Behavior Of Cellular Structures Research Articles

Behaviour of Cellular Structures under Impact Loading a Computational Study

New multiphysical computational models for simulation of regular open and closed-cell cellular structures behaviour under compressive impact loading are presented. The behaviour of cellular structures with fluid fillers under uniaxial impact loading and large deformations has been analyzed with the explicit nonlinear finite element code LS-DYNA. The behaviour of closed-cell cellular structure has been evaluated with the use of the representative volume element, where the influence of residual gas inside the closed pores has been studied. Open-cell cellular structure was modelled as a whole to properly account for considered fluid flow through the cells, which significantly influences macroscopic behaviour of cellular structure. The fluid has been modelled by applying a Smoothed Particle Hydrodynamics (SPH) method. Computational simulations showed that the base material has the highest influence on the behaviour of cellular structures under impact conditions. The increase of the relative density and strain rate results in increase of the cellular structure stiffness. Parametrical numerical simulations have also confirmed that filler influences the macroscopic behaviour of the cellular structures which depends on the loading type and the size of the cellular structure. In open-cell cellular structures with higher filler viscosity and higher relative density, increased impact energy absorption has been observed.

Thermal Post-Impact Behavior of Closed-Cell Cellular Structures with Fillers – Part I

The study describes the behavior of regular closed-cell cellular structure with gaseous fillers under impact conditions and consequent post-impact thermal conduction due to the compression of filler gas. Two dependent but different analyses types have been carried out for this purpose: (i) a strongly coupled fluid-structure interaction and (ii) a weakly coupled thermalstructural analysis. This paper describes the structural analyses of the closed-cell cellular structure under impact loading. The explicit code LS-DYNA was used to computationally determine the behavior of cellular structure under compressive dynamic loading, where one unit volume element of the cellular structure has been discretised with finite elements considering a simultaneous strongly coupled interaction with the gaseous pore filler. Closed-cell cellular structures with different relative densities and initial pore pressures have been considered. Computational simulations have shown that the gaseous filler influences the mechanical behavior of cellular structure regarding the loading type, relative density and type of the base material. It was determined that the filler’s temperature significantly increases due to the compressive impact loading, which might influence the macroscopic behavior of the cellular structure.

Evaluation of Thermal and Mechanical Filler Gas Influence on Honeycomb Structures Behavior

In this paper the behavior of hexagonal honeycombs under dynamic in-plane loading is described. Additionally, the presence and influence of the filler gas inside the honeycomb cells is considered. Such structures are subjected to very large deformation during an impact, where the filler gas might strongly affect their behavior and the capability of deformational energy absorption, especially at very low relative densities. The purpose of this research was therefore to evaluate the influence of filler gas on the macroscopic cellular structure behavior under dynamic uniaxial loading conditions by means of computational simulations. The LS-DYNA code has been used for this purpose, where a fully coupled interaction between the honeycomb structure and the filler gas was simulated. Different relative densities, initial pore pressures and strain rates have been considered. The computational results clearly show the influence of the filler gas on the macroscopic behavior of analyzed honeycomb structures. Because of very large deformation of the cellular structure, the gas inside the cells is also enormously compressed which results in very high gas temperatures and contributes to increased crash energy absorption capability. The evaluated results are valuable for further research considering also the heat transfer in honeycomb structures and for investigations of variation of the base material mechanical properties due to increased gas temperatures under impact loading conditions.

Behaviour of cellular structures with fluid fillers under impact loading

The paper describes experimental and computational testing of regular open-cell cellular structures behaviour under impact loading. Open-cell cellular specimens made of aluminium alloy and polymer were experimentally tested under quasi-static and dynamic compressive loading in order to evaluate the failure conditions and the strain rate sensitivity. Additionally, specimens with viscous fillers have been tested to determine the increase of the energy absorption due to filler effects. The tests have shown that brittle behaviour of the cellular structure due to sudden rupture of intercellular walls observed in quasi-static and dynamic tests is reduced by introduction of viscous filler, while at the same time the energy absorption is increased. The influence of fluid filler on open-cell cellular material behaviour under impact loading was further investigated with parametric computational simulations, where fully coupled interaction between the base material and the pore filler was considered. The explicit nonlinear finite element code ls-dyna was used for this purpose. Different failure criteria were evaluated to simulate the collapsing of intercellular walls and the failure mechanism of cellular structures in general. The new computational models and presented procedures enable determination of the optimal geometric and material parameters of cellular materials with viscous fillers for individual application demands. For example, the cellular structure stiffness and impact energy absorption through controlled deformation can be easily adapted to improve the efficiency of crash absorbers

Thermal expansion behavior in fabricated cellular structures

Thermal expansion behavior of cellular structures is of interest in applications where undesirable deformation and failure are caused by thermal expansion mismatch. This report describes the role of processing-induced effects and metallurgical aspects of melt-processed cellular structures, such as a bi-material structure designed to contract on heating, as well as uni-material structures of regular and stochastic topology. This bi-material structure utilized the principle of internal geometric constraints to alter the expansion behavior of the internal ligaments to create overall contraction of the structure. Homogenization design method was used to design the structure, and fabrication was by direct metal deposition by laser melting of powder in another part of a joint effort. The degree of porosity and grain size in the fabricated structure are characterized and related to the laser deposition parameters. The structure was found to contract upon heating over a short range of temperature subsequent to which normal expansion ensued. Also examined in this report are uni-material cellular structures, in which internal constraints arise from residual stress variations caused by the fabrication process, and thereby alter their expansion characteristics. A simple analysis of thermal strain of this material supports the observed thermal expansion behavior.

Topological and geometrical properties of crack patterns produced by the thermal shock in ceramics

We study the crack patterns produced by thermal shock in ceramic tableware. Cracks produced at sufficiently high-temperature gradients partition the surface plane of a ceramic material into cells forming a random two-dimensional space-filling cellular structure. The topological and geometrical properties of these structures are described and analyzed. The distribution of the number of cell sides, the topological correlations between adjoining cells, the probability distributions of a cell area and side length, the average area and perimeter of $n$-sided cells, and the distribution of vertex angles are determined. The results show that the studied cellular structures obey the Aboav-Weaire law [Metallography 3, 383 (1970); 7, 157 (1974)] and Desch's law [J. Inst. Metals 22, 241 (1919)]. The scaling behavior of cellular structures obtained at different temperature gradients of the thermal shock is also presented.

Deformability modelling of foam elongation

The relationships concerning the deformation of rigid cellular foams during elongation are reviewed under the conditions of a monotonous load increase. The cellular structure behavior is modelled by the operation of a certain piston structure. The proposed equation leads to a satisfactory approximation of the experimental stress-strain diagram for polystyrene and poly(vinyl chloride) foams.