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排序方式: 共有348条查询结果,搜索用时 15 毫秒
91.
92.
ZdenĚK Slanina Jean-Pierre FranÇlois Matthias Kolb Dirk Bakowies Walter Thiel 《Fullerenes, Nanotubes and Carbon Nanostructures》2013,21(2):221-230
C84 is treated as a system composed of 24 local minima whose energies, geometries, and vibrational frequencies are obtained from MNDO calculations. The predicted global minimum of D2d symmetry remains the most abundant species in the equilibrium isomeric mixture only till 276 K, being replaced by a D2 species beyond that point. However, a C 1 structure is prevalent in the high temperature limit. The calculated composition around a temperature of 1000 K is consistent with very recent NMR observations. 相似文献
93.
Abstract The vector concept of the propagation invariance is formulated to be applied to the stationary electromagnetic fields. The analysed exact solutions of the Maxwell equations are obtained on the basis of general propagation-invariant solutions of the scalar Helmholtz equation. A possible classification of the propagation-invariant fields based on quantities which appear in the complex Poynting theorem is proposed. The longitudinal periodicity of the electromagnetic field obtained due to the superposition of two Bessel beams is verified by a simple experiment. 相似文献
94.
Ivan Jirka Marta Vandrovcová Otakar Frank Zdeněk Tolde Jan Plšek Thomas Luxbacher Lucie Bačáková Vladimír Starý 《Materials science & engineering. C, Materials for biological applications》2013,33(3):1636-1645
β-Stabilized titanium (Ti) alloys containing non-toxic elements, particularly niobium (Nb), are promising materials for the construction of bone implants. Their biocompatibility can be further increased by oxidation of their surface. Therefore, in this study, the adhesion, growth and viability of human osteoblast-like MG 63 cells in cultures on oxidized surfaces of a β-TiNb alloy were investigated and compared with the cell behavior on thermally oxidized Ti, i.e. a metal commonly used for constructing bone implants. Four experimental groups of samples were prepared: Ti or TiNb samples annealed to 600 °C for 60 min in a stream of dry air, and Ti and TiNb samples treated in Piranha solution prior to annealing. We found that on all TiNb-based samples, the cell population densities on days 1, 3 and 7 after seeding were higher than on the corresponding Ti-based samples. As revealed by XPS and Raman spectroscopy, and also by isoelectric point measurements, these results can be attributed to the presence of T-Nb2O5 oxide phase in the surface of the alloy sample, which decreased its negative zeta (ζ)-potential in comparison with zeta (ζ)-potential of the Ti sample at physiological pH. This effect was tentatively explained by the presence of positively charged defects acting as Lewis sites of the surface Nb2O5 phase. Piranha treatment slightly decreases the biocompatibility of the samples, which for the alloy samples may be explained by a decrease in the number of defective sites with this treatment. Thus, the presence of Nb and thermal oxidation of β-stabilized Ti alloys play a significant role in the increased biocompatibility of TiNb alloys. 相似文献
95.
František Wald Zdeněk Sokol David Moore 《Journal of Constructional Steel Research》2009,65(8-9):1896-1903
Fire tests carried out on the eight-storey steel framed building at the Building Research Establishment’s Cardington laboratory have shown that the connections are subject to large axial force. These forces are the result of thermal movements of the structure during heating and cooling and in some cases can result in failure of the structure. This paper describes a fire test carried out on the steel frame at Cardington on 16th January 2003 and a fire test carried out on a structure in Ostrava on 16th June 2006. In both cases the tests were designed to measure the forces generated in the connections. 相似文献
96.
Our research focused on the membrane separation of wastewater resulting from the production of dried potato purée. Our aim was to investigate possibilities for recycling obtained retentate back to the actual production process, and, consequently, for reducing wastewater pollution. This paper describes trials of MF and RO membrane filtration of starch wastewater. The treated water contained starch, in either granulated or gelatinized form, and solids (fine pieces of potato skins). The trials were conducted in either one or two stages. We used a pilot plant equipped with a ceramic membrane with a filtration area of 0.35 m2 and pore sizes of 500 and 100 nm. We also tested an organic RO membrane (7410) in the laboratory.High permeate flux above 100 l/(m2 h) was measured for the 100 nm membrane, but with considerable fouling. Filtration through this membrane resulted in high COD and BOD5 rejection (approximately 60%), an effect which was increased by the subsequent RO filtration. The content of soluble carbohydrate, 0.011% in permeate (with 0.44% dry substance), was analyzed using high-performance anion-exchange chromatography coupled with pulsed amperometric detection. 相似文献
97.
Size-Effect Testing of Cohesive Fracture Parameters and Nonuniqueness of Work-of-Fracture Method 总被引:2,自引:0,他引:2
The cohesive crack model has been widely accepted as the best compromise for the analysis of fracture of concrete and other quasibrittle materials. The softening stress-separation law of this model is now believed to be best described as a bilinear curve characterized by four parameters: the initial and total fracture energies Gf and GF, the tensile strength ft′, and the knee-point ordinate σ1. The classical work-of-fracture test of a notched beam of one size can deliver a clear result only for GF. Here it is shown computationally that the same complete load-deflection curve can be closely approximated with stress-separation curves in which the ft′ values differ by 77% and Gf values by 68%. It follows that the work-of-fracture test alone cannot provide an unambiguous basis for quasibrittle fracture analysis. It is found, however, that if this test is supplemented by size-effect testing, all four cohesive crack model parameters can be precisely identified and the fracture analysis of structures becomes unambiguous. It is shown computationally that size-effect tests do not suffice for determining GF and ft′, which indicates that they provide a sufficient basis for computing neither the postpeak softening of fracturing structures nor the peak loads of a very large structure. However, if the size-effect tests are supplemented by one complete softening load-deflection curve of a notched specimen, an unambiguous calculation of peak loads and postpeak response of structures becomes possible. To this end, the notched specimen tests must be conducted in a certain size range, whose optimum is here established by extending a previous analysis. Combination of the work-of-fracture and size-effect testing could be avoided only if the ratios GF/Gf and σ1/ft′ were known a priori, but unfortunately their estimates are far too uncertain. 相似文献
98.
The size effect on structural strength is an important phenomenon with a very old history. Unfortunately, despite abundant experimental evidence, this phenomenon is still not taken into account in most specifications of the design codes for concrete structures, as well as the design practices for polymer composites, rock masses and timber. The main reason appears to be a controversy between two different theories of size effect, namely the theory based on energetic-statistical scaling and the theory based on ideas from fractal geometry. This paper aims to critically analyze these two theories, examine their hypotheses and point out the limitations, in order to help code-writing committees choose a rational basis for their work. The paper begins by reviewing the theory of energetic size effect and the efforts to explain the size effect by fractal geometry. The advantages and disadvantages in modeling the structural size effect by fractals are pointed out. Certain flaws in the fractal theory of size effect are illuminated and it is shown that various aspects of this theory lack a sound physical or mathematical basis, or both. The paper ends by recommending how engineering designers and code writers should take the size effect into account. 相似文献
99.
Zdeněk P. Ba?ant Ferhun C. Caner Mark D. Adley Stephen A. Akers 《Canadian Metallurgical Quarterly》2000,126(9):962-970
The formulation of microplane model M4 in Parts I and II is extended to rate dependence. Two types of rate effect in the nonlinear triaxial behavior of concrete are distinguished: (1) Rate dependence of fracturing (microcrack growth) associated with the activation energy of bond ruptures, and (2) creep (or viscoelasticity). Short-time linear creep (viscoelasticity) is approximated by a nonaging Maxwell spring-dashpot model calibrated so that its response at constant stress would be tangent to the compliance function of model B3 for a time delay characteristic of the problem at hand. An effective explicit algorithm for step-by-step finite-element analysis is formulated. The main reason that the rate dependence of fracturing must be taken into account is to simulate the sudden reversal of postpeak strain softening into hardening revealed by recent tests. The main reason that short-time creep (viscoelasticity) must be taken into account is to simulate the rate dependence of the initial and unloading stiffness. Good approximations of the rate effects observed in material testing are achieved. The model is suitable for finite-element analysis of impact, blast, earthquake, and short-time loads up to several hours duration. 相似文献
100.
Zdeněk Chlup Hynek Hadraba Lenka Slabáková Daniel Drdlík Ivo Dlouhý 《Journal of the European Ceramic Society》2012,32(9):2057-2061
Laminates with strong bonds between thin layers were examined in this work to explore the influence of developed internal stresses on the fracture behaviour. A set of laminates having different level of internal stresses were prepared. Alumina and zirconia were the model materials for evenly alternating layers. The electrophoretic deposition technique was used for manufacturing of the laminates. The basic mechanical properties as elastic modulus and flexural strength were determined for all prepared materials. The crack propagation changes due to effect of internal stresses, elastic mismatch and surface effects were investigated using modified single edge notched beam technique. An extensive fractographical analysis of fracture surfaces was undertaken using laser confocal microscopy. The changes of the crack direction when crack propagates through alternating layers under different angels were described. Further, the effect of the internal stresses level within individual layers was reported. 相似文献