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991.
992.
M. V. Yurkov 《Atomic Energy》2003,94(2):108-112
The program for developing free-electron x-ray lasers at TESLA started in 1994. The plan is to use the TESLA test accelerator to develop an x-ray laser with minimum wavelength 0.1–6 nm. The first phase of the project was successfully completed in 2001. At saturation, the laser produces ultrashort 30–100 fsec, gigawatt, radiation pulses. The wavelength can be tuned smoothly over the range 80–120 nm. 相似文献
993.
Inference of message sequence charts 总被引:1,自引:0,他引:1
Alur R. Etessami K. Yannakakis M. 《IEEE transactions on pattern analysis and machine intelligence》2003,29(7):623-633
Software designers draw message sequence charts for early modeling of the individual behaviors they expect from the concurrent system under design. Can they be sure that precisely the behaviors they have described are realizable by some implementation of the components of the concurrent system? If so, can we automatically synthesize concurrent state machines realizing the given MSCs? If, on the other hand, other unspecified and possibly unwanted scenarios are "implied" by their MSCs, can the software designer be automatically warned and provided the implied MSCs? In this paper, we provide a framework in which all these questions are answered positively. We first describe the formal framework within which one can derive implied MSCs and then provide polynomial-time algorithms for implication, realizability, and synthesis. 相似文献
994.
Kubat M. Hafez A. Raghavan V.V. Lekkala J.R. Wei Kian Chen 《Knowledge and Data Engineering, IEEE Transactions on》2003,15(6):1522-1534
Association mining techniques search for groups of frequently co-occurring items in a market-basket type of data and turn these groups into business-oriented rules. Previous research has focused predominantly on how to obtain exhaustive lists of such associations. However, users often prefer a quick response to targeted queries. For instance, they may want to learn about the buying habits of customers that frequently purchase cereals and fruits. To expedite the processing of such queries, we propose an approach that converts the market-basket database into an itemset tree. Experiments indicate that the targeted queries are answered in a time that is roughly linear in the number of market baskets, N. Also, the construction of the itemset tree has O(N) space and time requirements. Some useful theoretical properties are proven. 相似文献
995.
This paper studies a Kansas Department of Transportation welded plate girder bridge that developed fatigue cracks at small web gaps close to the girder top flange. Repair had been previously performed by softening the connection plate end with a slot retrofit, but cracks were recently found to have reinitiated at some of the repaired details and are again propagating. A comprehensive finite-element method study was performed to investigate the cracking behavior observed in the bridge and to recommend appropriate measures for future bridge retrofit. The analytical results show that stresses developed at the top flange web gaps could exceed yielding under the loading of an HS15 fatigue truck. The current slot repair used in the bridge was found to have introduced higher magnitude fatigue stresses in the web gap. To achieve a permanent repair of the bridge, it is recommended that a welded connection plate to flange attachment be used during future bridge retrofit. The web gap details should be able to withstand unlimited number of load cycles once this additional repair is performed. 相似文献
996.
Awareness of the construction environment can be improved by automatic three-dimensional (3D) sensing and modeling of job sites in real time. Commercially available 3D modeling approaches based on range scanning techniques are capable of modeling static objects only, and thus cannot model dynamic objects in real time in an environment comprised of moving humans, equipment, and materials. Emerging prototype video range cameras offer an alternative by facilitating affordable, wide field of view, dynamic object tracking at frame rates better than 1?Hz (real time). This paper describes a methodology to model, detect, and track the position of static and moving objects in real time, based on data obtained from video range cameras. Experiments with this technology have produced results that indicate that video rate 3D data acquisition and analysis of construction environments can support effective modeling, detection, and tracking of project resources. This approach to job site awareness has inherent value and broad application. In combination with effective management practices and other sensing techniques, this technology has the potential to significantly improve safety on construction job sites. 相似文献
997.
R. Boroch J. Wiaranowski R. Mueller-Fiedler M. Ebert J. Bagdahn 《Fatigue & Fracture of Engineering Materials & Structures》2007,30(1):2-12
The aim of this work is to characterize the strength properties of polycrystalline silicon (polysilicon) with the use of tensile and bending test specimens. The strength of thin polysilicon films with different geometry, size and stress concentrations has been measured and correlated with the effective size of the specimen and its stress distribution. The test results are evaluated using a probabilistic strength approach based on the weakest link theory with the use of STAU software. The use of statistic methods of strength prediction of polysilicon test structures with a complex geometry and loading based on test values for standard material tests specimen has been evaluated. 相似文献
998.
A. GEORGE A. JACQUES M. LEGROS 《Fatigue & Fracture of Engineering Materials & Structures》2007,30(1):41-56
Undoped FZ silicon single crystals were cyclically loaded in tension‐compression under plastic strain amplitude control in a temperature and strain rate domain where the glide of dislocations is still controlled by the lattice friction: 1073–1173 K and (1.5–6) × 10−4s−1; the plastic strain amplitude being varied from 6 × 10−4 to 10−2. Cyclic hardening curves display logarithmic then linear hardening and pass through a maximum before the peak stress per cycle stabilizes. The maximum and stabilized stresses decrease when temperature increases. Microscopical observations suggest that strain localization takes place near the maximum cyclic stress and beyond. But, contrary to what is observed in fcc metals, the maximum or saturation stress decreases when the strain amplitude per cycle increases. Several types of dislocation arrangements, rather different from those found in copper, but looking more similar to Ni fatigued at low temperatures, were revealed by transmission electron microscopy. Before mechanical saturation, edge dislocation dipoles gather in thick stripes forming more or less corrugated walls, depending on the amplitude per cycle, when viewed normal to the primary slip plane. Once the maximum stress is reached, it seems that parts of the microstructure cease to participate in the imposed plastic strain, while others concentrate it. In active areas, thick walls condense into much thinner ones, forming chevrons when viewed normal to the slip plane. In areas which seem to be inactive, dipolar walls anneal out leaving a rather homogeneous distribution of prismatic loops. Thin PSBs with a well‐characterized ‘ladder‐structure’ are very rare. Characteristic lengths of the observed patterns are given and briefly discussed using current theories of cyclic deformation. 相似文献
999.
Crack closure in fibre metal laminates 总被引:1,自引:0,他引:1
H.M. PLOKKER R.C. ALDERLIESTEN R. BENEDICTUS 《Fatigue & Fracture of Engineering Materials & Structures》2007,30(7):608-620
GLARE is a fibre metal laminate (FML) built up of alternating layers of S2-glass/FM94 prepreg and aluminium 2024-T3. The excellent fatigue behaviour of GLARE can be described with a recently published analytical prediction model. This model is based on linear elastic fracture mechanics and the assumption that a similar stress state in the aluminium layers of GLARE and monolithic aluminium result in the same crack growth behaviour. It therefore describes the crack growth with an effective stress intensity factor (SIF) range at the crack tip in the aluminium layers, including the effect of internal residual stress as result of curing and the stiffness differences between the individual layers. In that model, an empirical relation is used to calculate the effective SIF range, which had been determined without sufficiently investigating the effect of crack closure. This paper presents the research performed on crack closure in GLARE. It is assumed that crack closure in FMLs is determined by the actual stress cycles in the metal layers and that it can be described with the available relations for monolithic aluminium published in the literature. Fatigue crack growth experiments have been performed on GLARE specimens in which crack growth rates and crack opening stresses have been recorded. The prediction model incorporating the crack closure relation for aluminium 2024-T3 obtained from the literature has been validated with the test results. It is concluded that crack growth in GLARE can be correlated with the effective SIF range at the crack tip in the aluminium layers, if it is determined with the crack closure relation for aluminium 2024-T3 based on actual stresses in the aluminium layers. 相似文献
1000.
A. MONSALVE M. PÁEZ M. TOLEDANO A. ARTIGAS Y. SEPÚLVEDA y N. VALENCIA 《Fatigue & Fracture of Engineering Materials & Structures》2007,30(8):748-758
The S-N-P (stress, number of cycles, failure probability) curves for 2024 T3 and 7075 T7351 aluminium alloys were obtained. Previously, surface treatments of degreasing and different types of anodizing were applied to samples, evaluating the influence of these treatments on the fatigue life of the alloys. The determination of the S-N-P curves was done using Maennig's method. Rotary fatigue was used because this technique produces greater stress on the sample surfaces, the zone in which it is important to evaluate fatigue resistance. Both the transition range and the finite life range were evaluated, calculating the 1, 50 and 99% fracture probability. SEM was performed in order to characterize the fracture micromechanism. The conclusions were that Maennig's method is useful to evaluate fatigue life of these materials in a fast and efficient manner. Moreover, surface treatments produce a decrease in the fatigue life of both alloys, associating this effect with the surface damage produced on each sample during the treatment. 相似文献