中等应变率下泡沫铝的吸能特性

进行了不同密度、高度和压缩方向下泡沫铝的准静态压缩试验和中等应变率下(＜100 s-1)的冲击试验,研究了具有不同密度的闭孔泡沫铝在准静态压缩和冲击工况下的吸能特性.结果表明,泡沫铝是一种近似的各向同性结构,具有较高的单位质量吸能特性,是一种较好的吸能材料.在准静态和中等应变率冲击条件下,泡沫铝对应变率不敏感,其应力应变关系与应变率关系不大.不同的泡沫铝,其平台应力与密度之间的关系不同,在研究其性能时,必须测量应力-应变关系.泡沫铝的致密区对其吸能特性有很大的影响.     

泡沫铝芯三明治板材U型弯曲成形试验研究

研究了泡沫铝芯三明治板材U型弯曲工艺,建立了冲压弯曲试验系统,给出了泡沫铝芯三明治板材弯曲变形模式和载荷位移曲线。综合运用试验、塑性力学理论分析了三明治板材冲压弯曲宏微观协调变形机制,以及泡沫铝三明治板材冲压成形板面-泡沫铝芯间界面剥离、圆角半径处过度减薄、泡沫铝芯剪应力裂纹等主要成形缺陷。探讨了压边力和冲压成形板厚的控制规律。     

Experimental and numerical studies of dynamic axial compression of thin walled spherical shells

Axial compression experiments on aluminium spherical shells of radius to mean thickness ratio (R/t) values between 13 and 85 were conducted on a gravity drop hammer setup. Typical histories of their deformation, variation of shell thickness along the meridian, load–compression curves, energy absorbing capacity and mean collapse loads obtained from the experiments are presented. Influence of the R/t values of the shell on their modes of collapse and energy absorption capacities are discussed. The shells are numerically simulated and analysed in detail by using the finite element code FORGE2. The material was modelled as rigid-viscoplastic. The experimental and computed results are compared. Typical contours of equivalent strain, equivalent strain rate, different stress components and velocity distribution are presented. The dynamic response of the shells is compared with their static response.     

Compressive behaviour of closed-cell aluminium foams at high strain rates

It has been well established that ALPORAS® foams is a strain rate sensitive material. However, the strain rate effect is not well quantified as it is not unusual for strain rate to vary during high speed compression. Moreover, according to previous research, aluminium foams, especially ALPORAS® foams, behave differently at low and high strain rates. Therefore, different plastic deformation mechanisms are expected for low and high strain rate loadings as a result of micro-inertia of cell walls. In this paper, the strain rate effect on the energy dissipation capacity of ALPORAS® foam was investigated experimentally by using a High Rate Instron Test System, with cross-head speed up to 10 m/s. The compressive tests were conducted over strain rates in the range of 1 × 10^?3 to 2.2 × 10² s^?1, with each test being at a fairly constant strain rate. An energy efficiency method was adopted to obtain the densification strain and plateau stress. The effect of strain rate and the foam density was well presented by empirical constitutive models. The experimental data were also discussed with reference to the recent results by other researchers but with different range of strain rates. An attempt has been made to qualitatively explain the observed decrease of densification strain with strain rate.     

Modelling of aluminium foam core sandwich panels under impact perforation

Aluminium foam core sandwich panels are good energy absorbers for impact protection applications, such as light-weight structural panels, packing materials and energy absorbing devices. In this study, the high-velocity impact perforation of aluminium foam core sandwich structures was analysed. Sandwich panels with 1100 aluminium face-sheets and closed-cell A356 aluminium alloy foam core were modelled by three-dimensional finite element models. The models were validated with experimental tests by comparing numerical and experimental damage modes, output velocity, ballistic limit and absorbed energy. By this model the influence of foam core and face-sheet thicknesses on the behaviour of the sandwich panel under impact perforation was evaluated.     

Finite element analysis of closed-cell aluminium foam under quasi-static loading

Closed-cell aluminium foam was represented with a new type of repeating unit-cell (RUC) constructed from the tetrakaidecahedra structure. After that the thin shell geometrical parameters and material properties of aluminium foam were assigned to this unit-cell. Finite element studies were then conducted to evaluate the stiffness and mechanical response of this model under large strain. Our results are compared to cruciform-pyramidal and cubic-spherical unit-cell foam models that describe the load and global deformation response in-terms of unit-cell structure. We demonstrate that the plateau phase stress–strain characteristics of our model are more representative of real aluminium foam. It was also found that the crushing resistance and energy absorption capability of tetrakaidecahedral foam was higher than the cruciform-pyramidal and cubic-spherical foam models.     

Experiments on curved sandwich panels under blast loading

In this paper curved sandwich panels with two aluminium face sheets and an aluminium foam core under air blast loadings were investigated experimentally. Specimens with two values of radius of curvature and different core/face sheet configurations were tested for three blast intensities. All the four edges of the panels were fully clamped. The experiments were carried out by a four-cable ballistic pendulum with corresponding sensors. Impulse acting on the front face of the assembly, deflection history at the centre of back face sheet, and strain history at some characteristic points on the back face were obtained. Then the deformation/failure modes of specimens were classified and analysed systematically. The experimental data show that the initial curvature of a curved sandwich panel may change the deformation/collapse mode with an extended range for bending dominated deformation, which suggests that the performance of the sandwich shell structures may exceed that of both their equivalent solid counterpart and a flat sandwich plate.     

The Collapse Response of Sandwich Beams with Aluminium Face Sheets and a Metal Foam Core

Plastic collapse modes of simply supported and clamped sandwich beams have been investigated experimentally and theoretically, for aluminium face sheets and Alporas foam core. The effect of clamped boundary conditions is to induce axial stretching after the initial yield mechanism. Hence, face sheet ductility dictates the level of energy absorption of the beam. Numerical and analytical predictions are validated by the available experimental evidence.     

Crushing of axially compressed steel tubes filled with aluminium foam

Summary This study, with the emphasis on experiments, investigates the applicability of aluminium foam as filler material in tubes made of mild steel having square or circular cross sections, which are crushed axially at low loading velocities. In addition to the experiments finite element studies are performed to simulate the crushing behaviour of the tested square tubes, were a crushable foam material model is shown to be suitable for describing the inelastic response of aluminium foam with respect to the considered problems. The experimental results for the square tubes reveal efficiency improvements with respect to energy absorption of up to 60%, resulting from changed buckling modes of the tubes and energy dissipation during the compression of the foam material itself. The principal features as well as the changes of the crushing process due to filling can also be studied by the numerical simulations. A global failure mechanism due to a high foam density can be observed for filled circular tubes. Aluminium foam is shown to be a suitable material for filling thin-walled tubular steel structures, holding the potential of enhancing the energy absorption capacity considerably, provided the plastic buckling remains characterized by local modes.Dedicated to Prof. Dr. Dr. h. c. Franz Ziegler on the occasion of his 60th birthday     

Experimental and numerical studies of impact axial compression of thin-walled conical shells

Impact axial compression experiments on aluminium conical shells of semi-apical angles varying from 6.84° to 65.35° and the mean diameter to thickness (D/t) ratios varying from 22.32 to 79.29 were conducted on a gravity drop hammer set-up. Typical histories of their deformation, variation of shell thickness along the length, load–deformation curves, energy absorbing capacity, and mean collapse loads obtained from the experiments are presented. Influence of the semi-apical angles, D/t ratios, thickness, depth, and top and bottom diameter values of the shell on their modes of collapse and energy absorption capacities are discussed. The shells are numerically simulated and analysed in detail by using the finite element code FORGE2. The material was modelled as rigid-viscoplastic. The experimental and computed results are compared. Typical contours of equivalent strain, equivalent strain rate, different stress components and velocity distribution are presented. The impact response of the shells is compared with their static response.     

Adaptation of aluminium foam properties by means of precipitation hardening

Abstract

Aluminium foam samples based on four aluminium alloys were investigated with respect to their reaction to heat treatments, namely precipitation hardening treatments. Foam samples were produced according to the powder compact foaming or Fraunhofer process. 6000 and 7000 series alloys containing significant amounts of copper (6061, 7075) were compared to members of the same group with lower copper content (6082, 7020) as matrix alloys. Comparison was based on strength values and failure modes as reflected in the stress–strain curves obtained in quasi-static compression tests. Measurements were performed on samples without heat treatment and samples subjected to different precipitation hardening treatments. To evaluate the influence of quench sensitivity, the quench rate was varied for the alloys 6082 and 7020 by using air and water as quenchants.     

织物增强相对复合材料冲击压缩性质的影响

分别采用材料试验机（MTS）和分离式霍普金斯压杆（SPHB）测试系统研究二维平纹层压复合材料（2DWCs）、三维正交复合材料（3DOWCs）和三维编织复合材料（3DBCs）在准静态和高应变率压缩载荷下的力学响应和破坏形态。通过应力-应变曲线研究三种织物结构增强树脂基复合材料的应变率效应、应变率敏感性和能量吸收性质。通过破坏形态研究三种织物增强树脂基复合材料的破坏机制。结果显示:三种复合材料都具有明显的应变率效应。面外压缩时,2DWCs的压缩刚度、强度及应变率敏感性都最高,而3DBCs最弱;但当应变率大于1 500 s-1后,3DBCs能量吸收能力最强,2DWCs能量吸收能力最弱;2DWCs增强相和3DOWCs增强相以剪切断裂形式为主,3DBCs增强相以"坍塌"压扁形式为主。面内压缩时,2DWCs的面内压缩刚度最大而面内能量吸收能力最弱,3DOWCs的面内压缩强度最高,3DBCs的断裂应变最大、面内能量吸收能力及应变率敏感性最高,2DWCs增强相以分层破坏为主,3DOWCs和3DBCs以端面膨胀破坏为主。      

泡沫铝硅合金动态压缩力学性能及吸能特性研究

利用分离式霍普金森压杆研究了泡沫铝硅合金的动态压缩力学性能,得到了应变率为1400s-1~2500s-1的动态应力-应变曲线,且与准静态压缩实验结果进行了对比,分析了应变率对泡沫铝硅材料压缩强度和吸能特性的影响。动态压缩实验过程中,针对泡沫铝硅合金的低阻抗特点,采用LC4铝压杆和半导体应变片改进了测试装置和方法,保证了实验结果的可靠性。结果表明：应变率对泡沫铝硅合金的流动应力有着明显的影响,其流动应力随着应变率的增大而增大;由于惯性效应和胞孔的坍塌,在弹性极限处应力出现波动,且波动应力随应变率的增大而增大。该文还讨论了泡沫铝硅合金在不同应变率下的吸能效率。     

Debonding in foam-core sandwich panels

The strain energy release rate is used to give a criterion for debonding in structural sandwich beams with isotropic faces and a foam core. The critical strain energy release rate of the interface is measured on double-shear specimens and the results of the debonding analysis are compared with experiments on sandwich beams with aluminium faces and foamed polyurethane cores. The analysis describes debonding failure well. Comparison of the load for bebonding with that for other failure modes shows that debonding occurs only if relatively large cracks exist at the interface between the face and the core.     

Fracture of open- and closed-cell metal foams

Two closed cell aluminium foams and one open cell nickel-chromium foam were subjected to microstructural characterization, in situ fracture tests and fractography. The failure process of the open cell foam was observed to be rather ductile, while that of the closed cell foams was found to be brittle. The ductility was related to the purity of the nickel chromium alloy, resulting in necking to be the dominant source of energy dissipation during failure. The brittleness of the closed cell foams was related to the presence of precipitates and particles in the cell wall microstructure, limiting the amount of plastic dissipation. The embrittling phases were traced back to the alloy composition, viscosity enhancing additions and foaming agent.     

The Influence of Finite Geometry and Material Properties on Mixed-Mode I/II Fracture of Aluminium

Numerical calculations have been carried out to assess the influence of both finite geometry effects as well as material properties on mixed mode fracture of aluminium. These effects have been studied in close connection to experimental data for two aluminium alloys found in the literature. Interactions between the crack tip and the outer boundary have, for one of these alloys, been quantified in two ways. Firstly, by evaluating a number of non-singular stress on mixed mode fracture have been examined within the framework of a recently suggested effective plastic strain criterion. The other alloy was addressed in order to furnish a limited investigation concerning the sensitivity of this criterion with respect to material properties. The main conclusions arrived at in this paper are: (i) Boundary induced constraints may relocate the transition between different operative fracture modes and hence be responsible for scatter of experimental achieved under different testing conditions. (ii) The two alloys under consideration were predicted to behave very differently due to variations in the flow behaviour. Different behaviour was also confirmed by the experimental results. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characterisation of Strength Behaviour of Aluminium Foam Sandwiches Under Static Load

Abstract: Cellular metals, particularly aluminium foams, are increasingly used in automotive, railcar and aircraft industries due to their advantages such as low density, comparatively high stiffness, noise damping or non‐flammability. From this point of view a new powder‐compact forming and foaming technique has been developed to manufacture 3D‐aluminium foam sandwich (AFS) parts for components of railcars without glued joint between the foamed core layer and both cover sheets. Three different stages of foam expansion have been analysed to describe the material properties. We have characterised samples made of plane AFS panels by compression, bending and shear tests. The shear strain is optically measured by digital image correlation to estimate the shear modulus of foamed sandwiches. Furthermore, these experimentally determined values and curves are the basis for the verification and optimisation of finite element models by design of experiments. As a result of this work, recommendations could be derived for improving technological parameters.     

An experimental study on energy absorbing structures made of fabric composites

Egg-box shaped energy absorbing structures made of fabric composites were fabricated to find out the compressive characteristics and energy absorption capacity. Various stacking sequences and boundary conditions (unconstrained and bonded) were examined to investigate the stress–strain curves during compression. Failure modes of composite egg-box panels were observed and investigated correlating each step of meaningful collapsing behaviour. In order to check out the possibility as an ideal energy absorbers foam filled composite egg-box panels were fabricated and tested. From the test results it was found that the foam filled composite egg-box panels had good energy absorption capacity with smooth stress–strain curves which resembles the ideal energy absorber. The energy absorption per unit mass of composite egg-box panels made of different types of material and stacking sequences was calculated and compared with.     

Aluminium Foam Sandwich (AFS) Ready for Market Introduction

A novel production process for aluminium foam sandwich panels (AFS) is described. As an example for a serial application of AFS a support for a mobile telescope arm on a small lorry is presented and discussed.     

Quasistatic and dynamic crushability of polymeric foams in rigid confinement

An approach was developed for investigating the crushability behavior of epoxy-based, low-density structural polymeric foam (initial bulk density 0.81 g/cm³ was used for test illustration) under quasistatic and high strain rate conditions in rigid confinement. Quasistatic crushability tests were conducted in a steel confinement cell using an MTS material testing system and the high strain rate (dynamic) crushability behavior was investigated by placing a foam specimen in a steel confinement tube and then loading the specimen using two different split Hopkinson pressure bar systems, namely, a magnesium bar and steel bar. The dynamic deformation characteristics were obtained using a multi-step incremental loading procedure. It was found that these foams exhibited large uniform inelastic deformation during the confined loading. It is verified that multi-step incremental loading can be used to construct the complete stress–strain response curve for the specimens under both quasistatic and dynamic loading conditions. A phenomenological constitutive model was then applied to parametrically describe the crushability response and to determine the rate sensitivity of the foams. The rate sensitivity of yield stress was found to be around three under rigid confinement.