聚丙烯内聚区银纹断裂时间的预测

文中拟定的聚丙烯(PP)银纹断裂分离标准是根据银纹拉伸产生变形热引起内聚区熔融层的形成和增厚提出的。内聚区分离的数值模拟是基于活性层附近的银纹区和本体区内的非线性热传导和热对流,并伴随均一轴向银纹的力学拉伸。聚丙烯银纹断裂时间的预测是在假定常银纹增厚速率和常内聚力的条件下实现的。研究表明,活性层温度相同时PP材料银纹区和本体区的温度分布不同,前者的热传导较后者快得多。随着应力或拉伸速率增大,无论是银纹区还是本体区的温度分布均呈减小的趋势。随着应力增加或银纹增厚速率的增加,PP的银纹断裂时间呈递减的趋势。实验结果的比对证实了通过热反内聚模型的数值模拟程序可预测聚丙烯的银纹断裂时间。     

冲击拉伸过程中HDPE银纹内聚力的测定

热塑性聚合物经受稳定的银纹微纤拉伸时,银纹区的迅速增厚将会迅速地引起局部绝热升温现象.预测裂尖银纹的热力学分析可以预测结晶热塑性高聚物冲击断裂阻抗.热反内聚模型用来模拟冲击拉伸过程银纹寿命或反内聚时间.文中报道了一种新型的全切痕蠕变测试法可以在银纹增厚速率高达2m/s时测量平面银纹层的内聚力,在快速拉伸条件下高密度聚乙烯的内聚力和银纹寿命被测量,测试的结果与数值模型的预测结果是相一致的.     

含银纹的高聚物裂纹扩展

根据已有的实验结果,考虑银纹损伤演变规律,假设裂尖银纹模型,导出Ⅰ型裂纹准静态下裂纹尖端银纹增长的微分方向和稳态扩展的方程,研究了裂纹长度及其扩展速度随时间的演变规律,初步分析了银纹损伤区平均,应力的变化,对含Ⅰ型裂纹ＰＭＭＡ的寿命进行了探讨。     

分形维数和弹性模量衰减表征2D-C/SiC的拉伸蠕变损伤

真空条件对2D—C／SiC复合材料在1300℃和1500℃进行了高温拉伸蠕变试验，蠕变进行到0、0．5h、2h、10h、25h、50h中断试验，用SEM观察表面形貌，用盒维数法计算试样表面裂纹的分形维数；同时测量试样的弹性模量。结果表明，由于2D—C／SiC特有的蠕变损伤形式，所形成的损伤尺度都较短，其分形维数介于0～1之间。用分形维数和弹性模量衰减都可表征2D—C／SiC的蠕变损伤，两种损伤参量所描述的蠕变损伤总的发展趋势基本一致，即蠕变开始阶段损伤发展较快，随后进入缓慢发展的第二阶段。在第二阶段中，分形维数表征的损伤持续单调增加；而用弹性模量衰减表征的损伤在该阶段出现先下降随后升高的现象。以基体裂纹为主要损伤形式的条件下。分形维数主要反映蠕变试样局部的损伤，而弹性模量衰减反映的是蠕变试样整体的损伤。     

不同因素影响下混凝土立柱爆破效果的模型试验研究

钢筋混凝土建（构）筑物爆破拆除过程中,混凝土立柱最终的破碎效果不仅与炸药单耗、孔网参数等有关,还与立柱的受力状态有关。为研究截面应力和单位面积炸药量对不同强度混凝土立柱破碎效果的影响,利用自主研制的力学试验系统进行了15组爆破试验,从碎块筛分和分形维数两方面分析爆破效果。结果表明:对于相同强度的试件,截面应力一定时,随着单位面积炸药量的增加,分形维数整体上不断递增;但单位面积炸药量超过0.18 kg/m²后,增势有所放缓,截面应力为2 MPa和4 MPa的立柱趋势变化最为明显。单位面积炸药量一定时,随着截面应力的增加,碎块分形维数不断增大,当单位面积炸药量小于0.15 kg/m²时,0~2 MPa之间的截面应力增加会对分形维数有一定的抑制作用;当单位面积炸药量大于0.15 kg/m²时,截面应力的增加对分形维数起到促进作用。可为拆除爆破中的参数设计及优化提供理论依据,达到了控制爆破危害、改善爆破效果的目的。     

临界接触参数连续的粗糙表面法向接触刚度弹塑性分形模型

基于MB接触分形理论及其修正模型,在弹性和塑性接触变形机制基础上,考虑弹塑性过渡变形机制,建立了临界接触参数连续条件下的计及微接触面积分布域扩展因子影响的粗糙表面法向接触刚度弹塑性分形模型;数值仿真结果表明:弹塑性过渡变形机制对法向接触刚度影响明显,且考虑弹塑性过渡变形机制下的法向接触刚度大于仅考虑弹性和塑性接触机制下的对应法向接触刚度;无量纲法向接触刚度随着无量纲法向接触载荷的增大而增大且因分形维数取值不同而呈凸弧性的非线性关系(分形维数D=1.1~1.4)或近似线性关系(分形维数D=1.4~1.9),随着分形维数的增大而增大(分形维数D=1.1~1.5)及分形维数的增大而减小(分形维数D=1.5~1.9),随着分形粗糙度的增大而减小,随着塑性指数的增大而增大。     

金属断裂表面分形维数与组织、性能之间的关系

借助分形研究了拉伸条件下,分形维数 D_F 与拉伸性能之间的关系,讨论了显微组织对分形维数的影响,并把冲击韧性与分形维数之间的关系和拉伸性能与分形维数之间的关系进行了比较。实验结果表明,拉伸性能与分形维数只存在定性关系。延伸率与分形维数成正比,强度σ_b、σ_s与分形维数成反比,显微组织对分形维数有重要影响。断裂方式对分形维数、断裂性质以及两者的对应关系也有重要影响。     

冲击荷载作用下岩石破碎分形特征

为探究冲击荷载作用下岩石破碎分形特征,选取花岗岩和砂岩开展分离式霍普金森压杆(SHPB)岩石动力学试验,得到了不同应变率下岩石的应力-应变曲线、破碎特性、强度参数和能量参数;利用标准筛对破碎后的岩块进行筛分,获取了岩石破碎块度分布曲线,并基于碎块粒径分布的质量分形模型计算出分形维数D;最后分析了分形维数与加载参数、破碎特性和耗能特性之间的关系。结果表明,岩石在冲击荷载作用下的破碎块度分布符合分形规律;动态抗压强度随应变率增大而增大,两者满足乘幂函数关系;加载过程中岩石应变率越大,岩石破碎程度越深,分形维数越大;分形维数与岩石破碎耗能密度之间满足乘幂函数关系。采用分形维数可实现对岩石在冲击荷载作用下的破碎特性、力学特性和破碎耗能特性的定量研究。     

异步轧制Pb-Ca-Sn-Al合金晶界节点分形分析

为了确定晶界节点分形维数是否存在,以低碳钢冷轧薄板标样为研究对象,在MATLAB软件平台上自行开发了晶界分形维数计算程序,采用标准分形图形-koch曲线检验了自开发分形程序的准确性,利用该程序计算了3种标准下(不同晶粒延伸度)不同晶粒度级别的金相图片晶界节点分形维数,并研究了不同工艺条件下Pb-Ca-Sn-Al合金晶界节点的分形维数.结果表明:自开发的分形计算程序准确;晶界节点的分形维数存在;不同工艺条件Pb-Ca-Sn-Al合金晶界节点的分形维数与工艺参数之间存在定量联系.在一定范围内,随着压下率、异速比、退火温度的增加,晶界节点的分形维数随之增加;随退火时间的增加,Pb-Ca-Sn-Al合金晶界节点分形维数先上升后下降;退火张力增加,晶界节点分形维数下降.     

捻绳的分形维数对其拉伸性能的影响

分形结构维数是描述无序系统多级自相似结构特征的有效方式,并可以将捻绳的结构与其力学性质联系起来。通过观察捻绳的堆砌方式,并绘制了均等面积和圆形化的捻绳理论截面图,并求得了对应的分形维数;用WDW-20电子微机控制万能材料试验机测试了几种不同直径的丙纶、涤纶捻绳的拉伸性能。由此讨论了捻绳的断裂强度、初始模量、断裂伸长率和断裂比功与试样的整体分形维数的关系。结果表明,捻绳的整体分形维数与其断裂强度和初始模量呈线性负相关,而与断裂伸长率呈正相关关系。丙纶捻绳的整体分形维数与其断裂比功呈线性负相关,而涤纶捻绳的整体分形维数与其断裂比功呈线性正相关。     

Rubber-toughening of plastics

A new and quantitative method for studying rubber-toughening is presented: the extent of crazing in creep specimens is determined from time-dependent volume changes, which are compared with longitudinal strains in order to determine the contribution of other mechanisms. The results show that creep deformation in HIPS under high tensile stress consists essentially of elastic deformation followed by crazing. A simple model for the kinetics of crazing is developed, from which rate constants for initiation (k _I) and propagation (k _P) of crazes are obtained. Bothk _I, andk _P exhibit an Eyring-type dependence upon stress, with the same apparent activation volume of 5000 ų.     

Effects of the creep deformation on the fractal dimension of the grain boundaries in an austenite steel

The change in the fractal dimension of the grain boundaries during creep was investigated using an austenitic SUS304 steel at 973 K. The fractal dimension of the grain-boundary surface profile (the fractal dimension of the grain boundaries, D, 1 < D < 2) in the plane parallel to the tensile direction (in the parallel direction) and in the transverse direction, was examined on specimens deformed up to rupture (about 0.30 creep strain). Grain boundaries became serrated and the fractal dimension of the grain boundaries increased with increasing creep strain, because the density of slip lines which formed ledges and steps on grain boundaries increased as the creep strain increased. The increase in the fractal dimension due to creep deformation was slightly larger under the higher stress (118 MPa) than under the lower stress (98 MPa), while the increase of the fractal dimension with strain was a little larger in the specimens tensile-strained at room temperature (293 K) than in the crept specimens. These results were explained by the grain-boundary sliding and the diffusional recovery near grain boundaries, which lowered the increase of the fractal dimension with the creep strain. The fractal dimension of the grain boundaries in the parallel direction was slightly larger than that in the transverse direction in both creep at 973 K and tensile deformation at room temperature, especially at the large strains. This could be correlated with the shape change of the grains by creep or plastic deformation. Grain-boundary cracks were principally initiated at grain-boundary triple junctions in creep, but ledges, steps and carbide precipitates on serrated grain boundaries were not preferential nucleation sites for the cracks.     

The competition between shear deformation and crazing in glassy polymers

Whereas thin films of some polymers such as polystyrene readily form crazes when strained in tension, thin films of other polymers such as polycarbonate rarely exhibit crazing under the same testing conditions; the polymers that rarely craze tend to form regions of shear deformation instead. Polymers such as polystyrene-acrylonitrile which lie between these two extremes of behaviour may exhibit both modes of deformation. Thin films suitable for optical and transmission electron microscopy (TEM) of six such co-polymers and polymer blends have been prepared. After straining, the nature of the competition between shear deformation and crazing is examined by TEM. It is found that in these polymers many crazes have tips which are blunted by shear deformation. This process leads to stress relaxation at the craze tip, preventing further tip advance. In this way short, but broad, cigar-shaped crazes are formed. Examination of the deformation at crack tips in the same polymers shows more complex structures, the initial high stress levels lead to chain scission and fibrillation but as the stress drops, shear becomes the dominant mechanism of deformation and the stress is relieved further. Finally, at long times under stress, chain disentanglement may become important leading to fibrillation and craze formation again. The nature of the competition is thus seen to be both stress and time dependent. Physical ageing of these polymers, via annealing below T _g, suppresses shear leading to the generation of more simple craze structures.     

Crazing behaviour in oriented poly(ethylene terephthalate)

A study has been made of the two types of crazes formed in oriented sheets of poly(ethylene terephthalate). The crazes have been termed tensile crazes and shear crazes. The tensile crazes formed parallel to the initial draw direction (IDD) whereas the shear crazes formed in a direction close to that of the deformation bands observed when the material yields.The possibility of applying a yield criterion to shear craze formation has been examined and there appears to be fairly good agreement between theory and experiment. Measurements of crazing stress on the tensile crazes indicated that the criterion for tensile craze formation is not purely dependent on the component of stress normal to the extended chains.It is concluded that the two types of crazes are formed by two quite different mechanisms, although the exact nature of these mechanisms is still uncertain.     

Stress relaxation of high impact polystyrene

The stress relaxation behaviour of high impact polystyrene has been correlated with the microstructural changes observed in tensile tests. The inhomogeneity of plastic deformation, manifested as stress whitening, has been measured using microhardness tests. This method has been found to be sensitive to the amount of crazing in the material. The stress relaxation behaviour changed at the onset of crazing, but did not change appreciably as the volume fraction of crazes increased. An analysis of the relaxation in terms of a site population model based on White's approach suggests the macroscopic stress relaxation is related to the crazes in the boundary regions between the stress whitened and unwhitened material.     

Micromechanism of a deformation process before crazing in a polymer during tensile testing

Polymers are popularly used for housing and parts of machines and equipment. However, their mechanical properties, especially the deformation process, have not been clarified. During tensile testing, crazes are thought to be a source of microcracking and fracture, but the relation between the craze formation process and the deformation process before crazing is not understood. In the present work, scanning acoustic microscopy and X-ray diffraction were used to investigate the micromechanism before craze formation in polymethylmethacrylate (PMMA) and polycarbonate (PC). The velocity change of the surface acoustic wave and X-ray diffraction intensity indicated that molecular orientation occurred in a very small area from early stages of plastic deformation. From the results it was thought that texture was heterogeneous and anisotropic in a very small area, the shape of the area was spheroidal with a longer radius in the direction perpendicular to the applied stress, and the molecular chain in the area was oriented parallel to the stress axis. The area is thought to increase with increasing plastic strain.     

The effect of specimen size on the mechanical behaviour associated with crazing

Since crazes generally nucleate at the surface it is expected that the size of the specimen, as described by the ratio of surface area to volume, should affect the mechanical behaviour of polymers which deform primarily by crazing. The stress relaxation curves and the stress-strain curves of PS, PMMA, PTFE, and PC were measured in liquid nitrogen for specimens of different size which were machined from the same rod. The predicted size effect was observed in that the smaller (6.4mm diameter) specimens stress-relaxed faster and the stress to produce a given amount of craze deformation was lower than for the larger (12.7 mm diameter) specimens. The range of the tensile strength from 0 to size is also presented based on the stress to nucleate the first craze and on the tensile strength that is observed when no crazing occurs     

Rubber-toughening of plastics

Mechanisms of deformation in an ABS emulsion polymer were studied quantitatively by a uniaxial tensile creep method. Craze formation was measured in terms of volume strain, which was calculated from simultaneous observations of longitudinal and lateral strains, and shear deformation was measured in terms of lateral strain. The experiments showed that shear deformation predominated during the early stages of creep, but that the rate of shear deformation fell with time. At stresses below 27 MN m⁻², specimens reached extensions of 5% without significant craze formation: at higher stresses, crazing was observed at strains above about 21/2%. Rates of crazing increased with time and with stress, so that the contribution of crazing to creep was greatest during the later stages of the test, and at the higher stresses. The relevance of these results to engineering applications of ABS polymers is discussed.     

Microstructural aspects of tensile deformation of high density polyethylene

Abstract

The paper describes tensile flow behaviour and related microstructural aspects of injection moulded polyethylene. The flow stress increased with increase in strain rate indicating that the tensile flow behaviour is sensitive to strain rate. Also, the yield stress exhibited a linear relationship with strain rate and followed the thermal activation concept with the activation volume consistent with the thickness of the lamellar crystallite. The behaviour of mechanically induced surface damage on polyethylene was studied by SEM. At low strain rates, the deformation process was characterised by features that looked like deformation bands. With increase in strain rate and strain, the deformation bands developed into a distinct array of crazes and grew inwards, followed by tearing. Also, with increasing strain rate the crazes multiplied and secondary crazes were generated that were at an angle to the tensile axis. The examination of the morphology of the fracture surface of polyethylene at various strain rates provided an insight into the mode of fracture process. At low strain rates polyethylene exhibited a ductile type of fracture with extreme fibrillation and, at intermediate strain rates, crazing - tearing was the predominant mode of fracture at the edges, while fibrillar failure occurred in the mid-thickness of the fractured surface. But, at higher strain rates the percentage of fibrillation was relatively small in comparison to lower strain rates. The different modes of deformation processes can be represented in the form of mechanical deformation - strain diagrams, which provide a broad perspective of the deformation processes operating in the different regimes.     

Rubber-toughening of plastics

The effects of stress-and strain-history upon the mechanical properties of HIPS (highimpact polystyrene) and of blends containing HIPS and PPO^R resin have been studied in a number of different tests, including repeated creep testing of individual specimens and repeated tensile tests at constant strain-rate upon individual specimens. The results show that craze formation increases volume and lowers Young's modulus in specimens subjected to tensile strain, and that strained specimens recover only slowly towards the properties of the unstrained material. Recovery is accelerated by heating, or by immersion in alcohols. A given initial strain produces a greater reduction in modulus in HIPS, which deforms almost entirely by crazing, than in HIPS/PPO blends, which deform by a combination of crazing and shear band formation. The properties of strained specimens are dominated by the distinctive non-linear mechanical behaviour of crazes, and the problems of constructing models to represent this behaviour are discussed.