Variations In Cell Wall Thickness Research Articles

Effect of cell size on the response to solution and aging heat treatments of open-cell Al-4.5Cu (wt.%) foams

Open-cell Al-4.5Cu (wt.%) foams were produced by the replication casting technique in cell sizes of 2.00–2.38 and 3.35–4.75 mm. The fabricated foams were subjected to solution and aging treatments to assess the effect of such heat treatments on the microstructure and mechanical properties of the foams as a function of cell size. Solution and aging heat treatments were carried out at 535 °C for 5.5 h and 170 °C for 8 h, respectively. The porosity and relative density of all produced samples were estimated by He pycnometer. In addition, the average cell wall thickness was assessed by image analysis to correlate this variable with the response to heat treatments of the material. The microstructural evolution of the heat-treated samples was analyzed by means of scanning electron microscopy, scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and differential scanning calorimetry. The mechanical characterization of the studied samples was carried out using uniaxial compression tests and microhardness tests. It was found that the foams did present different responses to both solution and aging treatments as a function of cell size, attributing these outcomes to the cell wall thickness variations, which presumably conditioned the cooling rates after heat treatments, thereby influencing the resulting microstructures and precipitation of Al-Cu second phases.Graphical abstract

Dynamic crushing of tailored honeycombs realized via additive manufacturing

Enhancing the energy absorption characteristics of a material/structure without compromising its strength and stiffness has been a longstanding challenge in the pursuit of lightweight engineering. Here, we introduce a novel tailoring strategy where the wall thickness of the honeycombs is bi-linearly graded along the out-of-plane direction to tune their energy absorption and impact resistance by varying two design parameters, the gradation parameter α and the normalized taper length η'. Based on the proposed scheme, hexagonal honeycombs of the same mass and varying parameters [α, η'] were designed and realized via Digital Light Processing (DLP) additive manufacturing. Low-velocity out-of-plane impact tests and dynamic FE calculations were performed to examine the collapse response of geometrically tailored honeycombs and assess their energy absorption characteristics and collapse mechanisms in relation to those observed in conventional (non-tailored) honeycombs of the same mass. The measurements and predictions revealed that the bi-linearly wall-thickness tailored honeycombs consistently outperform their non-tailored counterparts when the impact energy is high, reporting an increase in energy absorption as high as 250%. Such remarkable enhancement in energy absorption is attributed to a transition in the underlying collapse mechanism from global buckling mode to progressive crushing of the cell-walls. We also examined the impact response of honeycombs with periodic variations in cell-wall thickness and found that the latter structures collapse rapidly in an unstable manner, similar to what observed in conventional honeycombs, leading to limited capacity to dissipate the impact energy. With careful selection of the design parameters [α,η′], we experimentally demonstrate that bilinearly wall-thickness tailored honeycombs can exhibit simultaneous improvements in energy absorption and impact resistance, providing new opportunities for expanding the property space of honeycombs and opening the door for a wide range of applications.

Effects of cell size and cell wall thickness variations on the strength of closed-cell foams

In this work, the effects of cell size and cell wall thickness variations on the compressive and shear strengths of closed-cell foams were investigated using Laguerre tessellation models. It is found that the compressive and shear strengths of closed-cell foams decrease as cell size and cell wall thickness variations increase, and the compressive strength reduces more significantly. At a given level of variation, the effect of cell size variation on strength reduction is comparable to that of cell wall thickness variation on them. In the foam studied (M130 foam), cell wall thickness has a larger dispersion than cell size, and therefore is the main contributor to the strength reduction of the foam. In comparison to compressive and shear stiffnesses, cell size and cell wall thickness variations reduce compressive and shear strengths more significantly. Cell size variation results in strain concentration at the end of foam samples, thereby reducing the compressive strength of closed-cell foams. Cell wall thickness variation leads to cell wall buckling and thus reduction in compressive and shear strengths of closed-cell foams. As cell size and cell wall thickness in foams become less uniform, the relationships of compressive and shear strengths to relative density become less linear.

Effects of cell size and cell wall thickness variations on the stiffness of closed-cell foams

This paper concerns with the micromechanical modelling of closed-cell polymeric foams (M130) using Laguerre tessellation models incorporated with realistic foam cell size and cell wall thickness distributions. The cell size and cell wall thickness distributions of the foam were measured from microscope images. The Young’s modulus of cell wall material of the foam was characterised by nanoindentation tests. It is found that when the cell size and cell wall thickness are assumed to be uniform in the models, the Kelvin, Weaire–Phelan and Laguerre models overpredict the stiffness of the foam. However, the Young’s modulus and shear modulus predicted by the Laguerre models incorporating measured foam cell size and cell wall thickness distributions agree well with the experimental data. This emphasizes the fact that the integration of realistic cell wall and cell size variations is vital for foam modelling. Subsequently the effects of cell size and cell wall thickness variations on the stiffness of closed-cell foams were investigated using Laguerre models. It is found that the Young’s modulus and shear modulus decrease with increasing cell size and cell wall thickness variations. The degree of stiffness variation of closed-cell foams resulting from the cell size dispersion and cell wall thickness dispersion are comparable. There is little interaction between the cell size variation and cell wall thickness variation as far as their effects on foam moduli are concerned. Based on the simulation results, expressions incorporating cell size and cell wall thickness variations were formulated for predicting the stiffness of closed-cell foams. Lastly, a simple spring system model was proposed to explain the effects of cell size and cell wall thickness variations on the stiffness of cellular structures.

Effects of cell shape and cell wall thickness variations on the elastic properties of two-dimensional cellular solids

The Voronoi tessellation technique and the finite element method are utilized to investigate the microstructure-property relations of two-dimensional cellular solids having irregular cell shapes and non-uniform cell wall thickness. Twenty finite element models are constructed for each type of honeycomb samples (specimens) to obtain the mean values and standard deviations of the effective elastic properties. Spatially periodic boundary conditions are applied to each specimen containing 360 complete cells. The simulation results indicate that the elastic moduli increase as cell shapes become more irregular, but decrease as cell wall thickness gets less uniform. The Poisson’s ratios are insignificantly affected by the presence of these two types of imperfections. The effect of the interaction between the co-existing cell shape and cell wall thickness imperfections on the elastic moduli is found to be weak. Bending remains as the dominant deformation mechanism in a loaded honeycomb having irregular cell shapes and/or non-uniform cell wall thickness. In addition, it is revealed that such imperfect honeycombs can be regarded as isotropic.

Influence of cell wall thickness variations on elastic stiffness of closed-cell cellular solids

The stiffness of closed-cell low-density cellular solids, or solid foams, is affected by “imperfections” such as non-uniform cell shape and size, wavy distortions of cell walls, variations in cell wall thickness, etc. The present paper focuses on the influence of non-uniform cell wall thickness on stiffness. Calculations are performed on one model with different degrees of thickness variations. The model used is the flat-faced Kelvin structure, which consists of 14-sided polyhedra in a bcc arrangement. The results indicate that the stiffness of closed-cell cellular solids is not very sensitive to thickness variations.

Influence of Imperfections on Effective Properties of Cellular Solids

ABSTRACTThe mechanical properties of cellular solids, or solid foams, is affected by “imperfections” such as wavy distortions of cell walls, variations in cell wall thickness, non-uniform cell shape, etc. The present paper is focused mainly on elastic stiffnesses of closed cell cellular solids. A “perfect” model is first discussed and shown to predict the behavior of PVC foams well. However, this model over-estimates the stiffnesses of aluminum foams. The relatively poor properties of the aluminum foam are believed to be caused by imperfections in the cells. The main body of the paper focuses on modeling different kinds of imperfections, and analyzing their impact on foam properties.

Gram characteristics and wall ultrastructure of Arthrobacter crystallopoietes during coccus-rod morphogenesis.

Arthrobacter crystallopoietes growing exponentially as cocci were changed to rods by adding succinate to the medium. Cells were sampled before, during, and after this transition for Gram-staining and ultrastructural studies. Cells were Gram stained by the standardized method of Bartholomew, and all samples were fixed and prepared for thin sectioning in an identical manner. Cocci were gram positive, and thin sections demonstrated a gram-positive type of cell wall having an average thickness of 31 nm. Cells sampled during morphogenesis appeared as cocci with most having a single rodlike projection. The coccus portion of these transition cells was gram positive and bound by a gram-positive type of wall having an average thickness of 29 nm. The rodlike projection of the transition cells appeared to be gram negative; it was also surrounded by a gram-positive type of wall, but its average thickness was only 22 nm. Gram-negative rods of the type species, Arthrobacter globiformis, were also examined and found to produce a gram-positive type of wall with a 19-nm average thickness. Evidence for the trilaminar region, characteristic of most gram-negative bacterial cell walls, was totally lacking in both species. These results suggest that variations in cell wall thickness may be an important contributing factor to the variable Gram-staining characteristics of this genus.

The fine structure of developing microsclerotia of Verticillium dahliae Kleb.

Electron micrographs of the early stages in microsclerotial development in v. dahliae Kleb. showed that hyphae became swollen and vacuolate and extruded melanizing particles into the interhyphal spaces of the microsclerotium. Peripheral microsclerotial cells were killed either by a process of autoparasitisation from adjacent hyphae or by autolysis. Variations in the thickness of the melanized material surrounding cells gave the superficial appearance of variations in cell wall thickness between individual cells though actual changes in cell wall thickness were not observed.