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排序方式: 共有1227条查询结果,搜索用时 15 毫秒
1.
2.
A single-feed dual frequency compact microstrip antenna with a shorting pin is described. This new antenna configuration gives a large variation in frequency ratio of the two operating frequencies, without increasing the overall size of the antenna 相似文献
3.
G. Babu Viswanathan Michael J. Mills Vijay K. Vasudevan 《Metallurgical and Materials Transactions A》2003,34(10):2113-2127
The effects of microstructure on the tensile properties and deformation behavior of a binary Ti-48Al gamma titanium aluminide
were studied. Tensile-mechanical properties of samples with microstructures ranging from near γ to duplex to fine grained, near- and fully-lamellar were determined at a range of temperatures, and the deformation structures
in these characterized by transmission electron microscopy (TEM). Microstructure was observed to exert a strong influence
on the tensile properties, with the grain size and lamellar volume fraction playing connected, but complex, roles. Acoustic
emission response monitored during the tensile test revealed spikes whose amplitude and frequency increased with an increase
in the volume fraction of lamellar grains in the microstructure. Analysis of failed samples suggested that microcracking was
the main factor responsible for the spikes, with twinning providing a minor contribution in the near-lamellar materials. The
most important factor that controls ductility of these alloys is grain size. The ductility, yield stress, and work-hardening
rate of the binary Ti-48Al alloy exhibit maximum values between 0.50 and 0.60 volume fraction of the lamellar constituent.
The high work-hardening rate, which is associated with the low mobility of dislocations, is the likely cause of low ductility
of these alloys. In the near-γ and duplex structures, slip by motion of 1/2<110] unit dislocations and twinning are the prevalent deformation modes at room
temperature (RT), whereas twinning is more common in the near- and fully-lamellar structures. The occurrence of twinning is
largely dictated by the Schmid factor. The 1/2<110] unit dislocations are prevalent even for grain orientations for which
the Schmid factor is higher for <101] superdislocations, though the latter are observed in favorably oriented grains. The
activity of both of these systems is responsible for the higher ductility at ambient temperatures compared with Al-rich single-phase
γ alloys. A higher twin density is observed in lamellar grains, but their propagation depends on the orientation and geometry
of the individual γ lamellae. The increase in ductility at high temperatures correlates with increased activity of 1/2<110] dislocations (including
their climb motion) and twin thickening. The role of microstructural variables on strength, ductility, and fracture are discussed.
This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented
at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint
Committee on Mechanical Behavior of Materials. 相似文献
4.
Long-period fibre gratings in specially designed birefringent few-mode fibres are used to demonstrate spectrally flat polarisers with bandwidths of 10 nm and polarisation extinction levels of 20 dB. This is enabled by the special dispersive properties of few-mode fibres, yielding fibre polarisers with performance approaching that of bulk-optic devices. 相似文献
5.
The optical recording of the conditions at the tips of dynamically moving cracks is often associated with a wave pattern which arises from what we call here radial waves. This observation is prevalent for tough solids, especially when the observation tools involve surface deformations such as the caustic method. These waves can modify the caustic shape and size and thus influence the interpretation of measurements. The quantification of the sources of these waves poses a very difficult problem so that a precise evaluation of that phenomenon is not yet possible. However, in an attempt to better understand the limitation imposed on the method of caustics by this set of waves this paper analyzes primarily the change of shape in the caustic associated with the passage of radial waves over the region of the initial curve of the caustic. The analysis is approximate in that the radial waves are assumed to be insensitive to the presence of the crack surfaces, but otherwise accounts for the three dimensional character of wave propagation in a plate of finite thickness.
Résumé On associe souvent l'enregistrement optique des conditions présentes à l'extrémité de fissures en cours de propagation spectaculaire, à une disposition ondulée, due à ce que l'on dénomme ici des ondes radiales. L'observation vaut surtout pour les corps à haute ténacité, et en particulier lorsque les méthodes d'observation tiennent compte des déformations de surface, comme la méthode des caustiques. Les ondes radiales peuvent altérer la forme et la dimension d'une caustique, et, de ce fait, influencer l'interprétation des mesures. Une évaluation précise de ce phénomène n'est pas encore possible, car la quantification des sources de ces ondes pose un problème trés délicat. Néanmoins, en vue de mieux comprendre les limites qu'impose à la méthode des caustiques la formation de ces ondes, on a analysé essentiellement le changement de forme de la caustique lorsque se développent des ondes radiales sur la région couverte par la courbe originale de la caustique. On procède à une approximation dans la mesure où on suppose que les ondes radiales sont insensibles à la présence des surfaces de la fissure, mais prennent par ailleurs en considération le caractère tridimensionnel de la propagation d'une fissure dans une plaque d'épaisseur finie.相似文献
6.
V. S. Ramachandran 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1972,22(11):1125-1138
The hydration characteristics of 3CaO.SiO2 or β2CaO.SiO2 are studied by an addition of 0.0, 0.1, 0.5 or 1.0% triethanolamine. The amount of Ca(OH)2 found at 1, 3, 7 or 28 days was in the order C3S + 0% TEA > C3S +0.1% TEA > C3S + 0.5% TEA > C3S+1.0% TEA, irrespective of whether lime was estimated by X-ray, DTA, TGA or chemical analysis. The rate of hydration, in terms of the disappearance of 3CaO.SiO2, showed that hydration proceeded faster in the presence of TEA after 1 day. Additions of TEA increase the induction period, promote the formation of a C-S-H with higher CaO/SiO2 ratio, increase the formation of non-crystalline Ca(OH)2 and enhance the surface area of the hydrated silicate product. 相似文献
7.
A technique that is sensitive to change in texture is more reliable than mechanical property measurements for the study of
recrystallization behavior in alloys where recrystallization is coupled with precipitation. Hence, ultrasonic velocity measurements
have been employed to characterize microstructural changes during annealing of cold worked Ti-modified austenitic stainless
steel where precipitation of TiC is known to retard recrystallization. Mechanical property measurements, such as strength
and hardness, could not distinguish the recovery and recrystallization regimes, and it was difficult to determine the exact
time or temperature for onset of recrystallization. The variation of velocity with annealing time or annealing temperature
exhibited a three-stage behavior. The velocity exhibited a slight increase in the recovery region followed by a sharp and
continuous decrease in the recrystallization region and reached saturation on completion of recrystallization. Based on the
microstructural investigations, the three stages were identified to be recovery, progress of recrystallization, and completion
of recrystallization. Velocity measurements could sense the onset, progress, and completion of recrystallization more accurately
compared with that of hardness and strength measurements. 相似文献
8.
9.
Logan Molyneux Krishnan Vasudevan Homero Gil de Zúñiga 《Journal of Computer-Mediated Communication》2015,20(4):381-399
Recent research suggests that social interactions in video games may lead to the development of community bonding and prosocial attitudes. Building on this line of research, a national survey of U.S. adults finds that gamers who develop ties with a community of fellow gamers possess gaming social capital, a new gaming‐related community construct that is shown to be a positive antecedent in predicting both face‐to‐face social capital and civic participation. 相似文献
10.
We present a randomized EREW PRAM algorithm to find a minimum spanning forest in a weighted undirected graph. On an n -vertex graph the algorithm runs in o(( log n)
1+
ɛ
) expected time for any ɛ >0 and performs linear expected work. This is the first linear-work, polylog-time algorithm on the EREW PRAM for this problem.
This also gives parallel algorithms that perform expected linear work on two general-purpose models of parallel computation—the
QSM and the BSP. 相似文献