Cell Wall Fracture Research Articles

High-temperature dynamic failure behavior and compressive mechanical properties of alumina porous ceramic with various pore sizes

The dynamic compressive mechanical properties of alumina porous ceramic with various average pore sizes are tested at strain rates and temperatures ranging from 0.01 s−1 to 2900 s−1 and 20 °C to 850 °C, respectively. The corresponding 3d Voronoi models are employed to simulate the meso‑characteristics of cells and failure characteristics of alumina porous ceramic in multiscale. The failure in macroscale of quasi-static compression is fragmentation, and that of dynamic compression is comminution. But in mesoscale, the failure, both of quasi-static and dynamic compression, is the fracture of cell walls. Its dynamic strength increases with the increase of strain rate, but decreases with the increase of temperature and average pore size. The increase in temperature and average pore size leads to a decrease in its strain rate sensitivity. Its Young's modulus is not sensitive to the strain rate, but increases with the decrease of average pore size, and decrease with the increase of temperature, which is presented as a temperature and average pore size dependent function. The strain rate and temperature effects are also presented by average pore size dependent functions, respectively. A constitutive model is proposed based on them. It shows that the constitutive model fits well with the experimental results.

Dynamic response of GFRP/Nomex honeycomb sandwich panels subjected to multiple hail impacts – An experimental study

AbstractNowadays, more and more aircraft components are made of composite sandwich structures, and ice impacts are prone to cause internal damage to composite sandwich structures, so it is crucial to study the impact of hail on composite sandwich structures. Since hail impacts may occur several times at a single point, an experimental approach was used to investigate the dynamic response of GFRP/Nomex honeycomb sandwich panels under multiple hail impacts. The dynamic response of the structure was investigated in terms of response rate and deflection‐profile curves using 3D digital image correlation methods, and the effects of impact energy, hail size, and number of impacts on the impact resistance of the honeycomb sandwich structure were explored. The results show that the dynamic response process of sandwich panels can be categorized into the ball‐and‐crown phase, the rebound phase, and the vibration phase. Under the same impact energy, the impact of small‐sized hailstones is more threatening to honeycomb sandwich panels. As the number of impacts increases, the maximum deflection value of the back panel first increases uniformly, and when the layered damage area reaches the threshold value, fiber stripping occurs, and the increment of the maximum deflection value increases significantly.Highlights Since hail impacts may occur several times in real situations, this study conducted multiple impact experiments for one impact site. The dynamic response and damage pattern of the GFRP/Nomex honeycomb sandwich panels under multiple hail impacts were also investigated. The dynamic response process of GFRP/Nomex honeycomb sandwich panels was categorized into three phases: ball‐crown stage, rebound stage, and vibration stage. With the increase of the number of impacts, fiber stripping and fiber break damage began to occur when the layered damage area of the panel reached the threshold value. The honeycomb core has three main failure modes: cell wall folds, cell wall fracture, and cell wall debonding at the TGPW interface. Small‐diameter hails pose a more significant threat of damage to honeycomb sandwich panels at the same impact energy.

Insights into cork weathering regarding colour, chemical and cellular changes in view of outdoor applications.

A comprehensive analysis of outdoor weathering and soil burial of cork during 1-year experiments was carried out with measurements of CIELAB color parameters, cellular observations by scanning electron microscopy, and surface chemical features analysed by ATR-FTIR and wet chemical analysis. Cork applied in outdoor conditions above and below ground retained its physical structure and integrity without signs of deterioration or fracturing. The cellular structure was maintained with some small changes at the one-cell layer at the surface, featuring cellular expansion and minute cell wall fractures. Surface color and chemistry showed distinct results for outdoor exposure and soil burial. The weathered cork surfaces acquired a lighter color while the soil buried cork surfaces became darker. With outdoor weathering, the cork polar solubles increased (13.0% vs. 7.6% o.d. mass) while a substantial decrease of lignin occurred (about 28% of the original lignin was removed) leading to a suberin-enriched cork surface. The chemical impact on lignin is therefore responsible for the surface change towards lighter colors. Soil-burial induced hydrolysis of ester bonds of suberin and xylan, and the lignin-enriched cork surface displayed a dark brown color. FTIR and wet chemical results were consistent. Overall cork showed a considerable structural and physical stability that allows its application in outdoor conditions, namely for building façades or other surfacing applications. Architects and designers should take into account the color dynamics of the cork surfaces.

Cell wall fracture mechanism in ultrasonic-assisted cutting of honeycomb materials

Revealing the ultrasonic cutting mechanism of honeycomb composite is important for determining the acoustic parameters of the ultrasonic system and selecting the parameters of the cutting process. Understanding more details of the stress on the cell wall from ultrasonic vibrating tool and the conditions for cell wall breakage is essential to study the machining mechanism. According to the evolution of contact state between the straight edge cutter and the honeycomb cell wall in a cycle, the cutting force acting on the cell wall is divided into three stages: transverse cutting load action, longitudinal cutting load action, and no cutting load action. The cell wall deflection and stress equations under transverse cutting load were established by applying elastic thin plate small deflection theory. The deformation and fracture characteristics of the honeycomb cell wall were analyzed by combining the analytical and the finite element model. The results showed that the ultrasonic vibration of the cutter greatly improved the stiffening effect of the cell wall and its fracture was caused by the deflection under the transverse cutting load, which exceeded the maximum allowable deformation after local stiffening. In addition, with only longitudinal cutting load, it was difficult to break the critical buckling state that leads to cell wall fracture.

Effect of imperfections on the actuation performance of lattice materials

The effects of five different types of imperfection (fractured cell walls, missing cells, cell wall waviness, cell wall misalignment, and non-uniform cell wall thickness) on actuation performance are numerically investigated for the Kagome lattice and two of its variants: Double Kagome (DK) and Kagome with concentric triangles (KT). The lattice materials of interest are excited by deploying a single linear actuator located at their centre. The actuation performance of the lattices is determined by measuring the energy spent by the actuator and the attenuation distance of the deformation induced by the actuator. The deformation localises in a narrow corridor approximately one unit cell-wide for all three lattices in the absence of imperfections. The finite element calculations show that the critical parameter determining the actuation performance is the stiffness along the actuation corridor rather than the macroscopic Young’s modulus of the lattice. The less stiff the actuation corridor is, the smaller the actuation energy and the larger the attenuation distance (except when there is a fractured or wavy cell wall or a missing cell along the actuation corridor that immediately attenuates the displacement field). When imperfections are randomly distributed outside the actuation corridor, cell wall misalignment and non-uniform cell wall thickness barely affect the actuation performance, although cell wall misalignment considerably reduces the macroscopic Young’s modulus of the lattice. The actuator feels the presence of fractured or wavy cell walls or missing cells, whether placed inside or outside the actuation corridor. These three types of imperfection cause the largest knock-down both in the macroscopic Young’s modulus of the block and the actuation energy while increasing the attenuation distance. The increase in the attenuation distance due to imperfections is, however, impotent, as the accompanying reduction in stiffness makes the lattice more vulnerable to failure. However, if a defect is introduced slightly beyond the attenuation distance of the perfect lattice as an intentional design feature, it is beneficial for the actuation performance without decreasing the macroscopic Young’s modulus.

Fracture and repair in a bio‐inspired self‐healing structure

AbstractMany self‐healing materials have been developed, but very few self‐healing structures. We designed a structure in the form of a cylinder required to resist bending. Taking inspiration from plant stems, it has a cellular structure including longitudinal vascular channels for the delivery of healing agents. The structure was found to be capable of absorbing energy effectively, by deformation and fracture of cell walls. The introduction of healing agents (a two‐part liquid adhesive) into the vascular channels allowed fractured cell walls to be repaired. The resulting structure was capable of near‐perfect self‐healing, restoring its original mechanical properties even after significant damage. A computer simulation (finite element analysis) successfully predicted the early‐stage deformation and the initiation of damage. We advocate this structure‐level approach as a more appropriate way to introduce self‐repair into engineering systems.

3D printed self-similar AlSi10Mg alloy hierarchical honeycomb architectures under in-plane large deformation

Self-similar hierarchical honeycombs with complicated cellular topologies are promising cores for lightweight sandwich structures ascribed to superior structural efficiency in terms of mitigating damage. In this paper, the in-plane compressive characteristics of AlSi10Mg alloy hierarchical honeycombs, fabricated using Selective Laser Melting (SLM) methodology, under large deformation are reported. The results suggest that higher hierarchies enhance the elastic modulus of honeycombs significantly, whereas the compressive strength is improved negligibly. There is a failure mechanism transition from cell wall bending dominated to cell wall fracture dominated with the increase of relative density and hierarchical order, and the failure mechanisms are related to the wall thickness with critical value of 0.345 mm. It is concluded that the in-plane failure of hierarchical honeycombs is dominated by the bending, axial compression and shear deformation of original unit cell walls. Besides, the parent material without heat treatment contributes to more excellent mechanical properties of hierarchical honeycombs owing to higher tensile strength and elastic modulus. This work enables to facilitate the structural design and optimization of high-performance hierarchical honeycomb metamaterials with desired properties and functions.

Honeycomb core failure mechanism of CFRP/Nomex sandwich panel under multi-angle impact of hail ice

The rudder of an airplane is fabricated as a sandwich panel and can potentially be damaged by hail ice. The dynamic responses of a CFRP/Nomex sandwich panel with barely any visible core damage under multi-angle impact of an ice sphere at various velocities are researched experimentally and by using numerical simulations at the mesoscale. The results show that the honeycomb core represents three types of failure modes: wrinkling, fracture of cell walls, and debonding of cell walls at the interface of two-glued-paper-walls (TGPW) due to resin failure. These failure modes exist at specific impact velocities and angles. This paper proposes a mesoscale numerical modeling method of a honeycomb structure to represent the debonding failure at the TGPW interface. In addition, the mechanical properties of these failures are revealed, as the wrinkling of the cell walls are caused by buckling and the fracture of the cell walls and debonding of the TGPW interface are caused by out-plane and in-plane shearing of cell walls. The results showed that the relationship between the denting and impact kinetic energy under multi-angle impact is linear with respect to the impact angle. The effects of impact angles on the contacting forces on a rigid wall and sandwich panel are presented as the amount of peak impact forces affected by the target material type. However, the intensity of the peak impact forces is rarely affected by the type of the target material. The peak impact force of nonvertical impact is greater than that of the normal impact when the normal components of the impact velocities are the same. This is because the impact angle affects the distribution of ice scatter fragments during the impact, and this generates different pressure distributions on the face sheet of the panel.

Decreased deposition and increased swelling of cell walls contribute to increased cracking susceptibility of developing sweet cherry fruit

Main conclusionDuring fruit development, cell wall deposition rate decreases and cell wall swelling increases. The cell wall swelling pressure is very low relative to the fruit’s highly negative osmotic potential.Rain cracking of sweet cherry fruit is preceded by the swelling of the cell walls. Cell wall swelling decreases both the cell: cell adhesion and the cell wall fracture force. Rain cracking susceptibility increases during fruit development. The objectives were to relate developmental changes in cell wall swelling to compositional changes taking place in the cell wall. During fruit development, total mass of cell wall, of pectins and of hemicelluloses increases, but total mass of cellulose remains constant. The mass of these cell wall fractions increases at a lower rate than the fruit fresh mass—particularly during stage II and early stage III. During stage III, on a whole-fruit basis, the HCl-soluble pectin fraction, followed by the water-soluble pectin fraction, the NaOH-soluble pectin fraction and the oxalate-soluble pectin fraction all increase. At maturity, just the HCl-soluble pectin decreases. Cell wall swelling increases during stages I and II of fruit development, with little change thereafter. This was indexed by light microscopy of skin sections following turgor release, and by determinations of the swelling capacity, water holding capacity and water retention capacity. The increase in cell wall swelling during development was due primarily to increases in NaOH-soluble pectins. The in vitro swelling of cell wall extracts depends on the applied pressure. The swelling pressure of the alcohol-insoluble residue is low throughout development and surprisingly similar across different cell wall fractions. Thus, swelling pressure does not contribute significantly to fruit water potential.

Characterization of initial and subsequent yield behaviors of closed-cell aluminum foams under multiaxial loadings

As one of widely-used multi-functional materials, metallic foams are often subjected to multiaxial stress states in practical applications; in which the yield behaviors under different stress states need to be better characterized. However, it is difficult to conduct multiaxial mechanical tests on foam materials, especially under arbitrarily proportional triaxial loadings. In this study, numerical modeling of the yield properties of aluminum (Al) foam is performed based upon 3D image-based models. Real structural models of the Al foam are reconstructed using microscopic X-ray computed tomography (micro-CT) technique for multiaxial characterization computationally. Numerical simulation reveals that plastic collapse and fracture of cell-walls first occur in a relatively weak zone under prescribed uni/tri-axial loadings, whereas happen in the central section under biaxial loadings. An initial yield criterion is defined based upon energy dissipation. A constant stress proportion coefficient can be obtained under the stress-controlled triaxial proportional loading and its effectiveness is also validated by comparing the stress-strain responses with those under velocity-controlled uniaxial and hydrostatic compressive loading. It is found that the initial yield points of foam specimens can be obtained in different stress states in the von Mises-mean stress space. The normalized yield surface is approximately independent of relative density and can be well fitted to a parabolic or an elliptic function. Comparisons between the numerical yield surface and three typical theoretical yield criteria suggest that the Deshpande and Fleck's criterion provides more precise predictions than Gibson and Miller yield criteria when a proper plastic Poisson's ratio is adopted. Furthermore, it is shown that the early stage yield surface of metallic foams can be evolved in a geometrically self-similar manner.

Preparation and properties of open-cell zinc foams as human bone substitute material

Foamed zinc was prepared by infiltration casting process. The mechanical properties and corrosion resistance of the samples were studied, and the feasibility of the foamed zinc as a bone implant material was discussed. All the compression stress-strain curves of open-cell zinc foams with various cell size (1–4 mm) and porosity (55%–67%) show three stages: elastic stage, plastic stage, and densification stage. The compression strength increases with decreasing density. The smooth stress-strain response indicates a progressively deformation of open-cell zinc foam. In addition, the cell wall or edge bending and fracture are the dominated mechanisms for failure of open cell zinc foam. The immersion test for determining the corrosion rate of open cell zinc foam was conducted in simulated body fluid. It was found that zinc foam with a small cell size and high porosity showed a higher corrosion rate. In addition, open-cell zinc foams can effectively induce Ca-P deposition in immersion tests, showing good bioactivity. Therefore, the open cell zinc foam prepared in this experiment has a good potential application as a human bone substitute material.

Collapse response of two-dimensional cellular solids by plasticity and cracking: application to wood

The competition between fracture and plasticity in periodic hexagonal honeycomb structures subjected to (i) intercell cracking, (ii) intrawall cracking and (iii) transwall cracking is examined, and their effect upon the macroscopic collapse response is explored using dedicated FEM analyses of unit cell configurations. These three cracking mechanisms are regularly observed in wood microstructures, and insight into their influence on the macroscopic collapse behavior is necessary for adequately designing timber structures against failure. The numerical results are presented by means of collapse contours in the hydrostatic-deviatoric stress space, illustrating the effects of wall slenderness, relative fracture (versus yield) strength, and the relative size of the plastic zone at the crack tip. Both the hydrostatic and deviatoric collapse strengths of the honeycomb strongly increase in the transition from brittle cell walls with low relative fracture strength to ductile cell walls with high relative fracture strength. This strength increase typically changes the shape of the collapse contour, and is the largest for transwall cracking, followed by intercell cracking and finally intrawall cracking. The ultimate collapse strength of the honeycomb is significantly more sensitive to the fracture strength than to the fracture toughness of the cell walls, and correctly approaches the plastic yield surface under increasing relative fracture strength. The numerical results may serve as a useful guideline in the experimental calibration of the local fracture and yield strengths of cell walls in wood.

Compressive behavior of Mg alloy foams at elevated temperature

It is the purpose of this work to investigate compressive behavior of closed-cell Mg alloy foams fabricated by melting method at elevated temperature. First, the Mg alloy foams were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) for microstructure, composition and phases, respectively. Then, the compressive tests were carried out on closed-cell Mg alloy foams at room temperature (25 °C), 200 °C, 400 °C and 500 °C. It is found that the effect of temperature on compressive behavior of the Mg alloy foams is different between high temperature and low temperature. The Mg alloy foams exhibit a brittle compressive behavior at room temperature by the fracture of cell walls. However, the pure brittle fracture mode transforms into a combination of brittle fracture mode and ductile fracture mode at temperature in the range of 200 °C–400 °C. This indicates a soften effect of high temperature on the foams matrix and the plasticity of the cell walls at low temperature. It is concluded that the compressive strength and energy absorption capacity of the closed-cell Mg alloy foams decrease with the increasing temperature. Furthermore, a new method of defining the densification strain is proposed based on the perspective of energy absorption efficiency.

Manufacturing and Performance Evaluation of Carbon Fiber–Reinforced Honeycombs

In this work, the manufacturing characteristics and a performance evaluation of carbon fiber–reinforced epoxy honeycombs are reported. The vacuum-assisted resin transfer molding process, using a central injection point, is used to infuse a unidirectional dry slit tape with the epoxy resin system Prime 20 LV in a wax mold. The compression behavior of the manufactured honeycomb structure was evaluated by subjecting samples to quasi-static compression loading. Failure criteria for the reinforced honeycombs were developed and failure maps were constructed. These maps can be used to evaluate the reliability of the core for a prescribed loading condition. Improvements in the load-carrying capacity for the reinforced samples, as compared with unreinforced specimens, are discussed and the theoretical predictions are compared with the experimental data. The compression test results highlight a load-carrying capacity up to 26 kN (~143 MPa) for a single hexagonal cell (unit cell) and 160 kN (~170 MPa) for cores consisting of 2.5 × 3.5 cells. The failure map indicates buckling to be the predominant mode of failure at low relative densities, shifting to cell wall fracture at relative densities closer to a value of 10−1. The resulting energy absorption diagram shows a monotonic increase in energy absorption with the increasing t/l ratio of the honeycomb core cell walls.

Structural Loading of Cellular Metals: Damage Mechanisms and Standardization Concepts

Structural loading of cellular metals is strongly affected by brittle fracture of cell struts and walls that exhibit tensile loads, e.g., during fatigue loading. The present paper summarizes results of compression, tension and cyclic loading experiments on various closed-cell metal foams and metal foam sandwiches (Alulight, Alporas, Foamtech, AFS) using various mechanical testing systems. The results were correlated with a thorough analysis of the cellular mesostructure and the cell strut/wall microstructure by means of scanning electron microscopy revealing defects, such as casting porosity and large Si precipitates in the Al-Si eutectic of aluminum cast alloy. The results of the work served for the definition of testing standards for compression testing (ISO 13314) and tensile testing (DIN 50099), which are outlined in the paper. Such standards and design guidelines are crucial for a successful implementation of cellular metals in innovative products in mechanical, automotive and energy engineering as well as in bioengineering.

Experimental study on in-plane compressive response of irregular honeycombs

Compared with regular honeycombs, irregular honeycombs are more representative of real foams, and thus more suitable for the study of foam mechanics. In this paper, the deformation and failure progression in the irregular honeycombs are investigated by analysing the images captured in order to gain an improved understanding on foam failure. Irregular honeycombs with varying cell wall thickness, cell size and cell shape were manufactured using a three-dimensional printer and tested under compression. The behaviour of irregular honeycombs is found to be different from that of regular honeycombs. In irregular honeycombs, cell walls start to fracture at some point, initially at a low speed from multiple locations. The global stress reaches its maximum value shortly after the first fracture of cell walls. Only a few cell walls buckle in the specimens with cells of irregular shape. Fracture is more likely to occur to thin and long cell walls aligned within a medium angle (around 30 to 60°) to the compressive load. However, the susceptibility of a cell wall is to fracture is also affected by its neighbouring cell walls. Strong and stiff neighbouring cell walls could shield load away and protect it from breaking. Because of this, it is better to think of a weak spot as a region, rather than an individual cell or cell wall. Overall, the more uniform cell wall size and thickness are, the better the mechanical performance of cellular solids is.

Adding tetrahydrofuran to dilute acid pretreatment provides new insights into substrate changes that greatly enhance biomass deconstruction by Clostridium thermocellum and fungal enzymes

BackgroundConsolidated bioprocessing (CBP) by anaerobes, such as Clostridium thermocellum, which combine enzyme production, hydrolysis, and fermentation are promising alternatives to historical economic challenges of using fungal enzymes for biological conversion of lignocellulosic biomass. However, limited research has integrated CBP with real pretreated biomass, and understanding how pretreatment impacts subsequent deconstruction by CBP vs. fungal enzymes can provide valuable insights into CBP and suggest other novel biomass deconstruction strategies. This study focused on determining the effect of pretreatment by dilute sulfuric acid alone (DA) and with tetrahydrofuran (THF) addition via co-solvent-enhanced lignocellulosic fractionation (CELF) on deconstruction of corn stover and Populus with much different recalcitrance by C. thermocellum vs. fungal enzymes and changes in pretreated biomass related to these differences.ResultsCoupling CELF fractionation of corn stover and Populus with subsequent CBP by the anaerobe C. thermocellum completely solubilized polysaccharides left in the pretreated solids within only 48 h without adding enzymes. These results were better than those from the conventional DA followed by either CBP or fungal enzymes or CELF followed by fungal enzyme hydrolysis, especially at viable enzyme loadings. Enzyme adsorption on CELF-pretreated corn stover and CELF-pretreated Populus solids were virtually equal, while DA improved the enzyme accessibility for corn stover more than Populus. Confocal scanning light microscopy (CSLM), transmission electron microscopy (TEM), and NMR characterization of solids from both pretreatments revealed differences in cell wall structure and lignin composition, location, coalescence, and migration-enhanced digestibility of CELF-pretreated solids.ConclusionsAdding THF to DA pretreatment (CELF) greatly enhanced deconstruction of corn stover and Populus by fungal enzymes and C. thermocellum CBP, and the CELF–CBP tandem was agnostic to feedstock recalcitrance. Composition measurements, material balances, cellulase adsorption, and CSLM and TEM imaging revealed adding THF enhanced the enzyme accessibility, cell wall fractures, and cellular dislocation and cell wall delamination. Overall, enhanced deconstruction of CELF solids by enzymes and particularly by C. thermocellum could be related to lignin removal and alteration, thereby pointing to these factors being key contributors to biomass recalcitrance as a barrier to low-cost biological conversion to sustainable fuels.

Microstructure change in wood cell wall fracture from mechanical pretreatment and its influence on enzymatic hydrolysis

Mechanical pretreatment is an effective process for chemical or biochemical conversion of woody biomass. The deconstruction features of the wood cell wall play an important role in its chemical or biochemical processing. In this work, we evaluated the wood cell wall fracture in the early stage of mechanical pretreatment process conducted with various initial moisture contents. Electronic microscopy (i.e., SEM and TEM) and confocal laser scanning microscopy (CLSM) were used to visualize the cellular structure changes due to cell wall fractures.Results reveal that the enzymatic digestibility of micronized wood produced from different initial moisture contents was improved by 2–6 folder than that of the raw material. The types of cell wall fractures after mechanical pretreatment were distinguished by the initial moisture contents of wood. In wood samples with lower moisture content, interwall fracture occurred predominantly at the middle lamella region, while intrawall fracture occurred primarily at inner cell wall layers, with sever breakage in wood fibers for high moisture content samples. Differences in the distribution of surface chemical composition also resulted from different cell wall fractures. Lignin preferentially covered the fracture surface of low-moisture content samples, while carbohydrates were more predominate in high-moisture content samples. These morphological and structural alternation contributed to improving enzymatic digestibility of micronized wood.Findings from this study demonstrate how mechanical pretreatment modifies the fracture features of wood cell wall for further chemical/biochemical reactions.

Cell wall swelling, fracture mode, and the mechanical properties of cherry fruit skins are closely related.

Cell wall swelling, fracture mode (along the middle lamellae vs. across cell walls), stiffness, and pressure at fracture of the sweet cherry fruit skin are closely related. Skin cracking is a common phenomenon in many crops bearing fleshy fruit. The objectives were to investigate relationships between the mode of fracture, the extent of cell wall swelling, and the mechanical properties of the fruit skin using sweet cherry (Prunus avium) as a model. Cracking was induced by incubating whole fruit in deionised water or by fracturing exocarp segments (ESs) in biaxial tensile tests. The fracture mode of epidermal cells was investigated by light microscopy. In biaxial tensile tests, the anticlinal cell walls of the ES fractured predominantly across the cell walls (rather than along) and showed no cell wall swelling. In contrast, fruit incubated in water fractured predominantly along the anticlinal epidermal cell walls and the cell walls were swollen. Swelling of cell walls also occurred when ESs were incubated in malic acid, in hypertonic solutions of sucrose, or in water. Compared to the untreated controls, these treatments resulted in more frequent fractures along the cell walls, lower pressures at fracture (p fracture), and lower moduli of elasticity (E, i.e., less stiff). Conversely, compared to the untreated controls, incubating the ES in CaCl2 and in high concentrations of ethanol resulted in thinner cell walls, in less frequent fractures along the cell walls, higher E and p fracture. Our study demonstrates that fracture mode, stiffness, and pressure at fracture are closely related to cell wall swelling. A number of other factors, including cultivar, ripening stage, turgor, CaCl2, and malic acid, exert their effects only indirectly, i.e., by affecting cell wall swelling.

Response of closed-cell aluminum foams under static and impact loading: Experimental and mesoscopic numerical analysis

This paper investigates the mechanical behavior and mesoscopic deformation of closed-cell aluminum foam under static and impact loading. Two groups of closed-cell aluminum foam specimens are prepared for static and dynamic tests, respectively. The configurations of the specimen (such as porosity and base material) are considered. The mechanical response and the energy absorption capability are studied. In order to reveal the mesoscopic deformation mechanism, a mesoscopic model is developed considering the randomness of pores in size and distribution. The dynamic response under low and high impact velocity is investigated based on the mesoscopic model. Insights into the deformation mode are conducted. The collapse and the fracture of cell-walls under different impact velocity are discussed. It reveals that the mesoscopic deformation is closely related to the impact velocity. Porosity is also an important aspect on the dynamic property. At low impact velocity, the plastic deformation can be found in the middle domain of the specimen. At high impact loading, a dynamic mode is formed, in which the collapse and fracture of cell-walls is mainly located near the impact end. At a very high impact velocity, a shock mode can be found, forming a strain localization at the impact end for the metallic foam. It leads to significant improvements on the dynamic plateau stress and the energy absorption capability. This originates from the inertia effects.