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1.
玻璃态高分子材料银纹力学研究进展   总被引:1,自引:0,他引:1  
银纹是玻璃态高分子材料所特有的一种现象, 它既是高分子材料的塑性变形和增韧机理, 又 是高分子材料的损伤机理, 还是连接高分子材料微观损伤与宏观破坏的桥梁. 银纹的萌生、 长大与断裂是高分子科学和固体力学所共同关注的难题. 过去几十年在试验、理论和数值模诸方面也取得了显著进展, 但较为系统的理论框架尚未 建立, 人们试图把细观力学的基本知识与高分子系统银纹化现象联系起来的设想才刚刚开始, 且尚未在指导高分子合金设计方面起到关键作用. 本文试图对近10多年来, 高分子银纹化的 研究进展予以介绍和评述. 首先简要介绍银纹的基本形貌、萌生判据与生长规律, 然后分别介绍银纹断裂力学、银 纹损伤力学和银纹细观力学的研究进展和成果, 最后概要介绍银纹分子动力学研究的最新进 展.  相似文献   

2.
联合应用力法与逐段刚化法推导出具有半刚性节点性质的弹簧节点梁单元和平面刚架弹簧单元的单元刚度方程. 该方法具有物理概念清楚,推导过程简便、巧妙的特点. 本文所建立的弹簧节点梁单元和平面刚架弹簧单元可以应用在具有半刚性节点性质的工程结构的承载力分析与计算中.  相似文献   

3.
本文研究了高分子材料的非线性拟断裂特性。拟断裂分析的模型是一个具有中心银纹的矩形板,在拉伸应力场作用下。银纹内的取向纤维束在计算中用表面边界力来表示。这种表面应力与张开位移是非线性相关,所以应用非线性有限元进行分析。计算结果给出了银纹表面的应力分布、张开位移曲线和银纹顶点前沿的塑性场。与一般的断裂问题相比较,发现银纹顶点附近的应力分布并不存在奇异特性,要求的初始屈服载荷提高。在相同的载荷作用下,塑服区的尺寸也远比弹塑性断裂问题小得多。  相似文献   

4.
高聚物细观损伤演化的研究进展   总被引:12,自引:1,他引:11  
罗文波  杨挺青  张平 《力学进展》2001,31(2):264-275
聚合物的银纹化损伤与断裂是一个复杂和重要的研究课题。简 述了银纹引发的(热)力学条件和银纹成核的微观机理。结合最新的研 究进展,对银纹向前扩展的弯月面不稳定机理、银纹增厚的蠕变机理 和界面转入机理作了较详细的分析与综合。考虑银纹细观结构中横系 的作用,对银纹结构模型、银纹微纤断裂判据、微纤断裂行为的分子 量和缠结密度相关性以及银纹与裂纹相互作用等问题进行了较详细的 综述。指出银纹生长和断裂的深入研究可望建立材料宏观断裂韧性和 材料细观结构以及微观参数之间的关联,为进行材料韧性的微观设计 提供一条可行的途径。并对今后这一领域的研究方向和重点进行了展 望。  相似文献   

5.
二维高亚音速Laval喷管流场的有限元计算   总被引:3,自引:2,他引:1  
吕文朝 《力学季刊》1995,16(2):121-127
本文使用了八节点曲四边形等参元,通过变分有限元法,对二维高亚音速Laval喷管位势流场进行了计算,结果是满意的,并和一维结果进行了对比。文中还使用了有限元法特有的局部线化理论,取单元中心点密度去处理迭代过程中单元系数阵的计算,其结果与通常的有限元法计算相一致,但计算时间却大大减少了。  相似文献   

6.
针对含有间断的非均匀材料的断裂问题,本文将虚节点多边形单元的形函数引入到扩展有限元(XFEM)中,提出了一种基于四叉树结构的动态网格细化方法,该方法可对间断面附近单元实现可调控的多层级细化,特别是对于裂纹扩展问题,可实现裂尖附近单元的动态网格细化与粗化。基于以上网格细化方法,本文提出了针对非均匀材质裂纹扩展问题的计算方法VP-XFEM。为验证算法的准确性与计算效率,针对含有孔洞及材料界面的断裂问题,本文给出了相应的算例。结果显示,与传统的一致性网格的XFEM相比,VP-XFEM能够明显改善计算精度与计算效率。  相似文献   

7.
颗粒填充复合材料强韧化效应的力学分析   总被引:9,自引:0,他引:9  
卢锡年 《力学学报》1995,27(5):619-623
颗粒填充复合材料中基体微损伤形式对材料韧性产生决定性影响。本文通过二相或三相材料中基体应力分布的分析计算,结合损伤萌生的力学条件,对颗粒填充的强韧化效应作出定性分析。计算结果表明,若颗粒刚度高于基体,随着颗粒模量的提高,开裂与银纹趋势逐步增强,由此可知单纯使用硬粒子填充难以实现增韧,但若粒子与基体间有柔性界面相存在,基体屈服趋势将随界面厚度迅速增长,它将在损伤引发机制的竞争中占据优势,成为损伤的主导形式,并由此可成功地实现材料的强韧化。  相似文献   

8.
二维弹性接触问题的接触面单元法   总被引:3,自引:0,他引:3  
杨耀文  刘正兴 《力学季刊》1996,17(3):201-210
本文基于虚功原理,推导了二维接触面单元的刚度矩阵,并引进预留单元的概念,避免了接触过程中由于接触面变化,节点和单元需要重新编号的麻烦。采用位移和应力联合控制的增量法控制加载过程,文中给出考题验证计算方法的有效性,并给出本文方法在汽检轮机中根与轮缘接触问题中的应用实例。  相似文献   

9.
Delaunay多边形单元的有理函数插值格式   总被引:11,自引:0,他引:11  
王兆清  冯伟 《力学季刊》2004,25(3):375-381
本文提出了基于Delaunay多边形化的多边形单元有理函数插值格式。给出了Delaunay多边形化的概念和Delaunay多边形单元有理函数插值形函数的计算表达式。与Delaunay三角化网格不同,Delaunay多边形化网格形成对区域的唯一剖分。Delaunay多边形单元有理函数插值是以Delaunay多边形的顶点作为插值点,构造的有理函数形式插值。Delaunay多边形单元有理函数插值克服了有限元方法中难以构造边数大于4单元多项式形式位移插值的困难。有理函数插值形函数在多边形单元的内部是无穷次光滑的,在多边形的边界上是线性的。在三角形单元和矩形单元上,有理函数插值分别等价于有限元的三角形面积坐标插值和四边形双线性插值。给出了Delaunay多边形有理函数插值在圆域温度分布插值近似中的两个算例。  相似文献   

10.
针对目前在概念车身设计中比较难解决的接头模拟仿真问题,采用解耦方法来实现参数化接头的建立和详细接头单元的刚度阵的求解,计算并转化出能够更加贴近于真实情况的参数化接头。本文详细介绍的参数化接头主要是由扭簧单元和球铰单元相互作用组成,并且全面地描述了参数化接头的模拟过程。通过实验研究,计算具有接头的车身刚度和不考虑接头情况的车身刚度,并与详细车身模型的结果进行对比,发现具有接头单元的概念车身模型更加贴近于详细车身模型的扭转刚度。结果显示,考虑接头因素可以大大提高概念设计的精度,能够更加准确的预测车身的结构性能,为今后的车身设计积累经验。  相似文献   

11.
一种高聚物银纹损伤演化的实验研究   总被引:4,自引:0,他引:4  
采用光学显微技术对高聚物蠕变条件下的银纹损伤引发和演化进行了实验观测.通过对银纹面密度的测量,定义一种损伤变量,得到了聚甲基丙烯酸甲酯不同蠕变时间和不同应力水平下的银纹损伤值,并给出了一种损伤演化模型.  相似文献   

12.
In a previous thermo-mechanical analysis [Estevez, R., Basu, S., van der Giessen, E., 2005. Analysis of temperature effects near mode I cracks in glassy polymers. Int. J. Fract. 132, 249–273] in which shear yielding of the bulk and failure by crazing were accounted for, we examined which of these two viscoplastic processes contributed to heat in mode I fracture. The present study completes this work by investigating the conditions for thermo-elastic cooling prior to crack propagation as reported experimentally by Rittel [Rittel, D., 1998. Experimental investigation of transient thermo-elastic effects in dynamic fracture. Int. J. Solids Struct. 35, 2959–2973] and Bougaut and Rittel [Bougaut, O., Rittel, D., 2001. On crack tip cooling during dynamic crack propagation. Int. J. Solids Struct. 38, 2517–2532] on high strain rate loading of PMMA. To this end, coupled thermo-mechanical finite element simulations are carried out by accounting for the thermo-elastic source, in addition to the heat sources related to shear yielding and crazing. The bulk as well as cohesive zone parameters for crazing realistically describe PMMA as they are obtained from detailed calibration experiments. Our results show that if significant thermo-elastic cooling has to be observed in the vicinity of the crack tip of a polymeric material, suppression of shear yielding as well as suppression of crazing is necessary. It seems that at these high strain rates a brittle fracture mechanism activated at very high stresses takes over from crazing, or at least that craze initiation occurs for stress levels very different to those for quasi-static conditions.  相似文献   

13.
14.
The following article proposes a damage model that is implemented into a glassy, amorphous thermoplastic thermomechanical inelastic internal state variable framework. Internal state variable evolution equations are defined through thermodynamics, kinematics, and kinetics for isotropic damage arising from two different inclusion types: pores and particles. The damage arising from the particles and crazing is accounted for by three processes of damage: nucleation, growth, and coalescence. Nucleation is defined as the number density of voids/crazes with an associated internal state variable rate equation and is a function of stress state, molecular weight, fracture toughness, particle size, particle volume fraction, temperature, and strain rate. The damage growth is based upon a single void growing as an internal state variable rate equation that is a function of stress state, rate sensitivity, and strain rate. The coalescence internal state variable rate equation is an interactive term between voids and crazes and is a function of the nearest neighbor distance of voids/crazes and size of voids/crazes, temperature, and strain rate. The damage arising from the pre-existing voids employs the Cocks–Ashby void growth rule. The total damage progression is a summation of the damage volume fraction arising from particles and pores and subsequent crazing. The modeling results compare well to experimental findings garnered from the literature. Finally, this formulation can be readily implemented into a finite element analysis.  相似文献   

15.
In many glassy amorphous polymers, localisation of deformation during loading leads to crazes. Crazes are crack like features whose faces are bridged either by fibrils or a cellular network of voids and fibrils. While formation of crazes is aided by the presence of surface imperfections and embedded dust particles, in this work, we focus on intrinsic crazes that form spontaneously in the volume of the material. We perform carefully designed molecular dynamics simulations on well equilibrated samples of a model polymer with a view to gaining insights into certain incompletely understood aspects of the crazing process. These include genesis of the early nanovoids leading to craze nucleation, mechanisms of stabilising the cellular or fibrillar structure and the competition between chain scission and chain disentanglement in causing the final breakdown of the craze. Additionally, we identify and enumerate clusters of entanglement points with high functionality as effective topological constraints on macromolecular chains. We show that regions with low density of entanglement clusters serve as sites for nanovoid nucleation under high mean stress. Growth occurs by the repeated triggering of cavitation instabilities above a growing void. The growth of the void is aided by disentanglement in and flow of entanglements away from the cavitating region. Finally, for the chain lengths chosen, scission serves to supply short chains to the growing craze but breakdown occurs by complete disentanglement of the chains. In fact, most of the energy supplied to the material seems to be used in causing disentanglements and very little energy is required to create a stable fibril.  相似文献   

16.
为探讨陶瓷封装双列直插器件在焊接后出现开裂的问题,应用云纹干涉法和Twyman/Green干涉法实时测试了该类型器件在焊接过程中的面内和离面变形情况,并将测试数据与有限元法相结合,评估了焊接过程中器件内部因印制板变形而产生的应力大小。由此对原先"焊接导致开裂"的说法做出了评价。同时,通过器件的变形测试,探讨了优化焊接工艺的方法。研究结果表明,焊接顺序对器件内部热应力和器件变形影响不大,而适当提高器件高度可有效减小焊接时的热应力。  相似文献   

17.
Two recently proposed developments of the Glass–Rubber constitutive model for glassy polymers treat the viscoplastic deformation as intrinsically anisotropic, and incorporate the kinetics of structural evolution. These features enable the model to capture better the distinctive features of glassy polymers’ constitutive response: post-yield strain-softening and strain-hardening and effects of pre-existing molecular orientation. They have been combined to form a new variant of the model, and the consequences for necking have been explored. Uniaxial extension of prismatic bars was simulated using the finite element method, employing a numerical implementation of the new model, with material parameters of polystyrene. Strain localization predicted with the new model was found to be systematically retarded as compared to predictions with the original (intrinsically isotropic) version of the model, for the same conditions. In particular, the effect of frozen-in molecular orientation was examined. This was found to retard strain localization for stretching parallel to the orientation direction, for both models. But the localization predicted with the new model was always significantly less pronounced than with the original model. Indeed, for sufficiently high pre-orientation (e.g. a uniaxial stretch of 2.2), localization could be effectively prevented with the new model, under conditions when otherwise failure by necking is predicted. Such results can all be explained in terms of a linear stability analysis. They suggest that all previous simulations of necking in glassy polymers made using intrinsically isotropic representations of polymer viscoplasticity may have over-predicted the rate of strain localization.  相似文献   

18.
A hybrid framework for inverse analysis of crack-tip cohesive-zone model is developed in this two-part paper to measure cohesive-zone laws of void growth in polymers by combining analytical, experimental, and numerical approaches. This paper focuses on experimental measurements of the cohesive-zone laws for two nonlinear fracture processes in glassy polymers, namely multiple crazing in crack-growth toughening of rubber-toughened high-impact polystyrene (HIPS) and crazing of steady-state crack growth in polymethylmethacrylate (PMMA) under a methanol environment. To this end, electronic speckle pattern interferometry (ESPI) is first applied to measure the crack-tip displacement fields surrounding the fracture process zones in these polymers. These fields are subsequently equilibrium smoothed and used in the extraction of the cohesive-zone laws via an analytical solution method of the inverse problem, the planar field projection method (P-FPM) [Hong, S., Kim, K.-S., 2003. Extraction of cohesive-zone laws from elastic far-fields of a cohesive crack tip: a field projection method. Journal of the Mechanics and Physics of Solids 51, 1267-1286]. Results show that the proposed framework of the P-FPM could provide a systematic way of finding the shape of the cohesive-zone laws governed by the different micro-mechanisms in the fracture processes. In HIPS, inter-particle multiple crazing develops and the craze zone broadens ahead of a crack-tip under mechanical loading. The corresponding cohesive-zone relationship of the multiple-craze zone is found to be highly convex, which indicates effectiveness of rubber particle toughening. It is also observed that the effective peak traction, 7 MPa, in the crack-tip cohesive zone of HIPS (30% rubber content) is lower than the uniaxial yield stress of 9 MPa, presumably due to stress multi-axiality effects. In contrast, in PMMA, methanol localizes the crack-tip craze, weakening the craze traction for craze-void initiation to about 9 MPa and the fibril pull-out stress to less than 6 MPa. This reduction in cohesive traction, coupled with a strongly concave traction-separation cohesive-zone relationship, signifies environmental embrittlement of PMMA. These experimentally determined cohesive-zone laws are compared with detailed numerical analyses of effective microscale-void growth ahead of a crack tip in Part II.  相似文献   

19.
Failure mechanisms in amorphous polymers are usually separated into two types, shear yielding and crazing due to the differences in the yield surface. Experiments show that the yield surface follows a pressure modified von Mises relation for shear yielding but this relation does not hold during crazing failure. In the past different yield conditions were used to represent each type of failure. Here, we show that the same damage model can be used to study failure under shear yielding and crazing conditions. The simulations show that different yield surfaces are obtained for craze and shear yielding if the microstructure is included explicitly in the simulations. In particular the breakdown of the pressure modified von Mises relation during crazing can be related to the presence of voids and other defects in the sample.  相似文献   

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