Vertex Effect in Strain-Softening Concrete at Rotating Principal Axes

The inelastic behavior of concrete for highly nonproportional loading paths with rotating principal stress axes is studied. Test cylinders are first loaded in compression under uniaxial stress and then torsion is applied at constant axial displacement. Proportional compressive-torsional loading tests are also carried out for comparison. The tests demonstrate that the response of concrete for load increments parallel in the stress space to the current yield surface is highly inelastic (i.e., much softer than elastic) in the peak load range and especially in the postpeak range. The classical tensorial models of plasticity type incorrectly predict for such load increments the elastic stiffness. The experiments are simulated by three-dimensional finite element analysis using the microplane model M4, in which the stress-strain relations are characterized not by tensors but by vectors of stress and strain on planes of various orientations in the material. It is shown that the observed vertex effect is correctly predicted by this model, with no adjustment of its material parameters previously calibrated by other test results. The experiments are also simulated by a state-of-the-art fracture-plastic model of tensorial type and it is found that the vertex effect cannot be reproduced at all, although an adjustment of one material parameter suffices to obtain a realistic postpeak slope and achieve a realistic overall response. What makes the microplane model capable of capturing the vertex effect is the existence of more than 60 simultaneous yield surfaces. Capturing the vertex effect is important for highly nonproportional loading with rotating principal stress axes, which is typical of impact and penetration of missiles, shock, blasts, and earthquake.     

Size Effect on Strength of Quasibrittle Structures with Reentrant Corners Symmetrically Loaded in Tension

The effect of V-notches (or reentrant corners) on fracture propagation has been analyzed for brittle materials, but not for quasibrittle materials such as concrete, marked by a large material characteristic length producing a strong size effect transitional between plasticity and linear elastic fracture mechanics. A simple size effect law for the nominal strength of quasibrittle structures with symmetrically loaded notches, incorporating the effect of notch angle, is derived by asymptotic matching of the following five limit cases: (1) Ba?ant’s size effect law for quasibrittle structures with large cracks for notch angle approaching zero; (2) absence of size effect for vanishing structure size; (3) absence of size effect for notch angle approaching π; (4) plasticity-based notch angle effect for vanishing size; and (5) the notch angle effect on crack initiation in brittle structures, which represents the large-size limit of quasibrittle structures. Accuracy for the brittle large-size limit, with notch angle effect only, is first verified by extensive finite-element analyses of bodies with various notch angles. Then a cohesive crack characterized by a softening stress-separation law is considered to emanate from the notch tip, and the same finite-element model is used to verify and calibrate the proposed law for size and angle effects in the transitional size range in which the body is not far larger than Irwin’s material characteristic length. Experimental verification of the notch angle effect is obtained by comparisons with Dunn et al.’s extensive tests of three-point-bend notched beams made of plexiglass (polymethyl methacrylate), and Seweryn’s tests of double-edge-notched tension specimens, one set made of plexiglass and another of aluminum alloy.     

Transport of nitric acid through the anion-exchange membrane NEOSEPTA-AFN

The paper presents the determination of mobility of H₃O⁺ and NO₃⁻ ions and diffusivity of the non-dissociated form of acid on the basis of experiments with nitric acid carried out in a batch mixed cell with an anion-exchange membrane NEOSEPTA-AFN, which have been completed by measurements of membrane conductivity. The dependencies of mobility of H₃O⁺ and NO₃⁻ ions upon the acid concentration in the membrane were approximated by the second degree polynomial, whose coefficients were determined by the numerical integration of the differential equation describing the time dependence of nitric acid in the cell followed by an optimizing procedure. Diffusivity of the non-dissociated form was calculated from mobilities of H₃O⁺ and NO₃⁻ ions. The model used is based on the Nernst-Planck electro-diffusion equation and takes into account ionic equilibrium between the species. Using all the experimental data obtained at various acid concentrations and rotational speeds ofthe stirrers, it was found that the transport characteristics mentioned above (except for mobility of H₃O⁺ ions) were affected by the nitric acid concentration in the membrane.     

The High-Temperature Wear and Oxidation Behavior of CrC-Based HVOF Coatings

Three commercially available chromium carbide-based powders with different kinds of matrix (Cr₃C₂-25%NiCr; Cr₃C₂-25%CoNiCrAlY and Cr₃C₂-50%NiCrMoNb) were deposited by an HVOF JP-5000 spraying gun, evaluated and compared. The influence of heat treatment on the microstructure and properties, as well as the oxidation resistance in a hot steam environment (p = 24 MPa; T = 609 °C), was evaluated by SEM and XRD with respect to their potential application in the steam power industry. The sliding wear resistance measured at room and elevated (T = 600 °C) temperatures according to ASTM G-133. For all three kinds of chromium carbide-based coatings, the precipitation of secondary carbides from the supersaturated matrix was observed during the heat treatment. For Cr₃C₂-25%NiCr coating annealed in hot steam environment as well as for Cr₃C₂-25%CoNiCrAlY coating in both environments, the inner carbide oxidation was recorded. The sliding wear resistance was found equal at room temperature, regardless of the matrix composition and content, while at elevated temperatures, the higher wear was measured, varying in dependence on the matrix composition and content. The chromium carbide-based coating with modified matrix composition Cr₃C₂-50%NiCrMoNb is suitable to replace the Cr₃C₂-25%NiCr coating in a hot steam environment to eliminate the risk of failure caused by inner carbide oxidation.     

Self‐Propelled Supercapacitors for On‐Demand Circuit Configuration Based on WS2 Nanoparticles Micromachines

The miniaturization of energy storage microcapacitors to develop portable electronic devices has been of high recent interest. Here, microsupercapacitors microrobot is fabricated using membrane template‐assisted electrodeposition of WS₂ nanoparticles (WS₂NPs)/polyaniline (PANI) and platinum (Pt) layers. The microrobot navigates in the microchannel and attaches itself as part of the electrical circuit. The attached WS₂NPs‐PANI/Pt microrobots enhance the capacitive behavior of the circuit significantly. The results presented in this work open the door for the development of smart and miniaturized functional micromotors that are able to self‐assemble to on‐demand circuits.     

Structural Diversity of Copper(II) Complexes with 9-Deazahypoxanthine and Their in Vitro SOD-Like Activity

Two structurally different copper(II) complexes of the compositions [{Cu(9dhx)(H₂O)₃}₂(µ-SO₄)₂] (1) and [Cu(9dhx)₂(H₂O)₂(NO₃)₂]·H₂O (2), involving 9-deazahypoxanthine (9dhx; 6-oxo-9-deazapurine; 9-deazahypoxanthine), have been prepared and characterized by elemental analysis, infrared and electronic spectroscopy, electrospray ionisation (ESI) mass spectrometry, thermogravimetric (TG) and differential thermal (DTA) analyses, and cyclic voltammetry. The X-ray structures of complexes 1 and [Cu(9dhx)₂(H₂O)₂(NO₃)₂] (2a) revealed the distorted octahedral geometry in the vicinity of the copper(II) atoms, with the NO₅ and N₂O₄ donor set, respectively. In the dimeric compound 1, the {Cu(9dhx)(H₂O)₃}₂ units are bridged by sulfate groups with the Cu···Cu separation being 5.3446(2) Å. In both structures the 9dhx ligands are coordinated through the N3 atoms of the pyrimidine moieties. The SOD-like activity of complexes 1 and 2 was evaluated in vitro showing moderate effect, with the IC₅₀ values equal to 18.20, and 53.33 μM, respectively.     

Thermodynamic Properties of He@C60

Abstract

Contributions to thermodynamic properties of the endohedral He@C₆₀, resulting from motion of He inside the cage, are calculated. The contribution to C _v shows a maximum at low temperatures. The maxima for ³He@C₆₀ and ⁴He@C₆₀ are at about 41 K and 30 K, respectively.     

Homogeneous nucleation rate at various initial supersaturations in a closed system

The rate of formation of nuclei in a closed system is studied by numerical solution of kinetic equations at various supersaturations. Depletion of vapor (decrease of supersaturation) due to the phase transition process is taken into account. At a relatively low value of initial supersaturations, the decrease of supersaturation is negligible. At somewhat higher initial supersaturations, the phase transition process is more intense and the decrease of supersaturation plays an important role. As a consequence the nucleation rate after reaching a maximum decreases with time and for sufficiently long times only the nuclei of certain size are formed. At high initial supersaturation the nucleation rate for subcritical cluster sizes is negative, i.e. the number of subcritical clusters tends to get reduced. Critical size increases with time due to lowered supersaturation.     

Fatigue Behavior of Bodies with Thermally Sprayed Metallic and Ceramic Deposits

This paper summarizes the basic results of fatigue testing of bodies with both metallic and ceramic thermally sprayed coatings. Three kinds of ceramic coatings (Al₂O₃, Cr₂O₃, and olivine) sprayed with DC plasma under identical conditions were investigated together with metallic Ni-5wt.%Al coatings sprayed by wire arc, DC plasma, and HVOF. The elastic modulus of the deposited coatings was investigated using four point bending and resonance method. Bending fatigue tests at resonance frequency were performed with cantilever beam specimens. The processes taking place during the fatigue test are identified and discussed. The morphology of the fracture surfaces was investigated together with microstructure and porosity of the coatings.