医用氧舱有机玻璃银纹研究现状

从拉应力、溶剂、紫外线等单一因素及以上因素共同作用情况下对医用氧舱有机玻璃银纹产生的机理进行了分析,并通过对国内外有机玻璃的断裂性能和疲劳寿命的研究现状进行对比分析,提出了建立一套基于银纹损伤密度的银纹扩展直至失效的评价系统,并将其运用于医用氧舱有机玻璃的银纹检测及预测银纹状态下有机玻璃的使用寿命的构想。     

有机玻璃溶剂银纹的产生及其预防

聚甲基丙稀酸甲酯俗称有机玻璃.具有很好的光学性能、热性能、机械性能和化学性能,对很多化学品都具有良好的稳定性.但容易受到某些溶剂的侵蚀,出现开裂和银纹现象,对有机玻璃的使用寿命带来严重威胁.本文就有机玻璃溶剂银纹的机理,讨论了溶剂对有机玻璃的影响.     

聚碳酸酯玻璃老化性能的研究

考察了自然老化对聚碳酸酯玻璃抗银纹性能的影响,从不同老化场所、不同溶剂等方面对抗银纹性能进行了讨论。     

聚硅氧烷涂层对飞机座舱玻璃抗银纹能力的改进

介绍了飞机座舱玻璃银纹产生原因和聚硅氧烷涂层性质,通过涂覆后板材性能测试证明,涂层可以提高座舱玻璃抗银纹的能力。     

塑料的弹性体增韧及其影响因素

综述了塑料增韧理论的发展,包括微裂纹理论、多重银纹理论、剪切屈服理论、空穴化理论、逾渗理论等,分析了影响塑料韧性的因素,包括聚合物基体、增韧剂和共混加工工艺等。韧性的聚合物基体以剪切屈服断裂为主,而脆性的聚合物基体以银纹化为主。增韧剂对聚合物韧性的影响包括增韧剂的种类、用量、相尺寸、粒子间距等,共混加工工艺即共混顺序也影响聚合物的韧性。正确地理解聚合物的增韧理论和影响因素,有利于合理地设计出高效的聚合物增韧体系。     

有机玻璃的生产工艺及性能改进

有机玻璃因具有极好的透光性能、良好的电绝缘性能和耐老化性,外观优美且易于成型等优点,广泛应用于各行各业。本文从改进生产工艺入手,解决了有机玻璃生产时的散热防爆聚问题,又针对其耐热性差、易产生银纹等缺点,总结了提高有机玻璃耐热和抗银纹性的方法。     

大型风电叶片灌注银纹问题研究与解决

海上及陆上低风速风电的发展促使风电叶片的长度和根部直径急速增大,随之而来的是超大型叶片根部灌注银纹问题的产生。研究表明叶片根部灌注的银纹问题主要发生在树脂灌注固化过程。本文通过研究调整叶片根部树脂灌注固化产生的内应力,减缓叶片后固化过程的内应力释放,有效地解决了大型风电叶片根部的灌注银纹问题。     

细观结构参量对共混物银纹化倾向影响的数值模拟

银纹化、剪切屈服、空穴化是现今被广为接受的橡胶增韧塑料共混体系韧性得以增加的主要原因。进行了分散相颗粒形状较规则的单相连续两相共混物细观结构参量对共混物银纹的产生和发展影响的研究。借助有限元方法系统模拟了共混体系橡胶粒子的分布形式、添加比例、粒子尺寸、两相间界面层性能、粒子形状等细观结构参量的改变对共混物银纹化的影响规律。其规律可以加深对银纹化这一重要的共混物增韧机理的了解,并对改善共混物的细观结构形式,以期设计出有较好增韧效果的高聚物材料有一定指导意义。     

ABS树脂增韧机理

介绍了国内外对丙烯腈-丁二烯-苯乙烯共聚物(ABS)增韧机理的研究情况,分析了ABS结构特点,讨论了ABS增韧机理,分析了橡胶空洞化对应增韧作用的贡献,研究了银纹、多重银纹及剪切带对增韧的作用,总结了橡胶数量、凝胶、粒径等对增韧效果的影响.     

塑料的环境应力纹裂

纹裂是热塑性塑料热力学性能中的一种重要的现象。纹裂可分为应力纹裂与环境应力纹裂。综述了银纹的组成、结构和特性。以及银纹产生的判据和生长动力学。     

Environmental stress cracking in impact polystyrene

Environmental stress cracking (ESC) measurements for various impact polystyrenes were performed using a constant load technique with the specimens in contact with a 50/50 solution of cotton seed oil and oleic acid. It was shown that ESC in impact polystyrene is controlled by the transport of the aggressive liquid through a pre-established dry craze structure where capillary pressure is the driving force. At moderate stress levels just above the critical stress for environmental cracking, there is an apparent incubation time for the dry craze formation. The craze incubation time is strongly influenced by thermal stresses induced by the gel particles. As a consequence, ESC is two-stage process involving both an incubation time and actual crack growth. Control of the craze structure to maximize fibril content is essential for good ESC resistance. The craze fibril content can be altered by variables such as gel particle size, matrix molecular weight, plasticizer content, and rubber content.     

Stress cracking of rigid polyvinyl chloride by plasticizer migration

Environmental stress cracking (ESC) (failure caused by crack and craze formation at a stress less than the yield stress) reduces the service life of many plastic products. This paper is concerned with ESC of rigid PVC products which are in contact with a plasticized PVC material. The ESC affect (as measured by elongation to break) is reduced at faster strain rates and by higher plasticizer viscosity, which suggests a mechanism requiring flow of plasticizer into a growing craze. Well fused (gelled) PVC made at a higher melt temperature slowed but did not eliminate environmental stress cracking. Rubber impact modifier added to the rigid PVC had no effect on ESC. Environmental stress cracking can be avoided by using flexible PVC that has a non-migrating plasticizer or by designing the product so that rigid PVC is not stressed while in contact with plasticized PVC.     

Prediction and evaluation of the susceptibilities of glassy thermoplastics to environmental stress cracking

Glassy, amorphous thermoplastics experience a nearly universal susceptibility to crack and craze formation in the combined presence of stress and a chemical environment. This susceptibility has been evaluated for styrene-acrylonitrile copolymers according to the factors which effect craze initiation and those which limit the rates of flaw propagation. Molecular weight, styrene-acrylonitrile ratio, rubber content, and rubber phase morphology are variables which were found to affect flaw initiation and propagation. Fundamental characterization of the chemical parameters which control the initiation of craze formation in polysulfone and polycarbonate is also presented. In particular, “3-dimensional” solubility parameters have been observed to be a reasonable means of characterizing susceptibility to environmental stress cracking.     

Direct evidence for the existence of a craze at the crack tip in environmental stress cracking of polyethylene

Liquid nitrogen fracture tests have been carried out to produce direct evidence of the existence of a voided region, the craze, ahead of the crack in environmental stress cracking of polyethylene. Evidence of crazing is presented for both low and high density polyethylenes.     

Environmental stress cracking resistance of blow moded poly(ethylene terephthalate) containers

A method is developed for determining the environmental stress cracking resistance (ESCR) of blow molded poly(ethylene terephthalate) (PET) containers. By this method, the internal chamber of a container is presurized in 68.9 kPa (10 psi) increments while the outside of the base is being exposed to an environmental stress cracking (ESC) agent. The base of the container is examined after each 68.9 kPa of pressurization if crazing has occurred. The process is continued until a threshold value of craze initiation pressure (CIP) can be determined. Low CIP for the type of containers tested generally corresponds to a high rate of field failures. The method does not only gage the susceptibility of different types of one-piece PET containers to ESC but also provides helpful information to improve the container designs.     

The micromechanics and microstructure of CO2 crazes in polystyrene

Although CO₂ at 1 atmosphere pressure is not a crazing and/or cracking agent for polystyrene (PS), we have established that it becomes one at higher pressure. Crazes grown from cracks in PS thin films in high pressure CO₂ are investigated using transmission electron microscopy (TEM). The fact that broken craze fibrils retract strongly upon exposure to high pressure CO₂ gas suggests that the primary effect of the CO₂ is plasticization, not surface energy reduction. Quantitative analyses of TEM micrographs of crazes grown at CO₂ pressures in the range 5 to 100 MPa at 34°C and 45°C have been carried out to find the craze fibril volume fractions v_f(x) and the surface displacements w(x) along each craze. From the fibril volume fraction profile along the craze, the dominant craze thickening mechanism of CO₂ crazes is shown to be the same as that for air crazes, i.e. the surface drawing mechanism, and not the fibril creep mechanism. The craze surface stress profile is computed from the craze surface displacements using a distributed dislocation analysis. These profiles all show a stress concentration at the craze tip which falls to a roughly constant value σ_b, over the rest of the craze. The fracture toughness G_Ic (and critical stress intensity factor K_Ic) for propagation of a crack in PS at these CO₂ pressures can also be computed. All these quantities (V_f, σ_b, G_Ic and K_Ic) show pronounced minima as a function of CO₂ pressure at 20 MPa, the same CO₂ pressure at which T_g of the polymer also reaches a minimum. These minima are more pronounced at 45°C than at 34°C. The G_Ic's and K_Ic's are depressed by orders of magnitude at the minimum, which corresponds to the qualitative observation that CO₂ becomes a severe cracking agent at these pressures. These observations provide additional confirmation that the major mechanism for the environmental crazing and cracking of PS by CO₂ is plasticization of the craze fibrils and surfaces.     

Mechanism of craze thickening during craze growth in polycarbonate

It has been shown previously that dry craze growth kinetics in bisphenol-A polycarbonate (PC) subjected to creep are compatible with a model in which craze growth occurs at constant aspect ratio, the rate-controlling process being craze thickening due to homogeneous creep of craze material. In the present work, craze thickening during growth was investigated by electron microscopy and by an optical interference method in order to check this hypothesis. Results indicate that thickening occurs by drawing in fresh material from the craze—matrix interface. The previous model is modified in order to take these findings into account.     

The effects of physical aging on the brittle fracture behavior of polymers

The effects of physical aging on the failure behavior of a typical brittle polymer, polystyrene, have been studied. Properties examined were creep rupture lifetimes, fatigue lifetimes, and environmental stress cracking in ethanol. Fractured samples were examined both optically and by scanning electron microscopy to determine the degree of crazing. It was found that a longer physical aging time produced shorter lifetimes in all cases. The main reason for this is the reduction in craze strength caused by a reduced toughness due to physical aging. A long aging time was found to delay craze formation, but once formed, these crazes were much less stable than those formed with a short aging time. The effects of aging are important on failure prediction criteria and on testing methodologies, and the implications are discussed.     

Craze morphology and molecular orientation in the slow crack growth failure of polyethylene

An optical microscopy study and a micro‐Raman spectroscopy study were carried out on polyethylene samples subjected to an environmental stress crack resistance (ESCR) test. The aim was to elucidate the molecular deformation mechanisms associated with the failure process. It has been shown that in the early stages of the ESCR test, in a regime of low local stress, failure in the craze occurs via a brittle process with limited ductility and with molecular orientation being detected. As the experiment progresses, however, extensive fibrillation takes place. The molecular orientation in these fibrils was found to be comparable to that measured in cold‐drawn samples. Moreover, the fibril molecular orientation decreased from the crack to craze tip and was found to be higher in the midrib part of the fibril (fibril failure point). As a consequence, fibril creep is the most likely mechanism of failure in the craze. Microscopy and Raman measurements showed that the extent of the brittle process is molecular weight‐dependent, that is, the brittle process seems to operate longer at higher molecular weights. These observations are in agreement with a previous work which showed that the molecular stress per macroscopic strain/stress decreases with increasing molecular weight, therefore holding the high molecular weight craze in a regime of low local stress for longer testing times. Fibrils spanning the craze are envisaged as the anchor points that hold the structure during the process of failure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 283–296, 2000     

Interpreting instrumented three point bend fracture tests on craze forming polymers

Abstract

Tests on three point bend loaded specimens, containing a sharp initial notch of measured length, are the basis of a standard method (ISO/CD 17 281) for measuring the toughness of a plastic under either quasi-static or impact loading. In some polymers the fracture surface reveals that a long, stable, coplanar craze has extended from the notch tip during loading. Doubts about how to treat this notch extension have sometimes confused interpretation of the test. Using a quasi-static Dugdale–Barenblatt cohesive zone model, this paper presents a simple correction to the standard linear elastic analysis for tests in which a craze length can be measured. The corrected toughness results are higher and linearity restrictions on their validity can be significantly relaxed. Results are presented from Charpy type impact fracture tests on polyethylene. The computed craze stress reveals a craze size dependence which is thought to reflect a two stage process of craze fibril extension.