金属泡沫填充薄壁圆管的轴压载荷-位移关系

将泡沫填充圆管的能量吸收视为泡沫与圆管两者之和, 基于包含偏心率效应的直链塑性铰模型和Reddy等对Alexander模型的改进结果, 对圆管的变形模式进行了更改, 以此来反映管壁与金属泡沫之间的相互作用效应, 导出了金属泡沫填充圆管的静、动态轴向平均压溃力的表达式. 通过理论预测与实验的对比, 发现理论预测偏低, 但与实验曲线的趋势保持一致, 比空管与金属泡沫的平均载荷之和略高一些. 此外, 泡沫填充圆管的平均压溃力随填充泡沫平台应力的增大而呈线性增加, 与已有研究结果及实际情况一致, 由此表明了模型的合理性.      

The role of plastic deformation on the impact behaviour of high aspect ratio aluminium foam-filled sections

The main objective of this work is to investigate the role of the plastic deformation of metal foams on the dynamic behaviour of aluminium foam-filled columns with respect to their energy absorbing capabilities. The influence of the cross-section shape as well as other parameters is thoroughly studied. A comparison with correspondent hollow-sections is performed concerning the dissipation of kinetic energy and the obtained deformed profiles. For this particular purpose, three-dimensional finite element modelling dynamic analyses are carried out using ABAQUS/Explicit in order to achieve an in-depth study of the structural crash behaviour, during which energy needs to be absorbed in a controlled manner. A comprehensive numerical study of the crush behaviour of aluminium foam-filled sections undergoing axial compressive loading is performed. The results obtained are also analysed with respect to the reduction in the length of the structural element and impact time, the effect of friction between the foam and the outer skin, the energy decomposition, the role of plastic deformation, the influence of the skin material and impact velocity, and the influence of the shape of the cross-section on the impact behaviour. A comparison with existing analytical expressions is made in order to corroborate the numerical results.     

Partition energy absorption of axially crushed aluminum foam-filled hat sections

The “interaction effect” between aluminum foam and metal column that takes place when foam-filled hat sections (top-hats and double-hats) are axially crushed was investigated in this paper. Based on experimental examination, numerical simulation and analytical models, a systemic approach was developed to partition the energy absorption quantitatively into the foam filler component and the hat section component, and the relative contribution of each component to the overall interaction effect was therefore evaluated. Careful observation of the collapse profile found that the crushed foam filler could be further divided into two main energy-dissipation regions: densified region and extremely densified region. The volume reduction and volumetric strain of each region were empirically estimated. An analytical model pertinent to the collapse profile was thereafter proposed to find the more precise relationship between the volume reduction and volumetric strain of the foam filler. Combined the superfolding element model for hat sections with the current model according to the coupled method, each component energy absorption was subsequently derived, and the influence of some controlling factors was discussed. According to the finite element analysis and the theoretical modeling, when filled with foam, energy absorption was found to be increased both in the hat section and the foam filler, whereas the latter contributes predominantly to the interaction effect. The formation of the extremely densified region in the foam filler accounts for this effect.     

内部填充泡沫铝的柱壳力学响应数值模拟

针对泡沫铝及其填充圆柱壳的变形模式复杂、理论分析困难的问题,在分析泡沫金属唯象本构模型的基础上,用Bilkhu/Dubois可压缩泡沫模型描述泡沫铝的力学行为,用随机几何缺陷描述结构的可能缺陷形式,采用有限元法对内部填充泡沫铝的圆柱壳结构在轴向载荷作用下的静、动态力学响应进行了数值模拟.数值模拟结果与实验结果较一致,表...     

泡沫铝夹芯双方管结构准静态轴压性能的实验研究

对几种泡沫铝夹芯方管结构的准静态轴压性能进行了实验研究。结果表明,较空管及泡沫铝夹芯单管结构,泡沫铝夹芯双管结构的承载能力及能量吸收效率明显得到增强。双管夹芯结构中外管撕裂模式的结构行程利用率和能量吸收效率高于相应的外管周期折叠模式。而泡沫铝四角填充方式提高了双管夹芯结构相同变形模式下的能量吸收效率和结构变形的稳定性。同时研究了内管管壁及材料强度对泡沫铝夹芯双管结构的影响。随着内管壁厚增大,双管夹芯结构的能量吸收效率提高,而内管材料强度影响不明显。     

泡沫铝填充金属薄壁圆管侧向压缩的实验和理论

利用实验研究与理论分析相结合方法研究了泡沫铝填充金属薄壁圆管在准静态侧向压缩下的力学响应。基于能量法,建立了泡沫铝填充圆管和金属薄壁圆管在侧向均匀压缩时的瞬时侧向力、平均侧向力和总吸能的理论公式。对泡沫铝填充管与金属薄壁圆管进行了准静态侧向压缩实验,并且将实验结果与理论公式进行了对比,结果表明理论预测值与实验结果吻合较好。基于建立的理论分析模型,研究了管的几何尺寸以及泡沫铝材料的密度对结构的瞬时侧向力、平均侧向力、总吸能和比吸能的影响。结果表明,在准静态侧向压缩下,泡沫铝填充管的总吸能大于对应的金属薄壁圆管;泡沫铝填充管的侧向压缩力和总吸能随管长度、壁厚和直径的增加而增大;当填充材料泡沫铝密度增大时,填充管的总吸能与侧向压缩力均增加。     

开孔泡沫铝填充圆管的准静态压缩行为

采用开孔结构泡沫铝填充到薄壁圆形铝管中,制备出开孔泡沫铝夹芯铝管,并进行压缩实验,研究了这种结构材料的压缩力学行为和变形特征以及材料的结构特征参数对压缩力学性能和能量吸收特性的影响。在压缩过程中,泡沫铝夹芯铝管的载荷-位移曲线呈现出弹性段、波动的屈服平台段和压实段3个阶段特征;铝管的径厚比及泡沫铝本身的参数和强度对填充管的屈服强度、平均压溃力和吸能特性均有着非常显著的影响。填充泡沫铝后铝管的压缩变形方式发生改变,管壁只发生向外翻折变形,产生的环状褶皱减少。     

泡沫铝夹芯双圆管结构的准静态轴向压缩性能研究

对泡沫铝夹芯双圆管结构的准静态轴压性能进行了实验研究,发现该新型结构的比质量吸能效率远远高于传统的泡沫铝夹芯单管,并接近甚至超过相应的空管结构;其内外管变形模式均与空管不同且受内外管组合的影响.本文讨论了它的变形机理,分析了外管壁厚对其压缩行为的影响,发现增大外管壁厚有利于增大结构的行程利用率,提高结构的比质量能量吸收...     

泡沫子弹冲击固支单梁的耦合分析模型

使用泡沫金属子弹进行冲击可以模拟爆炸载荷的作用,这一加载技术已被应用于防护结构的抗冲击性能测试中,然而泡沫子弹作用于被测试结构上的真实载荷以及二者间的相互作用过程尚不明晰.本文以泡沫子弹冲击固支梁的情形为例,开展了对该冲击过程的理论分析和数值模拟研究.基于泡沫材料的冲击波模型与固支单梁的结构冲击动力学响应模型,构建了描述泡沫子弹冲击固支梁过程的耦合分析模型.给出了不同响应阶段下子弹和单梁的动力学控制方程,并采用Runge-Kutta方法得到了方程的数值解.基于三维Voronoi技术,建立了泡沫子弹冲击固支单梁的有限元模型并进行了数值模拟.通过与有限元模拟结果的对比发现,相较于经典的脉冲加载模型,耦合分析模型能更好地预测泡沫子弹和单梁的速度变化规律,也能准确地预测子弹对单梁的真实冲击压强.当泡沫子弹的初始动量相同时,由于子弹自身的压溃行为,子弹的初始冲击速度、密度和长度的改变都会对冲击过程产生影响.最后,通过耦合分析模型分别分析了泡沫子弹的密度、长度、初速度对冲击压强的峰值、衰减速度和持续时间的影响,并针对具有不同特征的目标模拟载荷给出了泡沫子弹的筛选策略.所构建的耦合分析模型为研究泡沫...     

Aluminum foam-filled extrusions subjected to oblique loading: experimental and numerical study

The behavior of empty and foam-filled square thin-walled aluminum columns in alloy AA6060 subjected to quasi-static oblique loading was examined. Previous studies have shown that by introducing a load angle, the energy absorption decreases drastically compared to axial loading. One of the main objectives of the present investigation was to study the effect of introducing aluminum foam filler on the energy absorption. The square columns were clamped at one end and oblique load conditions were realized at the other end by applying a force with different angles to the centerline of the column. An experimental program was carried out where the main parameters were load angle, foam density and heat treatment of the extrusion material. Additionally, numerical analyses of the experiments were performed, mainly to verify a numerical model of the obliquely loaded foam-filled columns. The foam was modeled with the foam model of Deshpande and Fleck, together with a simple fracture criterion, which previously has been implemented as a user subroutine in LS-DYNA. The study shows that high-density aluminum foam filler increases the energy absorption considerably, but the specific energy absorption is lowered compared to the empty cross sections. Furthermore, the numerical analyses were able to predict the experimental results with reasonable accuracy.     

填充式泡沫铝防护结构的弹道极限

为了探索泡沫材料在空间防护上的应用,结合实验和数值模拟,绘制了弹速0.5~7.0km/s范围 内填充式泡沫铝防护结构的弹道极限曲线,并与Whipple结构进行了比较。结果表明:填充式泡沫铝防护结 构的防护性能优于同等面密度、总厚度的Whipple结构;孔隙率对填充式泡沫铝防护结构防护性能影响较小。     

泡沫铝合金填充圆管三点弯曲实验研究

用实验方法研究了三种不同管壁厚度、两种跨径的泡沫铝合金填充圆管的三点弯曲力学性能,得到了泡沫铝合金填充管结构承载过程中的三种变形模式,即压入、压入弯曲和管壁下缘拉裂破坏。给出了空管和泡沫铝合金填充管的载荷位移曲线,并进行了比较。实验发现泡沫铝合金填充管结构的承载能力随泡沫铝合金密度的增大而增大,但破坏应变则随之减小。结构承载力的相对提高量随着管壁厚度的减小和跨径的增大而增大。此外,分析了泡沫铝合金提高填充管结构承载能力的机理。泡沫铝合金填充使管壁压入量和管截面抗弯刚度的损失显著减小,从而提高了结构的抗弯能力。     

Size effects in the constrained deformation of metallic foams

The constrained deformation of an aluminium alloy foam sandwiched between steel substrates has been investigated. The sandwich plates are subjected to through-thickness shear and normal loading, and it is found that the face sheets constrain the foam against plastic deformation and result in a size effect: the yield strength increases with diminishing thickness of foam layer. The strain distribution across the foam core has been measured by a visual strain mapping technique, and a boundary layer of reduced straining was observed adjacent to the face sheets. The deformation response of the aluminium foam layer was modelled by the elastic-plastic finite element analysis of regular and irregular two dimensional honeycombs, bonded to rigid face sheets; in the simulations, the rotation of the boundary nodes of the cell-wall beam elements was set to zero to simulate full constraint from the rigid face sheets. It is found that the regular honeycomb under-estimates the size effect whereas the irregular honeycomb provides a faithful representation of both the observed size effect and the observed strain profile through the foam layer. Additionally, a compressible version of the Fleck-Hutchinson strain gradient theory was used to predict the size effect; by identifying the cell edge length as the relevant microstructural length scale the strain gradient model is able to reproduce the observed strain profiles across the layer and the thickness dependence of strength.     

Influence of geometrical parameters on the mode of collapse of a “pinched” rigid-plastic cylindrical shell

The modes of collapse of a rigid-plastic circular cylindrical shell subjected to centrally applied, opposed point loads in which the deformation is not confined to the vicinity of the loaded points are analysed. It is shown that the solution is analogous to that of an equivalent beam-foundation problem and the analogy is discussed. The results are compared with observations made during a series of experiments concerned with the crushing of aluminium tubes by opposed indenters.     

A constitutive model of aluminum foam for crash simulations

A new constitutive model for metallic foams is developed to overcome the deficiency of existing models in commercial finite element codes. The proposed constitutive model accounts for volume changes under hydrostatic compression and combines the hydrostatic pressure and von Mises stress into one yield function. The change of the compressibility of the metallic foam is handled in the constitutive model by allowing for shape changes of the yield surface in the hydrostatic pressure-von Mises stress space. The backward Euler method is adopted to integrate the constitutive equations to achieve numerical accuracy and stability. The model is implemented into LS-DYNA as a user-defined subroutine, verified with existing solutions, and validated with foam testing data. The verified and validated model is then utilized in the crushing simulations of foam-filled columns with square and hexagonal cross-sections. Two constitutive models are studied: the first using an exponential function to describe the relationship of plastic Poisson's ratio with respect to true strain and the second using linear interpolation function as an alternative approximation. The new foam model provides satisfactory prediction of crushing forces and deformed shapes compared to experimental results. Additionally, the new foam model was shown to have better numerical stability and accuracy than existing LS-DYNA built-in material models.     

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。     

中低速压缩加载下不同截面构型复合材料薄壁结构吸能特性及失效分析

为研究开剖面复合材料薄壁吸能结构的吸能特性,基于高速液压伺服试验系统,开展了开剖面复合材料薄壁结构轴向压缩试验,分析了截面构型、截面长宽比、触发模式及加载速度对其吸能特性的影响,揭示了其在压溃过程中的失效及吸能机理。研究结果表明,复合材料薄壁结构压溃过程中主要通过材料弯曲、分层、剪切破坏以及压溃区之间的摩擦吸能。截面构型对其吸能特性影响显著,其中,帽形及Ω形试件的平均压溃载荷较C形试件分别高出14.1%和14.6%,比吸能较C形试件分别高出14.3%和14.8%;截面长宽比对复合材料薄壁结构吸能特性的影响不如截面构型明显;触发模式主要影响吸能结构的初始压溃阶段,在降低峰值载荷方面,C形试件采用45°倒角触发效果更好,帽形试件采用15°尖顶触发效果更好;当加载速度从0.01 m/s提高到1 m/s时,C形、帽形及Ω形试件的平均压溃载荷分别下降了6.1%、10.9%和6.1%,比吸能分别下降了6.2%、11.0%和6.2%。     

薄壁管及其泡沫金属填充结构耐撞性的实验研究

对两种AA 6063T6铝合金薄壁空管(方/圆管)结构以及填充泡沫铝的5种不同几何截面的薄壁夹芯管(单方/圆管填充,双方/圆管填充,双方管四角填充结构)分别进行了准静态轴向压缩实验,研究了各种薄壁结构的变形模式和吸能性能,比较了反映不同结构耐撞性的各种参数,如比能量吸收和能量吸收效率因子等。同时,研究了各种填充结构的几何参数对结构耐撞性能的影响,发现填充结构内管的尺寸对结构的耐撞性影响显著。研究结果显示,圆管类型的结构平均压垮载荷、比质量能量吸收、单位行程能量吸收以及能量吸收效率因子都较方管类型结构高。泡沫填充单/双圆管结构由于其较高的压垮力效率和能量吸收效率,能够较平稳高效地吸能,作为耐撞性结构元件具有很大的优势。     

泡沫材料的应变率效应

对泡沫材料的应变率敏感性进行了系统深入的讨论,认定这种材料是应变率敏感材料,这种敏感性主要是由于泡孔的变形特性产生的。泡沫材料变形的局部化、微观惯性和致密性导致压垮应力明显提高,基体的应变率效应及泡孔的形状大小并不能对泡沫材料应变率敏感性起主导作用。     

Energy absorption of foam-filled circular tubes with braided composite walls

Braided glass-fibre/epoxy circular tubes with polymer foam cores are loaded in tension and in compression, and the energy of deformation is measured. Theoretical models of tube deformation are developed, and are used to predict the energy absorption as a function of tube wall strength, the ratio of tube wall thickness to tube diameter, and the density of the foam. The energy per unit mass and energy per unit volume are optimised with respect to the relative density and geometry. It is found that foam-filled braided circular tubes exhibit promising energy absorbing characteristics, due to a combination of energy absorption by the polymeric foam core and by the glass/epoxy braided tube.