随机源对颗粒增强金属基复合材料力学性能影响的多尺度分析及实验验证

20%(体积分数)SiC_p/2009Al复合材料的准静态拉伸实验结果表明，其失效应变等力学参数具有明显的分散性。采用多尺度有限元的方法研究了两类随机源对20%SiC_p/2009Al宏观力学性能的影响。通过对一系列含随机源的细观多颗粒代表体积单元模型进行准静态拉伸模拟，得到了材料失效应变和断裂能各自的随机分布特征及两者之间的相关性特征，然后结合宏观试样模型，实现了实际材料内不同区域材料性能随机性的表征。研究表明，宏观有限元结果与实验数据的吻合性较好，且由于考虑了随机源的影响，宏观有限元结果还能够很好地模拟失效应变等的分散性特征。     

数值模拟中细观泡孔构型对硅橡胶泡沫材料力学性能的影响

基于有限元方法模拟准静态单轴压缩过程中硅橡胶泡沫材料的力学响应,并基于硅橡胶泡沫材料的泡孔特征,建立具有均匀分布且球孔尺寸一致的周期性物理模型,采用多尺度结合有限元数值方法进行模拟。根据已有的实验数据,拟合了实体硅橡胶材料的唯像本构方程。多尺度模拟研究中,建立了四种不同的细观泡孔单元,模拟了四种细观泡孔单元在准静态单轴压缩条件下的力学响应。模拟结果表明,四种细观泡孔单元均具有典型的超弹性力学特性,不同泡孔构型能造成细观模型力学响应的显著差异,四种细观泡孔单元中应力最大差值超过300%。细观模型的力学性能通过数据传递机制传输到均匀化的宏观模型后,采用有限元方法对宏观模型进行数值模拟。结果表明:宏观模型具有典型的超弹性力学特性,不同的细观泡孔构型能导致宏观模型力学响应的显著差异,其中应力最大差值超过300%。     

连续石墨纤维增强铝基复合材料准静态拉伸损伤演化与断裂力学行为

针对连续石墨纤维增强铝基(CF/Al)复合材料,采用三种纤维排布方式的代表体积单元(RVE)建立了其细观力学有限元模型,采用准静态拉伸试验与数值模拟结合的方法,研究了其在轴向拉伸载荷下的渐进损伤与断裂力学行为。结果表明,采用基体合金和纤维原位力学性能建立的细观力学有限元模型,对轴向拉伸弹性模量和极限强度的计算结果与实验结果吻合良好,而断裂应变计算值较实验结果偏低。轴向拉伸变形中首先出现界面和基体合金损伤现象,随应变增加界面发生失效并诱发基体合金的局部失效,最后复合材料因纤维发生失效而破坏,从而出现界面脱粘后纤维拔出与基体合金撕裂共存的微观形貌。细观力学有限元分析结果表明,在复合材料制备后纤维性能衰减而强度较低条件下,改变界面强度和刚度对复合材料轴向拉伸弹塑性力学行为的影响较小,复合材料中纤维强度水平是决定该复合材料轴向拉伸力学性能的主要因素。     

加载速率及其突变对混凝土压缩破坏影响的数值研究

混凝土动态力学行为具有明显的率相关性,探讨了加载速率及其突变对混凝土压缩破坏模式及宏观力学性能的影响。考虑混凝土细观结构非均质性的影响,从细观角度出发将混凝土看作由骨料、砂浆基质及过渡区界面组成的三相复合材料。采用耦合应变率效应的塑性损伤本构关系模型来描述砂浆基质及界面的动态力学行为;认为骨料不产生断裂破坏,为弹性体。采用Monte Carlo法建立了混凝土二维随机骨料模型,首先对Dilger等混凝土动态压缩试验进行数值模拟,数值与试验结果的良好吻合证明了方法的可行性及细观参数选取的准确性。进而探讨了细观组分应变率效应的影响,对比了混凝土非均质模型与宏观均匀模型的应变率效应,最后分析了软化阶段加载速率突变对混凝土破坏模式及宏观应力-应变关系的影响,并得到了一些有益结论。     

基于材料静动态力学性能分析的SiCp/Al本构模型构建

基于SiCp/Al复合材料的静态和动态力学性能分析,构建SiCp/Al复合材料的本构模型。针对体积分数为20%的SiCp/2a14Al复合材料采用电子万能试验机进行准静态拉伸试验,研究材料的静态力学性能;采用霍普金森压杆试验进行不同温度(20～400℃)、不同应变率(500～3 000 s^-1)动态压缩试验,分析材料的动态力学性能。基于材料静态及动态下应力-应变试验数据构建SiCp/Al复合材料的Johnson-Cook(JC)本构模型,并通过遗传算法对模型进行优化。结果表明,SiCp/Al复合材料在准静态条件下表现出应变强化效应;在动态载荷条件下,材料流动应力随着应变速率的增加而增加,表现出应变率强化效应,这与碳化硅颗粒的体积分数有关;随着温度的增加,流动应力减小,表现为温度软化效应。最小二乘法拟合的JC模型与试验值的平均误差较大,经过遗传算法优化后模型误差减小,能够准确预测SiCp/2a14Al复合材料的流变行为。     

炭黑增强橡胶复合材料的大变形细观力学模型

为考察炭黑对橡胶复合材料超弹性力学行为的影响,首先,利用不同填充体积分数的炭黑增强橡胶复合材料的准静态力学试验数据,对现有的基于均质化方法的"变形放大"细观力学模型的大变形表征能力进行了评估。其次,在此基础上提出了新的"第一不变量放大"关系,并获得了较为合理的预测结果。最后,利用随机序列吸附算法建立了较接近材料真实细观结构的球形颗粒填充数值模型,进行了大变形情况下的三维数值模拟;为考察颗粒聚集效应的影响,还设置了颗粒均匀随机分布和团聚随机分布两种形式。计算结果与试验数据的对照表明:提出的三维细观数值模型已经能在一定程度上预测填充橡胶的大变形宏观力学行为,且颗粒团聚随机分布模型的预测能力更好一些。试验结果验证了该模型的合理性,所建模型为进一步的相关研究提供了参考。     

基于同步辐射X射线的SiC颗粒/Al复合材料变形损伤

通过原位X射线成像系统研究了两种SiC粒径配比（45 μm和（45+100）μm）对70vol% SiC颗粒（SiC_P）/Al复合材料变形损伤行为的影响。在准静态压缩加载下,利用X射线数字图像相关方法（XDIC）计算了SiC_p/Al复合材料在不同变形阶段的应变场分布。宏观应力-应变曲线表明,因颗粒尺寸引起的SiC_p/Al复合材料的强度差异较小,但粒径配比为45 μm的SiC_P/Al的延展性明显优于（100+45）μm的SiC_P/Al。细观应变场分析表明,粒径配比为（100+45）μm的SiC_P/Al比45 μm的SiC_P/Al更早出现变形损伤带,且前者在变形后期其应变场不均匀性更高。这是由于（100+45）μm SiC_P/Al中更早在大颗粒附近出现应变集中点,而且这些集中点会迅速长大和汇聚进而形成宏观裂纹,导致材料更早失效和破坏。因此,减小颗粒尺寸和促进颗粒均匀分布有利于提高颗粒增强金属基复合材料的延展性。断口回收分析表明:两种颗粒尺寸的SiC_P/Al复合材料的断裂模式都属于脆性断裂,且断口中都发现有颗粒破坏和界面脱粘现象存在。      

基于细观单元等效化方法的混凝土动态破坏行为分析

混凝土材料具有明显的应变率效应,对其力学性质增强机理的认识还不统一。在细观随机骨料模型基础上,采用特征单元尺度划分试件网格,推导了考虑材料拉/压强度应变率效应的细观单元等效本构关系,建立了非均质混凝土材料的细观单元等效化数值模型。基于二维模型对Dilger等混凝土动态压缩试验进行了数值模拟,获得的数值结果与试验数据及随机骨料模型结果吻合良好,证明了细观单元等效化方法的准确性;进而对三维混凝土试件动态单轴拉伸和压缩破坏模式及宏观力学性质的加载速率效应进行了研究。数值结果表明：随着加载速率的增加,混凝土裂纹(损伤)数量增大,混凝土破坏将耗散更多的能量,是混凝土动态强度提高的主要原因。     

孔隙率变化规律及其对混凝土变形过程的影响

混凝土内部随机分布的微裂纹和孔洞等细观缺陷影响其破坏机理及宏观力学性能,且孔隙率随着外荷载的变化而不断变化。该文中将含缺陷(微裂纹、各种空隙等)混凝土复合材料简化为空心球力学模型,基于弹性力学理论推导并获得了混凝土当前孔隙率与材料初始孔隙率及体应变之间的定量关系;推导并得到了含孔隙混凝土的有效弹性模量、有效泊松比及峰值应变等与孔隙率的定量关系,进而得到了单轴加载条件下含孔隙混凝土细观单元的等效多折线本构关系模型。最后,采用细观单元等效化力学模型,研究了单轴加载(拉伸和压缩)情况下不同孔隙率混凝土材料的破坏过程及宏观力学性能,探讨了孔隙率变化规律及其对混凝土变形过程的影响。     

一种通用的计算复合材料刚度的有限元方法

根据复合材料宏观有效模量的定义,本文通过在复合材料细观模型的边界施加六组特定形式的均匀边界条件,以三维有限元作为数值分析手段,对各种细观模型及增强相力学特性,可一次性全部解出复合材料的所有弹性系数。通过计算典型代表体元细观模型,验证了本文数值方法的准确性和优越性。     

纳米粒子在SiC/PTFE复合材料中分散三维仿真与分形表征

为了从理论上探讨纳米粒子在基体材料中的分布规律, 以纳米SiC质量分数为3%、 5%、 7%、 9%的SiC/PTFE(聚四氟乙烯)复合材料为例, 根据纳米SiC的半径(25 nm)、 密度(3.2 g/cm³)、 质量分数和基体材料的密度(2.2 g/cm³), 以10^-12 g为质量单位、 25 nm:1像素为比例尺, 建立了纳米粒子在基体中均匀／偏聚分布的三维仿真模型, 基于其盒维数定量表征了不同团聚／偏聚程度的纳米粒子的分散度, 并进行了力学实验验证。结果表明: 均匀分布下随着纳米SiC粒子半径的不断增加, 或体积分数的不断减小, 其盒维数也逐渐减小; 当SiC粒子半径超过100 nm时, 不再具有分形特性。偏聚分布下随着纳米SiC粒子(半径为50 nm)间距的不断加大, 或体积分数的不断减小, 或层状、 线状、 团状分布的依次改变, 其盒维数也逐渐减小; 相同体积分数下偏聚分布的盒维数低于均匀分布; 当粒子间距超过450 nm时, 不再具有分形特性。均匀分布下纳米SiC/PTFE复合材料的力学性能测试结果与其三维仿真模型的盒维数线性相关(|R|>0.9)。盒维数可定量表征纳米粒子的分散度, 并可用于预测纳米复合材料的宏观性能。     

Mechanical modeling of incompressible particle-reinforced neo-Hookean composites based on numerical homogenization

In this paper, the mechanical response of incompressible particle-reinforced neo-Hookean composites (IPRNC) under general finite deformations is investigated numerically. Three-dimensional Representative Volume Element (RVE) models containing 27 non-overlapping identical randomly distributed spheres are created to represent neo-Hookean composites consisting of incompressible neo-Hookean elastomeric spheres embedded within another incompressible neo-Hookean elastomeric matrix. Four types of finite deformation (i.e., uniaxial tension, uniaxial compression, simple shear and general biaxial deformation) are simulated using the finite element method (FEM) and the RVE models with periodic boundary condition (PBC) enforced. The simulation results show that the overall mechanical response of the IPRNC can be well-predicted by another simple incompressible neo-Hookean model up to the deformation the FEM simulation can reach. It is also shown that the effective shear modulus of the IPRNC can be well-predicted as a function of both particle volume fraction and particle/matrix stiffness ratio, using the classical linear elastic estimation within the limit of current FEM software.     

Shape and position effects of double holes on lateral buckling of cantilever composite beams

One of the most important mechanical behavior of composite beams subjected to certain external loads and boundary conditions is lateral buckling. The effects of hole dimension, shape and position, and beam thickness on the lateral buckling behavior of woven fabric laminated composite cantilever beams, having two square or two circular holes, were investigated. Firstly, the theoretical, experimental and numerical critical buckling loads of the beams without holes were found and compared with each other. It was shown that there is a good agreement among the theoretical, experimental and numerical results. ANSYS finite element (FEM) program was used for the numerical analyses. Therefore, the numerical analysis of some models with various hole dimensions, shapes (square or circular) and fiber directions were done by changing distance between the holes. It is concluded that the circular holes are advantageous compared to the square ones in terms of lateral buckling behavior.     

A novel method to prepare hydrophobic poly(tetrafluoroethylene) membrane,and its properties

Hydrophobic poly(tetrafluoroethylene) (PTFE) flat-sheet membranes were prepared by a novel method with polyvinyl alcohol (PVA) as membrane carrier. Factors affecting PTFE membrane pore structure and mechanical properties such as sintering temperature, sintering state, and the simulate model were investigated. Results show that it was better to form pore structure at the first sintering stage. Morphologies of membranes were observed by scanning electronic microscopy (SEM). Membrane microvoids among the fibrils net nodes were loose by fixed setting sintering while compact by relax setting sintering. Chemical constitution and thermal stability of PTFE/PVA blend membrane after sintering (m-PTFE) and PTFE membrane were investigated, and no obvious difference in chemical constitution could be found between m-PTFE and PTFE membranes. The m-PTFE membranes obtained are still strong hydrophobic by the determination of contact angle to water.     

多相复合陶瓷刀具材料残余热应力的有限元模拟

以SiC和(W,Ti)C颗粒增强Al₂O₃多相复合陶瓷刀具材料为基础,运用有限元方法详细研究了材料内部残余热应力的大小与分布形态。计算结果发现,单元模型取法、弥散相颗粒大小、分布及其含量均对多相复合陶瓷刀具材料中的残余热应力有较大影响。基体内不仅存在拉应力区,而且存在不同程度和范围的压应力区,拉、压应力区的结构形式与弥散相颗粒的分布方式密切相关。研究表明,残余热应力的存在与材料的力学性能和微观结构有着密切的关系。      

Mechanical and Reaction Properties of Al/TiH2/PTFE under Quasi‐Static Compression

TiH₂ is introduced into Al/PTFE reaction system for the first time. In order to study the mechanical and reaction properties of Al/TiH₂/PTFE composites, five kinds of mixtures with different TiH₂ contents and the contrasting TiH₂/PTFE mixture are prepared. The true stress–strain curves and reaction phenomena are recorded during the quasi‐static compression experiments; The reaction residues are analyzed with XRD to understand the reaction mechanism. The results show that the introduction of TiH₂ can significantly influence the strength of the composites; With the increasing of TiH₂ content, the mechanical strength is firstly increase and then decrease; When the content is the same, the reinforcing effect of TiH₂ particles on PTFE matrix is greater than that of Al. Violent reaction are observed in the four Al/TiH₂/PTFE composites with the content of TiH₂ below 20%, but not in TiH₂/PTFE. Furthermore, compared with Al/PTFE material, special flames are found during the reaction of composites with TiH₂, which become even more obvious with the increasing of TiH₂ content. The material reaction mechanism denotes that the high temperature of the crack tip result in the reaction of Al and PTFE, which activates TiH₂ to release hydrogen and generate TiC, resulting TiH₂ to fully release its energy.

     

Development of semianalytical axisymmetric shell models with embedded sensors and actuators

This paper presents the development of two semianalytical axisymmetric shell finite element models, which have the possibility of having embedded and/or surface-bonded piezoelectric ring actuators and/or sensors. A mixed finite element approach is used, which combines the equivalent single-layer higher-order shear deformation theory, to represent the mechanical behavior with a layerwise discretization in the thickness direction to represent the distribution of the electrical potential of each piezoelectric layer of the frusta conical finite element. The electrical potential function is represented through a layerwise discretization in the thickness direction and can be assumed linear or quadratic with two or six electrical potential ring nodes per piezoelectric layer. The displacement field and the electrical potential are expanded by Fourier series in the circumferential direction, considering symmetric and anti-symmetric terms. Several examples are presented and discussed to illustrate the accuracy and capabilities of both models.     

Finite element simulation and experimental study on mechanical behavior of 3D woven glass fiber composite sandwich panels

The results of finite element simulation followed by an experimental study are presented in order to investigate the mechanical behavior of three-dimensional woven glass-fiber sandwich composites using FE method. Experimental load–displacement curves were obtained for flatwise compressive, edgewise compressive, shear, three-point bending and four-point bending loads on the specimens with three different core thicknesses in two principal directions of the sandwich panels, called warp and weft. A 3D finite element model is employed consisting of glass fabric and surrounding epoxy resin matrix in order to predict the mechanical behavior of such complex structures. Comparison between the finite element predictions and experimental data showed good agreement which implies that the FE simulation can be used instead of time-consuming experimental procedures to study the effect of different parameters on mechanical properties of the 3D woven sandwich composites.     

钨合金动态力学性能的三维数值模拟研究

采用数值模拟的方法对钨合金在冲击载荷作用下的动态力学响应进行了研究。运用有限元动力分析程序建立了具有典型微观结构钨合金三维有限元单胞模型,对钨合金在拉伸载荷作用下的动态力学性能进行了数值模拟研究,分析了应变率对其力学性能的影响,给出了不同应变率条件下单胞模型的应力和应变分布云图,在与实验结果对比的基础上验证了该有限元模型的可靠性。