Strength of Metals at High Strain Rates

Results of metal testing in impact tension–compression and indentation are analyzed. As the analysis of these data demonstrates, the strength of metals increases greatly with strain rates and at moderate indentation rates. At high indentation rates, a decrease in the specific energy (per unit volume of a displaced material) necessary for the formation of a conical cavity was observed. The account of the effects of viscosity, temperature increase upon plastic deformation and its localization can be used to explain the above phenomena.     

PC/ABS在高应变率下的压缩大变形

目的对PC/ABS在高应变率下的压缩大变形行为进行实验研究与模拟。方法在应变率为1600~5000 s~(-1),温度为293~353 K的范围内,选用霍普金森压杆获取其在高应变率、高温下的大变形行为;选用DSGZ本构模型,模拟PC/ABS在高应变率下的大变形。结果 PC/ABS大变形行为强烈依赖于应变率和温度,屈服应力随应变率增加或温度降低而升高,大变形行为包括弹性、屈服、应变软化和应变硬化。结论DSGZ本构可准确模拟PC/ABS在高应变率、高温下的大变形行为。     

不同应变率下的高聚物银纹化分子动力学模拟

基于粗粒珠簧模型,采用分子动力学模拟了玻璃态高聚物在不同应变率下微空洞的产生、扩展及断裂演化过程,表明银纹产生的微空洞会引起周围的应力集中,微空洞迅速扩展并伴有少量合并;结合临界应力判据,给出了银纹萌生的微观构型;通过引入非仿射位移场并结合应力-应变响应关系,计算了参与率随应变的演化,证实了聚合物在银纹化破坏过程中具有应变率相关敏感性的线性粘弹性特征。与实验现象吻合较好。     

纯钛高应变率拉伸力学行为的实验研究

利用MTS809材料试验机和旋转盘式间接杆杆型冲击拉伸实验装置,对多晶纯钛进行了室温环境下应变率为0.001,0.01s-1的准静态和300,1400s-1的动态拉伸实验,获得了纯钛等温和绝热拉伸应力-应变曲线;实施了应变率为300s-1的冲击拉伸复元实验,获得了纯钛在高应变率下的等温应力-应变曲线。结果表明:纯钛的拉伸力学行为具有明显的应变硬化效应、应变率强化效应和绝热温升软化效应。采用修正的KHL模型较好地表征了纯钛在准静态和动态实验应变率范围内的拉伸力学行为。     

Tensile Behaviour and Strength Distribution of Polyvinyl-Alcohol Fibre at High Strain Rates

Mechanical properties of Polyvinyl-alcohol (PVA) fibre bundles were studied at three high strain rates (270/s, 600/s and 1500/s). It was found that, except for the elastic modulus, which remains unchanged, both the maximum stress and the failure strain show an apparent increase with strain rate. Two failure modes of PVA fibre under tensile impact were observed. A four-parameter Weibull function was adopted to describe the strength distribution of PVA fibre and the Weibull parameters were obtained by a fibre bundle testing method. Consistency between simulated and experimental results indicates that such a function and the method are valid and reliable.     

金属板料高速率拉伸试验及其硬化模型的综述

针对金属板料的动态试验,回顾了高应变速率下弹性杆与液压伺服型单轴试验的方法,介绍了高应变速率下双轴拉伸试验:动态液压胀形试验与动态十字形试件双拉试验。最后,根据不同应变速率下流动应力曲线的四大类型,总结了相应的率相关硬化模型。     

Modeling of Constitutive Behavior for Epon 828/T-403 at High Strain Rates

Experimental results on the uniaxial compressive stress-strainbehavior of Epon 828/T-403 over a strain rate range from 1.1 × 10^–4 to 5.2 × 10³ s^–1were simulated using a viscoelastic viscoplastic constitutive modeldeveloped by Hasan and Boyce (1995). An optimal combination of materialparameters for the constitutive model was determined by curve-fittingthe experimental results. A comparison between the modeling andexperimental results shows that this model, with proper parameters, iscapable of capturing the strain-rate effects, as well as the typicalpolymeric compressive stress-strain behavior, of Epon 828/T-403, whichincludes the stages of linearly elastic, nonlinearly elastic, yield-like(peak) behavior, strain softening, and nearly perfect flow. For all thestress-strain behavior modeled over seven decades of strain rate span,the maximum error between modeling and experimental results is less than25%. The temperature increase induced by the dissipated plasticwork is also computed and found to be between 3 and8°C at these strain rates.     

Strain Accumulation in Ti-6Al-4V during Fatigue at High Mean Stress

The effect of mean stress and frequency on the high cycle fatigue behavior of Ti-6Al-4V has been investigated. It has been shown that a transition in the fatigue behavior occurs at a stress ratio of approximately 0.7. Above this value, the material exhibits measurable strain accumulation and necking. Since Ti-6Al-4V is susceptible to room temperature creep, an empirical model was developed using static creep data in an attempt to predict the cyclic behavior of the material. The model was unable to account for the large amounts of strain seen experimentally. In addition, closer examination of the data revealed that the deformation was more closely related to the number of cycles than to time.     

聚碳酸酯拉伸力学性能的应变率相关性实验

在MTS809材料试验机和自行研制的旋转盘式间接杆杆型冲击拉伸试验机上,对聚碳酸酯板材进行了准静态0.001s-1、0.01s-1、0.1s-1和高应变率400s-1、800s-1、1700s-1加载条件下的单向拉伸试验,得到了不同应变率下聚碳酸酯板材的拉伸工程应力-工程应变曲线。试验结果表明:聚碳酸酯的拉伸力学行为呈现明显的非线性变形特征;在文中实施的0.001s-1～1700s-1应变率加载范围内,聚碳酸酯拉伸力学性能具有明显的应变率相关性,其屈服应力和与屈服应力相对应的失稳应变均随应变率的增加有显著的增大。     

超高强冷轧双相钢DP1000高应变速率下的拉伸性能

使用CMT4105型电子万能试验机和霍普金森拉杆（SHTB）装置研究了超高强冷轧双相钢DP1000在室温下的准静态和动态拉伸力学性能。结果表明:应变速率范围在0.0001—2250 s^-1,DP1000双相钢具有明显的应变速率敏感性,表现出较强的应变速率增强效应,强度随着应变速率的增加而增加;Johnson-Cook模型能够在一定程度上描述DP1000双相钢在高应变速率下变形行为,但由于应变速率敏感性在高应变速率下吻合程度较差;对Johnson-Cook模型的应变速率效应多项式进行二次化修正后,模型能很好地描述DP1000双相钢在高应变速率下的变形行为,平均可决系数从0.9434提高到0.9850。     

Flow Behavior of Fresh Very High Strength Portland Cement Pastes

Flow behavior of fresh very high strength portland cement pastes prepared at various water/cement ratios was studied by using a rate-controlled coaxial cylinder viscometer (Rotovisko-Haake 20, system M5-osc., measuring device MV2P with serrated surfaces). The tests were performed under continuous flow conditions. Experimental shear stress and shear rate data were fitted very satisfactorily with various models, one of which has already been proposed by the authors. Excellent results were achieved also by applying the Quemada equation. In addition, the influence of two different commercial superplasticizing agents (Concretan RX and Ergomix 1000, the former based on polycyclic copolymers with modified structures, carrying hydroxylated side chains, and the latter on a modified polyacrylic resin) was studied with the aim of determining their optimum dosage and verifying the effectiveness by comparing rheological results with those obtained in a previous work on an ordinary portland cement. The use of superplasticizers modified rheological behavior of the pastes; however, no value of optimum dosage was found for any additive, but only a superplasticizer concentration range within which pastes presented very low values of viscosity. Moreover, both superplasticizers showed a greater effectiveness when added to HSPC pastes rather then the OPC ones formulated with the same water/cement ratio.     

Thermal Degradation of Polymers at High Temperatures

Work on thermal degradation of polymers has previously been carried out at temperatures up to about 500° C. In the present work the range has been extended to 850° C. Polystyrene was pyrolyzed in a vacuum and also in helium at atmospheric pressure at 362° and at 850° C. Analysis of the volatile products indicates that higher temperatures and higher pressures cause a greater fragmentation of the volatile products. Samples of poly (vinylidene fluoride), polyacrylonitrile, and polytrivinylbenzene, were pyrolyzed in a vacuum at temperatures from 350° to 800° C. The more volatile products were analyzed qualitatively and quantitatively in a mass spectrometer. The less volatile products were tested for their average molecular weight by a microcryoscopic method.Rates of thermal degradation were also determined for the last three polymers. The activation energies in the temperature range 218° to 440° C were found to be 48, 31, and 73 kcal/mole, respectively, for poly(vinylidene fluoride), polyacrylonitrile, and polytrivinylbenzene.     

镁合金高速冲击载荷下的变形行为研究进展

概述了国内外对镁合金在高应变率加载条件下的力学行为及其高速变形时组织中绝热剪切带形成机制的研究现状,归纳了在镁合金动态力学行为及微观变形机制方面以及镁合金中形成绝热剪切带带内组织成因研究方面存在的不足。基于此,提出了镁合金动态力学行为及其在动态变形过程中微观组织演变机制的研究重点,并指出了进一步尚需开展的工作。     

较高应变速率下结构钢屈服强度的估计

本文给出了一种考虑应变速率效应时材料屈服强度近似求法，它依据两组低应变速率的拉伸试验数据，对其它应变速率下的屈服强度进行估计，并具有良好的精度。     

Spin Transport in Very Dilute 3He-4He at Very High B/T

We report preliminary results for pulsed-NMR measurements on a very dilute ³ He- ⁴ He mixture (x ₃ =900 ppm) in a 14.74 T field at temperatures down to 3 mK. The aim of the project is to investigate low temperature transverse spin relaxation and possible spin diffusion anisotropy. Spin-echo and T₁-type measurements were employed to measure the transverse and longitudinal spin diffusion coefficients. At high temperatures both spin diffusion coefficients are well explained assuming a quasiparticle interaction consistent with previous experiments. Conversely, at low temperatures the measured value of || is apparently limited to about ten by a poorly-understood process that accelerates spin-echo decay. This hampers an accurate determination of the transverse diffusion coefficient at the lowest temperatures reached.     

Dithiopheneindenofluorene (TIF) Semiconducting Polymers with Very High Mobility in Field‐Effect Transistors

The charge‐carrier mobility of organic semiconducting polymers is known to be enhanced when the energetic disorder of the polymer is minimized. Fused, planar aromatic ring structures contribute to reducing the polymer conformational disorder, as demonstrated by polymers containing the indacenodithiophene ( IDT ) repeat unit, which have both a low Urbach energy and a high mobility in thin‐film‐transistor (TFT) devices. Expanding on this design motif, copolymers containing the dithiopheneindenofluorene repeat unit are synthesized, which extends the fused aromatic structure with two additional phenyl rings, further rigidifying the polymer backbone. A range of copolymers are prepared and their electrical properties and thin‐film morphology evaluated, with the co ‐benzothiadiazole polymer having a twofold increase in hole mobility when compared to the IDT analog, reaching values of almost 3 cm² V^?1 s^?1 in bottom‐gate top‐contact organic field‐effect transistors.     

高温下的金属基复合材料双剪切试样应力应变有限元分析

对金属基复合材料双剪切试样的应力应变分布进行蠕变有限元计算分析,得到受剪区的应力应变随时间的分布规律。通过典型取向试样分析,表明受剪区的应力应变与纤维随机分布平面和加载方向夹角θ之间存在很大的相关性:随角度θ的减小,其蠕变应力分布越均匀;当角度θ=45°时,其稳态蠕变率γ·最大。双剪切试样能得到较均匀的剪应力区域,适合研究材料纯剪应力状态下的蠕变行为。     

RH真空精炼吹氩参数对循环流动影响的数值分析

为研究RH真空脱气过程中的流动行为,建立了描述气泡驱动下的RH循环气-液两相流动的数学模型.基于欧拉-欧拉两流体模型,利用计算流体力学(CFD)商业软件FLUENT6.0,对不同充气量条件下的循环流量进行了预测.计算结果与实验数据的比较表明两者具有较好的一致性.应用该模型对充气压强与循环流量、充气量与上升管内气相及液相速度分布关系进行了数值模拟,用以理解其中的流动规律,为工程技术改进提供参考.