43 resultados para eco-plasticity
em Chinese Academy of Sciences Institutional Repositories Grid Portal
Resumo:
陆地样带是国际地圈——生物圈计划(IGBP)研究中最引入注目的创新之一。目前,国际上已经设立了15条陆地样带,研究内容涉及环境梯度分析、气候变化对植被初级生产力的影响及环境变化、土地利用等与植被变化的对应关系等。沿该陆地样带分布较广的关键种生理适应性等方面对影响其生理功能形态结构的研究较少,特别是茎、叶等组织功能研究较少。 中国东北样带(NECT)是全球陆地样带的重要组成部分,多年来已开展了大量深入系统的研究工作,已成为我国生态学、地学等学科的重要研究平台。本研究以中国东北样带中西段广泛分布的重要关键种——羊草(Leymus chinensis)为研究对象,分析了羊草茎、叶显微结构的生态可塑性及其与水分利用效率的关系,进而阐述了羊草适应不同生境条件,特别是适应水分变化的机制,为揭示羊草及其种群、群落乃至以羊草为优势种或建群种的草地生态系统在全球变化背景下的发展趋势提供理论依据。 基于2001年7~8月第3次中国东北样带考察资料,采用高精度Olympus显微镜及C同位素分析技术(δ13C判别值),结合在野外取样过程中测定的样地土壤含水量和海拔高度,以及近十年各样地年降水量和年均温度气象资料,分析了羊草茎、叶显微结构和水分利用变化与环境因子的关系,以及以羊草为建群种或共建种的无牧和放牧样地群落生物量、物种多样性和植物功能型组成变化与环境因子的关系。 结果表明:羊草叶片表面及内部主要显微结构特征参数各样地间有不同程度的差异,其中气孔密度与降水量呈线性正相关。代表气孔开张程度的气孔长度和宽度变化与土壤含水量呈线性相关。叶表面角质层厚度与海拔高度变化关系较大,并以上表面角质层厚度变化最为明显,主要受海拔高度升高引起的紫外线照射增强的影响。运动细胞带宽度占叶面积比虽然与各环境因子关系不很密切,但温度变化的影响较突出,这一显微结构调整与气孔变化构成干旱——高温调节机制。叶片表面毛茸的变化也是非常显著的,但与各环境因子关系密切程度均不大,可以肯定的是在土壤水分状况较好的生境下羊草叶片表面毛茸密度及长度明显增加,而一些干旱生境中常表现为毛茸较少、较短,个别样地基本没有发育较好的毛茸。总体上看,羊草叶片对干旱化的形态结构调整以气孔密度和开张程度的变化最大,是羊草叶片调节水分利用效率的重要适应性生态可塑性调整。 与叶片相比,羊草茎横切面结构特征的变化与各环境因子关系的显著性不是很强,但各样地间的差异是比较显著的,许多结构调整可能与土壤养分条件的变化有一定关系,如茎秆粗度变化、基本薄壁组织厚度和中央空腔(髓腔)直径的变化等,但本研究未能涉及这方面内容,有待于进一步研究。 羊草水分利用效率与降水量和土壤含水量呈显著的负相关关系,即随降水量和土壤含水量增大羊草水分利用效率明显降低,蒸腾耗水增大,这一生理变化与显微结构的调整关系密切,特别是气孔密度与气孔宽度在水分较差生境中明显减小,从而有利于适应干旱环境,减少耗水量。表现比较突出的是非地带性林西样地,其降水量处于10个样地的中等偏低水平,但其δ13C判别值较低,达-26.063‰,与降水量较大的长岭、双辽样地几乎相当,并比相邻的林东和克旗样地明显低,其气孔密度、开张程度及叶脉后生导管直径均较高(大),但其土壤水分状况是最好的样地之一,尽管取样时不幸遇到雨天,但从其群落类型——羊草杂类草草甸,并伴生许多中、湿生种类上看,其生境的湿润程度是毋庸置疑的。这一非地带性样地中羊草结构的变化从另一侧面反映了羊草显微结构调整对水分环境的适应。δ13C判别值是一个非常敏感的参数,在分析植物水分利用效率及其相关领域的研究中应深入利用。 群落植物功能群组成与环境因子及群落初级生产力关系研究结果表明,丛生禾草生长型功能群、旱生和中旱生植物水分生态类型功能群具有明显的地带性变化规律,并与群落生物量变化关系密切,变异性较低,占群落生物量比例较大,可考虑作为植物功能型组合对无牧样地植被变化进行评估和预测。在放牧影响下,C4植物光合类型功能群呈现明显的地带性变化,并在群落中所起的作用明显增强,亦可考虑作为评估和预测植被变化的植物功能型组合。无牧样地与放牧样地研究结果均表明,按Raunkiaer划分的地面芽、地下芽、地上芽和一年生植物生活型功能群,其地带性变化不明显,或变异率高,或占群落生物量比例小,不宜作植物功能群组合对植被变化进行评估和预测。
Resumo:
A composite material containing uniformly distributed micrometer-sized Nb particles in a Zr-based amorphous matrix was prepared by suction cast. The resulting material exhibits high fractured strength over 1550 MPa and enhanced plastic strain of about 29.7% before failure in uniaxial compression test at room temperature. Studies of the serrations on the stress-strain curves and the shear bands on the fractured samples reveal that the amplitude of the stress drop of each serration step corresponds to the extent of the propagation of a single shear band through the materials. The composite exhibits more serration steps and smaller amplitude of stress drop due to the pinning of shear band propagation by ductile Nb particles.
Resumo:
The flow theory of mechanism-based strain gradient (MSG) plasticity is established in this paper following the same multiscale, hierarchical framework for the deformation theory of MSG plasticity in order to connect with the Taylor model in dislocation mechanics. We have used the flow theory of MSG plasticity to study micro-indentation hardness experiments. The difference between deformation and flow theories is vanishingly small, and both agree well with experimental hardness data. We have also used the flow theory of MSG plasticity to investigate stress fields around a stationary mode-I crack tip as well as around a steady state, quasi-statically growing crack tip. At a distance to crack tip much larger than dislocation spacings such that continuum plasticity still applies, the stress level around a stationary crack tip in MSG plasticity is significantly higher than that in classical plasticity. The same conclusion is also established for a steady state, quasi-statically growing crack tip, though only the flow theory can be used because of unloading during crack propagation. This significant stress increase due to strain gradient effect provides a means to explain the experimentally observed cleavage fracture in ductile materials [J. Mater. Res. 9 (1994) 1734, Scripta Metall. Mater. 31 (1994) 1037; Interface Sci. 3(1996) 169].
Resumo:
A new hardening law of the strain gradient theory is proposed in this paper, which retains the essential structure of the incremental version of conventional J(2) deformation theory and obeys thermodynamic restrictions. The key feature of the new proposal is that the term of strain gradient plasticity is represented as an internal variable to increase the tangent modulus. This feature which is in contrast to several proposed theories, allows the problem of incremental equilibrium equations to be stated without higher-order stress, higher-order strain rates or extra boundary conditions. The general idea is presented and compared with the theory given by Fleck and Hutchinson (Adv. in Appl. Mech. (1997) 295). The new hardening law is demonstrated by two experimental tests i.e. thin wire torsion and ultra-thin beam bending tests. The present theoretical results agree well with the experiment results.
Resumo:
Cowper-Symonds and Johnson-Cook dynamic constitutive relations are used to study the influence of both strain rate effect and temperature variation on the material intrinsic length scale in strain gradient plasticity. The material intrinsic length scale decreases with increasing strain rates, and this length scale increases with temperature.
Resumo:
Mode I steady-state crack growth is analyzed under plane strain conditions in small scale yielding. The elastic-plastic solid is characterized by the mechanism-based strain gradient (MSG) plasticity theory [J. Mech. Phys. Solids 47 (1999) 1239, J. Mech. Phys. Solids 48 (2000) 99]. The distributions of the normal separation stress and the effective stress along the plane ahead of the crack tip are computed using a special finite element method based on the steady-state fundamental relations and the MSG flow theory. The results show that during the steady-state crack growth, the normal separation stress on the plane ahead of the crack tip can achieve considerably high value within the MSG strain gradient sensitive zone. The results also show that the crack tip fields are insensitive to the cell size parameter in the MSG theory. Moreover, in the present research, the steady-state fracture toughness is computed by adopting the embedded process zone (EPZ) model. The results display that the steady-state fracture toughness strongly depends on the separation strength parameter of the EPZ model and the length scale parameter in the MSG theory. Furthermore, in order for the results of steady crack growth to be comparable, an approximate relation between the length scale parameters in the MSG theory and in the Fleck-Hutchinson strain gradient plasticity theory is obtained.
Resumo:
Ceramic/metal interfaces were studied that fail by atomistic separation accompanied by plastic dissipation in the metal. The macroscopic toughness of the specific Ni alloy/Al2O3 interface considered is typically on the order of ten times the atomistic work of separation in mode I and even higher if combinations of mode I and mode II act on the interface. Inputs to the computational model of interface toughness are: (i) strain gradient plasticity applied to the Ni alloy with a length parameter determined by an indentation test, and (ii) a potential characterizing mixed mode separation of the interface fit to atomistic results. The roles of the several length parameters in the strain gradient plasticity are determined for indentation and crack growth. One of the parameters is shown to be of dominant importance, thus establishing that indentation can be used to measure the relevant length parameter. Recent results for separation of Ni/Al2O3 interfaces computed by atomistic methods are reviewed, including a set of results computed for mixed mode separation. An approximate potential fit to these results is characterized by the work of separation, the peak separation stress for normal separation and the traction-displacement relation in pure shearing of the interface. With these inputs, the model for steady-state crack growth is used to compute the toughness of the interface under mode I and under the full range of mode mix. The effect of interface strength and the work of separation on macroscopic toughness is computed. Fundamental implications for plasticity-enhanced toughness emerge.
Resumo:
Based on detailed x-ray diffraction and transmission electron microscopy we have found body-centered-cubic (bcc) Ni upon room-temperature rolling of nanocrystalline (nc) face-centered-cubic (fcc) Ni. The bcc phase forms via the Kurdjumov-Sachs (KS) martensitic transformation mechanism when the von Mises equivalent strain exceeds similar to 0.3, much higher than accessible in tensile testing. The fcc and bcc phases keep either the KS or the Nishiyama-Wasserman orientation relationship. Our results provide insights into the deformation physics in nc Ni, namely, the fcc-to-bcc phase transformation can also accommodate plasticity at large plastic strains. (C) 2008 American Institute of Physics.
Resumo:
Material potential energy is well approximated by '' pair-functional '' potentials. During calculating potential energy, the orientational and volumetric components have been derived from pair potentials and embedding energy, respectively. Slip results in plastic deformation, and slip component has been proposed accordingly. Material is treated as a component assembly, and its elastic, plastic and damage properties are reflected by different components respectively. Material constitutive relations are formed by means of assembling these three kinds of components. Anisotropy has been incorporated intrinsically via the concept of component. Theoretical and numerical results indicate that this method has the capacity of reproducing some results satisfactorily, with the advantages of physical explicitness, etc. (c) 2007 Elsevier Ltd. All rights reserved.
Resumo:
An analytical method for determining slip shear rate under prescribed stress rate or prescribed strain rate has been presented on the basis of the incremental theory of crystal plasticity. The problem has been reduced to a quadric convex programming.In order to analyse the plastic response of crystals subjected to external load, two new extremum principles are proposed. They are equivalent to the boundary-value problem of crystal plasticity. By the new extremum principles, the slip shear rates are independent function which can be obtained from the variational equation.
Resumo:
Shear banding characterization of Zr64.13Cu15.75Ni10.12Al10 and Zr65Cu15Ni10Al10 bulk metallic glasses (BMGs) with significant difference in inherent plasticity and quite similar chemical composition was studied by depth sensitive macroindentaion tests with conical indenter. Well-developed shear band pattern can be found for both BMGs after indentation. Distinct difference in the shear band spacing, scale of plastic deformation region and the shear band branching in the two BMGs account for the different plasticity.
Resumo:
Shear banding characterization of Zr64.13Cu15.75Ni10.12Al10 and Zr65Cu15Ni10Al10 bulk metallic glasses (BMGs) with significant difference in inherent plasticity and quite similar chemical composition was studied by depth sensitive macroindentaion tests with conical indenter. Well-developed shear band pattern can be found for both BMGs after indentation. Distinct difference in the shear band spacing, scale of plastic deformation region and the shear band branching in the two BMGs account for the different plasticity.
Resumo:
Through a systematical analysis of the elastic moduli for 137 metallic glasses (MGs) and 56 polycrystalline metals, we use a simple model developed by Knuyt et al. [J. Phys. F: Met. Phys. 16 (1986) p.1989; Phil. Mag. B 64 (1991) p.299] based on a Gaussian distribution for the first-neighbor distance to reveal the short-range-order (SRO) structural conditions for plasticity of MGs. It is found that the SRO structure with dense atomic packing, large packing dispersion and a significant anharmonicity of atomic interaction within an MG is favorable for its global plasticity. Although these conditions seem paradoxical, their perfect matching is believed to be a key for designing large plastic bulk MGs not only in compression but also in tension.
Resumo:
本书系统地介绍了材料微尺度力学行为的尺寸效应实验现象,重点介绍了几种代表性的微尺度应变梯度塑性理论及对微尺度实验现象的解释;以及对裂纹尖端微尺度范围内解理断裂的应用。融会贯通的介绍了国内外学者的原创性工作和创新性学术思想。 全书共8章。第1章介绍了应变梯度塑性理论的应用背景及经典微极理论;第2章介绍了金属材料典型的微尺度力学实验现象;第3~7章介绍了几种典型的应变梯度理论及其应用;第8章介绍了应变梯度理论在微观断裂力学中的应用。 本书适合从事固体微尺度力学、先进材料的微结构设计与力学性能优化、微机电和微电子元件力学行为研究的科技工作者及工程师使用和参考,也可供力学专业及材料专业的高年级本科生和研究生阅读参考。