Foam Slabs Research Articles

Characterization of Flexural Properties of Syntactic Foam Core Sandwich Composites and Effect of Density Variation

Microballoons (hollow particles) of the same outer radius but with five different inner radius values are used to fabricate five types of syntactic foam slabs. These five types of slabs are used as the core material to fabricate sandwich composites. Three- and four-point bending and short beam shear strength tests are carried out to characterize the flexural behavior of syntactic foam core sandwich composites. The effect of change in microballoon radius ratio (ratio of the inner to the outer radius) on the flexural properties of the sandwich composites is also studied. The results show that in three- and four-point bending tests, the failure is governed by tensile properties of the foam core and the strength is not affected by the microballoon radius ratio. Shear failure takes place in short beam shear tests, which makes the microballoon radius ratio an important factor in determining the strength of the sandwich composite.

Experimental determination of the mechanical effects of mass density gradient in metallic foams under large multiaxial inelastic deformation

Mass density gradients at the scale of the cell size are an inherent property of metallic foams. Sandwich specimens are prepared from a continuous foam slab and then subjected to large combined compression and shear loads. The characteristic crushing nucleation envelope and the crushing plateau stress interaction surface which determine the amount of energy absorbed during crushing have been measured. In addition to ideal crushing which involves the continual collapse of the solid network structure, fracture-dominated crushing has been observed. The later mechanism occurs under large shears strains and involves the frictional sliding and interlocking of separated surfaces. The wide spread in the measured mechanical properties is only partially related to the density gradient at the microscale denoting that it is not enough to normalize the measured quantities with mass density functions. Other effects such as morphological changes and local variations in cell wall strength should be taken into account.

Gradient syntactic foams: Tensile strength, modulus and fractographic features

Syntactic foams are new age polymer composite materials. They find use not only in aerospace but also in naval applications as marine utility components. Due to their low density, lesser ingression by water and ability to withstand compressive strengths they are considered for sub-sea applications. Dispersing hollow glass microballoons in matrices like, for instance, thermosetting epoxy resins yields on curing the syntactic foam slabs. Due to the differences in densities of epoxy resin and the glass hollow microballoons, the former being heavier than the latter, the resins have a tendency to settle down. This settling leads to a gradation in the density across the height of the cast sections. This effect, however, is more conspicuous in rectangular slabs of higher thickness, for e.g., in 25 mm size mould used in this work. Hence, to study such distribution differences, 3 mm thick sections are sliced from the different sections along the thickness part of the slab from which tension test coupons are made and tested. From the data, both tensile strengths and moduli are calculated. These increased from 23.8 to 41.9 MPa and 2 to 2.47 GPa, respectively, as the volume% of microballoons in the sliced sections decreased from 43.9 to 25.9. Fractographic features, observed on the tension-failed samples, show that higher microballoons’ bearing samples (i.e., samples from the top portion of the slabs’ thickness) have formations of cavities. The lower microballoons’ bearing sample show well marked matrix deformation features. These and other findings are analyzed and the features observed are correlated with the mechanical test results.

Short‐beam three‐point bend test study in syntactic foam. Part III: Effects of interface modification on strength and fractographic features

AbstractSyntactic foam slabs having uncoated microballoons and paraffin oil surface‐treated microballoons were fabricated and tested for short‐beam three‐point bend test. The work points to the role of paraffin oil coating first weakening the interface between the microballoons and the matrix and hence lowering the efficiency of load transfer from matrix to the fillers (i.e., microballoons). This led to an overall decrement of 71% in the experimentally measured strength value compared to the deduced value for uncoated microballoons' specimens. The large strengths for uncoated microballoons specimens can be traced to the presence of the curvilinear marks in the matrix that, incidentally, are absent in the case of paraffin oil coated specimens. These observations are revealed distinctly in the microscopy of test‐failed specimens. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 687–693, 2005

Development and characterization of an inorganic foam obtained by using sodium bicarbonate as a gas generator

In the construction industry almost all of the insulating and expansive materials are organic foams. In this work, the production of an inorganic foam is described. Sodium bicarbonate is used as a gas generator. CO 2 gas is released when water is added to the mixture of sodium bicarbonate and β-hemihydrate gypsum powder (CaSO 4 · 1/2H 2O). Hence, stabilization of the foam is achieved when CO 2 gas is released and water is absorbed by β-hemihydrate gypsum powder, which subsequently is converted into calcium sulphate dihydrate (gypsum matrix). The bulk density and mechanical properties (compressive and flexural strength) of the inorganic foam and gypsum were determined. Microstructural characterization has been carried out by SEM and XRD, and a new sodium sulphate phase was identified in the gypsum foam due to this chemical reaction. Finally, thermal properties such as thermal conductivity and diffusivity were measured and it was observed how for the same heat flux, the thickness of an inorganic foam slab is 73.4% less than that of a concrete slab.

Flexural Wrinkling Strength of Lightly Profiled Sandwich Panels with Transverse Joints in the Foam Core

In Australia, sandwich panels are commonly made of flat or lightly profiled steel faces and expanded polystyrene foam cores, and are increasingly used as roof and wall claddings in industrial and commercial buildings. Flexural wrinkling is often the governing criterion in the design of these panels. The use of lightly profiled faces is expected to increase the flexural wrinkling stress considerably whereas the presence of joints between the polystyrene foam slabs in the transverse direction introduces a reduction to the flexural wrinkling stress. Therefore a series of full scale experiments and finite element analyses were conducted to evaluate the effects of lightly profiled faces and transverse joints on the flexural wrinkling stress of panels subjected to a lateral pressure loading. The results showed that properly designed lightly profiled panels provided considerable improvement to flexural wrinkling strength over flat panels whereas the presence of transverse joints caused a significant reduction to wrinkling strength. This paper presents the details of this investigation, the results and comparison with available theoretical and design solutions.

Numerical Simulation of Shock Wave Interaction with a Slab of Compressible Foam Attached to a Solid Wall

Numerical Simulation of Shock Wave Interaction with a Slab of Compressible Foam Attached to a Solid Wall

Experimental and numerical determination of SAR distributions within culture flasks in a dielectric loaded radial transmission line.

The effect of dielectric loading on the cell layer specific absorption rate (SAR) within a T-75 culture flask being irradiated within a transverse electromagnetic (TEM) cell was studied both experimentally and numerically. Direct thermal measurements of a T-75 containing 40 mL of culture medium and resting upon a 3-mm-thick slab of alumina ceramic (epsilon r = 9.6) revealed that, compared to the same flask resting upon a foam slab (epsilon r = 1.0) of the same thickness, the average SAR at the cell layer was increased roughly fourfold. This fourfold increase is significant experimentally because it allows biologists to perform experiments over a larger range of SAR values needed to determine possible dose-response curves without the costs and difficulties of a fourfold increase in amplifier power. Finite-difference time-domain (FDTD) simulations of the SAR distribution were in good quantitative agreement with the experimental measurements. It is concluded that FDTD modeling can be a cost effective and scientifically acceptable means of obviating the thermal measurement of SAR.

A Cellular Automata Approach To Cfd Flame Spread Modelling

This paper focuses on the growth and spread stage of full-scale fires in enclosures, where a localised flame spreads across a single fuel item, increasing the heat release rate, thereby resulting in increased fire hazard. The development of a numerical model for fire spread over a solid fuel surface, and its integration into a CFD model, is described. The conflicting demands of resolving the small scale flame front phenomena while simultaneously accounting for the large-scale enclosure flow phenomena are addressed. The surface of the fuel is descretised with a regular square array, and flame spread occurs as a series of ignitions of surface elements. Ignition of an element is determined by a combination of critical surface ignition temperature and cellular automata techniques. Regression of the combusting fuel surface is modelled, with a fine grid retained at the fuel surface by allowing the grid to collapse with the surface. A grid transformation is applied to restore orthogonality of the collapsed grid, for simple computation of the heat equation. The flame spread model has been designed to be independent of geometry, although experimental validation exercises are carried out for radial spread over horizontal surfaces only. Predictions are made for the burning of a slab of polyurethane foam in a full-scale multi-room experimental building-fire facility. The predictions compare favourably with the experiments, indicating the broad validity of the methods used in flame spread model.

Shear Properties on Aluminum Metal Foams Prepared by the Melt Route

ABSTRACTThe shear modulus and shear strength of AISi7Mg aluminum foam with 15% (vol) of 13tm SiC particles were determined through shear testing. A foam slab with a density of 0.31 g/cm3 was supplied by Hydro Aluminium.Four samples were tested according to ASTM C 273–61. The specimens were bonded to steel load plates. The relative displacement of the plates was measured using two extensometers. In order to evaluate the effect of the cell size distribution on shear properties, cell size and material distribution analyses were carried out for the metal foam slab in areas close to those from which the shear specimens were extracted.A fast failure was observed after the maximum shear load. The failure in the samples were located in the central section of the slab mainly because the lower density was located there.

Scale-up of Transformation of Negative Poisson's Ratio foam: Slabs

This paper presents an experimental study performed on the scale-up of processing of negative Poisson 's ratio foam. These materials become thinner under compression and fatter under tension, opposite to ordinary materials. The transformation process consists of triaxial compression combined with heat treatment. Transformed foam cells have a concave or "re-entrant" shape. Scaleup is achieved by use of a modified mould assembled around the foam slab and longer processing times. Foam with a distribution of cell sizes was transformed in small block and large slab forms. These blocks and slabs exhibited a negative Poisson's ratio.

Determination of Effective Diffusion Coefficients in Rigid Polyurethane Foam

The change in cell gas pressures over time in foam slabs and cylindrical foam samples was determined, and the effective diffusion coefficients of the gases were calculated. Seven different rigid polyurethane foams, blown with pentane or cyclopentane, were used. No systematic difference between the effective diffusion coefficients obtained from slabs and cylinders could be seen. The results were of the same order of magnitude as the values found in the literature. A discussion on time and sampling restrictions when measuring effective diffusion coefficients is presented along with recommendations on sample geometries and dimensions when evaluating diffusion of different gases. The thermal conductivity of the foams, calculated from the obtained effective diffusion coefficients, agreed well with the measured values of the thermal conductivity.

Density gradient effects on aluminium foam compression behaviour

The compression properties of an aluminium foam containing a nonuniform density gradient have been examined. Specimens were taken from various locations within the foam slab, and were tested in two directions. Measured foam properties were compared to calculated values using models derived by Ashby and Gibson [1]. The effect of the density gradient on the compression properties and also the energy absorption characteristics of the foam was found to be significant.

The impact of a shock wave on porous compressible foams

A phenomenological study of the processes occurring when a shock wave interacts with porous polyester and polyether foams has been undertaken. Plane shock waves generated in a shock tube were reflected off a slab of foam mounted against the back wall of the tube. Tests were conducted with an initial shock wave Mach number of 1.4 and a 70 mm thick slab of foam. The reduction in reflected shock wave strength and substantial increase in the back wall pressure over that for rigid wall reflection, found by other workers, were confirmed. Piezoelectric pressure transducers were used to record the pressure before, alongside and behind the foam specimen. Schlieren photographs of the flow were made and showed some features not previously reported. In particular it is shown that there is a flow of gas across the face of the foam at some point of the process. Previous investigations of this interaction process have assumed that the face of the foam is a contact surface. Short duration photographs of the distortion of the foam were taken, enabling the wave propagation in the foam material itself to be studied. It is established that the front of this compaction wave in the foam material moves at considerably lower velocity (- 90 m/s) than the gas wave as detected by the pressure transducers (- 200 m/s). This result contrasts with the assumption made in previous work that the two-phase medium behaves essentially as a homogeneous substance. A simple physical model based on a zone of compacted material in the foam acting as a high-resistance flow barrier, is proposed to explain the observed phenomena.

The Combustion of Rigid Polyurethane Foams

Abstract The effect of the porous structure on the flammability characteristics of rigid polyurethane foams is studied. It is found that the velocity of the downward flame spread over the surface of foam slabs increases as the oxygen content into the ambient flow increases and the effective density of PU foams decreases. The measurements of temperature profiles into the combustion wave made by Pt-PtRh thermocouples are used for the determination of the rate-controlling energy-transfer mechanisms.

Measurement of the coupling force between foam and fluid

The coupling impedance between porous foam and liquid resulting from their relative motion has been measured in an apparatus which excites torsional waves in an annulus of liquid-saturated foam. The apparatus consists of a foam slab sample cemented around a thin metal cylinder which in turn is mounted on a torsional rod of variable length. The resonant frequency and damping rate of torsional oscillations of the system when the cylinder is in air and when it is immersed in fluid are used to determine the shear loading on the cylinder from the foam for the two cases. For the low viscosity fluids tested, the coupling impedance determined from the experimental results was largely viscous with a small springlike component. The fact that the impedance was springlike rather than inertial suggests that bending of the small foam fibers may have been an important effect. [Work supported by NUSC.]