Effect of defects on elastic–plastic behavior of cellular materials (original) (raw)
Related papers
Effects of cell irregularity on the elastic properties of 2D Voronoi honeycombs
Journal of The Mechanics and Physics of Solids, 2001
Foams are more and more widely used in di erent areas. Most available mechanical models are usually based on idealised unit cell structures, and are not able to account for the natural variations in microstructure which are typical for most foam structures. The objective of this work has been to investigate how the cell irregularity a ects the elastic properties of 2D random foams. We have constructed periodical random structures with di erent degrees of irregularity, and applied ÿnite element analysis (FEA) to determine their e ective elastic properties. The results indicate that, the more irregular the 2D random foams, the larger will be their e ective Young's modulus and shear modulus, and the smaller will be their bulk modulus at a constant overall relative density. However, for varying degrees of irregularity, the foams remain isotropic and the Poisson's ratios are very close to 1. Both Young's modulus and Poisson's ratio of random Voronoi honeycombs having di erent degrees of regularity , decrease gradually with increasing relative density .
The voronoi tessellation technic is applied in order to model cellular solids with irregular cell geometry and variable strut sections. The ligaments are formed considering the volume and shape characteristics of the voronoi cells. This way, the strut cross section variability is linked to the adjacent cell topology. Moreover, cross sectional shape size is controlled through two independent parameters. The proposed methodology was applied to approximate a Ni foam with specific porosity, cell and strut size. A finite element model of the generated geometry is developed using software with the ability of computing large plastic deformations employing self contact. The latter function is essential for the simulation of the foam densification, since the contact between the struts is the dominating densification mechanism. The calculated simulation results of the foam compression exhibit satisfactory correlation with the derived experimental ones. Furthermore, the foam strain rate sensit...
Effects of cell irregularity on the high strain compression of open-cell foams
Acta Materialia, 2002
The high strain compression of low-density open-cell polymer foams has been modelled by finite element analysis. We used a Voronoi method to generate periodic structures with different degrees of randomness of the cell size and shape, then to investigate the influence of this randomness on the response of Voronoi open-cell foams to high strain compression. It is found that, although the reduced compressive stress-strain relationship and the Poisson's ratio vary in different directions for individual samples, the models are, on average, isotropic. A highly irregular foam has a larger tangential modulus at very low strains and a lower effective stress at high compressive strains than a more regular foam. The geometrical properties were investigated and used to predict the compressive stress-strain relationships for random open-cell foams with different degrees of cell regularity. For irregular low density foams, strut bending and twisting (the "springs-in-parallel" model) dominate the mechanical response at low strains and strut buckling (the "springs-in-series" model) becomes the main deformation mechanism at large compressive strains.
2014
Closed-cell polymer foams are well-known for their thermal capabilities, but works on the mechanical behavior of these materials are scarce, especially concerning the influence of the foam's microstructure. The objective of this study is to investigate the influence of the relative density and irregularity of Voronoi closed-cell foam structures on their elastic characteristics (such as the Young's modulus and the Poisson's ratio) and plastic characteristics (such as elastic limits and collapse stresses). New laws are proposed in order to approximate the macroscopic mechanical behavior of Voronoi closed-cell foams under uniaxial tension and compression.
A plasticity model for cellular materials with open-celled structure
International Journal of Plasticity, 2003
Based on a rigid-plastic material model that obeys the von Mises yield criterion, the plastic behavior of foams with an open-celled structure is studied in this paper using a single unit cell. An approximate continuum plasticity model is developed within the framework of the upper bound theorem of plasticity to describe the yield behavior of foams. The microscopic velocity fields are derived for the unit cell, which satisfy the incompressibility and the kinematic boundary conditions, and expressed in macroscopic rate of deformation. From the microscopic velocity fields, a macroscopic yield function is developed for foams under multi-axial stresses and includes the effects of the hydrostatic stress due to the void presence and growth. The dependency of the derived yield surfaces of foams on their relative densities is studied. The plastic behavior of foams is also studied numerically using the finite element method. The newly developed plasticity model is compared with the finite element analysis results and other available foam models and then correlated with the finite element results.
A. Rezaei Sameti, 2024
Aluminum foams are among the materials that have many applications in the construction of various building elements, including sandwich panels. This category of materials has unique features due to low density, the presence of small holes, sound insulation, thermal insulation, and corrosion resistance. In this paper, the Voronoi tessellation method is proposed to simulate the porous configuration of aluminum foams, which has the high capability to generate a porous structure with different densities. It is demonstrated that the Voronoi tessellation method can generate porous structures with different densities, hole sizes, and wall thicknesses stably. Moreover, the Voronoi tessellation method has a high speed and can be used to construct different sizes of aluminum foams. A comparison of the configurations obtained from the Voronoi tessellation method and experimental tests demonstrates the capability and competence of this method in generating the porous structure of the aluminum foam. In order to investigate the mechanical behavior numerically, the uniaxial tension test is applied to the aluminum nanofoams using the molecular dynamics (MD) method. The MD analysis is performed in the LAMMPS open-access software using the embedded-atom model (EAM) interatomic potential. The periodic boundary condition is imposed in all the boundaries of the atomistic model to satisfy the essential condition of the representative volume element (RVE) based on the homogenization theory. After minimization and relaxation of RVE, the uniaxial tension test is applied in an increment manner to reduce the strain rate effect. The evolution of the stress-strain curve, along with the stress contours, are presented for the aluminum nanofoam during the uniaxial tension test. Young’s modulus of nanofoam obtained by numerical analysis is compared to that of experimental data to confirm the accuracy of the computational modeling. Moreover, the results emphasize the high dependence of the mechanical behavior of aluminum nanofoams on the density and porosity. Keywords Sandwich panel aluminum foam Voronoi tessellation method atomistic simulation porous materials uniaxial tension test
Effect of Porosity and Cell Topology on Elastic-Plastic Behavior of Cellular Structures
Procedia Structural Integrity, 2019
In this work we study the mechanical behavior of Ti6Al4V cellular structures by varying the randomness in the cell topology from regular cubic to completely random and the porosity of the structure. The porosity of the structure is altered by changing the strut thickness and the pore size to obtain a stiffness value between 0.5-12Gpa. The geometrical deviation in the structures from the asdesigned values is studied by morphological characterization. The samples are subjected to compression and tensile loading to obtain the stiffness and the elastic-plastic behavior of the samples. Finite element modelling (FEM) is carried out on the as-designed structures for both tensile and compressive loading to study the effect of deviation between the as-designed and as-built structures. FEM is also carried out for as-built regular structures, by introducing the geometrical deviation to match the porosity of the as-built structures. Comparison of FEM and experimental results indicated that the effect of cell topology depends on the porosity values. Simulation results of as-built structures demonstrated the importance of defects in the structure.
Influence of wavy imperfections in cell walls on elastic stiffness of cellular solids
Journal of the Mechanics and Physics of Solids, 1998
The stiffness of open and closed cell low density cellular solids, or solid foams, is affected by "imperfections" such as non-uniform cell size (multi-dispersity), non-uniform cell wall thickness, wavy distortions of cell walls, etc. Metal foams generally have lower relative stiffnesses than, for example, expanded PVC based polymer foams, and a comparison of the morphologies suggests that the main difference between these cellular solids is wavy distortions of the cell walls of the metal foams. The influence of wavy distortions on stiffness is modeled in this paper. The concepts arc introduced through application to open cell materials. for which closed form solutions are obtained, primarily to illustrate the phenomenon. Closed cell materials arc analysed subsequently, and results that are considered to be in good agreement with experimental observations are obtained.