Properties Of Open-cell Foams Research Articles

Simulation of the dynamic properties of open cell foams by means of a macroscopic stochastic model

AbstractOpen cell foams are widely used in engineering which includes an application in areas, where vibration excitations can act on the structures. Therefore dynamical properties as eigenfrequencies and eigenmodes are of particular interest, which show a dispersion due to the stochastic characteristic of the microstructure. To avoid time consuming measurements a simulation algorithm will be presented which enables to estimate the distribution of the eigenfrequencies. In order to do this the microstructure has to be addressed as well as different weights of several specimen, which have an influence on the computed and measured eigenfrequencies.

Static stability and vibration behavior of graphene platelets reinforced porous sandwich cylindrical panel under non-uniform edge loads using semi-analytical approach

Buckling and free vibration characteristics of sandwich cylindrical panel with porous functionally graded graphene platelets (FG-GPL) core are investigated using semi-analytical approach. The effective mechanical properties are obtained by using properties of open cell foams and Halpin–Tsai micro mechanical model. The governing equations are obtained using Hamilton’s principle, considering a higher order theory to account the transverse shear and solved by Galerkin’s method. Effects of nature of in-plane edge load, distribution of porosity and GPL, porosity coefficient, GPL loading, core to total thickness ratio are analyzed in detail. It is shown that for a FG-GPL core sandwich cylindrical panel with high core thickness, even at higher amount of porosity the buckling resistance and free vibration frequency can be improved by properly tailoring both the GPL and porosity distribution. Moreover, a much variation in buckling and free vibration response with the type of in plane loading is observed and evident mode shape changes are observed with increase in aspect ratio. The cylindrical sandwich panel having a core with D-PD porosity variation and I-GPL-P pattern of GPL distribution has the maximum buckling resistance and free vibration frequency value.

Modelling open cell-foams based on the Weaire–Phelan unit cell with a minimal surface energy approach

The complex architecture of open cell foams has most often been described by Kelvin cell models. It has been shown that the accuracy to predict the elastic properties of open cell foams increases with an increasing level of detail and resemblance to real foam microstructures. However, the Kelvin cell does not possess pentagonal faces which are the most abundant within real open cell foams. Therefore this study focuses on the use of the Weaire–Phelan unit cell to model the elastic properties of an open cell polyurethane foam. Optical and scanning electron microscopy were used to characterise the architecture of the open cell foam. Surface Evolver software was used to minimize the surface energy and introduce the typical architectural characteristics of the open cell foam to the FE-model. The E-modulus and Poisson coefficient of the Kelvin and Weaire–Phelan cell show a similar behaviour as a function of density. The Weaire–Phelan cell predicts however a higher dependency of the shear modulus on the density. When the influence of the elongation of the cells in the rise direction of the foam and the uncertainty of the solid material properties of the polyurethane is taken into account, a good accuracy of the Kelvin cell and Weaire–Phelan structure based FE-models versus experimental compression tests is found.

NUMERICAL MODELING OF THE FRACTURE BEHAVIOR OF OPEN CELL FOAMS

The objective of this work is to numerically study the individual and mixed mode fracture behavior of open-cell foams. Three-dimensional tetrakaidecahedron model is adopted and finite element method is employed in analysis. The overall elastic properties of open-cell foams are numerically obtained. Then, modes I, II, III and mixed mode I–II fracture behavior of open-cell foams are numerically predicted respectively, in which both solid/hollow and square/circular strut cross-sections are considered. It is seen that the relationship between the overall critical stress intensity factor and the relative density of foam obeys power law for individual crack problems. Effects of the shape and the topology of strut cross-section are discussed in detail. For mixed mode I–II crack problem, the relationship between KI/KICand KII/KIICis obtained and the effect of the shape of strut cross-section is discussed.

The effect of continuous pore stratification on the acoustic absorption in open cell foams

This work reports new data on the acoustical properties of open cell foam with pore stratification. The pore size distribution as a function of the sample depth is determined in the laboratory using methods of optical image analysis. It is shown that the pore size distribution in this class of materials changes gradually with the depth. It is also shown that the observed pore size distribution gradient is responsible for the air flow resistivity stratification, which is measured acoustically and non-acoustically. The acoustical absorption coefficient of the developed porous sample is measured using a standard laboratory method. A suitable theoretical model for the acoustical properties of porous media with pore size distribution is adopted. The measured data for open porosity, tortuosity, and standard deviation data are used together with this model to predict the observed acoustic absorption behavior of the developed material sample. The transfer matrix approach is used in the modeling process to account for the pore size stratification. This work suggests that it is possible to design and manufacture porous media with continuous pore size stratification, which can provide an improvement to conventional porous media with uniform pore size distribution in terms of the attained acoustic absorption coefficient.

The influence of the stress relaxation and creep recovery times on the viscoelastic properties of open cell foams

AbstractThe aim of this work is to study the mechanical behavior of flexible polyurethane foams used in cushioning applications. In particular, the differences between slow recovery (SR) and fast recovery (FR) foams are highlighted. To characterize the flexible polyurethane foams, creep and hysteresis tests were performed at different strain rate, stress levels, and temperatures. Significant differences were observed between the SR and FR foams, particularly in terms of residual deformation after unloading, hysteresis area, and creep behavior at different stress levels. Creep compliance at different stress levels was compared with a Voigt‐Kelvin model. Stress–strain loading curves were compared with a phenomenological model originally modified to account for the strain rate dependence. In both cases, it is possible to show that the main differences observed in the behavior of the foams are due to the different relaxation and recovery times of the foams. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Mechanical Properties of Open Cell Foams: Simulations by Laguerre Tesselation Procedure

The Laguerre tessellation procedure is used for simulation of microstructures of open-cell foams. In contrast with the conventional Voronoii tessellation, the Laguerre one permits to simulate the foam microstructures with a given law of distribution of cell diameters. An original finite element method is developed for calculating the elastic properties: the ligaments are modelled as Timoshenko beams and each ligament is treated as one finite element. The size of the representative volume element for reliable calculations of the effective elastic properties is evaluated by computational experiments. Dependence of the properties on the cell size distributions and ligament shapes are analyzed.

Determination of spectral radiative properties of open-cell foam: hemispherical - directional spectral transmittance and directional - directional transmittance measurement

Spectral radiative properties (absorption coefficient, scattering coefficient, and phase function) of open-cell carbon foam have been determined experimentally. The identification method uses spectral transmittance and reflectance measurements and a prediction model based on a combination of geometric optics laws and diffraction theory. In the wavelength region 0.2-2 μm directional-hemispherical transmittance and reflectance measurements are used, and directional directional transmittance and reflectance measurements are used in the wavelength region 2-15 μm. Radiative properties are determined in the wavelength region from visible to infrared. Two approaches corresponding to two different types of measurements are compared for the determination of radiative properties. Moreover, experiments on a guarded hot-plate device are used to confirm that the proposed model is appropriate to predict the radiative heat transfer in such media.

Determination of Spectral Radiative Properties of Open Cell Foam: Model Validation

Spectral radiative properties (absorption coefe cient, scattering coefe cient, and phase function ) of open cell carbonfoamaredeterminedexperimentally. Theidentie cation method usesspectral transmittanceand ree ectance measurements and a prediction model based on a combination of geometric optics laws and of diffraction theory. In the wavelength region of 0.1 -2.1 πm, directional -hemispherical transmittance and ree ectance measurements are used, whereas directional -directional transmittance and ree ectance measurements are used in the wavelength regionof2-15 πm.Thus, radiativepropertiesaredeterminedin thewavelengthregionfromvisibletoinfrared.The two approaches corresponding to the two different types of measurement (directional -directional and directional - hemispherical ) are compared for the determination of radiative properties. Moreover, experiments performed on a guarded hot-plate-typedeviceareused to cone rm thattheproposed model is appropriate to predict the radiative heat transfer in such media. isputonthedeterminationofradiativepropertiesofopencellcarbon foam. The radiative properties of foam that are required for solving the radiative transfer equation are the spectral volumetric scatter- ing and absorption coefe cients and the spectral volumetric phase function. Recently, Baillis et al. 3 have adopted a new approach to determine such properties. Radiative properties were obtained from morphological data, such as porosity, particle sizes, and f s param- eter, and from solid hemispherical ree ectivity. Particle dimensions and porosity can be obtained from microscopic analysis, but solid hemispherical spectral ree ectivities are very dife cult to obtain di- rectly.Baillisetal. 3 havedeterminedsolidhemisphericalree ectivity andmorphologicalparameter f s withanidentie cationmethod.This method used spectral directional -directional experimental results of transmittance and ree ectance obtained for several measurement directions and for several wavelengths in the range 2 -15 l m. A good agreement was observed between experimental and theoret- ical results. These results are encouraging, but they do not permit validation of the model. Moreover, for high-temperature applica- tions, it is necessary to determine radiative properties in the region of visible and near-infrared wavelengths. In this paper, a device with an integrating sphere is used to measure spectral directional - hemispherical transmittance and ree ectance in the wavelength re- gion of 0.2-2.1 l m. The radiative properties predictive model and the identie cation method used to determine the unknown parameters are briee y de- scribed.Thenapplication toacarbonfoamsampleof98.75%poros- ity permits the study and validation of the model. This section rep- resents the most innovative part of this work. 1) Sensitivity of different parameters (porosity, particle di- mensions, etc. ) on hemispherical ree ectance and transmittance is studied. 2) Identie cation results obtained from the two approaches cor- responding to two different types of measurements (hemispherical or directional) are compared to each other. The values of f s iden- tie ed are also compared with the value of f s determined from a microscopic analysis. 3) Finally, experiments on a guarded hot-plate-type device allow comparison of the experimental and theoretical conductivities (ac- countingforradiativetransfer,obtainedfromtheradiativeproperties in the wavelength region of 0.2 -15 l m). Results are also compared with simpler models that neglect scattering.

A simple apparatus for measurement of the acoustical impedance of membranes, plates, and the bulk properties of porous materials

An apparatus has been built that can measure the acoustical impedance (Z) of a variety of acoustical objects including membranes, plates, and porous materials. It is also possible to use the technique to measure the complex bulk modulus (B) of air within a porous material. When B is combined with Z, a complete linear description of the bulk properties of porous material is obtained. The apparatus is very simple, consisting of two volumes, one of which is driven by a loud speaker at frequencies low enough so that the two volumes can be accurately modeled as lumped elements. It is shown that Z and B primarily depend upon on the pressure transfer function between the two volumes, which can be measured very quickly and accurately with modern analyzers. Due to the simplicity of the apparatus, it is felt that this technique is superior to methods previously used [J. D. McIntosh et al., J. Acoust. Soc. Am. 88, 1929–1938 (1990)], however it is limited to lower frequencies (<500 Hz with the current apparatus). Measured impedance data of membranes, plates, ports, perforated plates, and porous material is presented as well as bulk properties of open cell foam.