Laser Cladding of Cold-Work Tool Steel by Pulse Shaping

Repair welding of cold-work tool steels in cold is very risky and almost impossible by conventional processes. The application of pulse shaping in laser cladding with wire to avoid the solidification problems in relevant steel is demonstrated. The results show that sound remelting and/or cladding can be achieved by the right selection of laser parameters and pulse shape, i.e. long pulse duration, moderate pulse peak powers and ramped-down pulse shape. Despite the defects and softening in the cladding due to the formation of retained austenite, the cladding shows better wear resistance at lower loads compared to the heat-treated base material.     

NP-completeness of the energy barrier problem without pseudoknots and temporary arcs

Knowledge of energy barriers between pairs of secondary structures for a given DNA or RNA molecule is useful, both in understanding RNA function in biological settings and in design of programmed molecular systems. Current heuristics are not guaranteed to find the exact energy barrier, raising the question whether the energy barrier can be calculated efficiently. In this paper, we study the computational complexity of a simple formulation of the energy barrier problem, in which each base pair contributes an energy of −1 and only base pairs in the initial and final structures may be used on a folding pathway from initial to final structure. We show that this problem is NP-complete.     

Analytical model of thermal stresses in two- and three-component materials

The paper deals with an analytical model of thermal stresses in isotropic solid continuum represented by periodically distributed isotropic spherical particles in an isotropic infinite matrix, with or without an isotropic spherical envelope on the particle surface. The multi-particle-(envelope)-matrix system to represent a model system regarding the analytical modelling is applicable to two and three types of two- and three-component materials, respectively. The thermal stresses as functions of microstructural parameters (particle volume fraction, particle radius, inter-particle distance, envelope thickness) originate during a cooling process as a consequence of the difference in thermal expansion coefficients and as a consequence of dimension changes of mutually transforming crystalline lattices due to a phase transformation. Additionally, an analytical-(computational)-experimental method of the lifetime prediction based on the analytical model of the thermal stresses in a three-component material is also presented.     

Cluster points of sequences of fuzzy real numbers

Properties of cluster points of sequences of fuzzy real numbers are investigated in the paper. It is shown that some similar theorems like in the case of real sequences hold. On the other hand, some differences from real case are discussed.     

In vitro antifungal effect of black cumin seed quinones against dairy spoilage yeasts at different acidity levels

The antiyeast activity of the black cumin seed (Nigella sativa) quinones dithymoquinone, thymohydroquinone (THQ), and thymoquinone (TQ) were evaluated in vitro with a broth microdilution method against six dairy spoilage yeast species. Antifungal effects of the quinones were compared with those of preservatives commonly used in milk products (calcium propionate, natamycin, and potassium sorbate) at two pH levels (4.0 and 5.5). THQ and TQ possessed significant antiyeast activity and affected the growth of all strains tested at both pH levels, with MICs ranging from 8 to 128 μg/ml. With the exception of the antibiotic natamycin, the inhibitory effects of all food preservatives against the yeast strains tested in this study were strongly affected by differences in pH, with MICs of ≥16 and ≥512 μg/ml at pH 4.0 and 5.5, respectively. These findings suggest that HQ and TQ are effective antiyeast agents that could be used in the dairy industry as chemical preservatives of natural origin.     

The influence of doping on the Raman intensity of the D band in single walled carbon nanotubes

The D band in the Raman spectra of single walled carbon nanotubes is considered as an indicator of defects in carbon nanotubes. However, its dependence on charge-transfer doping is generally ignored, despite the studied samples are often naturally doped. We studied the intensity of the D band, the ratio of the intensities of the D band and TG band (I_D/I_TG) and the ratio of the intensities of the D and G′ band (I_D/I_G′) in the Raman spectra of the single walled carbon nanotubes in dependence on a doping level. We tested two laser excitation energies viz 2.41 and 1.92 eV, which are in resonance with semiconducting and metallic tubes, respectively in our sample. It is shown that the D band intensity is significantly attenuated in doped carbon nanotubes sample for both semiconducting and metallic tubes. The I_D/I_TG ratio is weakly dependent on doping for semiconducting tubes but for metallic tubes the I_D/I_TG ratio exhibits strong dependence on doping. The I_D/I_G′ ratio is suggested for evaluation of the defects in carbon nanotubes samples since it is less sensitive to doping both for semiconducting and metallic tubes. Nevertheless, for highly doped samples even the I_D/I_G′ ratio exhibits significant dependence on doping level.     

THERMAL STRESSES IN TRIAXIAL ANISOTROPIC PARTICLE–MATRIX SYSTEM

This paper deals with thermal stresses in an anisotropic particle–matrix system. The particle–matrix system is represented by one spherical triaxial anisotropic particle embedded in the infinite triaxial anisotropic matrix. The thermal stresses originate during a cooling process as a consequence of the difference in thermal expansion coefficients between the matrix and the particle.     

THERMAL STRESSES IN ISOTROPIC CELL-DIVIDED PARTICLE-MATRIX SYSTEM: SPHERICAL ANDCUBIC CELLS

Thermal stresses are studied in an isotropic particle-matrix system of homogeneously distributed spherical particles in an infinite matrix. The isotropic particle-matrix system is divided into cells containing the central spherical particle embedded in the matrix and is of dimensions equal to an interparticle distance. The cell surface is assumed to be acted on by nonzero stresses derived by a criterion of a minimum of the cell elastic energy of the thermal stresses. The thermal stresses originate during a cooling process as a consequence of the difference in thermal expansion coefficients between the matrix and the particle. The formulae for the thermal stresses acting in the isotropic cell-divided particle-matrix system for the ratio of a spherical particle volume to a cell volume v_p = 0 reduce to those for the isotropic particle-matrix system of one spherical particle embedded in an infinite matrix. The thermal stresses are derived for spherical and cubic cells, depending on the spherical particle distribution.