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The fracture of a known ductile polymer (polycarbonate) is investigated. It is shown that polycarbonate obeys the Dugdale model of ductile yielding by comparing measured plastic-zone sizes and displacements to the theoretical predictions. Strain-energy release rates are calculated and the critical value is shown to be one or two orders of magnitude higher than critical values for brittle polymers. Photoelastic photographs of the fracture process are presented and it is shown that the photoelastic effect definitely enhances the definition of the yield zone. Microscopic photoelastic photographs are also presented which are used to give a new interpretation to the photoplastic effect. Finally, suggestions for future work are presented.  相似文献   
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We present a neutron reflectivity study on interfaces in contact with flowing hexadecane, which is known to exhibit surface slip on functionalized solid surfaces. The single crystalline silicon substrates were either chemically cleaned Si(100) or Si(100) coated by octadecyl-trichlorosilane (OTS), which resulted in different interfacial energies. The liquid was sheared in situ and changes in reflectivity profiles were compared to the static case. For the OTS surface, the temperature dependence was also recorded. For both types of interfaces, density depletion of the liquid at the interface was observed. In the case of the bare Si substrate, shear load altered the structure of the depletion layer, whereas for the OTS covered surface no effect of shear was observed. Possible links between the depletion layer and surface slip are discussed. The results show that, in contrast to water, for hexadecane the enhancement of the depletion layer with temperature and interfacial energy reduces the amount of slip. Thus a density depletion cannot be the origin of surface slip in this system.  相似文献   
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The time scales for nuclear fission have been explored using both pre-and postfission neutrons and GDR gamma rays. Four systems were investigated: 133-MeV 16O + 176Yb and 208Pb and 104-MeV 4He + 188Os and 209Bi. Fission fragments were measured in coincidence with PPACs. The neutrons were detected using eight detectors from the DEMON array, while gamma rays were measured using the US BaF2 array. The pre-and postfission gamma rays were determined using moving source fits parallel and perpendicular to the fission fragment emission directions. The time scales for fission for the neutrons were determined using the neutron clock technique. The gamma-ray data were fitted using a statistical model calculation based on the code CASCADE. The results of the fits from both data types were used to extract nuclear friction coefficients, γ, and fission time scales. The γ values ranged from 7 to 20, while the fission times were (31–105)×10?21 s.  相似文献   
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RPA hole-particle wave functions for the lowest 2+ and 3? states in 208Pb were used to test the validity of effective nucleon-nucleon interactions for (p, p') excitations. Excellent agreement with experiment was obtained for the long-range part of the Hamada-Johnston S-state potential. Equally good agreement was found for the elastic scattering from 208Pb and the (p, n) transition to the analog state. It was found to be important to include an imaginary part in the interaction. The transitions to the two excited states of 208Pb were found to be almost pure isoscalar, but the transitions are dominated by the neutron excitations.  相似文献   
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We consider heavy-ion collisions with beam energies of a few hundred MeV/nucleon, because such collisions seem favorable for producing compressed nuclear matter. As an alternative to hydrodynamic ways of calculating such collisions, we are investigating a microscopic, rapid (and therefore economical) simulation method. There are two simplifications basic to this method: (i) using classical particle kinematics, and (ii) taking nucleonnucleon interactions into account via cross sections rather than explicit forces. Some other simplifications, concerning nuclear binding etc., are presently crude but will be improved. Even at normal density, nuclear matter is not so dilute; therefore our calculations show some sensitivity to details of the mechanism assume for nucleon-nucleon scattering. For head-on collision of two mass-235 nuclei, our present calculations yield maximum densities between 2 and 3 times that of normal nuclear matter.  相似文献   
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New experimental data has been obtained for the208Pb(α, α′) reaction induced by 160 MeV alpha particles, for inelastic scattering to forward angles. We use these data to investigate the applicability of the multistep scattering theory of Feshbach, Kerman, and Koonin for describing this reaction. The mechanism we study, following the work of Gadioli et al. [1], is of the incident alpha particle remaining intact throughout the scattering process, exciting nucleon particle-hole pairs through multistep process. We conclude that this mechanism, combined with compound nucleus decay at low emission energies, can account for much of the observed data. However, there are indications that other processes also contribute at energies above the compound nucleus emission regime, and we outline future theoretical analyses that are needed.  相似文献   
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The relationship between plastic thickness change and plastic isochromatics occurring beyond the plostic tensile instability point of a uniaxial tensile specimen is investigated. Mechanical thickness-change measurements and a hologram of thickness change are shown to be in close qualitative agreement with isochromatics in a region of gross plastic yielding. Analytical observations are discussed to aid the interpretation of the experimental results obtained. Finally, possible extensions and applications are discussed.  相似文献   
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