A preliminary study of cushion properties of a 3D printed thermoplastic polyurethane Kelvin foam

The cells in conventional packaging foams have random size and orientation, and the energy‐absorbing behaviour of these foams is determined by the collective contribution of different sizes of cells. In contrast to the random nature of stochastic foams, 3D printing technologies allow engineers to design and produce foams having engineered cellular structures. In this study, engineered cellular structures based on the classic Kelvin 1887 model were 3D printed in 30 × 30 × 30 mm thermoplastic polyurethane cubes with a repeating size of 216 unit cells. One hundred consecutive cyclic compression tests were performed to assess the 3D printed foam's resilience and energy absorption characteristics. The stress‐strain curve of the 3D printed thermoplastic polyurethane foam indicated viscoelastic behaviour and a Mullins effect indicative of resilient rubber. A long wave buckling mode was observed during cyclic compression cycles due to the Kelvin structure. The cushion factor computed from the stress‐strain curve was close to that of a metal spring with linear elasticity. The combination of the 3D printed foam's resilience, its much lower density than rubber, and the complete geometric freedom of the engineered cellular structures offer designers the potential to create high‐performance cushion materials tailored for packaging applications.     

Response of thin-skinned sandwich panels to contact loading with flat-ended cylindrical punches: Experiments,numerical simulations and neutron diffraction measurements

The response of aluminium foam-cored sandwich panels to localised contact loading was investigated experimentally and numerically using flat-ended cylindrical punch of four varying sizes. ALPORAS and ALULIGHT closed-cell foams of 15 mm thickness with 0.3 mm thick aluminium face sheets (of 236 MPa yield strength) were used to manufacture the sandwich panels. Face sheet fracturing at the perimeter of the indenter, in addition to foam cells collapse beneath the indenter and tearing of the cell walls at the perimeter of the indenter were the major failure mechanisms of the sandwich panels, irrespective of the strength and density of the underlying foam core. The authors employed a 3D model in ABAQUS/Explicit to evaluate the indentation event, the skin failure of the face sheets and carry out a sensitivity study of the panel's response. Using the foam model of Deshpande and Fleck combined with the forming limit diagram (FLD) of the aluminium face sheet, good quantitative and qualitative correlations between experiments and simulations were achieved. The higher plastic compliance of the ALPORAS led to increased bending of the sheet metal and delayed the onset of sheet necking and failure. ALULIGHT-cored panels exhibited higher load bearing and energy absorption capacity, compared with ALPORAS cores, due to their higher foam and cell densities and higher yield strength of the cell walls. Additionally, they exhibited greater propensity for strain hardening as evidenced by mechanical testing and the neutron diffraction measurements, which demonstrated the development of macroscopically measurable stresses at higher strains. At these conditions the ALULIGHT response upon compaction becomes akin to the response of bulk material with measurable elastic modulus and evident Poisson effect.     

FE modeling of the compressive behavior of porous copper-matrix nanocomposites

Compressive mechanical test and numerical simulation via finite element modeling have been employed on closed-cell copper-matrix nanocomposite foams reinforced by alumina particles. The FE analysis' purpose was to model the foam deformation behavior under compressive loading and to investigate the correlation between material characteristics and the compressive mechanical behavior. Exploring this, several foam samples with different conditions were manufactured and compression test was carried out on the samples. Scanning electron microscopy and image analysis have been performed on the foam samples to obtain the required data for the numerical simulation. The stress–strain curves exhibited plateau stress between 18 and 112.5 MPa and energy absorption in the range of 20.03–51.20 MJ/m³ for the foams with different relative densities. The foams exhibited enhanced mechanical properties to an optimum value, as a consequence of increasing the reinforcing nanoparticles, through both experimental tests and numerical simulation data. Also, the validated model of copper-matrix nanocomposite foams has been used to probe stress distribution in the foams. In addition, the results obtained by numerical simulation via ABAQUS CAE finite element modeling provided support for experimental test results. This confirmed that FEM is a favorable technique for predicting mechanical properties of nanocomposite copper foams.     

Mesophase AR pitch derived carbon foam: Effect of temperature, pressure and pressure release time

For the carbon foam production, mesophase pitch pellets are heated up in a reactor in an aluminum mold to specified pressures and finally pressure released to obtain green carbon foam samples. The green foams were then stabilized and carbonized. The effects of various temperatures, pressures and pressure release times on production of carbons foams are investigated. The samples are subjected to SEM, mechanical testing, mercury porosimetry analysis and bulk density determination for characterization. For the processing temperatures of 553, 556, 566 and 573 K, the densities of the foams produced were 380, 390, 410 and 560 kg/m³ respectively. The compressive strengths of the respective samples were increased from 1.47, to 3.31 MPa for the lowest and highest temperatures. The processing pressures were 3.8, 5.8, 6.8 and 7.8 MPa. The bulk density and the compressive strength of the carbon foams produced were changed from 500 to 580 kg/m³, and 1.87 to 3.52 MPa for the lowest and highest pressures respectively. Pressure release times of 5 s, 80 s, 160 s and 600 s are used to produce different carbon foam samples. The densities and the comprehensive strengths measured for the highest and lowest pressure release times changed from 560 to 240 kg/m³ and 3.31 to 2.16 MPa respectively. The pore size distribution of all of the products changed between 0.052×10^-6m and 120×10^-6m. Increase in temperature and pressure increased the bulk density and compressive strength of the carbon foams. The mercury porosimetry results show % porosity increase with increasing temperature and pressure. On the other hand, increase in pressure release time decreased the bulk density, compressive strength of the carbon foam.     

Effects of density and strain rate on properties of syntactic foams

Syntactic foams are characterized for high strain rate compressive properties using Split-Hopkinson Pressure Bar (SHPB) technique in this study. The results at high strain rates are compared to quasi-static strain rate compressive properties of the same material. Four different types of syntactic foams are fabricated with the same matrix resin system but different size microballoons for testing purpose. The microballoons have the same outer radius. However, their internal radius is different leading to a difference in their density and strength. The volume fraction of the microballoons in syntactic foams is maintained at 0.65. Such an approach is helpful in isolating and identifying the contribution of matrix and microballoons to the dynamic compressive properties of syntactic foams. Results demonstrate considerable increase in peak strength of syntactic foams for higher strain rates and increasing density. It is also observed that the elastic modulus increases with increasing strain rate and density. Scanning electron microscopy is carried out to understand the fracture modes of these materials and the density effect on high strain rate properties of syntactic foam.     

Compressive properties of closed cell Al alloy–fly ash particle composite foams

Abstract

The closed cell aluminium alloy–fly ash particle composite (Al/FA) foams containing 1·5 wt-% fly ash were manufactured by molten body transitional foaming process. The quasi-static compressive properties of Al/FA have been investigated. Results show the compressive stress–strain curves of Al/FA foams exhibit three regions, i.e. the elastic region, the plastic plateau region and the densification region. A linear relationship between the densification strain and the relative density was obtained. The relation between the plastic collapse stress and the relative density can be described with Gibson and Ashby’s model. The energy absorption capacities of the Al/FA foams gradually increase with increasing strain and relative density.     

开孔与闭孔泡沫铝的压缩力学行为

研究了开孔与闭孔两种胞孔结构不同、制备工艺不同的泡沫铝在准静态压缩载荷下的压缩响应曲线.结果表明:开孔与闭孔泡沫铝压缩应力-应变曲线均具有多孔泡沫材料明显的三阶段特征,即线弹性段、塑性屈服平台段及致密段;相对密度对泡沫材料的力学性能(如杨氏模量、屈服强度)有很大影响;在准静态下,开孔泡沫铝表现出明显的应变率效应,而闭孔泡沫不如开孔敏感;泡沫铝材料表现为弱的各向异性;胞孔结构影响两种泡沫材料的压缩响应曲线.     

Impact and dynamic response of high-density structural foams used as filler inside circular steel tube

Structural foams have good energy absorption properties and are effective in reducing the vulnerability of sandwich structures. This research investigated the impact and dynamic response of three different high-density polymeric structural foams; designated A, B and C for proprietary reasons. Foam-C had the lowest density out of the three; density of foam-B was approximately twice the density of foam-C, while the density of foam-A was about three times the density of foam-C. The cylindrical foam samples were initially impacted at different velocities in a DYNATUP Model 8250 instrumented impact test machine and their energy absorption was characterized from the resulting load–deflection data. Each of the three foams was then modeled as filler inside a circular steel tube of 0.8 mm thickness. Non-linear finite element analysis was performed under displacement controlled quasi-static compressive monotonic loading using PATRAN as pre-processor and ABAQUS Standard commercial software. The area under the load–deflection curve was calculated to obtain the absorbed energy and the crush loads for the three foam fillers were compared. Results indicate that foam-A having the highest density was more effective as filler inside the circular steel tube, with the intermediate density foam-B performing equally well under uni-axial compressive loading. Foam-C, which had the lowest density, was found to be ineffective as filler in this application due to large differences in stiffness between this foam and the enclosed steel tube.

A TA Instruments Model 983 DMA (dynamic mechanical analyzer) was used for obtaining the storage and loss modulus along with the damping and glass transition properties of the different density structural foams. Frequency multiplexing was also used in conjunction with the time–temperature superposition principle for characterizing the long-term behavior of these viscoelastic foams.     

Morphology,Oxidation, and Mechanical Behavior of Nanoporous Cu Foams

Nanoporous copper foams (65–80% porosity) are synthesized by dealloying different copper alloys, including Cu₂₀Zn₈₀, Cu₃₅Zn₆₅, and Cu₃₀Al₇₀. Ligament sizes, porosity, and oxide content are examined by scanning electron microscopy, Raman spectroscopy, and X‐ray powder diffraction to determine the nanoporous foam's structure and thermal stability. The pores and ligaments of the copper foams can range in size from 35 to 220 nm. Both parameters can be controlled by either the dealloying process or a subsequent heat treatment. The results show no copper oxide peaks for the as‐prepared samples; however, any heat treatments above 200 °C lead to the formation of Cu oxide (CuO and Cu₂O). These foams are shown to retain their structural integrity even after oxidation. A novel method is thus developed for synthesizing nanoporous Cu oxide foams by heat treating nanoporous copper.     

Development of rigid bio-based polyurethane foam reinforced with nanoclay

Integration of organic nanoclay into bio-based polyurethane (PU) foam is a promising alternative to enhance the foam’s properties via green technology. In this paper, modified diaminopropane montmorillonite (DAP-MMT) nanoclay was introduced into palm oil-based PU foam at different weight loadings, namely, 0, 2, 4, 6, 8, and 10 wt.%, in order to investigate the effects on the mechanical and thermal properties of the foam. Several tests and characterizations were carried out to study the surface morphology, density, compressive strength and thermal stability of the foam. It was found that foam exhibited an exfoliated or intercalated microstructure based on the DAP-MMT contents. The X-ray diffraction analysis showed that below 4 wt.%, the foams displayed exfoliated structures while beyond the value, the foams exhibited the intercalated morphologies. Closed cells with different cell sizes were observed when the DAP-MMT contents were varied. Meanwhile, thermal stability and compressive strength of foams increased with increasing DAP-MMT contents up to 4 wt.%, as shown by thermogravimetry analysis and compression test, respectively.     

Production of Metallic Foams From Ceramic Foam Precursors

A process has been developed for obtaining closed cell metallic foams using a ceramic foam precursor. In this approach, the major constituent of the ceramic foam precursor is iron oxide (Fe₂O₃), which is mixed with various foaming/setting additives. The mixture sets rapidly at room temperature to stabilize the foam generated by hydrogen release. The oxide foam is then reduced in a non‐flammable hydrogen/inert gas mixture to obtain a metallic foam with a cell diameter of 0.5–2 mm. Iron foams with a relative density of 0.23 were tested in compression and yielded an average compressive strength of ～ 34 MPa. The compressive stress‐strain curves obtained were typical of cellular metals. The normalized strengths of the metal foams obtained in the present study compare very favorably with that of steel foams produced by other techniques.     

Thermal and Electrical Conductivity Measurements on Aluminum Foams

Metal foams are one of the most interesting types of materials although there is limited information concerning their thermal and electrical conductivity. Closed cell different density Alporas foams are investigated, which has one of the most homogeneous cell size distribution recently. Comparative method has been chosen to determine the thermal conductivity of the samples in the function of the temperature at 30, 100, 200, 300, 400, 500 °C. For measuring the electrical conductivity of aluminium foams a special low frequency eddy current measuring apparatus was used. The ratio of thermal and electrical conductivity was calculated and shown an increasing function by the density of the foams.     

The compressive properties of expandable microspheres/epoxy foams

Expandable microspheres/epoxy foams with different densities and microstructures were prepared by changing the foaming temperature and the precuring extent. The microstructure of foams reveals a homogeneous distribution of cells at high precuring extent and high foaming temperature, while small cells size at high precuring extent and low foaming temperature. Furthermore, the compressive properties of epoxy foams were investigated. The compressive strength and modulus of the foam exhibited a power-law dependence with respect to density. By optimizing the foaming temperature and the precuring extent, epoxy foams with homogeneous cells and stable compressive property can be obtained. Fracture surface showed that deformed microspheres and less debris were observed at relatively high-density foams.     

Uniaxial and biaxial failure behaviors of aluminum alloy foams

Combined shear–tensile test have been performed on a closed-cell aluminum alloy foams with three relative densities over a wide range of loading rates in order to probe their failure behaviors under biaxial loading conditions. Quasi-static uniaxial compressive and tensile tests have also been conducted to investigate uniaxial failure behaviors of the aluminum alloy foams. The materials exhibit uniaxial failure stress asymmetry due to different failure mechanism in the uniaxial tensile and compression. Comparison is made between three phenomenological failure criteria and the measured failure stresses under different loading conditions to verify these criteria. The experimental failure surfaces of the aluminum alloy foams provide support for the three phenomenological failure criteria when suitable Poisson’s ratio is employed. The shape of the experimental failure surface in principal stress plane was not significantly influenced by variation in the relative density. The slight expansion of the failure surfaces with loading rate happened to be isotropic for this investigated closed-cell aluminum alloy foams in combined shear–tensile testes.     

Syntactic foams from copolymers based on epoxidized soybean oil

Syntactic foams are been increasingly used as core of sandwich panels due to their light weight and good mechanical properties. This investigation evaluates the compressive, flexural and thermo-mechanical properties of syntactic foams made by embedding randomly dispersed hollow glass microspheres in bio-based resins obtained by partial substitution of diglycidyl ether of bisphenol A (DGEBA) with epoxidized soybean oil (ESO). Volume fraction of glass microballoons was 0.55 in all foam formulation. Flexural and compressive strength values decreased simultaneously with increasing ESO content. Similar trend was observed for the flexural and compressive modulus and glass transition temperature. The work further showed that mechanism of failure mainly depended on the fracture of microballoons regardless the ESO content in the formulation. Results reported herein suggest that large fractions of DGEBA can be replaced by ESO with minor effect on mechanical and thermal properties.     

High temperature mechanical properties of low alloy steel foams produced by powder metallurgy

Cu–Ni–Mo and Mo based steel foams having different porosity levels for high temperature applications were produced by the space holder-water leaching technique in powder metallurgy. Steel powders were mixed with binder (polyvinylalcohol) and spacer (carbamide), and compacted. Spacer in the green compacts was removed by water leaching at room temperature and porous green compacts were sintered at 1200 °C for 60 min in hydrogen atmosphere. The successful application of foams at higher temperatures requires a good understanding of their high temperature mechanical properties. Compression tests were carried out on steel foams with different porosities at temperatures varying from room temperature to 600 °C in argon atmosphere. Effect of high temperature on compressive properties of the steel foams was investigated. It was found that the compressive strength of steel foams was greater at elevated temperatures than that at room temperature. This occurs across a range of temperatures up to 400 °C. Beyond this point the compressive strength decreased as the temperature increased. The reason for the enhancement of the compressive strength of Cu–Ni–Mo and Mo based steel foams is expected to be due to the effect of the dynamic age-hardening.     

Prediction of cushion curves of polymer foams using a nonlinear distributed parameter model

Polymer foams are commonly used in the protective packaging of fragile products. Cushion curves are commonly used within the packaging industry to characterize a foam's impact performance. These curves are two-dimensional representations of the deceleration of an impacting mass versus static stress. Cushion curves are currently generated from exhaustive experimental test data. This study represents the first time that the physics of the mass-cushion impact have been analysed by modelling the foam as nonlinear, continuous rod. Using a single mode of vibration and excluding the effects of damping, the maximum displacement during the impact can be obtained from a polynomial describing the maximum elastic energy in the foam. The displacements can be used to recover the amplitude of the deceleration shock pulse. Numerical and analytical analysis of the model with damping is considered in its ability to predict the shock pulse shape, duration, and amplitude at various static stresses, foam thickness, and drop heights as compared with experimental data. Furthermore, both the analytical and numerical results agree and are primarily within the expected lab-to-lab variability of 18% documented in ASTM D1596 - Standard Test Method for Dynamic Shock Cushioning Characteristics of Packaging Material.     

Fabrication of ferrous metallic foams by reduction of ceramic foam precursors

A process has been developed for obtaining closed cell metallic foams using a ceramic foam precursor. In the present study, the major constituent of the ceramic foam precursor was iron oxide (Fe₂O₃), which was mixed with various foaming/setting additives. The mixture set rapidly at room temperature, stabilizing the foam generated by hydrogen release. The oxide foam was then reduced by annealing at 1240^∘C in a non-flammable hydrogen/inert gas mixture to obtain a metallic foam with a relative density of 0.23 ± 0.017, and an average cell diameter of 1.32 ± 0.32 mm. The iron foams were tested in compression and yielded an average compressive strength of 29 ± 7 MPa. The compressive stress-strain curves obtained were typical of cellular metals. The normalized strengths of the metal foams obtained in the present study compare favorably with those of steel foams produced by other techniques.     

泡沫铝填充碳纤维增强树脂复合材料薄壁管的压缩变形行为与吸能特性

将填加造孔剂法制备的泡沫铝物理嵌入碳纤维增强树脂(Carbon fiber reinforced plastic,CFRP)复合材料薄壁管中,从而获得泡沫铝填充CFRP复合材料薄壁管的复合结构。针对CFRP薄壁管、泡沫铝和泡沫铝填充CFRP复合材料薄壁管分别开展准静态压缩试验测试其压缩和吸能性能,并在压缩过程中采用数字图像相关技术(Digital image correlation,DIC)同步分析其变形模式;进一步研究在不同环境温度下(25~150℃)泡沫铝填充CFRP复合材料薄壁管的压缩与吸能性能及失效模式。结果表明:泡沫铝作为填充芯材改变了CFRP复合材料薄壁管的压缩变形行为,由单一CFRP复合材料薄壁管的散射开花失效转变为泡沫铝填充CFRP复合材料薄壁管的纤维层断裂失效。同CFRP复合材料薄壁管相比,泡沫铝填充CFRP复合材料薄壁管的应力波动显著减小。随环境温度的升高,CFRP复合材料薄壁管、泡沫铝和泡沫铝填充CFRP复合材料薄壁管的压缩与吸能性能均不断降低,但泡沫铝与CFRP复合材料薄壁管之间的交互作用增强,泡沫铝对CFRP复合材料薄壁管的增强作用在高温下表现更为显著。      

Effect of Porosity and Cell Size on the Dynamic Compressive Properties of Aluminum Alloy Foams

The dynamic mechanical properties of open-cell aluminum alloy foams with different relative densities and cell sizes have been investigated by compressive tests.The strain rates varied from 700 s^-1 to 2600 s^-1.The experimental results showed that the dynamic compressive stress-strain curves exhibited a typical three-stage behavior:elastic,plateau and densification.The dynamic compressive strength of foams is affected not only by the relative density but also by the strain rate and cell size.Aluminum alloy foams with higher relative density or smaller cell size are more sensitive to the strain rate than foams with lower relative density or larger cell size.