高温处理后闭孔泡沫铝压缩性能及变形机理

目的 探究温度和孔隙率对闭孔泡沫铝材料压缩力学性能和变形机理的影响。方法 将孔隙率为84.3%～87.3%的泡沫铝试件在温度25～700 ℃内进行加热处理,对处理后的试样开展准静态压缩实验。结果 在准静态压缩条件下,闭孔泡沫铝材料在不同温度加热处理后的压缩应力–应变曲线均经历了3个阶段:弹性阶段、塑性平台阶段和密实阶段。孔隙率从87.3%减小到84.3%时,其弹性模量增大了44.4 MPa,屈服强度增大了0.39 MPa,平台应力增大了0.94 MPa。孔隙率为84.3%的泡沫铝,在25 ℃时,其弹性模量为141.4 MPa、屈服强度为4.25 MPa、平台应力为4.75 MPa;当加热温度为500 ℃时,弹性模量减小到了128.0 MPa、屈服强度减小到了4.22 MPa、平台应力减小到了4.51 MPa。结论 泡沫铝的弹性模量、抗压屈服强度和平台应力均随孔隙率的增加而减小;加热温度低于500 ℃以下时,泡沫铝材料力学性能变化很小,但屈服强度和弹性模量均小幅度降低;在压缩载荷下,泡沫铝的变形破坏模式呈现出先从试件铝基体较薄弱部分产生孔壁塑性变形、孔洞坍塌,并逐渐出现断裂压缩带,直至泡沫铝孔洞完全坍塌密实。     

闭孔泡沫铝缓冲性能及其变形失效机理研究

在闭孔泡沫铝的准静态压缩实验基础上,研究不同孔隙率下的力学性能和吸能性能,分析其压缩变形机理。结果表明,闭孔泡沫铝的压缩过程存在明显的3个阶段:线弹性阶段、塑性平台阶段和致密化阶段。随着孔隙率的增大,闭孔泡沫铝的屈服强度、弹性模量和压实应力均减小。在压缩过程中,吸能效率和理想吸能效率均是先上升后下降。孔隙率对吸能效率影响较大,对最大理想吸能效率影响不大。将理想吸能效率曲线和吸能效率曲线结合可以选择合适的缓冲材料,发挥其最佳吸能特性。闭孔泡沫铝在准静态压缩条件下有良好的塑性变形能力,变形呈逐层破坏的特征。     

应用SXR-CT技术研究闭孔泡沫铝微结构演化及变形分析

借助同步辐射硬X射线高强度、高能量、高准直、宽频谱以及可选能量等特点,对材料试件进行投影成像,并应用滤波反投影重建算法实现三维图像重建(Synchrotron X-Ray Computed Tomography).研究了闭孔泡沫铝在压缩过程中内部微结构的演化,得到了不同压缩状态下内部微结构图像,分析了闭孔泡沫铝在压缩过程中的变形及孔隙率变化.这些研究结果为泡沫铝制备工艺的改进和材料与结构的优化设计提供了有益的参考,并为泡沫铝压缩破坏机理的构建提供科学依据.     

不同高径比固体浮力材料的单轴压缩变形机制和能量耗散特征

将试验测试与数值模拟相结合,研究了不同高径比固体浮力材料在单轴压缩载荷作用下的变形机制和能量耗散特征。先使用MTS-45型万能材料试验机对五种不同高径比固体浮力材料试件进行单轴压缩试验,分析其力学响应特征和破坏模式;然后基于单轴压缩试验结果并使用ABAQUS有限元软件建立反映固体浮力材料宏观力学性能的数值分析模型,对比分析了不同高径比固体浮力材料在单轴压缩载荷作用下的变形演变机制和能量耗散历程。结果表明:固体浮力材料核心承载应力圆的扩展是压缩过程进入塑性平台阶段的一个标志,塑性平台阶段的变形特征以压缩膨胀为主。进入致密压实阶段后,随着高径比的增大试件变形由对称式双凹圆盘变形特征转变为非对称式滑移变形特征。高径比越小的固体浮力材料试件其破坏吸收的能量增加越快、峰值应力后的破坏越呈现出塑性剪切破坏特征。高径比越大,越呈现出压缩断裂破坏特征。     

碳纳米管增强开孔铝基复合泡沫的疲劳性能

采用原位化学气相沉积、短时球磨和填加造孔剂法相结合的工艺制备了碳纳米管(CNTs)/Al复合泡沫,研究了其在压缩-压缩循环载荷下的力学性能及失效机制。结果表明,CNTs/Al复合泡沫的应变-循环次数曲线经历线弹性、应变硬化及应变快速增长三个阶段。不同于泡沫铝的逐层坍塌变形失效模式,CNTs/Al复合泡沫疲劳失效的主要原因是大量剪切变形带的形成,试样出现快速的塑性变形。此外,CNTs含量为2.5wt%、孔隙率为60%的复合泡沫试样的疲劳强度相比于泡沫铝提高了92%。CNTs的均匀分布及增强相与基体材料之间良好的界面结合性保证了疲劳载荷能够以剪切力的形式从基体传递到CNTs上,使其充分发挥自身高强度、高韧性的特点,进而提高了疲劳性能。      

泡沫铝孔单元结构模型及压缩特性研究

泡沫金属是近年发展起来的一种新型结构功能材料.对熔体发泡法制备的泡沫铝进行压缩实验,测试了其压缩应力-应变曲线,探讨了压缩变形过程的机理.结果表明,泡沫铝压缩过程具有明显的三阶段变形特征,即弹性段、塑性变形平台段和压实段.压缩性能显示随泡沫铝孔隙率和孔径升高,屈服强度和压实强度下降,且随着颗粒增强剂SiCp的加入,泡沫铝强度得到较大提高.同时,初步建立起理想的孔单元结构模型,推导出孔隙率与孔结构(孔径、壁厚、孔分布)的关系.     

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。     

预制倒角对泡沫铝动态冲击变形及吸能的影响

为防止峰值过高导致的保护失效,对闭孔泡沫铝试件设置三种预制倒角(单倒角、双倒角、中部倒角),探究动态冲击过程中预制倒角对泡沫铝特征曲线、变形与吸能的影响。通过落锤设备进行低速冲击试验获取特征曲线,结合高速摄影设备、ARAMIS软件以及材料变形剖面分析了不同倒角试件的变形模式、能量吸收效率及其对峰值载荷的影响机理。结果表明:预制倒角使泡沫材料初始变形模式发生改变,有效降低了泡沫铝材料在动态冲击过程中的初始峰值载荷,其中单倒角降幅最为明显为26%;倒角种类决定平台区趋势及材料整体变形模式,中部倒角平台区呈硬化现象,载荷波动最明显;预置倒角对泡沫铝材料平台区的能量吸收效率无显著影响。     

泡沫铝环氧树脂互穿相复合材料压缩力学性能

通过一系列准静态压缩实验研究了纯泡沫铝、 纯环氧树脂及三种不同体积分数的空心玻璃微珠(HGB)泡沫铝-环氧树脂互穿相复合材料(IPC)等五种材料压缩的变形过程和破坏形貌, 分析了其破坏机制, 并对三种IPC进行了应力松弛实验。通过绘制应力-应变曲线, 分析了其变化规律, 得出了有效弹性模量、 屈服极限等力学性能及能量吸收特性。结果表明: 三种IPC的有效弹性模量、 屈服极限及比强度、 比刚度均较纯泡沫铝有较大的提高, 泡沫铝-环氧树脂的单位体积吸能率最大, 且吸能率随空心玻璃微珠体积分数的增加而减小。泡沫铝-环氧树脂IPC有效弹性模量的预测结果与实验值较为符合。应力松弛率随空心玻璃微珠体积分数增加而增大。     

泡沫铝填充薄壁铝合金多胞构件与单胞构件吸能性能研究

为研究泡沫铝填充薄壁铝合金多胞结构与单胞结构的吸能能力,该文基于有限元软件LS-DYNA建立了泡沫铝填充薄壁铝合金多胞结构与单胞结构的数值仿真。对经典薄壁圆管试验及泡沫铝填充薄壁圆管试验进行了数值模拟,分析表明该数值模型能够较好的模拟泡沫铝填充薄壁圆管在轴向冲击过程中的撞击力和变形发展。基于该模型对比研究了不同因素下泡沫铝填充薄壁铝合金多胞结构与单胞结构的轴向吸能特性,分析了其破坏模式、吸能机理和两者吸能效率。结果表明:在轴向冲击荷载作用下,泡沫铝填充薄壁铝合金的破坏模式为轴对称渐进折叠破坏模式,冲击力-位移曲线和变形模态图显示其变形过程分为3个阶段:弹性阶段、平台阶段和强化阶段。当冲击压缩距离为构件高度的80%时,7种不同参数下的泡沫铝填充薄壁铝合金多胞结构的吸能效率明显高于7种单胞结构,吸收的能量E和比吸能SEA都提高了50%以上,是一种优秀的吸能构件,可广泛应用于防护工程中。     

The mechanical behavior of foamed aluminum

Experiments have been carried out to investigate the mechanical behavior of foamed aluminum with different matrixes and states. It is found that the matrix composition has a significant influence over the deformation, failure and fracture of foamed aluminum. Like other cellular solid materials, Al foam shows a smooth compression stress–strain curve with three regions characteristic of plastic foams: linear elastic, plastic collapse and densification. AlMg10 foam has a serrated plateau and no densification, characteristic of brittle foams. AlMg10 foam has higher compressive and tensile strength but lower ductility than Al foam. The difference in the mechanical properties between Al foam and AlMg10 foam decreases as the relative density decreases, and when it is lower than roughly 0.15, no difference can be discerned. The mechanical properties in compression are clearly higher than those in tension, which can be explained in terms of dislocation theory and stress concentration behavior.     

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。      

Ultrasonic Torsion Welding of Sheet Metals to Cellular Metallic Materials

One of the most important requirements for finding new applications for cellular metals is to integrate them in complex technical structures. The metal foams have to be joined to each other, or to sheet materials, by suitable joining techniques. The main topics of this paper are the ultrasonic torsion welding of cellular metallic materials to sheet metals and the investigation of the mechanical properties of the joints. The basic materials of foams and sheet metals were different aluminum and iron alloys. Depending on the materials used, weldings with tensile shear strengths of up to 25 MPa were realized. Using aluminum foam sandwich (AFS) and sheet metals, successful weldings were performed before and after the foaming process. Furthermore, it was possible to perform a successful foaming process with the unfoamed AFS/sheet metal joints. Microscopic investigations showed that the ultrasonic welding technique allows the joining of the metal foams with sheet metals without significant deformation of the joining partners. The temperatures during the welding process in the interface were below the melting point of the foams and the sheet metals.     

Mechanical behavior of pure Al and Al–Mg syntactic foam composites containing glass cenospheres

Glass cenospheres were used as space holders for making aluminum matrix syntactic foams by pressure infiltration technique. The mechanical properties and failure behavior of cenospheres/Al syntactic foams with pure Al and Al–Mg alloys were investigated in the present work. The failure behavior of cenospheres in two syntactic foams was similar. However, the mechanical behavior of these two syntactic foams was different. Under compression process, the cenospheres/pure Al showed discontinuous shear band and drum shape, while cenospheres/Al–Mg exhibited continuous shear band and was divided by main shear zone. At the tensile state, the cenospheres in pure Al matrix syntactic foam debonded from the matrix, while the cenospheres in Al–Mg matrix syntactic foam was well-bonded and appeared to lamellar tearing. It is suggested that the difference of mechanical deformation behavior could be attributed to the matrix ductility and the forming of interfacial reaction product MgAl₂O₄ coatings.     

Compressive mechanical properties of closed-cell aluminum foam–polymer composites

The compressive mechanical properties of two kinds of closed-cell aluminum foam–polymer composites (aluminum–epoxy, aluminum–polyurethane) were studied. The nonhomogeneous deformation features of the composites are presented based on the deformation distributions measured by the digital image correlation (DIC) method. The strain fluctuations rapidly grow with an increase in the compressive load. The uneven level of the deformation for the aluminum–polyurethane composite is lower than that for the aluminum–epoxy composite. The region of the preferentially fractured aluminum cell wall can be predicted by the strain distributions in two directions. The mechanical properties of the composites are investigated and compared to those of the aluminum foams. The enhancement effect of the epoxy resin on the Young’s modulus, the Poisson’s ratio and the compressive strength of the aluminum foams is greater than that of the polyurethane resin.     

Semi-rigid biopolyurethane foams based on palm-oil polyol and reinforced with cellulose nanocrystals

In this study, water-blown biopolyurethane (BPU) foams based on palm oil were developed and cellulose nanocrystals (CNC) were incorporated to improve the mechanical properties of the foams. In addition, the foams were compared with petroleum polyurethane (PPU) foam. The foam properties and cellular morphology were characterized. The obtained results revealed that a low-density, semi-rigid BPU foam was prepared using a new formulation. CNC as an additive significantly improved the compressive strength from 54 to 117 kPa. Additionally, cyclic compression tests indicated that the addition of CNC increased the rigidity, leading to decreased deformation resilience. The dimensional stability of BPU foams was increased with increasing CNC concentration for both heating and freezing conditions.Therefore, the developed BPU nanocomposite foams are expected to have great potential as core material in composite sandwich panels as well as in other construction materials.     

闭孔泡沫铝压缩性能研究

闭孔泡沫铝作为一种新型多孔金属材料,被应用于各个领域,但其压缩力学性能受到孔隙率、孔洞结构参数、相对密度及材料基本力学性能等的影响,因此针对某闭孔泡沫铝企业研究出的一款新型产品,在确定其相关参数后进行10组试样的压缩力学试验,确定其应力-应变曲线,分析各段曲线意义和产生机理,并针对其特有的压缩力学性能,研究在外力作用下...     

Fabrication methods for auxetic foams

Auxetic materials have a negative Poisson's ratio, that is, they expand laterally when stretched longitudinally. Negative Poisson's ratio is an unusual property that affects many of the mechanical properties of the material, such as indentation resistance, compression, shear stiffness, and certain aspects of the dynamic performance. The unusual mechanical properties of auxetic foams are attributed to the deformation characteristics of re-entrant microstructures. One way of obtaining negative Poisson's ratio is by using a re-entrant cell structure. Auxetic foam was fabricated from a conventional polymeric foam. The fabrication method for making both small and large auxetic foam specimens is described. This revised version was published online in November 2006 with corrections to the Cover Date.