全文获取类型
收费全文 | 705618篇 |
免费 | 7737篇 |
国内免费 | 1541篇 |
专业分类
电工技术 | 13380篇 |
综合类 | 558篇 |
化学工业 | 107561篇 |
金属工艺 | 28245篇 |
机械仪表 | 22103篇 |
建筑科学 | 15482篇 |
矿业工程 | 4731篇 |
能源动力 | 17785篇 |
轻工业 | 55168篇 |
水利工程 | 8052篇 |
石油天然气 | 16189篇 |
武器工业 | 45篇 |
无线电 | 79734篇 |
一般工业技术 | 141952篇 |
冶金工业 | 130327篇 |
原子能技术 | 16747篇 |
自动化技术 | 56837篇 |
出版年
2021年 | 6241篇 |
2019年 | 6030篇 |
2018年 | 10425篇 |
2017年 | 10599篇 |
2016年 | 11109篇 |
2015年 | 6980篇 |
2014年 | 11880篇 |
2013年 | 31716篇 |
2012年 | 18503篇 |
2011年 | 25092篇 |
2010年 | 20177篇 |
2009年 | 22716篇 |
2008年 | 23220篇 |
2007年 | 22972篇 |
2006年 | 20116篇 |
2005年 | 18368篇 |
2004年 | 17671篇 |
2003年 | 17249篇 |
2002年 | 16717篇 |
2001年 | 16564篇 |
2000年 | 15751篇 |
1999年 | 15992篇 |
1998年 | 39497篇 |
1997年 | 27884篇 |
1996年 | 21534篇 |
1995年 | 16373篇 |
1994年 | 14611篇 |
1993年 | 14398篇 |
1992年 | 10716篇 |
1991年 | 10425篇 |
1990年 | 10236篇 |
1989年 | 9948篇 |
1988年 | 9412篇 |
1987年 | 8325篇 |
1986年 | 8209篇 |
1985年 | 9306篇 |
1984年 | 8357篇 |
1983年 | 7884篇 |
1982年 | 7144篇 |
1981年 | 7379篇 |
1980年 | 6911篇 |
1979年 | 6918篇 |
1978年 | 6812篇 |
1977年 | 7670篇 |
1976年 | 9747篇 |
1975年 | 6072篇 |
1974年 | 5847篇 |
1973年 | 5859篇 |
1972年 | 5063篇 |
1971年 | 4666篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
891.
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
892.
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
893.
R. RODRÍGUEZ-MARTÍN I. OCAÑA A. MARTÍN-MEIZOSO 《Fatigue & Fracture of Engineering Materials & Structures》2007,30(7):664-670
High-strength steels are used to increase the load carrying capacity of components. However, to guarantee a safe design, it is also necessary to combine high strength with adequate fracture toughness. In this paper, fracture toughness of three high-strength steels with yield strengths ranging from 460 to 890 MPa has been studied at very low temperatures. Taking into account experimental evidence, a new mechanism of cleavage at very low temperatures is proposed. This mechanism considers the possibility of reaching the ideal strength (the stress at which the lattice of a single crystal losses its stability) in the immediate vicinity of the fatigue crack tip. Moreover, a computational model able to calculate the external load needed to produce a catastrophic failure of these steels has been developed. 相似文献
894.
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
895.
L. T. MO M. HUURMAN S. P. WU A. A. A. MOLENAAR 《Fatigue & Fracture of Engineering Materials & Structures》2007,30(9):823-831
Tensile fatigue behaviours of bitumen–stone adhesion were investigated using a dynamic mechanics analyser under stress‐controlled mode at two temperatures of 5 and 25°C and various controlled‐stress levels. Failure characteristics including interfacial failure and cohesive failure were examined using image analysis of fracture surfaces. Finite‐element analysis on stress distributions was conducted under different temperatures, film thickness and interfacial bonding conditions. A Coulomb–Mohr like criterion in combination with shear and normal stresses is proposed to deal with the extreme thin adhesive layer, which can be further simplified into an adhesive zone without significant loss of accuracy for stress analysis. 相似文献
896.
A. Alt Dr.‐Ing. H. Mertens U. Arz L. Blessing C. Berger 《Materialwissenschaft und Werkstofftechnik》2007,38(5):402-409
High cycle fatigue of bolted connections Extensive tests regarding the influences on the fatigue of bolt‐nut‐connections of preloading with torsion, of preloading with yielding, of loading with superimposed bending and of the tested lot are processed. These influences are not yet known according to VDI 2230. New testing devices were designed for these tests, which allow a far less expensive operation and may easily be used for bolts of diameters up to M100 and testing frequencies up to 1000 Hz. The validity of fatigue resistance according to VDI 2230 is specified with respect to the test results. The determined influence of the tested lots is unexpectedly high. The manufacturing process of bolts should be improved to minimize this influence. 相似文献
897.
898.
899.
900.