基于Sherwood–Frost模型构建聚乙烯泡沫拉伸本构

目的 研究聚乙烯泡沫的拉伸力学性能,并构建聚乙烯泡沫的拉伸本构模型。方法 利用万能材料试验机对不同密度的聚乙烯泡沫进行不同拉伸速率的单轴拉伸实验,得到聚乙烯泡沫的拉伸应力–应变曲线;在Sherwood–Frost唯象本构模型框架的基础上,构建将密度和应变耦合的密度项,以及将应变率、应变和密度耦合的应变率项的拉伸本构模型。结果 聚乙烯泡沫在断裂前的拉伸力学特性为非线性弹性,表现出明显的应变率强化效应,现有的密度项和应变率项与实验数据的拟合精度较低,最大平均误差分别可达11.76%、7.90%。新构建的密度项和应变率项与实验数据拟合精度较好,最大平均误差分别为1.17%、1.92%。结论 新构建的拉伸本构模型能够更精确地描述聚乙烯泡沫单轴拉伸的应力应变关系,为聚乙烯泡沫的综合力学性能的进一步研究提供参考。     

Shear and mode II fracture of PUR foams

Polyurethane (PUR) foam materials are widely used as cores in sandwich composites, for packing and cushioning. They are made of interconnected networks of solid struts and cell walls incorporating voids with entrapped gas. The main characteristics of foams are lightweight, high porosity, high crushability, and good energy absorption capacity. Fracture toughness in mixed mode loading is of particular interest because foam cracking weakens the structure’s capacity for carrying loads.Present paper assesses the shear elastic (shear modulus) and mechanical (shear strength) properties of polyurethane foams. Also, three different types of specimens were used to determine mode I and mode II fracture toughness. The shear modulus, shear strength and fracture toughness increases with increasing foam density. Also the effect of loading direction and loading speed is investigated. The authors propose a micromechanical model to estimate fracture toughness based on the tensile strength of the solid material and the topology of the cellular structure.     

Comparison of tensile and compressive characteristics of vinyl ester/glass microballoon syntactic foams

The present study is focused on the synthesis and characterization of vinyl ester/glass microballoon syntactic foams. Tensile and compressive properties of vinyl ester matrix syntactic foams are characterized. Results show that the compressive strength and moduli of several syntactic foam compositions are comparable to those of the neat matrix resin. Due to the lower density of syntactic foams, the specific compressive properties of all compositions are higher than those of the neat resin. Similar trends are observed in the tensile properties. Mechanical properties of vinyl ester matrix syntactic foams are compared to well-documented mechanical properties of epoxy matrix systems. The comparison shows that low cost vinyl ester resins, which are extensively used in marine applications, can result in syntactic foams with comparable performance to epoxy matrix systems. In addition, tensile modulus is found to be 15–30% higher than the compressive modulus for all syntactic foam compositions. This difference is related to the possibility of particle fracture in the stress range where modulus is calculated in the compressive stress–strain curves.     

Fiber-reinforced composite foam from expandable PVC microspheres

The properties of composite foam based on PVC expandable microspheres reinforced with continuous aramid fibers are described. The foam was fabricated by infiltrating low-density non-woven fiber webbing with PVC microspheres. The assembly was subsequently heated to expand the foam. The resulting composite foam consisted of 10 wt% aramid fibers and had a density of 100 kg/m³. Mechanical properties, crack propagation, and microstructure of composite foams were evaluated and compared with properties of similar unreinforced foam and with commercial PVC foam of comparable density. The influence of fiber concentration, fiber architecture and bonding was investigated also. Properties were measured in tension, shear, compression, and flexure using standard ASTM test methods. The composite foam performance equaled or surpassed the performance of most thermoplastic foams commercially available. The tensile strength and modulus of the composite foam increased by factors of 6 and 8, respectively, and the shear strength and modulus increased by factors of 1.8 and 2.4. The composite foam also exhibited improved strain energy density and damage tolerance, and reduced notch sensitivity.     

Compressive fracture features of syntactic foams-microscopic examination

Syntactic foams made by mechanical mixing of polymeric binder and hollow spherical particles are used as core materials in sandwich structured materials. Low density of such materials makes them suitable for weight sensitive applications. The present study correlates various postcompression microscopic observations in syntactic foams to the localized events leading the material to fracture. Depending upon local stress conditions the fracture features of syntactic foam are identified for various modes of fracture such as compressive, shear and tensile. Microscopic observations were also taken at sandwich structures containing syntactic foam as core materials and also at reinforced syntactic foam containing glass fibers. These observations provide conclusive evidences for the fracture features generated under different failure modes. All the microscopic observations were taken using scanning electron microscope in secondary electron mode.     

纳米纤维增强闭孔泡沫材料的力学性能

为研究纳米纤维增强闭孔泡沫材料的力学性能,采用Voronoi随机泡沫模型对闭孔泡沫材料的细观几何结构进行模拟,并将纳米纤维随机分布在泡沫材料的胞壁中,利用改进的自动搜索耦合（ASC）技术将纤维单元与基体单元进行耦合,建立了能够反映纳米纤维增强闭孔泡沫材料细观结构的数值模型。在此基础上,进一步研究了泡沫模型随机度、相对密度以及纳米纤维长径比和质量分数对纳米纤维增强闭孔泡沫材料弹性模量与屈服强度的影响规律。结果表明:由所建立的数值模型得到的纳米纤维增强闭孔泡沫材料的弹性模量和屈服强度与实验值吻合较好;提高泡沫模型的随机度会使复合泡沫材料的弹性模量和屈服强度增加,而当随机度达到0.450以后,材料的弹性模量和屈服强度几乎不再发生变化;当相对密度在0.05~0.30范围内变化时,复合泡沫材料的弹性模量与屈服强度几乎随相对密度的增加呈线性增长;提高纳米纤维长径比和质量分数也会使材料的弹性模量和屈服强度得到提高,但当纤维长径比达到500以后,纤维长径比的增强作用逐渐减弱。所得结论对纳米纤维增强闭孔泡沫材料的制备具有重要意义。      

The deformation of brittle starch foams

There exists a theoretical model to describe the deformation of solid foams which relates the mechanical properties to the foam density and the cell-wall properties. Previous work has assumed that the wall properties are constant for a wide range of different density foams and can be characterized by the properties of the unfoamed material. In this paper we show that, when considering extruded starch foams, variation of the extrusion parameters in order to produce different bulk density foams has an effect on the cell-wall material: notably upon the crystallinity,T _g and wall density. Therefore, both the bulk foam and cell-wall mechanical properties were measured in order to test the full theory. For the relative fracture stress, excellent agreement was found between the predicted power law behaviour and the experimental results. However, the power law for the relative modulus is larger than the predicted value.     

环氧树脂复合泡沫材料的压缩力学性能

对空心玻璃微珠填充环氧树脂复合泡沫材料进行了准静态压缩实验, 研究了材料的宏观压缩力学性能, 并提出了弹性模量和屈服强度的预测公式。此外, 对压缩试件的断口进行了宏、细观观察, 研究了材料的压缩破坏机理。结果表明, 复合泡沫材料在压缩过程中, 具有普通泡沫材料的应力-应变曲线的典型特征, 在应变为2 %左右时材料发生屈服, 在应变大于30 %后发生破坏。此外, 材料的杨氏模量和强度均随密度的减小而下降, 预测公式给出的结果与实验值基本一致。压缩试件断口的宏、细观观察表明, 复合泡沫材料主要的破坏形式为剪切引起的弹塑性破坏。      

Elastic behavior of multi-scale,open-cell foams

The mechanical properties of cellular materials are still subject to numerous theoretical and experimental investigations. In particular, the impact of cell size on the foam’s elastic response has not been studied systematically mainly due to the lack of experimental techniques with which the cell size and relative density of materials can be varied independently. This paper presents the results of a study of the elastic behavior of open-cell foams as a function of relative density and the size of the interconnected, spherical pores. First, the chemical procedure allowed us to produce polystyrene open-cell foams in which the relative density and the average cell diameters were varied independently. The results of compression tests performed on these foams showed an unexpected influence of the cell diameter (at constant relative density) on the elastic response. The analysis of the microstructure of the foam revealed the presence of a complex nanostructure in the edge of the cells that appeared during the synthesis procedure. An analytical model (an extension of the Gibson–Ashby model) is presented, which takes into account the complex multi-scale structure of the foam and accurately describes the observed dependence of the measured Young’s moduli on cell size. This approach was confirmed further by a finite element numerical simulation. We concluded that the observed dependence of elastic modulus on cell size was due to the heterogeneous nature of the material that constitutes the walls of the cells.     

Mechanical,thermal and morphological characterization of cellulose fiber-reinforced phenolic foams

In this work, the compressive mechanical properties, thermal stability and morphology of cellulose fiber-reinforced phenolic foams were studied. The cellulose fiber-reinforced phenolic foam showed the greatest compressive mechanical properties by incorporating 2 wt.% of the reinforcement. The compressive modulus and strength of 2 wt.% cellulose fiber-reinforced phenolic foam were increased by 21% and 18%, respectively, relative to the unreinforced material. The addition of the cellulose fibers to the phenolic foam slightly decreased the thermal stability of the material. The study on the morphology of the cellulose-reinforced phenolic foams via Scanning electron microscopy (SEM) indicated a strong bonding between the fibers and phenolic matrix. In addition, the incorporation of the cellulose fibers into the foam resulted in a decreased cell size and increased cell density of the material. The incorporation of 2 wt.% of cellulose fibers into the phenolic foam led to obtain the material with the best features.     

Stiffness and energy dissipation in polyurethane auxetic foams

Auxetic open cell polyurethane (PU) foams have been manufactured and mechanically characterised under cyclic tensile loading. The classical manufacturing process for auxetic PU foams involves multiaxial compression of the conventional parent foam, and heating of the compressed specimens above the T_m of the foam polymer. Eighty cylindrical specimens were fabricated using manufacturing routes modified from those in the open literature, with different temperatures (135 °C, 150 °C), compression ratios and different cooling methods (water or room temperature exposure). Compressive tensile cyclic loading has been applied to measure tangent modulus, Poisson’s ratios and energy dissipated per unit volume. The results are used to obtain relations between manufacturing parameters, mechanical and hysteresis properties of the foams. Compression, both radial and axial, was found to be the most significant manufacturing parameter for the auxetic foams in this work.     

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.     

The mechanism of reinforcement of polyurethane foam by high-modulus chopped fibres

The morphology and fracture behaviour of polyurethane foams reinforced by short chopped fibres have been investigated. The presence of the fibres is shown to give rise to localized change in the foam morphology and the extent of this depends upon the fibre bundle size which is affected by the surface treatment. The changes in morphology are correlated with changes in the tensile properties of the foams at ambient and cryogenic temperatures. The systems are shown to be matrix limited with failure occurring remote from the interface which assumes a poassive role during tensile fracture. A critical fibre length for reinforcement of polyurethane foam, which depends on matrix shear strength and foam density, is defined.     

The influence of correlating material parameters of gradient foam core on free vibration of sandwich panel

For the sandwich panel with mass density gradient (DG) foam core, the Young's modulus of the core varies with the mass density along the thickness direction. To characterize the correlative effect of Young's modulus and mass density of the DG closed-cell foam material, a simplified formula is presented. Subsequently, based on a high-order sandwich plate theory for sandwich panel with homogeneous core, a new gradient sandwich model is developed by introducing a gradient expression of material properties. Finite element (FE) simulation is carried out in order to verify this model. The results show that the proposed model can predict well the free vibration of composite sandwich panel with the gradient core. Finally, the correlating effects of material parameters of the DG foam core on the natural frequencies of sandwich panel are investigated. It is found that the natural frequencies of sandwich panels decrease as the gradient changes of the DG foam cores increase under the condition of that the core masses keep constant.     

Viscoelastic properties of hollow glass particle filled vinyl ester matrix syntactic foams: effect of temperature and loading frequency

Viscoelastic properties of hollow particle-reinforced composites called syntactic foams are studied using a dynamic mechanical analyzer. Glass hollow particles of three different wall thicknesses are incorporated in the volume fraction range of 0.3–0.6 in vinyl ester resin matrix to fabricate twelve compositions of syntactic foams. Storage modulus, loss modulus, and glass transition temperature are measured and related to the microstructural parameters of syntactic foams. In the first step, a temperature sweep from ?75 to 195 °C is applied at a fixed loading frequency of 1 Hz to obtain temperature dependent properties of syntactic foams. In the next step, selected four compositions of syntactic foams are studied for combined effect of temperature and loading frequency. A frequency sweep is applied in the range 1–100 Hz and the temperature is varied in the range 30–140 °C. Time–temperature superposition (TTS) principle is used to generate master curves for storage modulus over a wide frequency range. The room temperature loss modulus and maximum damping parameter, Tanδ, are found to have a linear relationship with the syntactic foam density. Increasing volume fraction of particles helps in improving the retention of storage modulus at high temperature in syntactic foams. Cole–Cole plot and William–Landel–Ferry equation are used to interpret the trends obtained from TTS. The correlations developed between the viscoelastic properties and material parameters help in tailoring the properties of syntactic foams as per requirements of an application.     

Modeling the mode I fracture toughness of anisotropic low-density rigid PUR and PIR foams

Low-density foams have to possess a sufficient resistance to cracking in order to ensure the mechanical integrity of foam materials in service, even when not intended for load-bearing applications. In this study, mode I fracture toughness in the foam rise direction has been experimentally characterized for anisotropic rigid commercial polyurethane foams as well as for polyisocyanurate foams produced using polyols derived from rapeseed oil and filled with a montmorillonite nanoclay. Rectangular parallelepiped unit-cell based scaling relations expressing foam toughness via its relative density, cell dimensions, geometrical anisotropy, and the ultimate tensile stress of the base polymer have been employed for prediction of foam toughness. Assuming a brittle fracture of foam struts, a conservative estimate of toughness is obtained. It is demonstrated that considering the yielding of foam struts at the crack front as the criterion of crack extension provides a closer estimate of foam toughness.     

Synthesis of composite foam from thermoplastic microspheres and 3D long fibers

A method is described for preparing composite foam using expandable PAN-based microspheres reinforced with continuous fibers. Composite foams were produced by mixing expanded and non-expanded microspheres in select proportions, packing the dry microspheres into a fibrous preform in a closed mold, and heating the assembly to expand and weld the microspheres and fibers together. The composite foams exhibited mechanical performance and formability that surpassed the unreinforced foams. The tensile modulus and strength were increased by 750 and 400% respectively, and showed enhanced resistance to crack propagation compared with unreinforced foam samples. The improvement in compression properties was modest by comparison (<10%). Fiber performs were comprised of 3D, stochastically arranged long fibers, and typical fiber loadings were ∼8 wt%. Long fibers were deeply anchored in the foam and bridged crack wakes, resisting crack growth and delaying catastrophic failure of the foams during tensile tests.     

Tensile properties of open-cell nickel foams

Open-cell nickel foams with average pore size of 600 μm have been subjected to room temperature tensile tests to explore their tensile properties. Using a state of the art extensometer of non-contact type, foam properties as ultimate tensile strength (UTS), yield strength (YS) and the Young’s modulus (E) have been measured accurately. An extensometer of non-contact type was applied for the first time to help the foam’s mechanical properties to be determined accurately. The reason behind the usage of this kind of extensometer is to avoid completely any minor deformation that might be caused by the attachment of conventional extensometer to the sample’s surface prior to testing. The function of this extensometer is based on the usage of a laser (CCD) camera that detects and records the dimensional changes as soon as the load is applied. The fracture behavior of foam cells was observed to be ductile. Complete separation of struts or cell walls took place successively by necking.