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排序方式: 共有224条查询结果,搜索用时 15 毫秒
41.
In this paper we consider a special nonlinear total least squares problem, where the model function is of the form . Using the fact that after an appropriate substitution, the model function becomes linear in parameters, and that the symmetry
preserves the distances, this nonlinear total least squares problem can be greatly simplified. In this paper we give the existence
theorem, propose an efficient algorithm for searching the parameters and give some numerical examples.
Received: June 30, 1997; revised October 31, 1998 相似文献
42.
Nilsson C Viberg P Spégel P Jörntén-Karlsson M Petersson P Nilsson S 《Analytical chemistry》2006,78(17):6088-6095
Highly efficient reversed-phase capillary electrochromatography (CEC) separations (plate numbers up to 700 000/m), with electrospray ionization mass spectrometry detection were achieved utilizing novel dextran-coated polymer nanoparticles as a pseudostationary phase. A continuous full filling (CFF) technique in which nanoparticles are continuously introduced into the capillary was employed for separation of neutral analytes (dialkyl phthalates), utilizing an orthogonal electrospray interface to prevent nanoparticles from entering the mass spectrometer. CFF-CEC benefits from that an entirely fresh column is employed for every analysis, avoiding carryover effects associated with stationary-phase contamination. The highly efficient separations obtained were accomplished by optimizing the organic modifier concentration in the electrolyte and by using a high nanoparticle concentration (5 mg/mL), to improve interparticle mass transfer and gain sufficient retention. Nanoparticles, with an average diameter of 600 nm, were prepared by polymerization of methacrylic acid and trimethylolpropane trimethacrylate, which in turn were coated with dextran. These nanoparticles formed stable suspensions in electrolytes having broad ranges of polarities, enabling straightforward optimization of the reversed-phase conditions. 相似文献
43.
A. T. Kermanidis A. D. Zervaki G. N. Haidemenopoulos Sp. G. Pantelakis 《Journal of Materials Science》2010,45(16):4390-4400
Laser welding is increasingly used for the fabrication of lightweight and cost-effective integral stiffened panels in modern
civil aircraft. As these structures age in service, the issue of the effect of corrosion on their damage tolerance requires
attention. In this work, laboratory data on the influence of salt fog corrosion on the fatigue behavior of cladded 6156 T4
aluminum alloy laser welded specimens are presented. The experimental investigation was performed on 6156 T4 laser butt welded
sheets. Prior to fatigue testing the welded joints were exposed to laboratory salt fog corrosion exposure for 720 h. The results
showed that the clad layer offers sufficient corrosion protection both on base metal and the weld. Fatigue testing was followed
by standard metallographic analysis in order to identify fatigue crack initiation sites. Crack initiation is located in all
welded samples near the weld reinforcement which induces a significant stress concentration. Localized corrosion attack of
the clad layer, in the form of pitting corrosion, creates an additional stress concentration which accelerates crack initiation
leading to shorter fatigue life relative to the uncorroded samples. The potency of small corrosion pits to act as stress concentration
sites has been assessed analytically. The above results indicate that despite the general corrosion protection offered by
the clad layer, the localized attack described above leads to inferior fatigue performance, a fact that should be taken under
consideration in the design and maintenance of these structures. 相似文献
44.
A.T. Kermanidis A.D. Zervaki G.N. Haidemenopoulos Sp.G. Pantelakis 《Materials & Design》2010,31(1):42-49
The effects of temper condition and corrosion on the fatigue behavior of a laser beam welded Al–Cu–Mg–Ag alloy (2139) have been investigated. Natural aging (T3 temper) and artificial aging (T8 temper) have been applied prior to welding. Corrosion testing has been performed by exposing the welded specimens to a salt spray medium for 720 h. Aging influences the corrosion behavior of laser welds. In the T3 temper, corrosion attack is in the form of pitting in the weld area, while in the T8 temper corrosion is in the form of pitting and intergranular corrosion in the base metal. In the latter case corrosion is attributed to the presence of grain boundary precipitates. Corrosion degrades the fatigue behavior of 2139 welds. The degradation is equal for both the T3 and T8 tempers and for the corrosion exposure selected in this study corresponds to a 52% reduction in fatigue limit. In both cases fatigue crack initiation is associated with corrosion pits, which act as stress raisers. In the T3 temper, the fatigue crack initiation site is at the weld metal/heat affected zone interface, while for the T8 temper the initiation site is at the base metal. Fatigue crack initiation in uncorroded 2139 welds occurs at the weld toe at the root side, the weld reinforcement playing a principal role as stress concentration site. The fatigue crack propagates through the partially melted zone and the weld metal in all cases. The findings in this paper present useful information for the selection of appropriate heat treatment conditions, to facilitate control of the corrosion behavior in aluminium welds, which is of great significance for their fatigue performance. 相似文献
45.
A concept has been devised to assess the effect of existing corrosion damage on the residual tensile properties of structural alloys and applied for the magnesium alloy AZ31. The concept based on the use of a radial basis function neural network. An extensive experimental investigation, including metallographic corrosion characterization and mechanical testing of pre-corroded AZ31 magnesium alloy specimens, was carried out to derive the necessary data for the training and the prediction module of the developed neural network model. The proposed concept was exploited to successfully predict: the gradual tensile property degradation of the alloy AZ31 to the results of gradually increasing corrosion damage with increasing corrosion exposure. 相似文献
46.
Nanoscale Charge Percolation Analysis in Polymer‐Sorted (7,5) Single‐Walled Carbon Nanotube Networks
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Francesca Bottacchi Stefano Bottacchi Florian Späth Imge Namal Tobias Hertel Thomas D. Anthopoulos 《Small (Weinheim an der Bergstrasse, Germany)》2016,12(31):4211-4221
The current percolation in polymer‐sorted semiconducting (7,5) single‐walled carbon nanotube (SWNT) networks, processed from solution, is investigated using a combination of electrical field‐effect measurements, atomic force microscopy (AFM), and conductive AFM (C‐AFM) techniques. From AFM measurements, the nanotube length in the as‐processed (7,5) SWNTs network is found to range from ≈100 to ≈1500 nm, with a SWNT surface density well above the percolation threshold and a maximum surface coverage ≈58%. Analysis of the field‐effect charge transport measurements in the SWNT network using a 2D homogeneous random‐network stick‐percolation model yields an exponent coefficient for the transistors OFF currents of 16.3. This value is indicative of an almost ideal random network containing only a small concentration of metallic SWNTs. Complementary C‐AFM measurements on the other hand enable visualization of current percolation pathways in the xy plane and reveal the isotropic nature of the as‐spun (7,5) SWNT networks. This work demonstrates the tremendous potential of combining advanced scanning probe techniques with field‐effect charge transport measurements for quantification of key network parameters including current percolation, metallic nanotubes content, surface coverage, and degree of SWNT alignment. Most importantly, the proposed approach is general and applicable to other nanoscale networks, including metallic nanowires as well as hybrid nanocomposites. 相似文献
47.
V Krcméry J Trupl A Kunová S Spánik I Ilavská L Hel'pianska I Bezáková L Drgona E Oravcová M Studená J Lacka L Sevcíková P Koren E Kukucková K Stopková I Krupová S Grausová J Svec 《Canadian Metallurgical Quarterly》1996,97(11):647-651
Ninety nine patients with 101 bacteraemic episodes due to Ps. aeruginosa (PA) within 6 years were divided into two groups according to their resistance to imipenem-91 due to imipenem sensitive (ISPA) and 10 due to resistant (IRPA). Risk factors, the clinical course and the outcome were evaluated and compared. Acute leukaemia, prolonged neutropenia, previous therapy with amikacin, third generation of cephalosporins, imipenem and prophylaxis by quinolones were significantly more frequently associated with IRPA. Imipenem resistant PA bacteraemia were associated with higher incidence of septic shock (40% vs 19.8%, p < 0.02) and death (33.3%) than ISPA bacteraemias. Since 1992, when first IRPA appeared, the incidence of imipenem resistance increased tenfold, and in 1994, up to 10% of PA causing bloodstream infections in cancer patients in our center were imipenem resistant. (Tab. 3, Ref. 8.). 相似文献
48.
49.
Ohne Zusammenfassung 相似文献
50.
The changes in the dynamic structure during temperature-induced phase transition in D2O/ethanol solutions of poly(vinyl methyl ether) (PVME) were studied using NMR methods. The effect of polymer concentration and ethanol (EtOH) content in D2O/EtOH mixtures on the appearance and extent of the phase separation was determined. Measurements of 1H and 13C spin-spin and spin-lattice relaxations showed the presence of two kinds of EtOH molecules: besides the free EtOH expelled from the PVME mesoglobules there are also EtOH molecules bound in PVME mesoglobules. The existence of two different types of EtOH molecules at temperatures above the phase transition was in solutions with polymer concentration 20 wt% manifested by two well-resolved NMR signals (corresponding to free and bound EtOH) in 13C and 1H NMR spectra. With time the originally bound EtOH is slowly released from globular-like structures. From the point of view of polymer-solvent interactions in the phase-separated PVME solutions both EtOH and water (HDO) molecules show a similar behaviour so indicating that the decisive factor in this behaviour is a polar character of these molecules and hydrogen bonding. 相似文献