Nonlinear deformation of elastomeric foams

The nonlinear deformation of a porous foam-type elastomeric material is studied, both theoretically and experimentally. The elastomer is modeled by the neo-Hookean material. The one-dimensional compressive behavior of the foam is analysed by using certain kinematic assumptions. The stress required to compress the foam is predicted by the model in terms of the porosity of the foam and the single constant in the neo-Hookean stress-strain form. A particular silicone foam is used as a test of the theory. The neo-Hookean constant is evaluated from a test of the homogeneous elastomer. Hence the behavior of the corresponding foam is predicted theoretically and compared with experimental results. The general results are applicable to closed-cells foams of intermediate density.     

闭孔泡沫铝的塑性泊松比

在表征闭孔泡沫铝的力学性能中,塑性泊松比是较为重要的参数之一。本文应用Kelvin十四面体模型构建出不同相对密度的闭孔泡沫铝三维细观模型并采用LS-DYNA对所得细观模型进行单轴准静态压缩计算。数值模拟分析发现,随着轴向应变的增加,泡沫铝泊松比-轴向应变曲线呈倒S形,存在峰值和极小值,曲线变化规律与泡沫铝胞孔的变形有密切关系。根据泊松比-轴向应变曲线与胞孔变形之间的关系,给出了平均塑性泊松比的定义。计算结果显示,随着相对密度的提高,闭孔泡沫铝的平均塑性泊松比增大。当闭孔泡沫铝的相对密度低于0.1时,其平均塑性泊松比接近于零,计算中可以忽略;当闭孔泡沫铝相对密 度大于0.1时,其平均塑性泊松比随相对密度的增加而呈线性从0.17增加到0.5     

闭孔泡沫铝的高温局部压入力学响应

通过不同形状(平头和半球头)的压头在不同温度下对闭孔泡沫铝材料进行塑性压入实验,研究不同温度下闭孔泡沫铝的压入变形模式及载荷响应特性。并基于闭孔泡沫铝在高温下的准静态塑性压入载荷响应的实验结果,结合多种分析方法,(如量纲分析和有限元计算等),探索既考虑温度影响也包含压入深度影响的预测闭孔泡沫铝平头和半球头压入力学响应的经验公式。结果表明,本文得到的两种压头情况下的经验公式都能够较好地预测闭孔泡沫铝在不同温度下的压入力学响应。

     

闭孔泡沫铝材料十四面体模型的改进

基于对闭孔泡沫铝发泡过程更为合理的假设,提出了描述胞体结构的改进的十四面体模型,使之可以反映密度增大时质量集中于支柱和顶点的情况。采用有限元方法及耦合边界条件,研究了闭孔泡沫铝的相对弹性模量、泊松比等弹性特征与胞体参数的关系,给出了拟合的弹性模量的计算公式,并对模型在弹性压缩变形下应力分布进行了分析。通过与已有模型的比较表明,改进模型可以较好地模拟闭孔泡沫铝材料的弹性性能。     

Finite element micromechanics model of impact compression of closed-cell polymer foams

Finite element analysis, of regular Kelvin foam models with all the material in uniform-thickness faces, was used to predict the compressive impact response of low-density closed-cell polyethylene and polystyrene foams. Cell air compression was analysed, treating cells as surface-based fluid cavities. For a typical 1 mm cell size and 50 s^?1 impact strain rate, the elastic buckling of cell faces, and pop-in shape inversion of some buckled square faces, caused a non-linear stress strain response before yield. Pairs of plastic hinges formed across hexagonal faces, then yield occurred when trios of faces concertinaed. The predicted compressive yield stresses were close to experimental data, for a range of foam densities. Air compression was the hardening mechanism for engineering strains <0.6, with face-to-face contact also contributing for strains >0.7. Predictions of lateral expansion and residual strains after impact were reasonable. There were no significant changes in the predicted behavior at a compressive strain rate of 500 s^?1.     

基于十四面体模型的闭孔泡沫材料动态力学性能的有限元分析

应用有限元方法分析了基于十四面体模型的三维闭孔泡沫材料的动态力学性能。计算中所有十四面体具有相同的尺寸,主要研究了不同初始冲击速度、不同相对密度以及组成泡沫的机体材料的应变强化对泡沫材料的变形模态、平台力及密实化应变能的影响,尽可能全面地描述了泡沫材料的能量吸收特性。数值结果表明:冲击速度对泡沫模型的模态影响较大,特别受到高速冲击时,冲击端泡沫形成I形然后向支撑端传播;相对密度对能量吸收能力的贡献较大,密实化应变能随相对密度呈二次曲线变化;冲击速度、相对密度及机体材料的应变强化分别与坪应力呈线性关系。     

Measurements of pore scale flows within and exiting ceramic foams

LDA was used to measure the mean velocities and pore scale turbulence intensity of air flowing through ceramic foams having pore sizes between 0.8 and 2.5 mm in diameter. The stream-wise and transverse velocity components were measured near the exit plane, in a region between two foams separated by 5 mm, and within a section of ceramic foam. Mean velocities ranged from 0.25 m/s to 1.5 m/s, corresponding to pore Reynolds numbers between 20 and 200. A transition to a fully developed pore turbulence was achieved at pore Reynolds numbers above approximately 150.Support for this work was provided by the Texas Advanced Research Program and The Center for Energy Studies of The University of Texas at Austin.     

Stress-strain relations for elastomeric foams in uni-, bi- and tri-axial compression modes

Summary Based on suggestions given in [1], the uni-axial compressive stress-strain relations for elastomeric foams, which were developed in [2], have been extended to cover the bi- and tri-axial stress modes of compression for both open- and closed-cell elastomeric foams.The stress-strain relations for uni- and tri-axial stress compression modes were validated by comparing numerical predictions, which were based on them, to experimental results.     

Effect of high pressure on pore formation

A method of formation of duralumin plates from the melt under conditions of hydrostatic pressure is investigated. The mechanism of the action of high pressure on the structure of the metal during crystallization is discussed. Results from the measurement of density, strength, and specific electrical resistance are presented.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 6, pp. 110–113, November–December, 1970.     

Experimental study on the two-phase pressure drop in copper foams

Experiments were performed to study pressure drops in copper foams embedded in a rectangular copper channel. De-ionized water was used as the working fluid with mass fluxes of 30–200 kg/m² s, and inlet temperature of 40–80°C. The copper foam has the porosity of 0.88 and the pore densities of 30, 60 and 90 ppi (pores per inch). Both single-phase liquid flow and boiling two-phase flow are studied. Effects of mass fluxes, vapor mass qualities, and average pore diameters of metallic foams are investigated. It is found that friction factors for the single-phase liquid flow are mainly dependent on the Reynolds number and the average pore diameter of metallic foams. The friction factors are decreased with increases in the Reynolds numbers, and will approach 0.22 at high Reynolds numbers. For the boiling two-phase flow, two-phase pressure drops are increased with increases in the outlet vapor mass qualities, mass fluxes, and ppi values. The two-phase multiplier is increased with increases in the outlet vapor mass qualities and mass fluxes, and it is decreased with increases in the Martinelli parameter and will attain a constant value depending on the mass fluxes. The larger the mass fluxes, the larger the constant value is. An experimental correlation considering the effects of vapor mass qualities, mass fluxes, and average pore diameters of metallic foams is recommended, showing good accuracy to predict the two-phase pressure drops in metallic foams.     

Inflation analyses of a circular elastomeric membrane with hydrostatic pressure

Composite diaphragm inflation (CDI) experiment is a useful methodology for exploring the mechanics relation of cell-cell junctions within a living epithelial sheet. In this paper, a new methodology of CDI experiment has been put forward. The finite deformation of a circular polydimezhylsiloxane elastomeric membrane, which is used as the substrate for the culture of epithelial cells, under hydrostatic pressure is explored by the theory of non-linear continuum mechanics. The simplified governing equations with minimum amount of differential equations and boundary conditions are obtained and solved by multiple shooting method. The influences on the membrane by hydrostatic pressure have been discussed.     

冲击荷载作用下粘土孔隙水压的数值分析

为了分析冲击荷载作用下粘土的孔隙水压力的增长规律,为分析粘土介质的动态响应、动力固结加固提供理论依据,将冲击瞬态荷载简化为三角形荷载,通过编制瞬变动态弹塑性有限元程序,对冲击荷载作用下粘土孔隙水压的动态响应进行了模拟,得出了冲击荷载作用下粘土孔隙水压的增长规律,发现孔隙水压力峰值无论是随水平距离还是随深度的增加均呈指数形式衰减,和实际工程施工中孔隙水压的监测数据相吻合。     

Effects of microstructure on uniaxial strength asymmetry of open-cell foams

Based on the elongated Kelvin model, the effect of microstructure on the uniaxial strength asymmetry of open-cell foams is investigated. The results indicate that this asymmetry depends on the relative density, the solid material, the cell morphology, and the strut geometry of open-cell foams. Even though the solid material has the same tensile and compressive strength, the tensile and compressive strength of open-cell foams with asymmetrical sectional struts are still different. In addition, with the increasing degree of anisotropy, the uniaxial strength as well as the strength asymmetry increases in the rise direction but reduces in the transverse direction. Moreover, the plastic collapse ratio between two directions is verified to depend mainly on the cell morphology. The predicted results are compared with Gibson and Ashby''s theoretical results as well as the experimental data reported in the literature, which validates that the elongated Kelvin model is accurate in explaining the strength asymmetry presented in realistic open-cell foams.     

Analytical solutions for inflation of pre-stretched elastomeric circular membranes under uniform pressure

Elastomeric membranes are frequently used in several emerging fields such as soft robotics and flexible electronics. For convenience of the structural design, it is very attractive to find simple analytical solutions to well describe their elastic deformations in response to external loadings. However, both the material/geometrical nonlinearity and the deformation inhomogeneity due to boundary constraints make it much challenging to get an exact analytical solution. In this paper, we focus on the inflation of a prestretched elastomeric circular membrane under uniform pressure, and derive an approximate analytical solution of the pressure–volume curve based upon a reasonable assumption on the shape of the inflated membrane. Such an explicit expression enables us to quantitatively design the material and geometrical parameters of the pre-stretched membrane to generate a target pressure–volume curve with prescribed peak point and initial slope. This work would be of help in the simplified mechanical design of structures involving elastomeric membranes.     

Predicting pressure drop in open-cell foams by adopting Forchheimer number

Interconnected struts arranged in 3-D foam structures pose a challenge in understanding fluid flow, which is significantly different from that in traditional porous media. Different flow regimes (Darcy, transition and weak inertia regimes) and thus, different flow laws in open-cell foams are used. The impact of characteristic lengths’ choices based on both, morphological and hydraulic parameters on flow law formulation has been studied. Ambiguities in definitions and measurements of several key parameters have been shown and limitations in the use of some parameters have been pointed out.An equivalent Reynolds number in the form of Forchheimer number (F_o) has been proposed to establish the friction factor relationship in order to avoid any morphological ambiguities. This number takes into account hydraulic characteristics of viscous and inertia regimes simultaneously. It has been observed that when F_o < 0.1, the flow through open-cell foams remains in the Darcy regime while the occurrence of weak inertia regime dominates when F_o > 1. Transition regime occurs in a narrow range of flow velocity when 0.1 < F_o < 1. The limits of transition for regime identification are found to be independent of foam morphologies. The form drag coefficient varies in relation with foam morphological parameters and is not a “universal” constant.Empirical correlations have been derived to predict hydraulic characteristics and friction factor data for different strut shapes and porosities. An excellent agreement has been obtained between predicted and numerical/experimental flow data.     

Influential factors of loess liquefaction and pore pressure development

The liquefaction of loess under dynamic loading is studied experimentally with a dynamic triaxial test system. The effects of over-consolidation ratio (OCR), saturation degree and the frequency of dynamic loading upon loess liquefaction are investigated. The development of pore pressure within loess samples is also discussed. Based on the experimental results, the empirical relationship between pore pressure ratio and loading cycle number ratio is established for normal consolidated saturated loess.The project supported by the National Natural Science Foundation of China (50178005)     

闭孔泡沫金属变形模式的有限元分析

运用有限元软件ABAQUS/Explicit模拟了三维Voronoi闭孔泡沫金属在不同的冲击速度下的变形行为。随着冲击速度的提高,得到了3种变形模式:准静态均匀模式、过渡模式和冲击模式,并以相对密度和冲击速度为坐标建立了变形模式图。引入应力均匀性指标和变形局部化指标,确定了模式转化的临界速度,并与已有的冲击速度预测公式进行了比较。根据临界速度的数值和理论结果,提出了一种确定锁定应变的方案,结果介于压实应变和完全密实应变之间。     

Estimation of adsorption-induced pore pressure and confinement in a nanoscopic slit pore by a density functional theory

This study aims at characterising the adsorption-induced pore pressure and confinement in nanoscopic pores by molecular non-local density functional theory (DFT). Considering its important potential industrial applications, the adsorption of methane in graphitic slit pores has been selected as the test case. While retaining the accuracy of molecular simulations at pore scale, DFT has a very low computational cost that allows obtaining highly resolved pore pressure maps as a function of both pore width and thermodynamic conditions. The dependency of pore pressure on these parameters (pore width, pressure and temperature) is carefully analysed in order to highlight the effect of each parameter on the confined fluid properties that impact the solid matrix.