儿茶素-胞嘧啶分子间相互作用的密度泛函研究

采用密度泛函理论的B3LYP方法,在6-31+G*基组水平上研究了儿茶素-胞嘧啶分子间相互作用机制,得到稳定的儿茶素-胞嘧啶复合物11个.计算结果表明氢键对于复合物的稳定性起着重要的作用,并且当复合物形成2个或更多的氢键时,氢键的类型及强度共同决定着复合物的稳定性.我们还应用了分子中的原子(AIM)理论和自然键轨道(NBO)理论对这11种复合物中氢键的性质和特征进行了分析.通过研究发现,所有的氢键复合物进行基组重叠误差(BSSE)校正后的相互作用能为-17.35～-43.27kJ/mol,相互作用能主要由氢键所贡献.振动分析显示,氢键的形成使相对应键的对称伸缩振动频率减小,说明这些复合物中形成的氢键都是正常的红移型氢键,与实验结果相一致.     

Manipulation of Single‐Stranded DNA by Using an Artificial Site‐Selective DNA Cutter Composed of Cerium(IV)/EDTA and Phosphonate–Oligonucleotide Conjugates

An artificial site‐selective DNA cutter to hydrolyze single‐stranded DNA at a desired site was prepared from Ce^IV/ethylenediamintetraacetic acid (EDTA) and two ethylenediamine‐N,N,N′,N′‐tetrakis(methylenephosphonic acid)–oligonucleotide conjugates. By using this cutter, the sense strand of a blue fluorescent protein (BFP) gene was selectively cut at a predetermined site in the chromophore‐coding region. The upstream fragment obtained by the site‐selective scission was ligated with the downstream fragment of the closely related green fluorescent protein (GFP) gene so that the 5′‐ and 3′‐end portions of the chromophore came from the BFP fragment and the GFP fragment, respectively. The recombinant gene was successfully expressed in E. coli and the chimeric chromophore emitted green fluorescence as expected.     

A multi-field incremental variational framework for gradient-extended standard dissipative solids

The paper presents a constitutive framework for solids with dissipative micro-structures based on compact variational statements. It develops incremental minimization and saddle point principles for a class of gradient-type dissipative materials which incorporate micro-structural fields (micro-displacements, order parameters, or generalized internal variables), whose gradients enter the energy storage and dissipation functions. In contrast to classical local continuum approaches to inelastic solids based on locally evolving internal variables, these global micro-structural fields are governed by additional balance equations including micro-structural boundary conditions. They describe changes of the substructure of the material which evolve relatively to the material as a whole. Typical examples are theories of phase field evolution, gradient damage, or strain gradient plasticity. Such models incorporate non-local effects based on length scales, which reflect properties of the material micro-structure. We outline a unified framework for the broad class of first-order gradient-type standard dissipative solids. Particular emphasis is put on alternative multi-field representations, where both the microstructural variable itself as well as its dual driving force are present. These three-field settings are suitable for models with threshold- or yield-functions formulated in the space of the driving forces. It is shown that the coupled macro- and micro-balances follow in a natural way as the Euler equations of minimization and saddle point principles, which are based on properly defined incremental potentials. These multi-field potential functionals are outlined in both a continuous rate formulation and a time-space-discrete incremental setting. The inherent symmetry of the proposed multi-field formulations is an attractive feature with regard to their numerical implementation. The unified character of the framework is demonstrated by a spectrum of model problems, which covers phase field models and formulations of gradient damage and plasticity.     

The issues of the uniqueness and the stability of the homogeneous response in uniaxial tests with gradient damage models

We consider a wide class of gradient damage models which are characterized by two constitutive functions after a normalization of the scalar damage parameter. The evolution problem is formulated following a variational approach based on the principles of irreversibility, stability and energy balance. Applied to a monotonically increasing traction test of a one-dimensional bar, we consider the homogeneous response where both the strain and the damage fields are uniform in space. In the case of a softening behavior, we show that the homogeneous state of the bar at a given time is stable provided that the length of the bar is less than a state dependent critical value and unstable otherwise. However, we also show that bifurcations can appear even if the homogeneous state is stable. All these results are obtained in a closed form. Finally, we propose a practical method to identify the two constitutive functions. This method is based on the measure of the homogeneous response in a situation where this response is stable without possibility of bifurcation, and on a procedure which gives the opportunity to detect its loss of stability. All the theoretical analyses are illustrated by examples.     

非均匀材料损伤破坏的多尺度耦合理论

为研究微损伤对非均匀材料宏观破坏的影响,建立了非均匀材料的三维微单元强度分布模型,在平均场模型和集团分担模型的基础上,根据模拟计算结果建立了邻近单元加权分担模型;采用细观统计的方法建立了微观损伤造成宏观破坏的概率函数及跨尺度敏感性函数.结果表明:材料的损伤分数和名义应力越大,破坏集团之间的距离越近,宏观破坏概率就越大;...     

局部裂纹损伤简支梁的曲率模态特性

将裂缝损伤简化成矩形凹槽,采用delta函数表示简支梁的裂纹损伤位置,得到了全梁范围内截面转动惯量和单位长度质量的表达式,建立了局部裂缝损伤简支梁的横向自由振动方程.利用摄动方法给出了裂纹摄动项的一般表达式,根据摄动项和完整梁都同时满足边界条件的特点,将一阶和二阶摄动项都表示成完整梁模态的线性组合,结合delta函数的性质,最终获得了受损简支梁的特征值和模态振型的解析表达式.最后,通过数值计算得到结构模态参数,对比了一阶摄动和二阶摄动对计算结果的影响,分析了不同阶固有频率和模态曲率的变动量,为简支梁的损伤监控和检测提供了理论依据.     

陶瓷靶抗射流侵彻的理论模型及数值模拟

以球型空腔膨胀理论为基础,提出了一个计算陶瓷靶板阻力的损伤模型,该模型考虑了损伤因子对陶瓷靶板弹道性能的影响.结合不可压缩流体力学理论,对射流侵彻陶瓷靶板的侵彻速度进行了理论值计算,并与未考虑损伤的侵彻速度进行了比较,该模型的计算结果更接近实验结果.建立了射流侵彻陶瓷靶板的数值计算模型,对铜射流侵彻陶瓷靶的动态破坏过程进行了研究,讨论了药型罩的锥角、壁厚对射流侵彻结果的影响,结果表明:相同锥角的药型罩,壁厚对陶瓷靶板孔径的影响较小;同壁厚的药型罩,随着锥角的增大,侵彻孔径增大.侵彻速度的数值模拟结果与理论结果进行了比较,得到了较好的一致性.