高聚物银纹断裂的研究现状

银纹化是高聚物的一种非线性变形方式，对高聚物的增韧设计十分重要。考虑银纹细观结构特征的银纹生长和断裂规律是研究银纹增韧机制的核心内容。结合国内外最新的研究进展，对承载高聚物中的银纹断裂及其与裂纹扩展的相互作用等问题作了较全面的总结和分析，并对今后的研究重点和趋势作了简要展望。     

高聚物形变屈服与断裂机制

综述了有关高聚物形变机理及断裂机制的研究工作，重点指出了聚合物形变的切变屈服和银纹化屈服，及由银纹发展至裂缝的微观机制，介绍了利用连续介质的弹性力学和缺陷结构的弹性力学理论来解决高分子材料在形变过程中同力学性能有关的问题。     

MBS对PVC的增韧作用及其机理的探讨

本文通过扫描电镜观察了ＰＶＣ／ＭＢＳ共混物银纹的发展和冲击断面形貌,并测定了银纹体的密度变化。采用光散射法分析银纹和剪切带比例。合成不同玻璃化温度的ＭＢＳ考察它对ＰＶＣ增韧的影响,进行有关ＭＢＳ对ＰＶＣ增韧机理的探讨。     

改性聚丙烯材料裂尖银纹损伤研究

本文考虑了改性聚丙烯(PP)材料的线粘弹特性，基于银纹是聚合物材料的一种非线性变形方式，对改性PP材料裂尖应力银纹损伤进行实验研究。采用云纹实验方法，得到了改性PP材料裂尖延长线上由于银纹损伤引起的应变的分布，对裂纹尖端扩展进行了扫描电镜(SEM)观察实验，将两组实验进行对照分析，通过定性分析提出了不同于其他学者提出的考虑裂尖银纹损伤的裂纹扩展规律。     

航空有机玻璃环境—应力开裂的研究：2.取向的影响

本文对比了定向和非定向三号航空有机玻璃在乙醇作用下的应力－溶剂银纹临界应力σｃ，银纹形貌及吸水和紫外光辐照对这两种材料的影响，并就取向对航空有机玻璃抗环境－应力开裂能力的影响及机理进行了分析。     

用单轴拉伸实验研究HIPS的银纹增韧现象

高抗冲聚苯乙烯 (HIPS)是通过颗粒填充进行改性以提高其力学性能的工程塑料 ,通过单轴拉伸实验对其银纹损伤及增韧现象做初步探讨 ,发现银纹增韧机理是橡胶增韧中主导因素 ,分析得到受拉HIPS的弹塑性损伤本构方程的一般形式 .     

航空有机玻璃环境－应力开裂的研究2．取向的影响

本文对比了定向和非定向三号航空有机玻璃在乙醇作用下的应力－溶剂银纹临界应力σｃ、银纹形貌及吸水和紫外光辐照对这两种材料的影响，并就取向对航空有机玻璃抗环境－应力开裂能力的影响及机理进行了分析     

利用银纹诱发时间预测承载塑料的使用寿命

根据Findley双曲正弦蠕变方程和Zhurkov方程,推导出材料诱发银纹时间和材料断裂时间的关系式。PVC在空气和10％H_2SO_4中的实验结果和这一关系式很好吻合。利用这一关系,即可由短期蠕变实验去预测承载塑料的长期使用寿命。     

光伏组件蜗牛纹现象成分研究和应对策略的现状

蜗牛纹是一种光伏组件表面银栅线变暗的变色现象,对光伏组件的外观会造成严重影响,并进一步引发业主对组件产品质量的担忧,自报道以来,蜗牛纹现象已经受到了研究机构和组件厂商的广泛关注。介绍了蜗牛纹的概况,针对蜗牛纹组成成分调研了三类研究结果——单质银、碳酸银以及乙酸银和碳酸银。最后,对改进封装材料和推广双玻组件这两种蜗牛纹解决策略进行了讨论和分析。     

脆性高聚物的银纹化增韧设计

银纹化是脆性高聚物的一种典型的非线性变形方式。银纹在其引发、生长和断裂过程中消耗大量能量 ,对高聚物的增韧设计十分重要。考虑银纹细观结构特征的银纹生长和断裂规律是研究银纹增韧机制的核心内容。根据国内外近期的若干研究进展 ,基于对承载高聚物中的银纹断裂及其与裂纹扩展的相互作用等问题的分析 ,从理论上探讨将材料断裂韧性与其微观控制参数 (如分子量 ,缠结密度等 )联系起来 ,寻求对脆性高聚物进行微观增韧设计的途径和方法     

Nonsteady crack and craze behavior in PMMA under cyclical loading: III. Effect of load history on cohesive force distribution on the craze

Measurements of crack opening and craze profiles are made under a range of loading histories including cyclical deformations that lead to nonsteady crack propagation histories. Of particular interest is the comparison of the distribution of traction transmission of a newly formed craze relative to a cyclically stressed one as it approaches the slow-down phase. Real time, interferometric measurements provide precise and multiple craze profiles during individual cycles. Cyclic deformations reduce the stiffness of a craze in its center resulting in a stress drop as part of the craze strength evolution; also, its thickness changes nonuniformly during the acceleration/retardation phases of the advancing craze/crack. The implications are, that for quasi-statically formed crazes the craze material can be reasonably well characterized by a stress-strain relation, while that is no longer readily true for a cyclically deforming crack, since rate dependent characteristics of the craze and bulk material intervene. Cracks unloaded as part of cyclical deformation histories exhibit crack closure (compression) near the trailing end of the craze. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of varying the polyethylene content and the co-polymer content on crazing in polystyrene—low-density polyethylene blends

The effect of varying the low-density polyethylene content and the polystyrenepolyethylene block co-polymer content on the rates of craze initiation and craze growth in polystyrene/low-density polyethylene blends has been studied. it was found that the parameters in the Eyring rate coefficients for craze initiation and craze growth are not dependent on the low-density polyethylene content. However, the rates of craze initiation increased with increasing low-density polyethylene content. This is explained tentatively by a new model for craze initiation. It is argued that effective crazes are only formed within clusters of low-density polyethylene particles that have overlapping stress-concentration fields. The dependence of the rate of craze initiation on the volume-fraction of dispersed phase that follows from this cluster model is qualitatively in agreement with experimental results. PS-PE co-polymer addition gives rise to changes in the Eyring parameters of the rate coefficients of craze initiation and craze growth. This may be a consequence of changes in morphology near the interface and of the different stress state at the interface.     

On the interaction between a full craze and a near-by circular inclusion in glassy polymers

Crazing is the common reason in polymer composites to cause failure. So far research work on craze phenomenon has been done only in homogenous polymer materials. The first time in our current study, the stress analysis has been carried out on the interaction between a circular inclusion and a full craze in polymer composites. A craze can be treated as a crack with fibrils bridging the two crack surfaces. The forces applied by the bridging fibrils to the crack surfaces (pulling the two surfaces closer) depend on the crack opening displacement, while the crack opening displacement is directly related on the forces applied by the bridging fibrils. To solve this dilemma, an iterative procedure is created and introduced to solve the formulated singular integral equations. The influences of the inclusion's elastic properties on the craze thickness profile and the stress intensity factors are investigated in details. It is found that the case with a “stiffer” inclusion produces smaller craze thickness. Also, the craze thickness profile is strongly affected by the inclusion size and the craze-inclusion distance. The shielding effect of a stiffer inclusion on the craze is discussed by evaluating the stress intensity factors at both craze tips.     

Micromechanics of solvent crazes in polystyrene

Double exposure holographic interferometry (DEHI) is used to determine the strain energy release rate, craze opening displacement profile, and craze stress profile ofn-heptane and methanol crazes growing from cracks in polystyrene.n-heptane crazes have strain energy release rates (SERRs) close to those of cracks and their stress profile is almost crack-like in that the tensile stress across the craze falls almost to zero. On the other hand, the SERRs of methanol crazes are only 30 to 55% the SERR of a crack depending on stress intensity factorK _I of the precrack from which they are grown. The stress profile of the methanol craze shows it to be strongly load-bearing away from the craze tip, apparently as a result of the strain hardening of the craze fibrils. The stress concentration in front of the methanol craze tip is only 40% of that in front of then-heptane craze tip. The opening displacements of the methanol craze are almost as large as those of a crack very near its tip but are much less than those of a crack at large distances behind the tip. The Dugdale model of a strip yielding zone provides a poor representation of the craze opening displacements of the growing methanol craze. Dry (static) methanol crazes have larger opening displacements in response to an incremental tensile strain at moderate prestrains than at either low or high prestrains, suggesting that the craze fibrils undergo a yielding/strain-hardening process as the strain is increased similar to that observed in polycarbonate crazes by Kopp and Kambour. Dryn-heptane crazes do not show this response but rather open linearly with increasing prestrain. The opening displacement for long (dry)n-heptane crazes is almost crack-like whereas the largest opening of a dry methanol craze is only 20% of that of a crack. Dry methanol crazes break at aK _IC that is 40% of theK _IC of precracked but uncrazed specimens. The strongest (shortest) dryn-heptane crazes fail at only 7% ofK _IC of uncrazed specimens and theK _IC of the dryn-heptane crazes decreases markedly with increasing craze length.     

Microstructure of crazes in solvent-crazed polycarbonate thin films

Crazes have been produced in polycarbonate by atomizing acetone over uniaxial stressed solvent cast thin films. Transmission electron microscopy shows that two types of craze orientation exist. Crazes are formed perpendicular to the applied stress direction and at approximately 55° to the applied stress direction. The craze structures observed at 55° to the applied stress direction are suggested to result from cavitation of pre-existing deformation bands. The internal morphology of each type of craze is similar, and the fibre diameter, void size and craze width of each type depends on the amount of craze development. The structure of the craze-matrix interface is, however, different for each type of craze. Necking and work-hardening of the fibres occurs at the normal craze-matrix interface, whereas a rough undrawn interface exists for the angular crazes.     

Craze formation and growth in anisotropic polymers

Craze formation and craze growth in anisotropic polymers has been studied as a function of the degree of anisotropy and the relation between the testing direction and the primary orientation direction. Tests on PMMA and PC indicate that both the morphology and orientation of the crazes are senstivie functions of testing direction. Crazes form in directions which arenot orthogonal to the principal tensile stress, and the data clearly show that craze growth occurs in directions governed by the major principal strain. The fracture process is identical in nature to that in isotropic polymers, i.e. craze formation, crack nucleation within the craze and subsequent crack propagation through the craze. Thus, the angle of fracture coincides with the craze angle rather than occurring perpendicular to the principal tensile stress.     

Micromechanical and thermodynamical aspects of environmental crazing

The present study aims 1) to investigate theoretically the relation between the craze microstructure and the basic materials parameters such as the yield stress and the surface energy and 2) to provide a detailed thermodynamic treatment of a single isolated craze in glassy polymer tested in an aggressive liquid environment. Based on the assumption that the craze tip is somewhat blunted by small scale yielding and on the Taylor meniscus instability as the mechanism responsible for the propagation of the leading edge of the craze, the detailed micromechanical analysis is used to provide estimates of the critical opening displacement of the craze for growth initiation, mean fibril spacing, mean fibril diameter and fibril volume fraction at the craze tip. The influence of aggressive liquid environments on the yield stress and the surface energy is discussed together with predicted changes in the craze microstructure. The thermodynamic analysis starts with the recognition that induced high negative pressures around the craze tip can increase the solubility of a liquid at this site by several orders of magnitude. As a consequence the local density of thermodynamic potential drops significantly. This unbalanced fall in thermodynamic potential provides an additional driving force for the craze advance. It is shown that a corresponding release of external load is required to preserve the overall balance of the specimen with craze.     

Environmental stress cracking of PVC: Effects of natural gas with different amounts of benzene

Craze initiation, craze growth and ultimate fracture were studied in PVC (Polyvinylchloride) and PVC-CPE (PVC blend with 10% chlorinated polyethylene) under a constant load in air and in natural gas enriched with benzene. The craze initiation results are similar for PVC and PVC-CPE. The craze initiation stress is decreased dramatically at high benzene concentrations ( 25000 p.p.m.). Preferential and enhanced sorption of benzene molecules near surface inhomogeneities, the craze initiators, are held responsible for this phenomenon. Initial crazegrowth rates in natural gas enriched with benzene increase compared to those in air. However, in all the environments studied, a limited logarithmic craze growth is observed, and growth seems limited to this constant logarithmic interval. The logarithmic craze growth and the termination of craze growth are attributed to a reduction of the stress at the craze tip. The failure mode of PVC and PVC-CPE in air and in gas with 5000–6000 p.p.m. benzene is yielding. A brittle branch appears in the failure curve (failure stress against loading time) in the more concentrated benzene vapours. This branch is attributed to a reduction in the stability of the crazes.