Constitutive model of discontinuous plastic flow at cryogenic temperatures

FCC metals and alloys are frequently used in cryogenic applications, nearly down to the temperature of absolute zero, because of their excellent physical and mechanical properties including ductility. Some of these materials, often characterized by the low stacking fault energy (LSFE), undergo at low temperatures three distinct phenomena: dynamic strain ageing (DSA), plastic strain induced transformation from the parent phase (γ) to the secondary phase (α′) and evolution of micro-damage. The constitutive model presented in the paper is focused on the discontinuous plastic flow (serrated yielding) and takes into account the relevant thermodynamic background. The discontinuous plastic flow reflecting the DSA effect is described by the mechanism of local catastrophic failure of Lomer–Cottrell (LC) locks under the stress fields related to the accumulating edge dislocations (below the transition temperature from the screw dislocations to the edge dislocations mode T₁). The failure of LC locks leads to massive motion of released dislocations accompanied by the step-wise increase of the strain rate. The response of stress is defined in terms of four stages within each serration cycle. In the fourth stage, the strain rate sensitivity model with temperature playing the role of relaxation parameter is applied. Identification of parameters of the constitutive model is based on the experimental data collected during a campaign of tensile tests carried out on copper and stainless steel samples immersed in liquid helium (4.5 K), by means of a unique equipment developed at CERN.     

Absolute enantioselective separation: optical activity ex machina

The paper describes methodology of using three independent macroscopic factors affecting molecular orientation to accomplish separation of a racemic mixture without the presence of any other chiral compounds, i. e., absolute enantioselective separation (AES) which is an extension of a concept of applying these factors to absolute asymmetric synthesis. The three factors may be applied simultaneously or, if their effects can be retained, consecutively. The resulting three mutually orthogonal or near orthogonal directors constitute a true chiral influence and their scalar triple product is the measure of the chirality of the system. AES can be executed in a chromatography-like microfluidic process in the presence of an electric field. It may be carried out on a chemically modified flat surface, a monolithic polymer column made of a mesoporous material, each having imparted directional properties. Separation parameters were estimated for these media and possible implications for the natural homochirality are discussed.     

Concise and efficient synthesis of E-stereoisomers of exo-cyclic carbohydrate enones. Aldol condensation of dihydrolevoglucosenone with five-membered aromatic aldehydes Part 1

Stereoselective synthesis of exocyclic enones 10–17 via a base-catalyzed direct aldol condensation between dihydrolevoglucosenone 1 and heterocyclic aldehydes 2–9 is described. The reaction is performed under mild conditions and is applicable to variety of heterocyclic aldehydes. E-Steroisomers of exo-cyclic enones are the only products. They are very easy to isolate and were formed in good to excellent (72–88%) yield. ¹H NMR, ¹³C NMR analyses provide the structural assignment and absolute stereochemistry of the condensation products. Single-crystal X-ray diffraction of condensation product 12, additionally supports the structural assignment.     

One-pot synthesis of carbohydrate exo-cyclic enones and hemiketals with 6,8-dioxabicyclo-[3.2.1]octane moieties. Serendipitous formation of a spironolactone when 2-pyridinecarboxaldehyde is used as the reactant. Part II

A library of exo-cyclic carbohydrate enones 2–13 were prepared via a base-catalyzed, highly stereoselective aldol condensation of dihydrolevoglucosenone 1 (cyrene) with various aromatic aldehydes. In the case of 4-chlorobenzaldehyde as a reactant, under conditions of the aldol condensation, exo-cyclic enone 9 and a secondary product 9A was isolated. When 2-pyridinecarboxaldehyde was used as a reactant, an unexpected spironolactone 19 was exclusively observed. ¹H NMR, ¹³C NMR, MS analyses and single-crystal x-ray diffraction of the representative enone 9, the secondary product 9A, hemiketal 18, and spironolactone 19 unequivocally confirms the structural assignments. Mechanistic insights leading to the synthesized products are also proposed and discussed.     

Collisional broadening and shift of Doppler-free two-photon thallium lines perturbed by rare-gas atoms

The extended Omont-Ueda-Kaulakys treatment of collisional effects on quasi-Rydberg states, in which the perturbation of the lower state is taken into account is applied to thallium-rare gas systems. The pressure broadening and shift coefficients of two-photon transitions in thallium involving the 6P_1/2 -n P_1/2,3/2 (n = 9-14) states are calculated and compared with experimental data obtained by Hermann et al. [Eur. Phys. J. D 1, 129 (1998)].     

Shakedown and Inadaptation Mechanisms of Bellows Subject to Constant Pressure and Cyclic Axial Force

Abstract

In the design of pipelines, a thermal expansion of the pipes is usually compensated for by a thin-walled, flexible shell of revolution, called a bellows. The process of cyclic loading of the structure may result in the formation of a sequence of plastic strain fields in the shell, which often leads to the collapse of the structure. Therefore, the question of whether the structure shakes down or collapses under the combined, cyclic loading is of particular importance to engineers

The Reissner-type equations for the perfectly elastoplastic model of the shell are formulated on the basis of the geometrically nonlinear third-order theory. Various mechanisms of plastic collapse (e.g., maximal load or formation of plastic hinge) are discussed for the quasistatically loaded S-type bellows, as well as for the bellows subjected to cyclic, complex loadings. The analogy between these cases, as far as the failure modes are concerned, is explained. The adaptation (shakedown) and inadaptation (nonsymmetric alternating plasticity, incremental collapse) domains for the particular case of the S-type bellows (C-type geometry) acted upon by internal pressure and variable repeated axial force are presented.     

Temperature effects on the width,shift and asymmetry of 748.8 nm self-broadened neon line

The influence of temperature on the shape of the self-broadened 748.8 nm Ne line emitted from the low pressure glow discharge of neon was investigated with a Fabry-Perot interferometer. Detailed analysis showed departures of the registered line shapes from the ordinary Voigt profile. These departures were consistent with fits of the speed-dependent asymmetric Voigt profile to experimental profiles. The values of the pressure broadening, shifting and collision-time asymmetry were determined and compared with those calculated in the adiabatic approximation for the van der Waals and Lennard-Jones potentials.     

Controllability and Stabilization in Elasticity, Heat Conduction and Thermoelasticity: Review of Recent Developments*

The aim of this paper is to review developments in exact and approximate controllability as well as stabilization of elastic, thermoelastic, and thermo-viscoelastic bodies. Heat equations are also discussed.     

Asymmetry and speed-dependent effects on the 748.8 nm self-broadened neon line

Using an interferometric method, detailed analysis of the self-broadened 748.8 nm Ne line shapes emitted from the glow discharge of neon was performed, where departures from the ordinary Voigt profile were observed. These departures are shown to be consistent with fits of the speed-dependent asymmetric Voigt profile to experimental profiles. It was shown that neglecting the correlation between Doppler and collisional broadening may cause errors in determined line shape parameter values. Coefficients of the pressure broadening, shift and collision-time asymmetry are determined and compared with those calculated in the adiabatic approximation for the van der Waals and Lennard-Jones potentials.