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Discrete fine-scale models, in the form of either particle or lattice models, have been formulated successfully to simulate the behavior of quasi-brittle materials whose mechanical behavior is inherently connected to fracture processes occurring in the internal heterogeneous structure. These models tend to be intensive from the computational point of view as they adopt an “a priori” discretization anchored to the major material heterogeneities (e.g. grains in particulate materials and aggregate pieces in cementitious composites) and this hampers their use in the numerical simulations of large systems. In this work, this problem is addressed by formulating a general multiple scale computational framework based on classical asymptotic analysis and that (1) is applicable to any discrete model with rotational degrees of freedom; and (2) gives rise to an equivalent Cosserat continuum. The developed theory is applied to the upscaling of the Lattice Discrete Particle Model (LDPM), a recently formulated discrete model for concrete and other quasi-brittle materials, and the properties of the homogenized model are analyzed thoroughly in both the elastic and the inelastic regime. The analysis shows that the homogenized micropolar elastic properties are size-dependent, and they are functions of the RVE size and the size of the material heterogeneity. Furthermore, the analysis of the homogenized inelastic behavior highlights issues associated with the homogenization of fine-scale models featuring strain-softening and the related damage localization. Finally, nonlinear simulations of the RVE behavior subject to curvature components causing bending and torsional effects demonstrate, contrarily to typical Cosserat formulations, a significant coupling between the homogenized stress–strain and couple-curvature constitutive equations. 相似文献
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In this study we show that some discussions of finite-deformation single-crystal plasticity are conceptually flawed in their focus on a set referred to as the intermediate configuration. Specifically, we prove that what is usually referred to as the intermediate configuration is not a configuration but instead a vector space that we term the intermediate space. We argue that when applied to single crystals this intermediate space represents the lattice. 相似文献
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There has been much research and speculation recently on the nature of radiation induced defects in zirconium and its alloys, and in particular on the absence of voids at high fluences and temperatures in the range 0.3 to 0.5 T m (T m is the absolute melting temperature). Wolfenden and Farrell1 have reviewed the evidence and suggest that α-Zr has so far resisted void formation during neutron irradiation because of: (a) the absence of a dislocation (loop or tangle) structure and/or (b) a low insoluble gas (e.g. helium) content. 相似文献
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《Optimization》2012,61(5-6):439-457
For the many-to-one matching model with firms having substitutable and q-separable preferences we propose two very natural binary operations that together with the unanimous partial ordering of the workers endow the set of stable matchings with a lattice structure. We also exhibit examples in which, under this restricted domain of firms' preferences, the classical binary operations may not even be matching 相似文献
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可持续能源的迅速发展,使绿色清洁的氢能源成为热点。质子交换膜(PEM)水电解是一项很有前途的技术,可高效生产高纯度氢气。IrO_(2)作为质子交换膜(PEM)水电解槽阳极氧析出反应(OER)的商用电催化剂,既能在强酸性、高强度腐蚀条件下保持稳定,又表现出优异的催化性能。然而,由于Ir的稀缺性和昂贵的价格,提高Ir基催化剂的OER活性,开发低Ir催化剂就显得至关重要。对其反应机理的认知是当前的研究热点之一,也是设计优异的OER催化剂的关键所在。因此,首先从OER机理出发,对目前被广泛认可的吸附物逸出机理(AEM)和晶格氧逸出机理(LOER)两种反应机理进行了研究。随后,根据所提出的这两种机理,介绍了OER催化剂设计的基本准则,即调控Ir基催化剂的电子结构,改善反应中间物种在催化活性位点上的吸附能,从而提高OER催化活性。并从催化剂的结构设计、形貌控制、载体材料3个方面简单概述了最近OER催化剂的研究进展。最后,在已有研究的基础上,提出了目前OER催化剂面临的困难与挑战,这为以后相关的研究指明了方向。 相似文献
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《中国化学快报》2021,32(11):3613-3618
Spatial isolation of different functional sites at the nanoscale in multifunctional catalysts for steering reaction sequence and paths remains a major challenge. Herein, we reported the spatial separation of dual-site Au and RuO2 on the nanosurface of TiO2 (Au/TiO2/RuO2) through the strong metal-support interaction (SMSI) and the lattice matching (LM) for robust photocatalytic hydrogen evolution. The SMSI between Au and TiO2 induced the encapsulation of Au nanoparticles by an impermeable TiOx overlayer, which can function as a physical separation barrier to the permeation of the second precursor. The LM between RuO2 and rutile-TiO2 can increase the stability of RuO2/TiO2 interface and thus prevent the aggregation of dual-site Au and RuO2 in the calcination process of removing TiOx overlayer of Au. The photocatalytic hydrogen production is used as a model reaction to evaluate the performance of spatially separated dual-site Au/TiO2/RuO2 catalysts. The rate of hydrogen production of the Au/TiO2/RuO2 is as high as 84 μmol h−1 g−1 under solar light irradiation without sacrificial agents, which is 2.5 times higher than the reference Au/TiO2 and non-separated Au/RuO2/TiO2 samples. Systematic characterizations verify that the spatially separated dual-site Au and RuO2 on the nanosurface of TiO2 can effectively separate the photo-generated carriers and lower the height of the Schottky barrier, respectively, under UV and visible light irradiation. This study provides new inspiration for the precise construction of different sites in multifunctional catalysts. 相似文献