闭孔泡沫铝在动态加载下的压缩力学行为研究

为研究闭孔泡沫铝的动态压缩力学响应过程,基于典型泡沫铝试样的孔型和分布情况构建了Voronoi模型,根据实验结果验证了模型的准确性。基于LS-DYNA分析了目前泡沫铝常用的Kelvin模型和Voronoi模型之间的差异性,研究了加载速度、基体应变率效应和压缩惯性效应对闭孔泡沫铝变形模式和应力水平的影响规律。研究结果表明:Voronoi模型应力-应变水平和变形模式与实验结果拟合较好,内部结构比单胞阵列的Kelvin模型更趋真实合理;在低速压缩下,泡沫铝惯性效应基本上可以被忽略,而高速压缩下,受压缩惯性效应影响,泡沫铝平台应力随着加载速度的增大而增大;当考虑泡沫铝基体应变率效应时,泡沫铝平台应力水平会得到有效的改善,且泡沫铝整体呈现应变率效应。     

梯度泡沫金属的冲击吸能特性

基于闭孔泡沫铝的显微CT扫描信息,考虑胞孔的不规则形貌及胞孔分布的不均匀性,以及胞孔尺寸和壁厚沿泡沫高度的梯度分布,建立了梯度泡沫金属材料的二维细观有限元模型,分析了梯度泡沫金属材料在动态压缩过程中的变形、塑性波的传播和能量变化特征。对于平均相对密度0.3、平均梯度系数0.4的梯度泡沫铝,低速（10 m/s）加载时,梯度泡沫金属在变形的整个过程中吸收的总能量均低于均匀泡沫金属;高速加载时,梯度泡沫金属沿负梯度方向压缩的早期吸能比均匀泡沫金属有优势,而且速度越高,优势越明显。     

三维机织复合材料的一种梁单元细观力学模型

根据三维机织复合材料中纤维束排列和变形的周期性特点,推导了一种细观梁单元模型。该模型考虑了纤维束的拉 (压) 弯耦合效应和纤维束之间的相互作用,可以描述纤维束和基体中的细观应力分布,并得到宏观的力学性能。针对一种典型的三维机织复合材料,首先根据编织参数,确定其细观几何结构,取最小周期的一段纤维束作为分析胞元,用上述细观梁单元分析了该段纤维束在面内拉伸荷载下的细观应力分布,计算出平均模量, 并用材料试验和细观实体有限元对本模型进行了检验,结果与本文的预测吻合良好。研究表明,拉、弯耦合效应引起的纤维束中的细观弯曲应力同平均轴向应力相比,不可忽略。      

碳纤维增强形状记忆聚合物复合材料的热-力学行为建模与影响因素碳纤维增强形状记忆聚合物复合材料的热-力学行为建模与影响因素

采用变形梯度分解的方法,基于热力学内变量理论,构建了适用于描述碳纤维增强形状记忆聚合物（CF/SMP）复合材料热-力学行为的热黏弹性本构模型。模型中考虑了材料的结构松弛效应和应力松弛效应,且适用于有限变形条件。依据该模型研究了一种CF/SMP薄板在受到单向均布载荷作用且处于平面应力状态时的碳纤维有效应变影响因素。理论上证实了虽然碳纤维的容许应变很小,但合理取向的纤维分布形式能使其应用于有限变形条件下而不被破坏。此外,分析了该CF/SMP形状记忆热-力学循环过程中形状记忆效应（SME）的影响因素。结果表明,碳纤维含量的增大和纤维倾斜角的减小会导致CF/SMP刚度增大,从而降低其形状固定率。此外,碳纤维体积含量和温度变化率对升温回复阶段也存在一定影响。上述研究方法和结果能对单向连续CF/SMP的设计与应用提供一定理论指导。      

泡沫填充蜂窝材料动态力学性能的物质点法模拟

为了研究泡沫填充蜂窝材料(FFH)在动态加载下的力学响应和吸能效果,采用物质点法建立了FFH的细观物质点模型。泡沫细观物质点模型的应力-应变曲线与理论模型和实验结果吻合较好,FFH细观物质点模型的变形失效模式与实验结果一致。研究发现,填充泡沫和蜂窝分别通过塑性变形和屈曲变形吸能,填充泡沫对蜂窝吸能效果增强效应显著。获得了填充泡沫密度和加载应变率对FFH变形损伤和吸能效果的影响。填充泡沫密度增加,FFH动态力学性能提高,吸能总量增加,蜂窝吸能增加。填充泡沫增强了蜂窝的屈曲强度,促进蜂窝抵抗更多的变形。FFH的应力-应变曲线对加载应变率敏感,其吸能效果受加载应变率一定程度的影响,但总量变化不超过15%。吸能总量和组分吸能比例由FFH整体结构决定,与加载应变率无关。      

基于SR-CT技术的泡沫铝力学性能

基于同步辐射计算机断层技术（简称SR-CT技术）,对泡沫铝材料试件进行三维无损重建,获取泡沫铝材料试件的内部体结构。在此基础上,建立泡沫铝试件的三维仿真模型,并对该模型进行模拟计算,以研究泡沫铝材料在压缩过程中的变形情况、米塞斯应力分布情况及其弹塑性区域的分布。研究结果表明,基于同步辐射计算机断层技术重建结果建立的三维...     

单轴压缩载荷下闭孔泡沫铝的变形机制

基于X射线计算机断层扫描技术,重构了能够反映闭孔泡沫铝真实细观结构的三维有限元模型。采用数值模拟与试验测试相结合的方法,研究了泡沫铝在准静态单轴压缩载荷作用下的力学响应及其变形机制,重点关注了平台阶段及致密化阶段的变形模式。结果表明:试件中变形带的出现是压缩过程进入平台阶段的一个标志,此时棱杆和孔壁的变形以塑性弯曲为主;平台阶段,棱杆及孔壁的变形逐渐向塑性起皱与塑性屈曲转变;伴随致密化阶段的发生,变形带内部的胞孔严重坍塌,呈‘双凹圆盘’状。闭孔泡沫铝细观结构变形模式的数值模拟与试验结果相符,验证了该模型的有效性,为进一步研究各相关物理量(相对密度、加载速率等)及变形机制对其宏观吸能性能的影响奠定了基础。     

三维编织复合材料弹性性能数值预测及细观应力分析

基于考虑纤维束面接触的细观结构模型, 引入周期性位移边界条件, 采用细观有限元方法建立了材料的弹性性能预报模型。模型数值结果与试件实测数据吻合较好, 证明了该模型的合理有效性。经详细分析单胞在典型工况载荷作用下的细观应力分布及变形, 表明模型体现了周期性相邻单胞表面力和位移的连续性, 能获得单胞更为合理的细观应力应变场。      

中低应变率下闭孔泡沫铝动态力学性能研究

为探索闭孔泡沫铝的动态力学性能与吸能特性,基于万能材料试验机和高速液压伺服材料试验机在常温下分别对闭孔泡沫铝在准静态和中应变率下(0.001~100s^-1)的动态力学性能进行了测试,分析了不同应变率、不同相对密度和不同泡沫铝基体特性下闭孔泡沫铝的应力应变曲线特征和吸能特性变化。研究结果表明:中低应变率下的纯铝基体泡沫铝并不具备应变率效应,高脆性、相对密度较小的泡沫铝具备更好的吸能特性,塑性和脆性基体泡沫铝变形带分别呈现“V”形和“X”形,脆性基体泡沫铝同样不具备应变率效应。     

含双尺度界面复相陶瓷的细观界面滑移应力

基于复相陶瓷显微特征和双尺度界面特性,分析含双尺度界面复相陶瓷内的细观界面滑移应力。首先,基于复相陶瓷宏观、细观和纳观弹性性能,计算双尺度界面复相陶瓷产生弹性变形时的细观平均应力场。然后,在纳观界面位移和应力连续基础上,提出了界面应变模型,确定了纳观界面附近纤维和基体内的位移函数,考虑界面应变的突变值与界面模量间的比例关系,根据纳观界面特性和纤维分布形式,确定出弹性变形条件下外载传递到细观界面上的切应力。最后,基于压痕实验测得复相陶瓷细观界面滑移的屈服切应力,得到细观界面滑移应力的理论计算公式并进行了定量分析。结果表明,复相陶瓷内纳观界面弹性模量越小或泊松比越小时,细观界面越易滑移,复相陶瓷越易产生塑性变形。     

Impact of melt rheology on zein foam properties

Zein, the main protein fraction in maize, is left as a by-product from bio-ethanol production. The protein has been investigated as a material for a long time, but mainly in the form of films. In contrast, foamed zein is presented in this article. Zein foams may perhaps be used, e.g. as trays for biodegradable food packages or as scaffolds for tissue engineering. A batch method for manufacturing solid foams was successfully developed, the foams being manufactured by evaporation of solvent from zein resins. In order to be suitable for foam formation, a resin must possess gas-retaining properties, which can be predicted by extensional rheology. The presence of plasticizer in some of the resins decreased their extensional viscosity, and this in turn affected the foaming process. Although all the resins displayed strain-hardening behaviour, there was coalescence of pores in all the foams. Insufficient extensional viscosity resulted in the collapse of pore walls during foam expansion. Structure analysis showed, e.g. that most pores were elongated along the main axis of the mould in which the foams were manufactured. The plasticizer content in the resins had no significant effect on the mechanical properties of the foams.     

Constitutive modeling of polymeric foam material subjected to dynamic crash loading

This paper describes detail work on constitutive law modeling of low-density polymeric foam materials. Selected experimental results on low-density polyurethane (PU), polypropylene (PP), and polystyrene (PS) foams are presented. A rate-dependent hydrodynamic constitutive equation is presented for rigid polymeric foams. Focus has been placed on modeling of strain rate dependency and temperature effect on polymeric foams subjected to high rate impact loading. Numerical implementation procedure for the constitutive model is described. The constitutive model has been implemented into finite-element program as a user-defined material subroutine. Numerical examples are provided to validate the model under simple and complex loading conditions.     

Experimental determination of the thermal conductivity of three-phase syntactic foams

Syntactic foams are attractive for applications that require materials with high impact strength and low thermal conductivities. Because syntactic foams are manufactured by dispersing hollow microspheres in a resinous matrix, their characteristics are functions of the type and relative amounts of these materials. In this work, a discussion of an experimental approach to measure the thermal conductivity of three-phase syntactic foams (hollow carbon microspheres in a porous APO-BMI binder, analysis of the data and the comparison to predictive models are presented. The thermal conductivity of three-phase syntactic foams is measured using a Holometrix^© steady-state heat flow meter. The experimental data are found to be accurate to within a reasonable range of experimental error and are compared to three of the more reliable predictive models that have been used successfully to estimate the thermal conductivity of similar foams. It is observed that the model predictions at lower temperatures are more accurate as compared to those at higher temperatures. Also, that a model based on the concept of self-consistent field theory better predicts the thermal conductivity of syntactic foams than one based on resistance-in-series. Sensitivity studies indicate a strong dependency of the thermal conductivity of the three-phase foams on the thermal conductivity of the carbon used in the microspheres.     

Prediction of the effect of artificial aging heat treatment on the yield strength of an open-cell aluminum foam

The effect of artificial aging on the compression yield strength of an open-cell AA6101 foam is studied using both experimental and modeling approaches. Isothermal calorimetry is used to analyze the precipitation kinetics of the foam. The modeling work combines the established approaches for predicting the yield strength of open-cell metallic foams as a function of the relative density and normalized strength, as well as the age hardening behavior of AA6101 alloy. The foam yield strength is related to the evolution of precipitate content during aging and is modeled for artificial aging at 180 and 220 °C. It is shown that the model predictions match very well with the experimentally determined yield strength values. The overall results suggest that the presented analytical and modeling approaches can effectively be used to predict the precipitation hardening behavior and/or optimize processing and properties of AA6101 foams.     

考虑界面效应的复合泡沫塑料弹性性能数值仿真预测与试验研究

通过对不同空心陶瓷微珠含量的环氧基复合泡沫塑料进行准静态拉伸实验, 研究了填充微珠的体积分数对复合泡沫塑料弹性模量和泊松比的影响。基于其细观结构特征, 利用三维立方单胞有限元模型模拟了细观应力/应变场; 将内聚力单元引入细观有限元模型, 以此来模拟空心微珠与基体材料之间界面相的力学行为。将有限元预测结果以及两种传统的细观解析法与实验数据对比, 发现基于界面理想粘接假设的有限元模型和传统细观解析法均过高估计了复合泡沫塑料的弹性模量和泊松比; 复合泡沫塑料的弹性性能强烈地依赖于界面相的力学性质, 只有考虑界面效应的细观有限元模型才能给出较为精确的预测, 从而验证了文中细观建模方法的合理性。     

煤基炭泡沫孔结构调控

以肥煤镜质组富集物为前驱体, 采用高压渗氮法制备煤基炭泡沫, 研究了发泡温度、发泡压力和发泡时间对炭泡沫孔结构的影响。利用SEM观察炭泡沫的孔胞形貌, 同时利用Nano Measurer分析软件统计SEM照片孔胞直径分布和孔喉直径分布以及平均孔径。结果表明: 微孔塑料成核理论可以定性解释炭泡沫的孔结构变化趋势。发泡温度的升高导致成核密度增加, 同时导致气体在胶质体的溶解度降低, 不利于孔胞长大。发泡压力的增大导致炭泡沫的孔胞密度增加, 临界成核半径降低, 同时加剧了热聚合反应, 导致胶质体的粘度增大, 不利于孔胞长大。发泡时间的延长会使热聚合更加充分, 影响胶质体粘度, 进而影响孔结构。     

Effect of carrier fluid viscosity on retention time and resolution in gravitational field-flow fractionation

Gravitational field-flow fractionation (GrFFF) is a useful technique for fast separation of micrometer-sized particles. Different sized particles are carried at different velocities by a flow of fluid along an unobstructed thin channel, resulting in a size-based separation. They are confined to thin focused layers in the channel thickness where force due to gravity is exactly opposed by hydrodynamic lift forces (HLF). It has been reported that the HLF are a function of various parameters including the flow rate (or shear rate), the size of the particles, and the density and viscosity of the liquid. The dependence of HLF on these parameters offers a means of altering the equilibrium transverse positions of the particles in GrFFF, and hence their elution times. In this study, the effect of the viscosity of the carrier fluid on the elution behavior (retention, zone broadening, and resolution) of micrometer-sized particles in GrFFF was investigated using polystyrene (PS) latex beads as model particles. In order to change the carrier liquid viscosity without affecting its density, various amounts of (hydroxypropyl) methyl cellulose (HPMC) were added to the aqueous carrier liquid. It was found that particles migrate at faster rates as the carrier viscosity is increased, which confirms the dependence of HLF on viscosity. At the same time, particle size selectivity decreased but peak shape and symmetry for the more strongly retained particles improved. As a result, separation was improved in terms of both the separation time and resolution with increase of carrier viscosity. A theoretical model for plate height in GrFFF is also presented, and its predictions are compared to experimentally measured values.     

Dynamic response and simulations of nanoparticle-enhanced composites

The dynamic modulus and damping, low-velocity impact and high-strain (Hopkinson bar) response of nanoparticle-enhanced composites and fly ash based fire resistant structural foams have been characterized. Molecular Dynamic (MD) simulations were also used for obtaining the elastic constants for different matrices reinforced with single and multi-wall carbon nanotubes (MWCNT). Experimental results on the dynamic and impact response of nylon 6,6 with MWCNT, vinyl ester with nanoclay and graphite platelets, and Eco-Core^® foams are presented; along with some of the predictions from molecular simulations.     

The wet Kelvin model for air flow through open-cell polyurethane foams

Computational Fluid Dynamics (CFD) was used to compute the air-flow permeability K for laminar flow, for wet Kelvin foam models, as a function of cell size and cell face hole size. The predictions were compared with experimental data for a range of open-cell polyurethane (PU) foams. This suggests that the foam permeability is a function of the area of largest hole in the cells. The predictions are almost the same as those for dry Kelvin foams, showing that the face hole size and cell size are the main factors that determines foam permeability.     

Investigation on the influence of cell shape anisotropy on the mechanical performance of closed cell aluminium foams using micro-computed tomography

The mechanical behaviour of closed-cell aluminium foams made by both powder metallurgy (LKR) and liquid state (Hydro) processes is investigated. Hydro foams exhibit a significant anisotropy in their mechanical behaviour. The transverse direction stands out as the most favourable one in terms of strength. In contrast, LKR foams show an almost isotropic compressive behaviour. Both foams perform at a level far below the theoretical predictions. The reduced values are a result of imperfections and defects in the cellular microstructure. X-ray microfocus computed tomography (μCT) is therefore used for internal investigation of the foam cell structure. 2D and 3D quantitative image analyses have been performed on μCT images to characterise the morphometric parameters of the foams. The main parameters of interest are cell size, cell size distribution and cell features information. A preferred cell orientation in Hydro foams is observed along the normal and the transverse directions of the specimen. This cell shape anisotropy is quantified using the dimensions of the three axes of the equivalent ellipsoids. The orientation of the cells is well characterised by pole figures of the three axes of equivalent ellipsoids. The influence of this geometrical anisotropy on the mechanical behaviour of the foam is discussed.