Strain gradient theory based on a new framework of non-local model

A new framework of non-local model for the strain energy density is proposed in this paper. The global strain energy density of the representative volume element is treated as a non-local variable and can be obtained through a special integral of the local strain energy density. The local strain energy density is assumed to be dependent on both the strain and the rotation-gradient. As a result of the non-local model, a new strain gradient theory is derived directly, in which the first and second strain gradients, as well as the triadic and tetradic stress, are introduced in the context of work conjugate. For power law hardening materials, size effects in thin metallic wire torsion and ultra-thin cantilever beam bend are investigated. It is found that the result predicted by the theoretical model is well consistent with the experimental data for the thin wire torsion. On the other hand, the calculation result for the micro-cantilever beam bend clearly shows the size effect.

高压扭转Cu试样微观组织的热稳定性分析

通过高压扭转对Cu试样施加不同程度的变形,利用OM,TEM及差示扫描量热仪(DSC)对变形组织微观结构及其热稳定性进行了分析.在较小的变形程度下,变形组织为高位错密度的位错胞、亚晶组织,试样的变形储能随变形量的增大而增大,在切应变等于13时达到最大,为0.91 J/mol,DSC曲线显示的放热峰随变形量的增大向低温方向偏移;进一步变形,动态回复加剧,高位错密度的亚晶组织逐渐演化成无位错的等轴状晶粒组织,试样的变形储能减小,组织的稳定性提高.显微硬度随退火温度的提高而减小,晶粒的明显长大导致显微硬度急剧减小.出现明显晶粒长大的温度较DSC曲线显示的放热峰起始温度低45℃左右,这主要是由于变形组织的回复再结晶过程是退火温度与时间的函数,降低处理温度并延长处理时间能达到与高温短时处理相同的效果.

转速对高压扭转Cu试样的组织与性能的影响

通过高压扭转(HPT)技术在不同转速条件下实现了Cu试样的晶粒细化.利用光学显微镜(OM),透射电镜(TEM)及显微硬度计观察并测试了组织的结构与性能,并基于有限元计算了变形诱导试样的温升,研究了转速对Cu试样的组织细化与性能的影响.结果表明:转速由1/3r.min-1增大至1r.min-1,经1圈扭转变形,试样温度由40.8℃升高到54.1℃,变形组织均为100~600nm的高位错密度位错胞/亚晶组织,显微硬度由初始态的52HV0.05增大至140HV0.05;经16圈扭转变形,试样温度由50.4℃升高到97.4℃,组织细化到200nm.慢速扭转变形试样晶内位错密度高,微观组织处于严重变形状态;而快速扭转试样晶内衬度均匀,位错较少,微观组织经历明显的动态回复,显微硬度较慢速扭转变形试样低6%

Conformational and orientational analyses of 2,2 '-bithiophene under interaction of external electric field constructed by point charges

The fully relaxed single-bond torsional potentials and orientation-related rotational potentials of 2,2'-bithiophene (BT) under the interaction of an external electric field (EF) constructed by point charges have been evaluated with semi-empirical AMI and PM3 calculations. The torsional potentials are sensitive to both EF strength and direction. While the EF is parallel to the molecular long axis, the torsional barrier around C-x-C-x' bond obviously rises with increasing the EF strength, whereas the relative energies of syn and anti minima show a slight change. The interaction between the EF and the induced dipole moment has been proposed to elucidate this observation. On the other hand, the relative energy difference between the syn and anti minima shows an obvious change, while the EF is perpendicular to the molecular long axis. This feature has been ascribed to the interaction between the EF and the permanent dipole moment of BT. Furthermore, conformational and orientational analyses in two dimensions have been carried out by changing the torsional and rotational angles in the different EF. The conformation and orientation of a gas-phase BT in the EF are governed by both the above factors.

Synthesis and characterization of (phenolphthalein/4,4 '-thiodiphenol) PES copolymers

Copolymers based on monomers phenolphthalein (PP)/4,4'-thiodiphenol (Bis-T)/4,4'-dichlorodiphenylsulfone (DCDPS) were prepared by a route involving the toluene, N-methyl-2-pyrrolidone and anhydrous potassium carbonate synthesis. The range of optimum reaction temperature was between 185 and 195 degrees C. The copolymers were characterized by C-13 NMR, differential scanning calorimetry (DSC) and torsion braid analysis. It was found that all of the copolymers were random and homogeneous and their glass transition temperatures (T-g) decreased linearly with an increase of Bis-T contents in the copolymers. The thermal stability determined by thermogravimetry analysis in air atmosphere indicated that the copolymer had better resistance to thermo-oxidative degradation. Dynamic mechanical measurement showed that (PP/Bis-T) PES copolymers containing 0-50 mol% of Bis-T components had two secondary relaxations. (C) 1998 Elsevier Science Ltd. All rights reserved.

Crystal packing of poly(ether ketone ketone)

The crystal structure of poly(ether ketone ketone) (PEKK) is predicted by using Cerius2 software according to the wide angle X-ray diffraction (WAXD) experiment result. The predicted structure has a planar zigzag chain conformation between ether oxygen and ketone carbons in an orthorhombic lattice. Average zigzag angle is 126 degrees and average torsion angle is 30.32 degrees. The WAXD powder pattern calculated from the crystal packing model is in good agreement with the experiment result.

线形脑啡肽(N—Tyr~1—Gly~2—Gly~3—Phe~4—Leu~5)的NMR溶液构象研究

本文在Bruker AM-400 NMR谱仪上,在不同温度下研究了线形脑啡肽(N-Tyr~1-Gly~2-Gly~3-Phe~4-Leu~5)在DMSO中的NMR溶液构象。由NMR测试结果,得到了NH化学位移温度梯度系数、扭转角φ、χ＇约束和~1H-~1H NOE距离约束,用目标函数法计算了脑啡肽的溶液构象,分析了优势边链构象。研究结果指明了多肽骨架的柔变性且处于构象平衡中。

一种车用扭杆测试扭床

结合生产应用实际,根据扭杆在实际应用中的情况,利用扭床进行模拟,从而测试扭杆的机械特性及相关的各项指标,为扭杆生产提供可靠的质量保证。

Simulations of Two-Dimensional Foam under Couette Shear

null Sponsorship: Financial support is acknowledged from the University of Wales Aberystwyth Senate Fund, the Ulysses France-Ireland Exchange Scheme and EPSRC (EP/D014956/1, EP/D048397/1, EP/D071127/1).

SK1 of graded division algebras

The reduced Whitehead group $\SK$ of a graded division algebra graded by a torsion-free abelian group is studied. It is observed that the computations here are much more straightforward than in the non-graded setting. Bridges to the ungraded case are then established by the following two theorems: It is proved that $\SK$ of a tame valued division algebra over a henselian field coincides with $\SK$ of its associated graded division algebra. Furthermore, it is shown that $\SK$ of a graded division algebra is isomorphic to $\SK$ of its quotient division algebra. The first theorem gives the established formulas for the reduced Whitehead group of certain valued division algebras in a unified manner, whereas the latter theorem covers the stability of reduced Whitehead groups, and also describes $\SK$ for generic abelian crossed products.

Unitary SK1 of unitary and graded division algebras

The reduced unitary Whitehead group $\SK$ of a graded division algebra equipped with a unitary involution (i.e., an involution of the second kind) and graded by a torsion-free abelian group is studied. It is shown that calculations in the graded setting are much simpler than their nongraded counterparts. The bridge to the non-graded case is established by proving that the unitary $\SK$ of a tame valued division algebra wih a unitary involution over a henselian field coincides with the unitary $\SK$ of its associated graded division algebra. As a consequence, the graded approach allows us not only to recover results available in the literature with substantially easier proofs, but also to calculate the unitary $\SK$ for much wider classes of division algebras over henselian fields.

K-Theory of Azumaya Algebras over Schemes

Let X be a connected, noetherian scheme and A{script} be a sheaf of Azumaya algebras on X, which is a locally free O{script}-module of rank a. We show that the kernel and cokernel of K(X) ? K(A{script}) are torsion groups with exponent a for some m and any i = 0, when X is regular or X is of dimension d with an ample sheaf (in this case m = d + 1). As a consequence, K(X, Z/m) ? K(A{script}, Z/m), for any m relatively prime to a. © 2013 Copyright Taylor and Francis Group, LLC.

Changes in modal parameters of a steel truss bridge due to artificial damage

This study investigated how damage changes the modal parameters of a real bridge by means of a field experiment which was conducted on a real steel truss bridge consecutively subjected to four artificial damage scenarios. In the experiment, both the forced and free vibrations of the bridge were recorded, the former for identifying higher modes available exclusively and the latter for lower modes with higher resolution. Results show that modal parameters are little affected by damage causing low stress redistribution. Modal frequencies decrease as damage causing high stress redistribution is applied; such a change can be observed if the damage is at the non-nodal point of the corresponding mode shape. Mode shapes are distorted due to asymmetric damage; they show an amplification in the damaged side as damage is applied at the non-nodal point. Torsion modes become more dominant as damage is applied either asymmetrically or on an element against large design loads. © 2013 Taylor & Francis Group, London.

Severe plastic deformation of Al–Zn alloys

In this work, the R&D work mainly focused on the mechanical and microstructural analysis of severe plastic deformation (SPD) of Al–Zn alloys and the development of microstructure–based models to explain the observed behaviors is presented. Evolution of the microstructure and mechanical properties of Al–30wt% Zn alloy after the SPD by the high–pressure torsion (HPT) has been investigated in detail regarding the increasing amount of deformation. SPD leads to the gradual grain refinement and decomposition of the Al–based supersaturated solid solution. The initial microstructure of the Al–30wt% Zn alloy contains Al and Zn phases with grains sizes respectively of 15 and 1 micron. The SPD in compression leads to a gradual decrease of the Al and Zn phase grain sizes down to 4 microns and 252 nm, respectively, until a plastic strain of 0.25 is reached. At the same time, the average size of the Zn particles in the bulk of the Al grains increases from 20 to 60 nm and that of the Zn precipitates near or at the grain boundaries increases as well. This microstructure transformation is accompanied at the macroscopic scale by a marked softening of the alloy. The SPD produced by HPT is conducted up to a shear strain of 314. The final Al and Zn grains refine down to the nanoscale with sizes of 370 nm and 170 nm, respectively. As a result of HPT, the Zn–rich (Al) supersaturated solid solution decomposes completely and reaches the equilibrium state corresponding to room temperature and its leads to the material softening. A new microstructure–based model is proposed to describe the softening process occurring during the compression of the supersaturated Al–30wt% Zn alloy. The model successfully describes the above–mentioned phenomena based on a new evolution law expressing the dislocation mean free path as a function of the plastic strain. The softening of the material behavior during HPT process is captured very well by the proposed model that takes into consideration the effects of solid solution hardening and its decomposition, Orowan looping and dislocation density evolution. In particular, it is demonstrated that the softening process that occurs during HPT can be attributed mainly to the decomposition of the supersaturated solid solution and, in a lesser extent, to the evolution of the dislocation mean free path with plastic strain.

Large amplitude motions in the S1 state of formic acid /

A detailed theoretical investigation of the large amplitude motions in the S, excited electronic state of formic acid (HCOOH) was done. This study focussed on the the S, «- So electronic band system of formic acid (HCOOH). The torsion and wagging large amplitude motions of the S, were considered in detail. The potential surfaces were simulated using RHF/UHF ab-initio calculations for the two electronic states. The energy levels were evaluated by the variational method using free rotor basis functions for the torsional coordinates and harmonic oscillator basis functions for the wagging coordinates. The simulated spectrum was compared to the slit-jet-cooled fluorescence excitation spectrum allowing for the assignment of several vibronic bands. A rotational analysis of certain bands predicted that the individual bands are a mixture of rotational a, b and c-type components.The electronically allowed transition results in the c-type or Franck-Condon band and the electronically forbidden, but vibronically allowed transition creates the a/b-type or Herzberg-Teller components. The inversion splitting between these two band types differs for each band. The analysis was able to predict the ratio of the a, b and c-type components of each band.