a die with a smaller cone angle is beneficial for closing the voids.

A novel model of pressure drop for airflow penetrating randomly packed cylindrical particles

Pressure drop is the key factor related to performance and energy consumption of fixed bed. However, the packing of cylindrical particles produces anisotropic gaps and an extremely complex airflow field and makes the formation mechanism of flow resistance undefined. This study conducted a comprehensive investigation on how such factors as the void fraction, diameter, and aspect ratio of cylinders on the flow resistance, in terms of experimental and numerical methods. With the DEM-CFD simulation, the laminar and transitional flow pattern was acquired, and the critical Reynolds number was determined. The expression of void fraction in the inertia term of the original Ergun equation was confirmed not applicable to cylindrical particles, which was proved to be the reason why previous predictions cannot accurately give the flow resistance of packed cylindrical particles. Finally, a new prediction model for the pressure drop of airflow penetrating the packed cylindrical particles was developed.

Effects of void morphology on fracturing characteristics of porous rock through a finite-discrete element method

A systematic approach for generating a stochastic void geometry with controllable shape features is proposed for more realistic numerical modelling of porous rock materials. The numerical model (a finite-discrete model) is established by integrating zero-thickness cohesive elements with finite solid elements, to trace the progressive fracturing and failure behaviours of porous rocks. A series of Brazil split tests are conducted on the synthetic specimens that are generated using 3D printing technique, to validate the proposed numerical model. By comparing fracturing patterns and load-displacement curves, it is demonstrated that the proposed hybrid numerical method has promising performances to capture the mechanical responses of porous rocks. In addition, the influences of void aspect ratio and orientation on the mechanical and fracturing responses of porous rocks are discussed. By counting the total damage proportion (Pt) and shearing damage ratio (Ps) of cohesive elements, the hybrid method offered a micro insight to reveal the mechanism of strength variation induced by different characteristics of void morphology. Modelling results demonstrate that better mechanical properties of porous rocks correspond to higher values of both Pt and Ps. Comparison between modelling results and data from literature also validates the efficiency and accuracy of the proposed hybrid approach.

Low-Temperature Hydrothermal Growth of ZnO Nanowires on AZO Substrates for FACsPb(IBr)3 Perovskite Solar Cells.

Electron and hole transport layers (ETL and HTL) play an essential role in shaping the photovoltaic performance of perovskite solar cells. While compact metal oxide ETL have been largely explored in planar n-i-p device architectures, aligned nanowires or nanorods remain highly relevant for efficient charge extraction and directional transport. In this study, we have systematically grown ZnO nanowires (ZnO NWs) over aluminum-doped zinc oxide (AZO) substrates using a low-temperature method, hydrothermal growth (HTG). The main growth parameters were varied, such as hydrothermal precursors concentrations (zinc nitrate hexahydrate, hexamethylenetetramine, polyethylenimine) and growing time, in order to finely control NW properties (length, diameter, density, and void fraction). The results show that ZnO NWs grown on AZO substrates offer highly dense, well-aligned nanowires of high crystallinity compared to conventional substrates such as FTO, while demonstrating efficient FACsPb(IBr)3 perovskite device performance, without the requirement of conventional compact hole blocking layers. The device performances are discussed based on NW properties, including void fraction and aspect ratio (NW length over diameter). Finally, AZO/ZnO NW-based devices were fabricated with a recent HTL material based on a carbazole moiety (Cz–Pyr) and compared to the spiro-OMeTAD reference. Our study shows that the Cz–Pyr-based device provides similar performance to that of spiro-OMeTAD while demonstrating a promising stability in ambient conditions and under continuous illumination, as revealed by a preliminary aging test.

Parametric study on vertical void configurations for improving ventilation performance in the mid-rise apartment building

Double-loaded affordable apartments, which are commonly seen in tropical developing countries, suffer from poor cross-ventilation, particularly on the leeward side of the buildings. This study aims to parametrically evaluate closed-vertical void configurations to improve the ventilation performance of the double-loaded apartment building using the validated CFD models. The configurations include the aspect ratio of the void, pilotis size and fin size with various wind directions. The results showed that the most influential parameter for enabling natural ventilation by augmenting pressure coefficient differences on the windward and leeward sides of the building was the fin size, followed by wind direction and aspect ratio. Wind direction influenced the windward side more than the leeward side, whereas aspect ratio influenced the leeward side over the windward side. It was concluded that the mass flow rates on both sides of the building could be maximised by optimally combining the void's fin size and aspect ratio under the specific prevailing wind directions. These findings would help create sustainable design guidelines for improving natural ventilation by incorporating a closed-vertical void in the mid-rise apartment buildings in the tropics.

VOID EVOLUTION BEHAVIOR AND CLOSURE CRITERION INSIDE LARGE SHAFT FORGINGS DURING A FORGING PROCESS

In this study, the effects of different void positions, void shapes and sizes on the evolution of voids were discussed in detail using experiments and simulations. The results show that the influence of the void size on the void closure can be ignored, while the void position and void shape have a great influence on the closure of a void. Considering the complexity of the void-shape change in a forging process, we proposed a quantitative expression of the void-shape coefficient, which is affected by the effective stress and effective strain. Meanwhile, the void-shape evaluation parameter, defined as a function of the stress deviator, effective strain and effective stress, was proposed to describe the changes in the void aspect ratio. Finally, WHF (wide die heavy blow) forging experiments were conducted using a 5MN hydraulic press to verify the numerical-simulation results. Based on the experimental and simulation results, a new mathematical model for void-closure determination was established during a forging process of large shaft forgings. The experimental results were consistent with the simulation results, showing that the void-closure model can accurately determine whether a void is closed or not.

VOID EVOLUTION BEHAVIOR AND CLOSURE CRITERION INSIDE LARGE SHAFT FORGINGS DURING A FORGING PROCESS

In this study, the effects of different void positions, void shapes and sizes on the evolution of voids were discussed in detail using experiments and simulations. The results show that the influence of the void size on the void closure can be ignored, while the void position and void shape have a great influence on the closure of a void. Considering the complexity of the void-shape change in a forging process, we proposed a quantitative expression of the void-shape coefficient, which is affected by the effective stress and effective strain. Meanwhile, the void-shape evaluation parameter, defined as a function of the stress deviator, effective strain and effective stress, was proposed to describe the changes in the void aspect ratio. Finally, WHF (wide die heavy blow) forging experiments were conducted using a 5MN hydraulic press to verify the numerical-simulation results. Based on the experimental and simulation results, a new mathematical model for void-closure determination was established during a forging process of large shaft forgings. The experimental results were consistent with the simulation results, showing that the void-closure model can accurately determine whether a void is closed or not.

Effects of void shape and orientation on the elastoplastic properties of spheroidally voided single-crystal and nanotwinned copper

ABSTRACT Void nucleation, growth and coalescence have been identified as the leading cause of ductile damage in metallic materials. To understand the underlying deformation and damage mechanisms, extensive theoretical, experimental and simulation efforts have been attempted on spherically voided metals. In this work, molecular dynamics simulations are performed to analyze the uniaxial straining deformation behaviours of both single-crystal and nanotwinned copper materials embedded with a preexisting spheroidal void. The coupling effects among twin boundary, spheroidal void aspect ratio and orientation on unidirectional elastoplastic behaviours are systematically examined. The dislocation-induced plastic deformation mechanism is also examined and compared with the one due to a perfectly spherical cavity. Simulation results show that elastic modulus increases with both spheroidal void aspect ratio and orientation. So do the yield stress, the first peak stress and the plasticity index. Another peak stress exists for most cases, except for a prolate void embedded in nanotwinned specimens. The slope between peak stresses decreases with both the spheroidal aspect ratio and orientation. The incorporation of a twin boundary results in lower elastic modulus, higher yield strength and smaller plasticity index. For an oblate void, the twin boundary gives rise to more severe strain softening behaviour. The dislocation extraction algorithm illustrates that the continuous nucleation, propagation and reaction of dislocations emanated from both the void front and twin boundary are responsible for the ductile damage of spheroidally voided crystals. The lower dislocation densities found in nanotwinned specimens indicate the desired suppression effects of twin boundary on dislocation activities.

Dynamic response of ductile materials containing cylindrical voids

The fracture of ductile materials is often the result of the nucleation, growth and coalescence of microscopic voids. In this thesis, we mainly focus on the dynamic void evolution in porous media containing cylindrical voids. This study covers a problem that is of particular interest in many areas of research (e.g. development of shock mitigation devices for civil or military applications). Owing to the development of additive manufacturing, the processing of porous material with cylindrical voids is an option to create lightweight materials having interesting properties in terms of energy dissipation. Therefore, our work aims at describing the dynamic response of architectured materials such as honeycomb structures. In dynamic loading, microvoids sustain an extremely rapid expansion which generates strong acceleration of particles in the vicinity of cavities. These micro-inertia effects are known to play a significant role in the macroscopic response and the development of damage in porous media. In fact, the overall macroscopic stress is found to be the sum of two contributions: a static term (micro-inertia independent term) and a dynamic term (micro-inertia dependent term), the latter being related to the microstructure (e.g. size and aspect ratio of voids). In our work, a cylindrical shell is adopted as a Representative Volume Element (internal and external radii a and b, length 2l) for the porous material. The static term is derived from a yield function available in the literature. The dynamic stress is evaluated analytically using a trial velocity field for cylindrical voids combined with the multi-scale approach developed in the literature in LEM3. It is shown that the dynamic stress is scaled by the mass density, two characteristic lengths of the voids, the porosity, the macroscopic strain rate tensor and the time derivative of the strain rate tensor. An important outcome of the model is the differential lengthscale effect which exists between in-plane and out of plane components of the macroscopic stress. Namely, it is observed for axisymmetric loading that in-plane dynamic stress components are only related to the void radius a while the out of plane stress component is linked to a and the length of the RVE, l. In the thesis, we present the dynamic response of the porous medium when subjected to various loading conditions: spherical loading, axisymmetric plane strain loading, uniaxial loading and biaxial loading. While for plane strain loading under quasi static condition, the overall axial stress is spherical, in dynamic conditions, the inertia contribution hinders the overall stress tensor from being spherical. Another important result of the proposed theory is the effect of the void length, which does not exist in quasi static conditions where the overall response is solely modulated by the porosity. The case of thin cylinders under dynamic loading reveals a peculiar damage kinetics. In fact, the damage developed in such porous materials results from an increase of the void radius and a reduction of the external radius. The void collapse for uniaxial as well as for biaxial loadings are new observations. The analytical model predictions are validated based on comparisons with finite element calculations (Abaqus/Explicit).

Micromechanical modelling of edge failure in 800 MPa advanced high strength steels

Sheared edge failure is one of the major problems associated with the forming of advanced high strength steels (AHSS) such as dual-phase (DP) steels. To improve the performance of AHSS in industrial forming operations, ferritic-bainitic complex-phase (CP) steels have been developed and are gaining attention in academia as well as industry. The present work aims to investigate the influence of microstructure on micro-void (damage) evolution during the edge stretching of CP800 and DP780 steels and develop a micromechanics-based fracture model to predict edge failure for both reamed and sheared holes. Three-dimensional damage histories were obtained using x-ray microtomography on a series of hole tension specimens interrupted at different strain levels. These experiments considered a tensile specimen with a sheared hole at the center of specimen (sheared edge condition) or reamed (ideal edge condition). Void damage measurements, such as void area fraction, number of voids, void diameter, and void aspect ratio, were conducted and the results are compared to the reamed specimens to isolate the sheared edge effect on damage. The void measurements were used to implement a stress-state dependent model for nucleation, and coupled with the Ragab (2004) model for void growth and the Benzerga and Leblond (2014) model for void coalescence to develop a damage-based material model. Higher damage accumulation was observed behind the sheared edge in comparison the reamed edge at a given strain for both materials considered. An uncoupled anisotropic damage-based fracture model was formulated in a LS-DYNA user-defined material subroutine. As an alternative to computationally expensive multi-stage sheared edge stretching simulations, the measured strain-distribution of the sheared edge was mapped into a finite-element model to predict sheared edge failure. The proposed model was validated for the hole tension testing simulations and found to predict failure accurately for both the CP800 and DP780 for both the edge conditions.

Modeling mesoscale energy localization in shocked HMX, Part II: training machine-learned surrogate models for void shape and void–void interaction effects

Surrogate models for hotspot ignition and growth rates were presented in Part I, where the hotspots were formed by the collapse of single cylindrical voids. Such isolated cylindrical voids are idealizations of the void morphology in real meso-structures. This paper therefore investigates the effect of non-cylindrical void shapes and void-void interactions on hotspot ignition and growth. Surrogate models capturing these effects are constructed using a Bayesian Kriging approach. The training data for machine learning the surrogates are derived from reactive void collapse simulations spanning the parameter space of void aspect ratio (AR), void orientation ($\theta$), and void fraction ($\phi$). The resulting surrogate models portray strong dependence of the ignition and growth rates on void aspect ratio and orientation, particularly when they are oriented at acute angles with respect to the imposed shock. The surrogate models for void interaction effects show significant changes in hotspot ignition and growth rates as the void fraction increases. The paper elucidates the physics of hotspot evolution in void fields due to the creation and interaction of multiple hotspots. The results from this work will be useful not only for constructing meso-informed macro-scale models of HMX, but also for understanding the physics of void-void interactions and sensitivity due to void shape and orientation.

Closure of Internal Porosity in Continuous Casting Bloom During Heavy Reduction Process

To investigate the closure behavior of internal porosity (also referred to as internal void) in continuous casting bloom during heavy reduction (HR) and thus provide theoretical guidance for minimizing this kind of internal defect more effectively with HR, a three-dimensional (3D) mechanical model was developed based on the predicted temperature field by a 2D heat transfer model. With this 3D mechanical model, closure behaviors of internal porosity in continuous casting bloom during HR at and after the strand solidification end under different process conditions were numerically studied. It was found that the void axis length decreased significantly along the bloom thickness direction and increased slightly along the casting and bloom width directions after HR, and the influence of the initial void size on the void closure was not obvious. With a decrease of temperature difference between the bloom surface and center, HR efficiency for minimizing internal void decreased, while the required reduction force significantly increased. Compared with blooms with a uniform temperature distribution of 1100 °C, the void closure index after HR implemented at the strand solidification end was increased by ~ 25 pct. Compared with a conventional flat roll, the application of a convex roll during HR could contribute to minimizing the internal porosity more effectively and significantly enhance the reduction capacity of the withdrawal and straightening units. The void closure index of ηs and ηv (where ηs and ηv were defined based on the variation of the void aspect ratio and the void volume, respectively) was closely related to the equivalent strain (eeq) and the hydrostatic integration parameter (Q), respectively, and two mathematical equations were derived to quantitatively describe the relationship of ηs − eeq and ηv − Q.

Void Growth and Coalescence in Porous Plastic Solids With Sigmoidal Hardening

Abstract This paper presents an analysis of void growth and coalescence in isotropic, elastoplastic materials exhibiting sigmoidal hardening using unit cell calculations and micromechanics-based damage modeling. Axisymmetric finite element unit cell calculations are carried out under tensile loading with constant nominal stress triaxiality conditions. These calculations reveal the characteristic role of material hardening in the evolution of the effective response of the porous solid. The local heterogeneous flow hardening around the void plays an important role, which manifests in the stress–strain response, porosity evolution, void aspect ratio evolution, and the coalescence characteristics that are qualitatively different from those of a conventional power-law hardening porous solid. A homogenization-based damage model based on the micromechanics of void growth and coalescence is presented with two simple, heuristic modifications that account for this effect. The model is calibrated to a small number of unit cell results with initially spherical voids, and its efficacy is demonstrated for a range of porosity fractions, hardening characteristics, and void aspect ratios.

Void Formation and Strain-Induced Martensitic Transformation in TRIP780 Steel Sheet Submitted to Uniaxial Tensile Loading

This work aimed to analyze the damage behavior of cold rolled TRIP780 steel sheet submitted to interrupted uniaxial tensile tests performed along the rolling direction. The formation of voids is investigated as a function of the straining level using digital image analysis of scanning electron micrographs to obtain the measures of void density, void area fraction, void aspect ratio and mean void size. The volume fractions of both ferrite/martensite and retained austenite constituents were obtained from X-ray diffraction measurements. An abruptly decrease of retained austenite was observed at early stages of deformation followed by a slow saturation. The resulting strain-induced martensite is responsible for improving the formability of the TRIP780 as observed by instantaneous strain-hardening exponent. In the lower strain range, growth and coalescence of existing microvoids prevailed at both in-plane directions whereas nucleation of microvoids was also observed along the loading direction. Conversely, nucleation prevailed at the transverse direction in the intermediate strain range whereas growth and coalescence were predominant aligned to the loading direction. At larger strain levels, growth and coalescence of microvoids prevailed at both directions. The microvoids were initially found around inclusions and at the interface of ferrite-martensite phases and lastly also at the ferrite matrix.

Variable Poisson’s ratio materials for globally stable static and dynamic compression resistance

A novel porous mechanical metamaterial with variable Poisson’s ratio (VPR) is proposed for compression and impact applications. Constructed by stacking layers of varying void aspect ratio, VPR structures combine the collapsing penetration resistance of auxetic materials with the lateral expansion of positive Poisson’s ratio (PPR) materials. The concept is analyzed via numerical simulation of both compression and impact loading of VPR structures composed of silicone rubber. In addition, compression testing is performed with deformation maps obtained through digital image correlation. Results indicate that VPR structures, compared with uniform porous PPR structures, are resistant to global buckling instabilities. Moreover, in low-energy impact events, VPR structures respond to the impulse with a comparable force spread over a longer time than uniform PPR structures of the same porosity. At impact energies above approximately 45 J, the considered silicone VPR structures transition to a regime of more sharply peaked force response. At all energies, VPR structures deform more smoothly than their porous PPR counterparts.