Cell Wall Bending Research Articles

Characterization of close-celled cellular aluminum alloys

The deformation behaviour of two different types of aluminium alloy foam are studied under tension, compression, shear and hydrostatic pressure. Foams having closed cells are processed via batch casting, whereas foams with semi-open cells are processed by negative pressure infiltration. The influence of relative foam density, cell structure and cell orientation on the stiffness and strength of foams is studied; the deformation mechanisms are analysed by using video imaging and SEM (scanning electronic microscope). The measured dependence of stiffness and strength upon relative foam density are compared with analytical predictions. The measured stress versus strain curves along different loading paths are compared with predictions from a phenomenological constitutive model. It is found that the deformations of both types of foams are dominated by cell wall bending, attributed to various process induced imperfections in the cellualr structure. The closed cell foam is found to be isotropic, whereas the semi-open cell foam shows strong anisotropy.

Measuring the Deformation of Cells within a Piece of Compressed Potato Tuber Tissue

For the successful mathematical mechanical modelling of living plant tissues, relationships between cellular deformations and tissue deformation need to be investigated. In previous work these relationships have often been assumed. In this paper the deformation of living cells within potato tuber tissue is measured using light microscopy and image analysis and is analysed in relation to applied tissue deformations. The cell wall deformation was found to depend upon the orientation of the cell wall faces with respect to the global axes of the tissue and the applied tissue deformation. Some faces experienced compression, which reduced their surface area; others were deformed in bi-axial tension, thus increasing their surface area. These deformations were successfully related to the global tissue deformations, using a simple constant volume affine deformation model, up to compressive deformations of 20% of specimen height. Some deviation from the model was observed due to the bending of cell walls in compression.

Effect of imperfections on the yielding of two-dimensional foams

The influence of each of the six different types of morphological imperfection—waviness, non-uniform cell wall thickness, cell-size variations, fractured cell walls, cell-wall misalignments, and missing cells—on the yielding of 2D cellular solids has been studied systematically for biaxial loading. Emphasis is placed on quantifying the knock-down effect of these defects on the hydrostatic yield strength and upon understanding the associated deformation mechanisms. The simulations in the present study indicate that the high hydrostatic strength, characteristic of ideal honeycombs, is reduced to a level comparable with the deviatoric strength by several types of defect. The common source of this large knock-down is a switch in deformation mode from cell wall stretching to cell wall bending under hydrostatic loading. Fractured cell edges produce the largest knock-down effect on the yield strength of 2D foams, followed in order by missing cells, wavy cell edges, cell edge misalignments, Γ Voronoi cells, δ Voronoi cells, and non-uniform wall thickness. A simple elliptical yield function with two adjustable material parameters successfully fits the numerically predicted yield surfaces for the imperfect 2D foams, and shows potential as a phenomenological constitutive law to guide the design of structural components made from metallic foams.

Microstructural design of cellular materials—II: Microsandwich foams

The linear elastic behaviour of foams is primarily controlled by bending of cell walls. The mechanical performance of foams can be improved by increasing the bending stiffness of the cell walls. Recently, our group has developedmicrosandwich foams in which the cell walls are microsandwich elements. The improved bending stiffness of the walls resulting from the sandwich microstructure increases their performance indices. Here, we describe a finite element analysis of the Young's moduli and Poisson's ratio of microsandwich foams. We then use the analysis to perform a parametric study of the way in which the performance of the microsandwich foams depends on microstructural parameters. Finally we compare the results of the analysis with data for three microsandwich foams. The analysis describes the data well.

The elliptic paraboloid failure criterion for cellular solids and brittle foams

Cellular solids and brittle foams are increasingly finding application in constructions mainly as core materials for loaded sandwich structures where the loading of the structure generates multiaxial stress states on them. It has been established that the principal mechanism of deformation is based on the cell-wall bending and closed-cell as well as open-cell foams present similar stiffnesses. Therefore simple relations are found for their tensile, compressive and shear strengths and their elastic properties. It has been established in this paper that the modes of failure of such materials can be satisfactorily described by the elliptic paraboloid failure criterion for the general orthotropic body. Then, as soon as the yield or failure stresses in simple tension and compression are measured along the three principal stress directions of the material the failure locus is unequivocally defined and all the properties of the material under any complicated load can be accurately established. However, since these materials fail in the compression-compression-compression octant of the principal stress space by elastic buckling, the EPFS-criterion is cut-off by an ellipsoid surface, with intercepts along the principal axes the respective compressive failure stresses. The criterion thus established yields satisfactory results. It has been tested among others in a reticulated vitreous carbon foam as well as in an aluminium foam. The experimental results for these foams existing in the literature are fitting better with this universal criterion than any other considered, thus indicating the validity of the elliptic paraboloid failure criterion also for this type of materials.

A note on the anisotropy and fabric of highly porous materials

The dependence of the orthotropic elastic constants of a highly porous material upon the stereological parameters characterizing the anisotropy of the porous microstructure has been considered in two recent papers in this journal. In the first paper [1] dimensional arguments were employed to develop to a relationship between ratios of the orthotropic elastic constants and ratios of the mean intercept lengths for a class of cell wall bending models of highly porous materials. In the second paper [2] the general tensorial form of the relationship between the orthotropic elastic constants and the mean intercept length was described without reference to a specific form or type of porous microstructure. The purpose of this note is to observe that the particular relationships obtained from the class of cell wall bending models used in the first paper are proper special cases of the general relationships given in the second paper.

Multiplication of Pseudomonas syringae pv. phaseolicola“in planta”

AbstractIn the compatible combination of the halo blight disease of bean Pseudomonas phaseolicola was able to colonize large areas of the intercellular space of leaves, such that these confluent water congested areas became visible as water‐soaked spots. Most of the plant cell walls in the infected region maintained their normal shape, even when the cytoplasm had collapsed. Some inward bending of plant cell walls preceded their rather slow degradation and final replacement by bacterial masses. Neighbouring plant cells appeared to be metabolically active.In resistant leaves no indications of active bacterial attachment or encapsulation could be observed. However, bacteria appeared to be more densely packed in resistant leaves, and relatively more plant cells completely collapsed as compared with susceptible leaves.From 8—14 days after inoculation, the bacterial concentration did not change much in susceptible or resistant leaves, indicating the absence of bactericidal components. Even Pseudomonas pisi snowed some multiplication in bean leaves (immune reaction), but its growth stopped earlier than that of P. phaseolicola. in the resistant cultivars, probably due to a different mechanism of resistance. Although less bacteria were determined in the intercellular washing fluid (IF) compared with leaf homogenates, the high bacterial concentrations in the IF supported our observation that an effective encapsulation of bacteria in resistant leaves did not occur.