The mechanical behavior of closed-cell aluminum foam composites under different compressive loadings has been investigated. Closed-cell aluminum foam composites made using the liquid metallurgy route were reinforced with multiwalled carbon nanotubes (CNTs) with different concentrations, namely, 1%, 2%, and 3% by weight. The reinforced foams were experimentally tested under dynamic compression using the split Hopkinson pressure bar (SHPB) system over a range of strain rates (up to 2200 s −1 ). For comparison, aluminum foams were also tested under quasi-static compression. It was observed that closed-cell aluminum foam composites are strain rate sensitive. The mechanical properties of CNT reinforced Al-foams, namely, yield stress, plateau stress, and energy absorption capacity are significantly higher than that of monolithic Al-foam under both low and high strain rates.

A comprehensive study has been conducted to develop proper test methods for accurate determination of failure strengths along different material directions of closed-cell polymer-based structural foams under different loading modes. The test methods developed are used to evaluate strengths and failure modes of commonly used H80 polyvinyl chloride (PVC) foam. The foam's out-of-plane anisotropic and in-plane isotropic cell microstructures are considered in the test methodology development. The effect of test specimen geometry on compressive deformation and failure properties is addressed, especially the aspect ratio of the specimen gauge section. Foam nonlinear constitutive relationships, strength and failure modes along both in-plane and out-of-plane (rise) directions are obtained in different loading modes. Experimental results reveal strong transversely isotropic characteristics of foam microstructure and strength properties. Compressive damage initiation and progression prior to failure are investigated in an incremental loading–unloading experiment. To evaluate foam in-plane and out-of-plane shear strengths, a scaled shear test method is also developed. Shear loading and unloading experiments are carried out to identify the causes of observed large shear damage and failure modes. The complex damage and failure modes in H80 PVC foam under different loading modes are examined, both macroscopically and microscopically.

Hollow particulate composites are lightweight, have high compressive strength, are low moisture absorbent, have high damping materials, and are used extensively in aerospace, marine applications, and in the manufacture of sandwich composites core elements. The high performance of these materials is achieved by adding high strength hollow glass particulates (microballoons) to an epoxy matrix, forming epoxy-syntactic foams. The present study focuses on the effect of volume fraction and microballoon size on the ultrasonic and dynamic properties of Epoxy Syntactic Foams. Ultrasonic attenuation coefficient from an experiment is compared with a previously developed theoretical model for low volume fractions that takes into account attenuation loss due to scattering and absorption. The guidelines of ASTM Standard E 664-93 are used to compute the apparent attenuation. Quasi-static compressive tests were also conducted to fully characterize the material. Both quasi-static and dynamic properties, as well as coefficients of attenuation and ultrasonic velocities are found to be strongly dependent upon the volume fraction and size of the microballoons.

The increase of the luminosity of the Large Hadron Collider (LHC) by 2020 requires the upgrade of the ATLAS inner tracker experiment. Expected to be used as support structures in the design of the inner tracker, the thermal and mechanical properties of POCOFOAM and ALLLCOMP foam needed to be well understood and dimensionally stable in order to allow efficient cooling and accurate track reconstruction. Thermal conductivities of these foams were measured experimentally together with the Young's modulus, yield, and shear stresses of POCOFOAM at low stress. Thermomechanical measurements of POCOFOAM were also achieved. This paper describes briefly the measurement systems used and reports the results obtained.

Stress relaxation behavior of thermosetting polyurethane (PU) solid and foams were investigated in tensile mode using a dynamic mechanical analyzer (DMA). PU solid samples were manufactured in a closed mold to avoid any foam formation, whilst PU foam samples were manufactured inside a woven using a silicone mold. Samples with rectangular cross-section were subjected to a predetermined amount of tensile strain and the tensile force was recorded as a function of time. Relaxation modulus was determined for different temperatures up to near the glass transition temperature. It was found that the viscous part becomes more dominant with increasing test temperature. Although the stress relaxation behavior of PU solid and foam were found similar at lower temperature, the relaxation behavior of the foam was influenced by the cellular structure especially at higher temperature due to the combination of gas expansion and cell wall softening. Different stress relaxation models such as Maxwell model, Burgers model, Generalized Maxwell (GM) model, and Stretched exponential model were employed to predict the relaxation behavior of PU solid and foams. It was found that the GM model (with three or more elements) and the Stretched exponential model were in good agreement with the experimental data in predicting the stress relaxation behavior of both solid and foams. The predicted relaxation time and equilibrium modulus were found to decrease with increase in temperature.

Cure kinetic behavior was studied for both thermosetting polyurethane (PU) solids and foams. The effects of cure temperature, foam density, and carbon nanofiber (CNF) contents were examined. Cure studies were performed experimentally by measuring the evolution of complex shear modulus as a function of time using an advanced polymer analyzer operating in dynamic shear mode. Isothermal cure behavior of PU solid and foams was investigated at four different temperatures, namely, 25 ° C , 45 ° C , 60 ° C , and 80 ° C and at three different amounts of CNF, namely, 0.01%, 0.05%, and 0.1% by weight. The cure data were analyzed by using an autocatalytic cure kinetic model. The cure behavior of both solid and foam was found to be temperature dependent. Addition of CNF was also found to affect the cure behavior of the PU foam. It was observed that the PU foam with 0.1% CNF shows the highest polymerization reaction compared with the neat foam. It was also observed that the reaction rate constants follow an Arrhenius dependence on temperature, whereas the reaction orders remain fairly constant. A simple predictive model using the reaction orders indicated that the maximum cure reaction rate was occurred at 37.5% conversion.

Light weight high strength composites can be obtained by reinforcing resin with fillers such as hollow or solid particles and fibers. Composites were fabricated using microballoons (hollow particles) called syntactic foams. These foams can be used in various low density applications such as buoyancy aid materials for deep sea exploration and aerospace vehicles. These foams are usually utilized as light weight core materials for sandwich structures. The present study explores the procedure to fabricate functionally gradient syntactic foams (FGSFs) and further analyze their mechanical properties. The FGSFs produced are gradient structures consisting of four layers with four different types of microballoons, namely, S22, S32, S38, and K46, each having different wall thickness. The volume fraction of all microballoons is maintained constant at 60% to maintain light weight structures. Several FGSF specimens having similar density are fabricated with different layer arrangements. The different layers are integrated before major solidification takes place. Quasistatic compression testing is then performed on the cured FGSF samples using MTS-810 servohydraulic machine. Compressive strength and energy absorption values for each arrangement are compared. The stress plateau in integrated FGSF composites extends from 10% to 60% strain compared with plain syntactic foams. The integrated FGSF shows increment in yield strength and energy absorption compared with adhesively bonded FGSF. It is found that the compressive strength and energy absorption of integrated FGSF composites can be varied based on arrangement of the layers.

The effect of nanoclay on the high strain rate mechanical properties of syntactic foams is studied. Two types of microballoons with different wall thicknesses are used in fabrication of plain and nanoclay syntactic foams. Plain syntactic foams are fabricated with 60% volume fraction of glass microballoons. 1%, 2%, and 5% volume fractions of Nanomer I.30E nanoclay are incorporated to produce nanoclay syntactic foams. High strain rate test using split Hopkinson pressure bar (SHPB) apparatus is performed on all types of plain and nanoclay syntactic foams. Dynamic modulus, strength, and corresponding strain are calculated using the SHPB data. Quasistatic test is also performed and results are compared with the dynamic SHPB results. The results demonstrate the importance of nanoclay and microballoon wall thickness in determination of syntactic foam dynamic properties. It is found that at a high strain rate, the strength and modulus of composite foams having K46 microballoons increase due to addition of 1% volume fraction of nanoclay. However, in composite foams having S22 microballoons, the increase in strength is not significant at a high strain rate. Further increase in nanoclay volume fraction to 2% and 5% reduces the strength and modulus of composite foams having S22 microballoons. Difference in wall thickness of microballoons is found to affect the strength, modulus, strain energy, and deformation of composite foams. Composite foams fabricated with thicker walled microballoons (K46) show comparatively higher values of strength, modulus, and strain energy compared with thin walled (S22) microballoons. Scanning electron microscopy shows that crack propagation behavior is distinct at different strain rates.

The design of metal-polymer foam adhesion and load transfer characteristics is carried out in this research work. The metal inserts are used as the load transfer component, while the foam is used as the structural element of the system. The inserts are embedded in the foam during the foaming process. Flexural testing was conducted on different metal foam configurations to establish the typical interaction trends. The load-deflection response and the mode of failure of the structure were documented. Moduli of elasticity of the system for various geometries and embedded lengths were evaluated, and behavior patterns were gleaned. Rectangular, circular, and triangular (taper-/wedgelike) inserts were used. Results show that simple taper inserts embedded in foam slabs perform better than the other shapes. Finite element analyses of the interaction under different loads were carried out. The modeling results coincided with the experimental ones hence validating the model.

This paper presents the results of the combined experimental investigation and digital image correlation (DIC) analysis of the fatigue failure of open cell aluminum foams. Compression–compression cyclic loads were applied to foam specimens under the as-fabricated condition. Following characterization of the S-N curve behavior, the macroscale deformation of the tested foam under fatigue was recorded using an in-situ digital camera. The deformation sequence was then analyzed using DIC technique. It was found that foams failed with an abrupt strain jump when shear bands were formed, and serious deformation up to more than 30% was developed in the center of the shear band. The ex-situ scanning electron microscopy analysis indicated that the abrupt strain jump was due to the microscale damage accumulation in struts where surface cracks were formed and propagated.

The effect of density (relative densities 0.33 to 0.90) on the impact behavior of microcellular polycarbonate (PC) was investigated. Cell size and foaming gas content were also considered. Flexed-beam Izod impact tests were conducted and the impact strength of these foams appears to be a strong function of both density and cell size. The impact strength was observed to improve over the unprocessed polycarbonate’s impact strength for foams with relative densities of 60 percent and above. In terms of cell size, the impact strength increased with increasing cell size at a given density.

Compressed open-cell solid foams frequently exhibit spatially inhomogeneous distributions of local stretch. The theoretical aspects of this deformation habit have not been clearly elucidated. Here we briefly discuss the energetics of the problem to show that the stretch inhomogeneity stems from the nonconvexity of the underlying potential. We also perform displacement field measurements using the Digital Image Correlation technique, and discuss the results in light of the theory. [S0094-4289(00)01904-6]

The high strain tensile deformation of open-cell foams is analyzed, using a Kelvin foam lattice model. The stretching, bending, and twisting of elastic cell edges is analyzed, and the deformed cell shapes predicted. The stress-strain relation and Poisson’s ratio are predicted for strains up to 40% for tension in the [100] and [111] directions of the BCC lattice. The latter prediction is closest to stress-strain curves for polyurethane foams, especially when the cell shape anisotropy is taken into account. The change from edge bending to extension as the main deformation mechanisms, for strains exceeding 20% increases the slope of the stress-strain curve. A comparison is made with irregular cell structure models. [S0094-4289(00)01001-X]

In this study, mechanical properties of three types of polymeric foams (polypropylene (PP), polystyrene (PS), and polyurethane (PU) foams) are investigated. Focus has been placed on the strain rate and temperature effects on these foams under large deformations. Selected experimental results from uniaxial compression, hydrostatic compression, and simple shear tests are presented. A phenomenological hydrodynamic elastoplastic constitutive law is developed to model these polymeric foams. Numerical implementation and validation of the constitutive model are also described.

Based on thermodynamic considerations and experimental data on isothermal compression of elastomeric foams, a simple equation of state of flexible polymeric foams was developed. The developed equation of state is, in fact, a simple universal function relating the thermodynamic properties of the material of which the skeleton of the foam is made and the foam porosity. It was shown that the Hugoniot adiabat of foams, whose porosity is less than 0.3 and which are exposed to moderate shocks, could be expressed in a form similar to that of bulk solids, i.e., D = C o + Su, where D is the shock front velocity, C o is the speed of sound, u is the particle velocity and S is the maximum material compressibility.

Both conventional and negative Poisson ’s ratio foams exhibit dispersion of acoustic waves, as well as cut-off frequencies, at which the group velocity tends to zero. This macroscopic behavior is attributed to micro-vibration of the foam cell ribs. The purpose of this article is to develop a micromechanical model of the cut-off frequency. This model is based on the resonance of ribs which may be straight, curved, or convoluted. As foam cell ribs become more curved, the cut-off frequency decreases. Therefore foams with curved or convoluted ribs are expected to provide superior performance in applications involving the absorption of sound.

While the use of plastics has been growing at a significant pace because of weight reduction, ease of fabrication of complex shapes, and cost reduction resulting from function integration, the engineering applications of plastics have only become important in the past fifteen years. An inadequate understanding of the mechanics issues underlying the close coupling among the design, the processing (fabrication), and the assembly with these materials is a barrier to their use in structural applications. Recent progress on some issues relating to the engineering uses of plastics is surveyed, highlighting the need for a better understanding of plastics and how processing affects the performance of plastic parts. Topics addressed include the large deformation behavior of ductile resins, fiber orientation in chopped-fiber filled materials, structural foams, random glass mat composites, modeling of thickness distributions in blow-molded and thermoformed parts, dimensional stability (shrinkage, warpage, and residual stresses) in injection-molded parts, and welding of thermoplastics.

Based on the general shape of the curves describing experimental compressive stress-strain relations of flexible porous materials (e.g., flexible foams) which are known to depend on their relative density, a general mathematical functional dependence of the stress on the strain is proposed. The general function includes two constants. Using experimental and empirical information the values of these constants are determined, so that a final compressive stress-strain relation, in which the relative density is a parameter, is obtained. Good agreement is found when the presently developed empirical stress-strain relations are compared to experimental ones for a wide range of relative densities. The proposed compressive stress-strain relations are then used to derive shock Hugoniot relations for flexible porous materials. With the aid of these relations one can investigate the dynamic behavior of foams struck head-on by shock waves. The finally obtained empirical shock Hugoniot relations are found to be similar to experimental relations which are proposed in the literature. In addition, a mathematical investigation of the asymptotic behavior of the shock Hugoniot relations is conducted. The results of this investigation are found to agree excellently with actual experimental data.

Novel polycarbonate (PC) foams with bubbles on the order of 10 μm and cell nucleation densities between 1 and 10 billion cells per cubic centimeter of foam have been produced using carbon dioxide as the blowing agent. The size and number of bubbles can be controlled to produce a wide range of foam densities. This paper presents the results of an experimental study of the tensile behavior of these unique microcellular foams. It was found that the tensile strength of microcellular PC foams is proportional to the foam density. The strength is less than that predicted by the rule of mixtures, suggesting that the microcellular structure is inefficient in carrying the tensile load. The saturation of PC by CO 2 was found to reduce the tensile strength of the virgin material by approximately 20 percent. This showed that the sorption of a very high concentration of gas molecules by the polymer must be considered when characterizing and modelling the microcellular foam mechanical properties. The relative tensile modulus of microcellular foam was found to increase as the square of the foam’s relative density over the range of densities explored.