温度对聚氯乙烯缺口冲击强度及断面粗糙度的影响

研究了温度对聚氯乙烯ＰＶＣ缺口冲击强度，断口形貌特征及断面粗糙度的影响，结果表明，ＰＶＣ缺口冲击强度和断面粗糙度参数ＲＳ聚温度变化，在脆化温度Ｔｂ处存在极小值，前者取决于真实断裂表面积大小，后乾受控于局部高弹变形及裂纹分叉扩展综合作用，低温冲击断面上易观察到裂纹分叉扩展形成的弧形条纹及分层。     

玻纤增强尼龙6的断裂研究

通过对不同玻纤含量的玻纤增强尼龙复合材料(GFPA)的性能及断口形貌的研究,得出GFPA的宏观力学性能的变化可由断面形貌特征来定量表征。拉伸强度随拉伸断口断面平坦区面积与断面总面积之比AP/A0的变小而提高,Izod缺口冲击强度随GFPA的断面粗糙度参数RS的提高而线性提高。断面形貌的变化与玻纤在基体树脂中的应力集中作用及对裂纹扩展的阻碍作用有关。     

PA6/POE-g-MAH共混体系缺口冲击断裂机理研究

研究了PA6/POE-g-MAH共混体系缺口冲击断裂的裂纹萌生、裂纹扩展、冲击断面形貌及紧邻断面下方截面上的相形态.结果显示:随着POE-g-MAH质量分数增加,PA6/POE-g-MAH共混体系缺口冲击断裂发生脆韧转变,裂纹源由"中心辐射型"转变为"线型",裂纹扩展形貌由岩石状脆性断裂特征转变为具有横纹形貌的韧性断裂特征.在紧邻韧性断面下方的截面上可观察到网状形貌,可能是由被刻蚀掉的POE-g-MAH和冲击时基体微观破坏共同组成.在脆性断裂的截面上则无明显的网状形貌.PA6/POE-g-MAH共混体系的断面及截面形貌变化与其脆韧转变的逾渗行为完全对应.DSC分析认为POE-g-MAH提高POE-g-MAH/PA6共混物的缺口冲击韧性与其对PA6晶体结构的改变没有关系.     

PC/ABS共混合金冲击断口形态和强度EI

研究了不同原料和配比的PC/ABS共混合金冲击破坏断面形态和缺口冲击强度之间的关系。用注射成型方法制备缺口冲击样条,用扫描电镜观察断面形态,用简支梁测定冲击强度,结果表明:冲击强度得到提高的合金断面是PC和ABS部分断面形态的加合和增强。其特征形态是微坑、抛物线和裂纹带分枝。冲击强度较PC和ABS低的合金断面,其形态既不同于PC也不同于ABS。原料和配比对合金断面形态和强度有影响,其中配比的影响最大。     

PC/ABS共混合金冲击断口形态和强度

研究了不同原料和配比的PC/ABS共混合金冲击破坏断面形态和缺口冲击强度之间的关系。用注射成型方法制备缺口冲击样条,用扫描电镜观察断面形态,用简支梁测定冲击强度,结果表明:冲击强度得到提高的合金断面是PC和ABS部分断面形态的加合和增强。其特征形态是微坑、抛物线和裂纹带分枝。冲击强度较PC和ABS低的合金断面,其形态既不同于PC也不同于ABS。原料和配比对合金断面形态和强度有影响,其中配比的影响最大。     

用线型缺口冲击试样测定NDT温度可行性的探讨

从分析ＮＤＴ温度的实质出发，提出以线型缺口冲击试样进行系列温度的冲击试验测定ＮＤＴ温度的简便方法，并在１６Ｍｎ及ＳＡ２９９材料的试验上得到了验证，同时结合１２Ｃｒ１ＭｏＶ钢利用示波冲击试验机对冲击试验的裂纹形成功及裂纹扩展功进行分析，得出决定材料的动态冷脆特征温度的因素是裂纹扩展功。     

用示波冲击法测定42CrMo钢的冲击韧性

用示波冲击法测定了在冲击载荷下,淬回火态42CrMo钢的载荷-挠度曲线,并将冲击总功分成裂纹萌生功和裂纹扩展功;研究了回火温度对42CrMo钢回火组织与冲击韧性的影响。结果表明42CrMo钢在回火过程中,随回火温度升高,弹性功基本不变,裂纹萌生功略有升高,而裂纹扩展功迅速增加,42CrMo钢对缺口敏感,随回火温度升高,缺口敏感性降低,淬火态42CrMo钢经500—590℃回火,其组织由板条马氏体和条状及粒状θ-Fe_3C组成;冲击断口为准解理与韧窝混合断裂,随回火温度升高,韧窝增多,准解理减少。     

Al－Li合金缺口疲劳短裂纹行为的研究

研究了不同时效状态的Ａｌ－Ｌｉ合金短裂纹的扩展规律，缺口曲率半径对短裂纹扩展规律的影响．在相同的时效温度下，裂纹扩展速率ｄａ／ｄＮ随时效时间增加而加快；钝缺口萌生裂纹的ｄａ／ｄＮ在缺口应力场内变化不大，然后随着应力强度因子的增加而逐渐变大，而锐缺口萌生裂纹的ｄａ／ｄＮ在裂纹萌生后很快下降到最小值，然后又逐渐回升．     

Al—Li合金缺口疲劳短裂纹行为的研究

研究了不同时效状态的Ａｌ－Ｌｉ合金短裂纹的扩展规律，缺口曲率半径对短裂纹扩展规律的影响，在相同的时效温度下，裂纹扩展速率ｄａ／ｄＮ随时效时间增加而加快，钝缺口萌生裂纹的ｄａ／ｄＮ在缺口应力场内变化不大，然后随着应力强度因子的增加而逐渐变大，而锐缺口萌生裂纹的ｄａ／ｄＮ在裂纹萌生后很快下降到最小值，然后又逐渐回升。     

温度对微孔发泡PP和HDPE材料冲击性能的影响

通过对比微孔发泡PP、HDPE以及相应的未发泡PP、HDPE在不同实验温度下的Izod冲击强度,研究实验温度对微孔发泡PP、PE材料冲击性能的影响。结果表明:在实验温度范围内微孔发泡PP、HDPE以及相应的未发泡PP、HDPE相比较,随实验温度的降低,两者的Izod冲击强度的变化规律存在差异;通过对不同实验温度的冲击断口SEM分析,微孔对PP、HDPE冲击强度的作用机理为:一是裂纹扩展时微孔周围的树脂变形(及孔的变形)消耗能量,二是微孔的存在松弛了裂纹尖端应力集中,并会诱使主裂纹分解成次生裂纹,使裂纹扩展的方向和方式都发生变化,表现为裂纹扩展的阻力,三是微孔的引入减小了试样(材料)的有效承载面积。     

垂直轮廓剖面迹线法测定聚合物材料断面粗糙度的研究

运用垂直轮廓剖面迹线法测定了聚合物材料的断面轮廓线粗糙度参数ＲＬ及聚合物材料冲击强度和断口表面粗糙度参数Ｒｓ的关系。实验结果指出：聚合物的面轮廓线粗糙度与试样形状、断裂性质及断口形貌等因素有关。缺口试样断口形貌均衡分布时，试样两表面的测量值相等；断试样表面和心部测量结果相同；韧断试样表面测量结果大于心部测量结果。无缺口试样冲击脆断时，试样表面测量结果与断裂源位置有关，心部测量结果一般大于两表测量平     

Dynamic crack bifurcation in PMMA

An investigation of the branching characteristics of small PMMA single edge notched tensile (SENT) specimens is presented. The influence of notch depth and specimen thickness was examined and it was found that branching only occurred for thicker specimens and very short notch depths. The location at which successful branching occurred was very consistent for a given notch depth. Subsequently, however, a statistical variation of branching patterns was observed.A series of simulations was then performed to provide further insight into these tests and in particular to examine the evolution of the fracture process region ahead of the running crack. A finite volume/cohesive zone formulation was used to model micro-crack nucleation and dynamic interaction in the process zone. The cohesive strength and fracture resistance were estimated from unnotched tensile tests and the application of LEFM to the notch test data. Even though a very simple criterion was used to govern the insertion and subsequent behaviour of the cohesive surfaces in the model, many of the experimental observations were reproduced, including high frequency oscillations in crack velocity, the substantial increase in the fracture surface area due to the formation of subsurface micro-cracks, and the location at which successful branching took place.     

Fracture of slightly plasticized polyvinyl chloride

Impact tests and fracture toughness tests using compact tension specimens were carried out on a number of slightly plasticized PVC compositions. These measurements, together with calculations from the craze thickness profile were used to determine the fracture mechanisms operating in the various tests. The marked decrease in the impact strength of PVC on addition of small amounts of a conventional plasticizer was found to be due to the plasticizer decreasing the stress intensity necessary to nucleate a craze at the notch tip of the impact specimen. The fracture toughness of the compact tension specimens which failed by a crazing mechanism increased with increasing plasticizer content. It is thought that the fracture of these specimens is controlled by the stress intensity necessary to propagate the pre-existing crack/craze system through the material.     

Environmental stress cracking of PVC and PVC-CPE

The fracture toughness of Polyvinylchloride (PVC) and PVC modified with 10% chlorinated polyethylene (PVC-CPE) was studied in vapour and in liquid environments by crack growth measurements on single-edge notch specimens under three-point bending at 23°C. In addition, some results obtained in air at lower temperatures are presented. The fracture toughness is quantified by a stress intensity factor leading to failure after a given loading period. It is shown that for a given slow crack growth rate at 23 °C, the environment hardly affects the fracture toughness of PVC. In contrast, the slow crack growth in PVC-CPE at 23 °C is accelerated by the presence of benzene vapour, n-octane/benzene mixtures and gas condensate. A decrease in temperature results in an increase in fracture toughness, both for PVC and for PVC-CPE. A Dugdale model to describe the craze ahead of the crack was used to analyse the observed changes in fracture toughness.     

AN ANALYSIS OF FRICTIONAL EFFECTS ON CYLINDRICAL MODE III FATIGUE CRACK PROPAGATION SPECIMENS

Abstract— A model predicting the magnitude of frictional effects from fracture surface roughness on mode III fatigue crack growth is presened. Analysis of published data indicates that fracture surface roughness of the order of micrometers or less is enough to account for mode III fatigue crack growth retardation observation for increasing crack lengths for growth at constant Δ K . The model suggests that high strength materials will exhibit a greater resistance to shear crack growth than low strength materials. It also suggests that the resistance to shear crack growth will be more prominent at low nominal applied shear stress. The results of the analysis suggest that the concept of similitude does apply to mode III fatigue crack growth when the effects of friction on the stress intensity factor are included.     

EFFECT OF SPECIMEN GEOMETRY ON FATIGUE CRACK GROWTH IN PLANE STRAIN—I. CONSTANT AMPLITUDE RESPONSE

Abstract— The influence of specimen geometry on crack growth and crack closure response was determined for BS4360 50B steel and 6082-T6 aluminium alloy. Specimens were sufficiently thick for plane strain conditions to prevail along most of the crack front. After an initial crack growth transient from the sharpened notch, and steady state conditions are attained, the growth rate and closure responses are independent of specimen geometry. At growth rates above the near-threshold regime the cyclic crack openings exceed the fracture surface roughness for the steel, but are much less than the surface roughness for the aluminium alloy. This suggests that roughness-induced crack closure plays a dominant role for the aluminium alloy but not for the steel. Finally, the effect of mean stress upon closure response is presented for the steel.     

The Fractography and Crack Patterns of Broken Glass

The topographical features which appear on the fracture surfaces of broken glass objects and the resulting crack patterns which develop are Nature’s documentation of the fracture event. They are considered after a brief discussion of glass strength. Strength is central to the fracture surface features for it determines the strain energy release rate and the dynamics of crack extension. The surface features known as the mirror, the mist, and the hackle are illustrated and addressed through the principles of fracture mechanics and associated energy criteria. Quantitative aspects of the fracture process such as the stress level at fracture for a glass object are directly related to the size of the fracture mirror. The concept of a fracture mirror constant is related to the strength. Formation of the mist and hackle surface regions are also fundamentally addressed, as is crack branching. Distinctive crack patterns that evolve during fracture, that is the traces of the cracks intersecting the glass free surfaces, are described. Dicing fragmentation of high-strength tempered glass and the long sword-like shards of low-strength annealed glass fracture are contrasted through their strain energies. Characteristic cracking patterns are reviewed for several common glass fractures including those for pressure breaks, both bottle explosions and flat glass window failures from wind pressure whose basic similarities are described. The patterns of crack branching or forking, the branching angles and the crack length prior to forking, are also discussed. Other glass crack patterns such as those from impact and thermal stress are also considered.     

Influence of nanometre-sized notch and water on the fracture behaviour of single crystal silicon microelements

The influence of a notch and a water environment on the quasi‐static and fatigue fracture behaviour was investigated in single crystal silicon microelements. The tests were conducted in smooth and notched microcantilever beam samples. Smooth specimens were prepared by micromachining (photo‐etching) of (110) silicon wafers. For some specimens, a nanometre‐sized notch was machined 100 μm away from the sample root by using a focused ion beam system. A machining condition was optimized, and the V‐shaped notch was successfully introduced. The radius of curvature of the notch, measured by an atomic force microscope (AFM), decreased with an increase in notch depth, and ranged from about 20 to 100 nm. Single‐crystal Si microelements deformed elastically until final failure, which was of a brittle nature. The maximum fracture strength of a smooth microcantilever specimen reached about 7.7 GPa, which was higher than that obtained in millimetre‐sized single crystal Si samples. However, the fracture strength decreased with an increase in notch depth, even though the notch depth was of the order of a nanometre. This means that a nanometre deep notch, which is often regarded as surface roughness in ordinary‐sized mechanical components, caused a decrease in the fracture strength of Si microelements. The fracture initiated at the notch, and then the {111} crack propagated in the direction normal to the sample surface. Fatigue tests were also conducted in laboratory air and in pure water at a stress cycle frequency of 0.1 Hz and a stress ratio of 0.1. In laboratory air, no fatigue damage was observed even though the surface was nanoscopically examined by an AFM. However, when the fatigue tests were conducted in pure water, the fatigue lives in water were decreased. Crack formation on the {111} plane was promoted by a synergistic effect of the dynamic loading and the water environment. Atomic force microscopy was capable of imaging the nanoscopic cracks, which caused failure in water.     

EFFECT OF TEMPERATURE ON FATIGUE CRACK GROWTH IN THE POLYMER ABS

Abstract— Fatigue crack growth in a commercial grade ABS over the temperature range - 50°C to 80°C has been studied. An Arrhenius type relationship between fatigue crack growth rate and absolute temperature was found to describe the experimental data. At Δ K = 1 MPa√m, the activation energy for crack growth in the temperature range −50°C to 19°C is 3.47 kJ/mole and in the temperature range 30°C to 80°C it is 19.63 kJ/mole. The two different activation energies were found to be associated with the roughness of the fracture surfaces. The roughness of the fracture surfaces is discussed in relation to modes of fatigue crack growth. In the low temperature range (− 50°C to 19°C) the fracture surfaces were found to be rather coarse, whereas in the high temperature range (30°C to 80°C) they were found to be somewhat smooth. These different roughnesses were deduced to be due to different modes of crack branching influenced by crazing. A "stress intensity factor"-biased Arrhenius equation for fatigue crack growth successfully predicts growth rates at various temperatures.