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EXPERIMENTAL AND ThEORETICAL INVESTIGATION OF PSEUDOELASTIC hYSTERESIS IN ShAPE MEMORY ALLOYS

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Acta Metallurgica Sinica(English Letters) 1999年第3期12卷 235-240页

作者： Gong, S.K. xu, h.B. Beijing Univ of Aeronautics and Astronautics Beijing China

The pseudoelastic hysteresis loop associated with phase equilibrium and the stress relaxation on the yield line were investigated, and the theoretical description was confirmed by deformation controlled load-deformation measurements on CuZnAl single crystals. It was found that in a loading-unloading process, the states of shape memory alloys run through a hysteresis loop, and the points that trigger the internal yield and recovery inside the pseudoelastic hysteresis loop define a line that traverses the hysteresis loop from the upper left to the lower right corner. This line was interpreted as a line of unstable phase equilibrium according to the thermodynamic arguments, which determines also the tendency for stress relaxation on the yield line section and for a stress growth on the recovery line section.

关键词： Aluminum copper alloys hysteresis Stress relaxation

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影响青藏高原植被生产力的极端气候阈值

中国科学：地球科学 2024年第6期54卷 1996-2007页

作者：朴正刚李湘怡徐浩王锴汤舒畅阚飞洪松柏北京大学城市与环境学院北京100871

多种与水分和热量相关的极端气候事件对青藏高原植被生长造成了不利影响.然而,目前尚不清楚多大程度的极端气候胁迫水平会导致植被生长出现强烈响应.为了填补这一知识空白,我们综合利用水文气象数据和卫星遥感获取的植被指数,检测了2001... 详细信息

多种与水分和热量相关的极端气候事件对青藏高原植被生长造成了不利影响.然而,目前尚不清楚多大程度的极端气候胁迫水平会导致植被生长出现强烈响应.为了填补这一知识空白,我们综合利用水文气象数据和卫星遥感获取的植被指数,检测了2001~2018年间影响植被生产力对干旱、极端高温和极端低温响应的两个关键气候阈值(反映植被生产力响应发生突变和对植被损害达到最大).结果表明,当土壤湿度下降到正常水平的(-1.41±0.6)个标准差(σ)以下时,植被生产力对干旱事件的响应频率会迅速增加;当土壤湿度低于(-2.98±0.9)σ时,干旱对植被生产力造成的损害达到最大.区域上极端低温的阈值平均值为(-1.44±0.5)σ和(-2.53±0.8)σ,而极端高温的阈值平均值为(1.34±0.4)σ和(2.31±0.4)σ.这表明,极端高温比极端低温更易引发植被生产力的强烈响应.此外,我们发现极端温度事件中土壤水分亏缺的复合效应降低了极端低温和极端高温的阈值.基于地球系统模式预测,到2100年,青藏高原的西南部和东北部的部分地区将有极高的风险受到超过阈值的极端气候事件影响.本研究基于极端气候阈值识别了青藏高原地区具有高影响力的极端气候事件,提高了对不同极端气候事件植被生长响应的认识,为评估极端气候事件对区域生态系统的影响提供了研究范例.

关键词：青藏高原极端气候阈值植被生产力

青藏高原及其周边二叠纪综合地层、生物群以及古地理和古气候演化

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中国科学：地球科学 2024年第4期54卷 1125-1170页

作者：沈树忠张以春袁东勋徐海鹏琚琦张华郑全锋罗茂侯章帅南京大学地球科学与工程学院内生金属矿床成矿机制研究国家重点实验室关键地球物质循环前沿科学中心南京210023 中国科学院南京地质古生物研究所现代古生物学和地层学国家重点实验室南京210008 中国科学院大学南京学院南京210049 中国矿业大学资源与地球科学学院徐州221116

二叠纪是青藏高原各块体演化的关键时段,发生了一系列重大地质和生物事件,当时横跨南北半球统一的泛大陆演化到了鼎盛阶段,阻断了全球东西向的赤道暖流的同时,在泛大陆的东侧形成了一个半封闭的、具有重要历史意义的特提斯洋暖池,是古-... 详细信息

二叠纪是青藏高原各块体演化的关键时段,发生了一系列重大地质和生物事件,当时横跨南北半球统一的泛大陆演化到了鼎盛阶段,阻断了全球东西向的赤道暖流的同时,在泛大陆的东侧形成了一个半封闭的、具有重要历史意义的特提斯洋暖池,是古-中生代之交生物与环境演变的引擎.青藏高原二叠纪地层分布广泛、化石丰富,从南到北由于跨越不同的纬度带体现出不同的特点.雅鲁藏布江以南的喜马拉雅特提斯带当时位于冈瓦纳大陆印度板块北缘,整个二叠纪沉积含有典型的冈瓦纳冷水型生物群;雅鲁藏布江缝合带内的灰岩外来体时代集中在瓜德鲁普世(Guadalupian,中二叠世)中、晚期至早三叠世,古生物地理与喜马拉雅特提斯带有显著差异,最有可能为新特提斯洋中的海山型沉积.基墨里(Cimmerian)微地块群在乌拉尔世(Cisuralian,早二叠世)早期阿瑟尔期-萨克马尔期(Asselian-Sakmarian)全部位于冈瓦纳大陆北缘,均发育冰海相沉积及冈瓦纳型冷水生物群.乌拉尔世中期(约290~280Ma)基墨里微地块群由于印度板块北缘Panjal Traps等大火成岩省岩浆活动与冈瓦纳大陆裂解,并以不同的速度向北漂移.可以分为两个条带,由南羌塘、保山和Sibuma地块组成的北基墨里微地块群漂移速度较快,在亚丁斯克期(Artinskian)就已经出现大量碳酸盐岩沉积和暖水生物群,因此,它们最先从冈瓦纳大陆裂解并进入低纬度暖水区域;而由拉萨、腾冲和Irrawaddy地块组成的南基墨里微地块群,漂移速度相对较慢,在空谷(Kungurian)晚期才开始出现大规模碳酸盐岩沉积,并延续至乐平世(Lopingian,晚二叠世),瓜德鲁普世和乐平世地层中含暖水生物为主的动物群,因此,这些地块至少在空谷晚期已经从冈瓦纳大陆北缘分离,裂解时间稍晚于北基墨里微地块群;这种沉积和古生物地理的时空演变差异性表明乌拉尔世的裂解事件造成了中特提斯洋和新特提斯洋的开启和形成.更北的北羌塘地块由于整个二叠纪地层中均含有典型的暖水生物群,与华南地区、思茅地块和印支地块等可以比较,属于古特提斯洋北部的块体.因此,二叠纪中、晚期特提斯域形成了古、中、新特提斯多岛洋共存的局面,是当时构造活动最活跃的地区和全球生物多样性的中心.此外,青藏高原多个块体在二叠纪位于南半球中高纬度地区,完整记录了晚古生代大冰期、亚丁斯克期全球变暖和二叠纪末快速变暖等重大古气候事件,对于阐明板块运动和全球气候演变对生物群时空分布的影响具有举足轻重的作用.

关键词：二叠纪特提斯洋青藏高原生物群古地理

NEUROCOMPUTING ThEORY AND NUMERICAL SIMULATION ON ELASTICITY

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Acta Mechanica Solida Sinica 1999年第3期12卷 269-278页

作者： Sun, Dh Sun, XF hi, Q xu, h Xiamen Univ Dept Electromech Engn Xiamen 361005 Peoples R China Sw Jiaotong Univ Dept Appl Mech & Engn Chengdu 610031 Peoples R China NE Univ Technol Inst Engn Mech Shenyang 110006 Peoples R China

A new finite element analysis principle based on neural networks is proposed. It can realize the finite element analysis in real time. The neural computation of a simple structure was simulated on digital computer, the effect of input parameters on convergent speed and precision was analyzed. Before solving a certain FE problem, the linking weight matrix of the neural net, i.e., the global stiffness matrix of the structure analyzed, must be defined. In this paper, a method which uses the BP net to compute the element stiffness matrix in real time is proposed.

关键词： neural networks elasticity finite element real-time computation

Stability and nonlinear vibrations of a flexible pipe parametrically excited by an internal varying flow density

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Acta Mechanica Sinica 2020年第1期36卷 206-219页

作者： W.D.Xie X.F.Gao W.h.xu State Key Laboratory of hydraulic Engineering Simulation and Safety Tianjin UniversityTianjin30072China

Pipes are often used to transport multiphase flows in many engineering *** total fluid flow density inside a pipe may vary with time and *** this paper,a simply supported pipe conveying a variable density flow is modeled theoretically,and its stability and nonlinear vibrations are investigated in *** variation of the flow density is simulated using a mathematical *** equation governing the vibration of the pipe is derived according to Euler-Bernoulli beam *** the internal flow density varies with time,the pipe is excited *** stability of the pipe is determined by Floquet *** simple parametric and combination resonances are *** a higher mass ratio(mean flow mass/pipe structural mass),higher flow velocity,or smaller end axial tension,the pipe becomes unstable more easily due to wider parametric resonance *** the subcritical flow velocity regime,the vibrations of the pipe are periodic and quasiperiodic for simple and combination resonances,***,in the supercritical regime,the vibrations of the pipe exhibit much richer dynamics including periodic,multiperiodic,quasiperiodic,and chaotic behaviors.

关键词： Pipe Varying flow density Parametric excitation Stability Nonlinear vibrations

Freeze-thaw Resistance of Concrete in Seawater

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武汉理工大学学报 2010年第17期32卷 108-111页

作者： ShUI Z h xu h G ZhAN B J School of Materials Science and Engineering Wuhan University of TechnologyWuhan 430070China Transportation Bureau of Xinganmeng Inner Mongolia Ulanhot 137400China

The effect of freeze-thaw cycles in seawater on the performance of concrete was studied in this paper. Concrete samples with different water binder ratio and different component were prepared,and were then put into fresh water and synthesized seawater. After the experiments of freeze-thaw cycles,the mass loss,relative dynamic elastic modulus (RDEM) and compressive strength of each sample were tested. The results obtained reveal that with the increase of the water binder ratio,the resistance ability of freeze-thaw impact in seawater of concrete decrease dramatically. When the concrete contain 15% fly ash and 20% slag,its resistance ability to the freeze-thaw impact in seawater is the optimal. Compared to the seawater corrosion,the impact of freeze-thaw cycles to the properties of concrete is severer.

关键词： freeze-thaw damage seawater mass loss relative dynamic elastic modulus

COORELATION ANALYSIS OF SURFACE PRESSURE FLUCTUATIONS ON ThREE EQUISPACED CYLINDERS

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Journal of hydrodynamics 2001年第2期13卷 55-62页

作者： Dong, L.Y. xu, Y.h. huang, D.Q. Inst. of Appl. Math. and Mech. Shanghai Univ. Shanghai 200072 China

The fluctuating pressures on the surface of three circular cylinders in equispaced arrangement were measured simultaneously in the medium subcritical regime (Re=3×104). The distributions of their rms values and circumferential correlation were obtained in the spacing range from 1.25 to 6.0. The spatial correlation of pressures of two pressure taps on two different cylinders at the same azimuth was investigated as well. Owing to different flow patterns in various arrangements and spacing, these correlation curves of pressure show different distributions. The lift fluctuation can be estimated qualitatively from the circumferential coefficient curve with the knowledge of the rms pressure distribution. Some critical spacing for each arrangement were found that the flow regimes and the corresponding pressure correlation take a sudden change. Flow interference between cylinders in each arrangement was also discussed with various flow regimes.

关键词： Cylinders (shapes) Experiments Flow interactions Flow patterns Pressure distribution Pressure measurement

Macroscopic and microscopic mechanical behaviors of climbing tendrils

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Acta Mechanica Sinica 2019年第3期35卷 702-710页

作者： Q.Guo J.J.Dong Y.Liu X.h.xu Q.h.Qin J.S.Wang Tianjin Key Laboratory of Modern Engineering Mechanics Department of MechanicsTianjin UniversityTianjin 300054China State Key Laboratory of Nonlinear Mechanics (LNM) Institute of MechanicsChinese Academy of ScienceBeijing 100080China Research School of Engineering Australian National UniversityCanberraACT 2601Australia

Tendril-bearing climbing plants must recur to the tendril helices with chiral perversion or dual chirality for climbing and to obtain sun exposure. Despite researchers' prolonged fascination with climbing tendrils since Darwin's time and even earlier, why the soft and slender tendrils can bear heavy loads such as the self-weight of a plant or additional load caused by rain remains elusive. In this paper, we take towel gourd tendrils as an example and investigate the macroscopic and microscopic mechanical behaviors of tendrils through experiments and simulations. Our study indicates that the tendril filament exhibits rubber-like hyperelastic behaviors and can particularly endure large elongation, which is mainly attributed to the superelasticity of the cellulose fibril helix contained in the cell wall. Combination of the tendril helical structure with dual chirality or chiral perversion at a macroscale and a cellulose filament helix at a subcellular level creates superior elasticity for biological species relying on support and climbing. This study provides deep insight into the structure-property relationship of climbing tendrils, and the relationship is useful for the bioinspired design of composite systems with superior elasticity.

关键词： Climbing tendril Mechanical behaviors Structure-property relationship Large elongation

Enhanced localized and uniform corrosion resistances of bulk nanocrystalline 304 stainless steel in high-concentration hydrochloric acid solutions at room temperature

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Journal of Materials Science & Technology 2018年第12期34卷 2498-2506页

作者： S.G.Wang M.Sun Y.h.xu K.Long Z.D.Zhang Shenyang National Laboratory for Materials Science Institute of Metal ResearchChinese Academy of Sciences

The localized and uniform corrosion resistances of bulk nanocrystalline 304 stainless steel（NC-304 SS）produced by severe rolling technique, and its conventional polycrystalline 304 stainless steel（CC-304 SS）counterpart, were investigated in high-concentration hydrochloric acid solutions at room ***-304 SS can scarcely suffer from localized corrosion in 4 mol/L and 5 mol/L hCl solutions during 5-day immersion tests, and in 1-3 mol/L hCl solutions during thirty-five-day immersion tests. The corrosion rate of NC-304 SS was also less than that of CC-304 SS during these immersion tests. The improved localized and uniform corrosion resistances of NC-304 SS were explained in terms of the adsorption and chemical activity of Cl;on NC-304 SS and CC-304 SS characterized by X-ray photoelectron spectroscopy,and the valence electron configurations of NC-304 SS and CC-304 SS were characterized by ultra-violet photoelectron spectroscopy rather than conventional electrochemical results.

关键词： Stainless steels Localized corrosion Severe rolling technique X-ray photoelectron spectroscopy Ultra-violet photoelectron spectroscopy