Craze yielding and fracture mechanism in PE/PS/PE laminated films

Mechanical behaviour of laminated films of PS and PE, especially the altered mechanical properties of PS in the laminated state, were studied as functions of PE volume fraction. It was observed that crazing in PS can be modified by laminating layers of PE to both sides. Elongation at break and hence fracture energy increased conspicuously when PE volume fraction increased. Craze yielding stress in PS layers also increased with increasing PE volume fraction because craze formations in PS layers were suppressed by the reduction of tensile stress concentration effect at craze tips. Craze initiations were always found at the free side edges of the laminated films, which can be correlated with the transverse interlaminar shear stress concentrations existing at the edges of the laminated films caused by the difference in Poisson's ratios between PS and PE.     

Craze and yield zones in the fracture of rigid PVC

Two techniques have been used to examine the reported discrepancy between the measured fracture toughness of rigid PVC and that calculated from its crack tip craze. The use of a generalized model of a cohesive zone produced no better agreement between calculation and experiment than the Dugdale model used previously. The discrepancy was probably caused by the coexistence with the craze of a yield zone which was observed by optical and scanning electron microscopy.     

Orowan mechanism for abrupt localized flow and fracture initiation in metals ahead of an yielding notch

Plane strain elastic-plastic stresses are determined in Mises yielding solid at the root of an yielding crack like notch. This external edge notch is infinitely deep, and has a small finite (fixed) flank angle with a small tip root blunting radius. A boundary value type approach has been followed throughout, to solve this famous Orowan-lrwin problem. Firstly, a fictitious elastic stress field is calculated, considering a misfit in the bulk volume loading; these elastic stress expressions are valid when the notch is fully loaded. Secondly, the plastic stresses are determined in the compressibility gradient, maintaining the continuity of stresses and their derivatives at the yielded-unyielded interface. Our calculations reveal that: Orowan mechanism is fairly dominant below the notch root, as well as on ± 45° planes. It is concluded that the flow-localization in the Mises solid is due to a reverse slip, caused by the sudden release of a favourable critical mismatch stress concentration. Some elastic strain energy density is seen to be getting released from the bulk volume, while unloading the misfit load. The mismatch has been created entirely due to the compressibility-incompressibility difference, as suggested by Orowan.

Following Orowan, it is shown here that, before the onset of a stable crack extension, the increase in stress concentration at the notch tip root, is directly proportional to the strength of mismatch strain-localization below the notch, and inversely proportional to the plane strain plastic zone size on the crack extension plane. For a large scale yielding situation, compressive stresses and pure distortion regions are seen to occur at a far field within the plastic enclave.     

Mixed-mode delamination fracture in laminated composites

Quasi-static fracture of laminated composite plates with circular or elliptic delaminations due to transverse shear stresses is considered. The system is modeled as two laminated plates bonded everywhere except over the area of the disbond. A laminated plate theory with the effects of surface tractions and shear deformations is used for each plate and the problem is reduced to a set of integral equations. Because of the nature of the model, stress singularities along the disbond periphery appear as concentrated line forces, which are different from those obtained from three-dimensional elasticity analysis. Physically meaningful quantities such as strain energy release rates for mode I, II and III types of deformation are evaluated. Failure loads for catastrophic fracture are computed based on firstly a point criterion and secondly a physically realistic growth pattern. Results are compared with test data for disbonds located in the midplane. For such disbonds the mode I component of energy release rate is absent and, therefore, fracture is due to mode II and III interactions.     

Effect of thickness on fracture criterion in general yielding fracture mechanics

In this research work, the effect of thickness on fracture criterion is studied for extra deep drawn (EDD) steel sheets. Experimental results are generated on fracture toughness of EDD steel sheets using compact tension specimens and a ‘maximum load’ as a fracture criterion. Critical crack tip opening displacement (CTOD) is found with the help of three methods: plastic hinge model (PHM), crack flank opening angle (CFOA) and finite element model (FEM). The fracture toughness is found to increase with increase in thickness of specimens. The fracture behaviour exhibited characteristics of general yielding fracture mechanics.     

Chemical stability of laminated AlN/AlN:H films

Highly adherent and chemically stable coatings of aluminum nitride are produced by ion beam sputter deposition of graded concentration films of AlN:H_x, where x ranges to zero (AlN) at the outer surface of the film. These layered structures exhibit the good adhesion of AlN:H while maintaining the chemical stability of A1N. On the basis of scanning tunneling microscopy and scanning electron microscopy, films deposited on room temperature substrates (metal or mica) form layers that are smooth (structures smaller than 6 nm) and that remain so for periods longer than one month. Fourier transform spectroscopy indicates that they are chemically unchanged by exposure to air for two months.     

Experimental and finite element analysis of fracture criterion in general yielding fracture mechanics

Efforts made over the last three decades to understand the fracture behaviour of structural materials in elastic and elasto-plastic fracture mechanics are numerous, whereas investigations related to fracture behaviour of materials in thin sheets or general yielding fracture regimes are limited in number. Engineering simulative tests are being used to characterize formability and drawability of sheet metals. However, these tests do not assure consistency in quality of sheet metal products. The prevention of failure in stressed structural components currently requires fracture mechanics based design parameters like critical load, critical crack-tip opening displacement or fracture toughness. The present attempt would aim to fulfill this gap and generate more information thereby increased understanding on fracture behaviour of sheet metals. In the present investigation, using a recently developed technique for determining fracture criteria in sheet metals, results are generated on critical CTOD and fracture toughness. Finite element analysis was performed to support the results on various fracture parameters. The differences are within 1 to 4%. At the end it is concluded that magnitude of critical CTOD and/or critical load can be used as a fracture criterion for thin sheets.     

Mechanical and fracture properties of ternary PE/PA6/GF composites

This article studies the mechanical properties of short fiber reinforced polymer blends comprised of a soft thermoplastic matrix (polyethylene, PE), a rigid dispersed thermoplastic phase (polyamide-6, PA6) and glass fiber reinforcement. These ternary composites are designed as a model system to investigate the impact of the mutual interactions of the three phases on the composite mechanical properties. For this purpose two types of fibers are used, dispersed-phase and matrix-phase compatible fibers, respectively.     

Preparation of multifunctional wound dressings with composite PVA/PE films

Journal of Materials Science - The waterproof and breathable wound dressings have undergone continuous and substantial evolution over time. Polyethene films have been widely used to fabricate...     

聚合物基PTC热敏导电材料的性能及机理研究

针对PTC材料体系，提出了“PTC强度可提高空间”与“体系提高PTC强度的能力”两个概念，以低密度聚乙烯(LDPE)、高密度聚乙烯（HDPE)和乙烯-醋酸乙烯共聚物(EVA)作为基体，以导电炭黑作为填充导电粒子，制备了聚合物基复合型PTC材料，详细分析了炭黑含量、EVA含量对PTC材料性能的影响．研究表明，所提出的有关概念可用于分析材料的PTC效应机理及各种因素的影响，有助于进一步对聚合物基PTC热敏导电材料作用机制的理解及研究；炭黑含量影响材料电阻率和PTC强度，炭黑含量取在逾渗区电阻率偏小处可得到较高的PTC强度；EVA含量不同时材料电阻率和PTC强度都有先减小后稍增大的现象。     

Cascadating fracture in a laminated tempered safety glass panel

The macrofracture pattern of a laminated, tempered safety plate glass panel in which failure was initiated by a sharp object was statistically analyzed. The crack density decreased linearly with distance from the fracture origin as did the average distance between successive crack branching events. The average bifurcation half angle, however, remained at about 21°, independent of distance from the fracture origin. This half angle is very similar to that reported for the branching of an individual primary crack in a fracture mechanics specimen. The two branching half-angles are negatively correlated, approaching a maximum of 45° in the extreme.The curvatures of the new crack paths after bifurcation were also observed to be independent of the distance from the fracture origin and followed the general expression where the coefficient is a random variable and is a constant equal to 0.035. It was possible to describe the repetitive crack branching through the concept of a fractal tree with a fractal dimension of 2.58, a value similar to some geological processes. Applying the fractal tree concept, the multiple cascadation phenomenon was reproduced for the range of fractal dusts of z 4, beyond which degeneration related to excessive selfoverlapping occurred.{fx49-1}     

Characterization of the interlaminar fracture toughness of a laminated carbon/epoxy composite

Delamination between layers is an important problem in applications of fiber reinforced composite laminates. Tests were carried out to determine the interlaminar fracture toughness of AS4/3501-6 (carbon/epoxy) composite laminates using mixed-mode bending tests. Analysis of the test specimens in terms of mode I and mode II energy release rates showed good agreement between methods based on beam equations, compliance measurements, and detailed finite element analyses. The results showed that the critical mode I energy release rate for delamination decreased monotonically with increasing mode II loading. This is in contrast to some results in the literature. Various analytic representations of the mode interaction from the literature were compared, and shown to fit the data with reasonable accuracy.     

Properties of layered structured PE/PP and PB-1/PP films having ultra-thin layers

Highly oriented polymer films containing ultra-thin layers of polyethylene/polypropylene (PE/PP) or polybutene-1/polypropylene (PB-1/PP) were investigated with transmission electron microscopy, differential scanning calorimetry, optical birefringence methods and mechanical testing. The determination of the microstructures of the above systems before and after melting and recrystallization of the low-melting component revealed the existence of epitaxially crystallized PE lamellae in the PE/PP system, whereas no epitaxy was observed in the PB-1/PP system. However, if PB-1 was sandwiched between two layers of the polypropylene, a non-isotropic arrangement of the recrystallized PB-1 crystals was detected. The mechanical properties, as well as thermal behaviour and optical birefringence of these systems, were determined and the results were related to the specific morphologies present in the different samples. Reinforcement mechanisms resulting from epitaxially crystallized PE lamellae are discussed.     

聚苯乙烯/富勒烯复合材料抗紫外老化性能

运用紫外加速老化熔融共混法制得C60和C70含量（质量分数）分别为1%、3%、5%的聚苯乙烯（PS）/C60和PS/C70复合膜,考察了复合膜的抗紫外线老化性能。利用红外光谱（FTIR）分析了聚苯乙烯的老化前后结构变化,用示差扫描量热法（DSC）和差热法（DTA）测试了各试样老化前后的玻璃化转变温度、分解温度,结果表明,加入C60和C70后,PS/C60和PS/C70复合材料的耐紫外老化性能大大提高,添加量为5%时复合材料的抗老化性能较好。     

Effects of yielding and size upon fracture of plates and pressure cylinders

The investigation on the yielding at crack tips, its implication on fracture, size effect and its relation to the Griffith criterion of fracture and to the failure of pressure cylinders, and finally the correlation of fracture of plates with that of cylinders are presented herein.By means of the formula derived for the yielding zones at crack ends, it is shown that for failure besides the fulfilment of the Griffith condition, the yielding zones should attain critical dimensions and that in cylinders of all material and in flat plates of high strength material the width of the yielding zone is the same for a given size over a wide range of stresses.The size effect for plates of high strength material is expressed by a formula relating the Griffith stress to the applied stress, critical crack length and the width of specimen: but in the case of more ductile material different formula is derived on the basis that in different sizes of plates the yielding zones have the same width for crack lengths proportional to the width of specimens. The size effect obtained for cylinders not only yields the solution for failure mechanism of cylinders but furnishes the means of obtaining the rate of strain energy release when the formula is applied to test results.Finally a correlation between the fracture of flat plates and that of pressure cylinders on the basis of strain energy release is established. By means of this correlation and with the formula for failure of cylinders the computing of failing stresses for cylinders of different radii is made possible.The test data used to substantiate theory consists of four materials. They are two aluminum alloys, stainless steel and a titanium alloy. The width of flat plate specimens in tests is in the range from 2.25 in. to 30 in. and the radii of cylinders are from 3.6 in. to 15 in.

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Zusammenfassung Dieser Bericht handelt über eine Untersuchung des plastischen Fließens an den Extremitäten einer Einkerbung, sein Einfluß auf das Bruchverhalten, über Größeneinflüsse und ihre Zusammenhänge mit dem Kennwert von Griffith, dem Bruch von Druckzylindern Bowie über den Bruch von Blechen und von zylindrischen Hüllen.Mit Hilfe von zur Bescreibung des Fließverhaltens an den Rißrändern abgeleiteten Formeln wird gezeigt, daß die plastischen Bereiche sowohl die Griffith-bedingungen erfullen als auch kritische Abmessungen annehmen müssen. Außerdem wird bewiesen, daß in Zylindern aus beliebigen Werkstoffen und in Blechen aus hochfestem Material für gegebene Abmessungen, die Breite der plastischen Zone über einen weiten Bereich von Spannungen die Gleiche bleibt.Im Falle von Stahlblechen wird der Größeneffekt durch eine Formel beschrieben, welche das Verhältnis der Griffith-Spannung zur angelegten Spannung, die kritische Rißlänge Bowie die Breite des Probenstabes in Betracht zieht. Im Falle von duktileren Werkstoffen hingegen ergibt sich eine andere Formulierung welche begründet ist auf der Similität der Breite der plastischen Zonen für verschiedene Blechabmessungen wenn die Rißlängen der Breite der Probenstäbe proportional sind. Der für Zylinder erhaltene Größeneffekt gibt nicht nut Auskunft über den Bruchmechanismus, sondern ermöglicht es außerdem das Ausmaß der Freisetzung von Verformungsenergie zu bestimmen wenn man die Formel auf Versuchsergebnisse anwendet.Zum Schluß wurde, auf Grund der freigesetzten Verformungsenergie, eine Korrelation zwischen den Bruchbedingungen an flachen Blechen und denjenigen an Zylindern aufgestellt.Dank dieser Korrelation und unter Anwendung der Bruchformel für Zylinder ist es möglich die Bruchspannung für Zylinder verschiedener Abmessungen zu berechnen.Die Versuchsergebnisse; welche es ermöglichten these Theorie zu prüfen, beziehen sich auf vier Werkstoffe: zwei Aluminiumlegierungen, em rostfreier Stahl und eine Titanlegierung. Die Breite der Flachproben lag zwischen 2,25 und 30 Zoll und die Halbmesser der Zylinder zwischen 3,6 and 15 Zoll.

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Résumé Le mémoire est relatif à une étude de l'écoulement plastique aux extrémités d'une entaille, à ses implications sur la rupture, aux effets dimensionnels et à leurs relations avec le critère de rupture de Griffith et avec la rupture de cylindres de pression et finalement, aux corrélations entre la rupture de tô1es et celles de corps cylindriques.En recourant aux formules décrivant les zones d'écoulement plastique aux extrémités d'une fissure, on démontre que les zones plastiques doivent atteindre une dimension critique, outre la nécessité de remplir la condition de Griffith. On démontre aussi que daps les cylindres en matériau quelconque et dans les tôles planes en matériau à haute résistance, la largeur de la zone plastique est la même pour une dimension donnée, sur une large gamme de contraintes.L'effet dimensionnel dans le cas des tôles en acier à haute resistance s'exprime par une formule comportant le rapport de la contrainte de Griffith à la contrainte appliquée, la longueur critique de fissure et la largeur de l'éprouvette. Dans le cas de matériaux plus ductiles, néanmoins, une formulation différente est trouvée, qui se base sur la similitude de largeur des zones plastiques pour différentes dimensions de tôles, lorsque les longueurs des fissures sont proportionnelles à la largeur des éprouvettes. Dans le cas des cylindres, l'effet dimensionnel calculé rend non seulement compte du mécanisme de lent rupture, mais permet en outre d'obtenir le taux de libération d'énergie de déformation si la formule est appliqu6e aux résultats d'essais.Enfin, on a établi une correlation entre les conditions de rupture de tôles planes et celles de cylindres de pression sur la base de la libération d'energie de deformation.Grâce à cette corrélation, et en appliquant la formule de rupture des cylindres, il est possible de calculer les contraintes de ruine pour des cylindres de divers diamètres.Les données expérimentales ayant permis de vérifier la théorie sont relatives à quatre matériaux: deux alliages d'aluminium, un acier inoxydable et un alliage de titane. La largeur des éprouvettes planes était comprise entre 2,25 et 30 pouces et les rayons des cylindres considérés s'échelonnaient entre 3,6 et 15 pouces.

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Some results of this paper were originally presented under the title Fundamentals of Fracture in Metallic Sheets at the Fifth U. S. National Congress of Applied Mechanics, held in Minneapolis, Minnesota, 1966.

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Editorial Note: Through an administrative oversight, this paper, originally accepted for publication two years ago, inadvertently never reached publication. The paper is now published with apologies to the author.     

Computer simulation study of the atomistic mechanism of deformation and fracture initiation in thin fcc metal films

A molecular dynamics computer simulation was performed using a potential of embedded atom method (EAM) of copper so as to study the atomistic mechanism of plastic deformation in thin metal films. When a thin film of Cu crystal is elongated by more than 8%, small islands of surplus atoms start to nucleate between (111) planes. The formation of new (111) islands occurs by the movement of atoms along the 111 direction from two successive (111) planes. Multiplication of these (111) planes serves as the mechanism of elongation, releasing the accumulated elastic stress. Vacancies and small clusters, thereof, are left behind at the positions from which atoms have migrated and also at the part of newly nucleated (111) planes in which atoms were not filled completely. Stacking fault tetrahedra (sft) are nucleated directly in the deformed region as a result of movement of atoms along 111 on the tetrahedral (111) cap.     

FEP和多孔PTFE复合膜铁电驻极体的压电性及电荷动态特性

报道了将致密的FEP和多孔PTFE交互层叠在一起，采用热粘合的方法制备出具有孔洞结构的聚合物复合膜，然后经电晕极化处理使该复合膜成为铁电驻极体。最后用准静态方法测量该铁电复合膜的压电系数d33，并通过热刺激电流放电（TSD）电流谱和等温衰减研究了FEP和多孔PTFE复合膜铁电驻极体的电荷动态特性。结果表明：该复合膜的压电系数d33可达200～500pC/N，在0～12kPa的压强范围内呈现出良好的线性，并且在90℃下老化160min后仍保持在原来的43％且趋于稳定；FEP和多孔PTFE复合膜铁电驻极体电荷在热激发脱阱后复合途径主要有2种：一种是沿着固体介质的表面迁移与异性电荷复合，另一种是穿过固体介质层与异性电荷复合。在低温区（7512附近）前者占优，高温区（120℃附近）后者占优。