塑料型材挤出定型冷却系统设计的数值模拟概述

综述了国内外利用数值模拟技术优化设计型材挤出定型冷却系统的研究进展,指出了基于数值模拟技术的定型冷却系统的设计方法,并展望了今后的研究趋势。     

Effects of the transverse stress on biaxial fatigue crack growth predicted by plasticity-corrected stress intensity factor

The transverse stress has an important effect on the biaxial fatigue crack behavior. However, the experimental evidence has provided conflicting indications: it is sometimes considered to increase, decrease or have no effect. These complex phenomena cannot be rationally explained by the existing mechanical models. The effect of the transverse stress on the fatigue crack growth behavior is still one of the most puzzling questions in biaxial fatigue. Physically, this effect is a transverse stress induced plasticity phenomenon. In this paper, a plasticity-corrected stress intensity factor (PC-SIF) is proposed to describe the effect of transverse stress on biaxial fatigue. By use of this new crack driving force some important phenomena associated with transverse stress are predicted. Comparisons with experimental results showed that the PC-SIF as an effective mechanical parameter is capable of predicting the effects of the crack length, the stress level, cyclic stress ratio, biaxial stress ratio and phase difference on the biaxial fatigue crack growth. Consequently, the alleged conflicting experimental results have been rationally explained by the PC-SIF.     

Plastic coupled instabilities of I-shaped steel beams

The paper presents the case of plastic instability of I-shaped beams where the rotation capacity is a very important characteristic in order to assure the required ductility in plastic or seismic design. The failure of such beams can be due to the local plastic plate buckling of compression flange and local plate buckling of the web in flexural compression, produced in-plane or out-of-plane. In the same time the failure can occurs by coupling of these two local buckling modes. In order to study the plastic buckling of beams and to determine the available rotation capacity which is required by the plastic or seismic design, the collapse plastic mechanism theory was developed and a specialized computer program DUCTROT-M was elaborated. The parametrical analysis reveals the paramount importance of the web to thickness ratio and furthermore the sensitivity in out-of-plane buckling of the European hot-rolled sections.     

The behavior of statically-indeterminate structural members and frames with cracks present

Crack stability is discussed as affected by their presence in statically-indeterminate beams, frames, rings, etc. loaded into the plastic range. The stability of a crack in a section, which has become plastic, is analyzed with the remainder of the structure elastic and with subsequent additional plastic hinges occurring. The reduction of energy absorption characteristics for large deformations is also discussed. The methods of elastic-plastic tearing instability are incorporated to show that in many cases the fully plastic collapse mechanism must occur for complete failure.     

Nanoindentation characterised plastic deformation of a Al0.5CoCrFeNi high entropy alloy

Abstract

The plastic deformation of a high entropy alloy Al_0.5CoCrFeNi was investigated by instrumented nanoindentation over a broad range of strain rates at room temperature. Results show that the creep behaviour depends on the strain rate remarkably. In situ scanning images showed a significant pile up around the indents, demonstrating that a highly localised plastic deformation occurred in the process of nanoindentation. Under different strain rates, contact stiffness and elastic modulus basically remain unchanged. However, the hardness decreases as indentation depth increases due to indentation size effect. For the same maximum load, serrations became less prominent as the loading rate of indentation increased. Similar serrations have been observed in the current alloy upon quasi-static compression.     

Antoni Gaudí and Frei Otto: Essential Precursors to the Parametricism Manifesto

As Senior Architect to the Basilica of the Sagrada Família in Barcelona, Mark Burry has been ‘thinking parametrically’ for almost his entire career. Here he describes how his longstanding role overseeing the completion of Antoni Gaudí's masterpiece has afforded unique insights into the work of a great geometer and parametric thinker. Burry places the contribution of Gaudí alongside that of Frei Otto – the other eminent 20th-century Proto-Parametricist.     

高分子乳液型养护剂的研制及应用

通过优化养护剂组分,研制出有效保水率高、在新鲜混凝土面成膜快的高分子乳液型养护剂,室内对比研究了三种养护方式（标准养护、自然养护和养护剂养护）对混凝土强度、耐久性和塑性收缩、开裂的影响,并采用该养护剂在干旱风沙地区进行了工程应用。结果表明,与标准养护相比,养护剂养护下混凝土7d 前强度不降低,56d强度仅降低约5%,90d 自然碳化深度为0,氯离子渗透性略有增加;成膜养护剂能有效降低混凝土塑性收缩和开裂程度,用于干旱风沙地区混凝土养护具有明显的技术经济性能优势。     

Acoustic emission from the initiation of plastic deformation of Polyethylenes during tensile tests

The acoustic emission (AE) technique has been used in the aim to detect and to observe the initiation of plasticity and damage of several Polyethylenes (PE) during tensile tests. The detection of acoustic signals originating from the deformation of the material is challenging since PE samples strongly attenuate ultrasonic waves. A weak acoustic activity has been recorded during the tests. The use of two types of PE specimens and two methodologies based on the elimination and the discrimination of spurious sources has shown that most of the AE signals truly originate from the plastic deformation of materials. We also observed that the acoustic activity increases with strain rate. Besides, some AE signals are located along the specimen length during tensile tests at high strain rate. The acoustic activity increases strongly with the crystallinity. Micro-cavities, formed before the yield point of PE samples with high crystallinity in particular, likely initiate the release of acoustic energy. In addition, some signals are collected on PE samples, which do not exhibit a formation of cavitation. Hence, the shearing of crystallites and/or the fragmentation of crystalline lamellae may also be a source of the release of acoustic energies.     

Cavitation during deformation of semicrystalline polymers

Cavitation phenomenon is observed during deformation in many semicrystalline polymers above their glass transition temperature. Numerous voids (cavities) both nanometer and micrometer size are formed inside amorphous phase between lamellae during deformation of a polymer. The cavitation is observed only in tension, never during compression or shearing. Most often used methods of voids detection are: microscopies (SEM, TEM, AFM and light microscopy), small angle X-ray scattering and measurements of density. Usually the voids are detected close to yielding or at yielding, strongly suggesting that yielding is often caused by cavitation. However, there is a competition between two processes: breaking of amorphous phase leading to cavitation and plastic deformation of lamellar crystals. Which process occurs first depends on the relation between compliances of those two phases. If the crystals are weak and defected their deformation occurs (mostly by chain slips mechanism) without cavitation. If the crystals in a polymer are thick and more perfect then the barrier for their deformation, represented by shear yielding stress, is increased and the cavitation sets in first and yielding is determined by the stress needed for cavitation. Further deformation involves deformation of crystals due to rapid local change of stress around voids. The influence of different morphological factors: crystal thickness, crystallinity degree, arrangement of crystalline elements (e.g. in spherulites), morphology of amorphous phase (free volume, entanglements, tie molecules) were analyzed. Experimental factors, such as temperature of deformation and rate of deformation influence remarkably the formation of cavities. Cavitation is generated at points where a high local triaxial state of stress is developed. Triaxiality of stress can be amplified by a notch, even very mild notch with large radius of curvature stimulates generation of cavities. Evolution of nano-cavities into micro-cavities and change of their shapes with increasing deformation were evidenced by SAXS. Initially voids are oriented perpendicularly to deformation direction, however, with increasing elongation they become oriented along deformation direction. Stress whitening is visual sign of cavitation and is caused be light scattering either by microvoids or by assemblies of nanovoids.     

基于绿色制造的金属塑性精成形

金属塑性成形加工在产品制造过程中具有举足轻重的地位。但传统的加工方法存在以下不足：材料利用率很低;能源浪费严重;振动、噪声对操作工人以及周边环境产生严重危害和污染。本文综合分析了传统金属塑性成形加工在资源、能源及环境等方面存在的普遍症结,提出了节约资源、能源、环境保护,确保可持续发展是金属塑性成形理论与技术发展的重大课题。实现绿色塑性成形加工,应从技术、工艺和设备三方面入手,走金属塑性精成形之路。