全文获取类型
收费全文 | 4099篇 |
免费 | 196篇 |
国内免费 | 77篇 |
专业分类
化学 | 2680篇 |
晶体学 | 34篇 |
力学 | 174篇 |
综合类 | 3篇 |
数学 | 585篇 |
物理学 | 896篇 |
出版年
2023年 | 35篇 |
2022年 | 27篇 |
2021年 | 57篇 |
2020年 | 67篇 |
2019年 | 66篇 |
2018年 | 57篇 |
2017年 | 36篇 |
2016年 | 96篇 |
2015年 | 86篇 |
2014年 | 118篇 |
2013年 | 220篇 |
2012年 | 269篇 |
2011年 | 293篇 |
2010年 | 158篇 |
2009年 | 161篇 |
2008年 | 246篇 |
2007年 | 229篇 |
2006年 | 235篇 |
2005年 | 203篇 |
2004年 | 185篇 |
2003年 | 163篇 |
2002年 | 125篇 |
2001年 | 103篇 |
2000年 | 95篇 |
1999年 | 56篇 |
1998年 | 53篇 |
1997年 | 37篇 |
1996年 | 79篇 |
1995年 | 60篇 |
1994年 | 57篇 |
1993年 | 50篇 |
1992年 | 44篇 |
1991年 | 38篇 |
1990年 | 31篇 |
1989年 | 26篇 |
1988年 | 28篇 |
1987年 | 25篇 |
1986年 | 20篇 |
1985年 | 48篇 |
1984年 | 29篇 |
1983年 | 30篇 |
1982年 | 24篇 |
1981年 | 38篇 |
1980年 | 29篇 |
1979年 | 25篇 |
1978年 | 30篇 |
1977年 | 27篇 |
1976年 | 20篇 |
1975年 | 25篇 |
1974年 | 37篇 |
排序方式: 共有4372条查询结果,搜索用时 15 毫秒
41.
42.
沟谷坡面风沙起动规律研究 总被引:3,自引:0,他引:3
根据坡面风沙受力情况,以滚动模式建立了坡面风沙起动的风速公式.计算分析了相同风向条件下不同坡度风沙的起动风速和相同坡度条件下不同风向风沙的起动风速,并与平面风沙起动计算风速和实测风速进行了比较.计算分析表明,在风向一定时,坡面风沙更容易起动,风沙起动速率随着坡度的变陡而变小.在坡度一定时,风沙起动风速受到风向的影响很大,起动风速随风力与水平面垂线夹角的增大而增大.与平面相比,当风力与水平面垂线夹角小于90°时,坡面风沙起动流速比平面为小.在当风力与水平面垂线夹角大于90°时,起动风速比平面为大.当坡面为平面时,风沙起动公式与平面拜格诺起动公式完全一致. 相似文献
43.
44.
An analysis of examining the validity of a unified approach proposed earlier by the authors for the fatigue crack propagation (FCP) of engineering materials to include PMMA and PVC is described. The proposed formulation has been shown capable of characterizing a diversified range of materials with a master FCP diagram and expressed as da/dN = A(ΔG)m/(Gc − Gmax).An experimental program is undertaken to measure fatigue growth rate with the standard compact tension specimen. The FCP results are for the first instance analysed for each material using the unified formulation. The validity of the formulation for producing a master FCP diagram is verified when the fatigue crack growth rates of the materials are successfully characterized in one master diagram, yielding an excellent coefficient of correlation of 0.993. No such success is attained using a number of conventional FCP laws considered most acceptable to characterize polymeric materials. 相似文献
45.
The erosion process which occurs when an air cushion vehicle (ACV) passes over certain types of surface material has been investigated. Experiments were conducted in laboratory apparatus which simulates conditions under the edge of an ACV skirt.
Tests were carried out on three samples of dry erodible materials for a variety of cushion parameters. Representative photographs and data on time rates of erosion are presented. The results indicate that erosion rates are proportional to cushion pressure to the power 3/2 and that skirt angle, hoverheight and time since start of a run are of secondary importance. The results and analysis indicate that erosion rates are independent of particle size when this exceeds about 0.1 mm. 相似文献
46.
J.Y. Wong 《Journal of Terramechanics》1991,28(4):269-288
This report presents a review of recent developments in the study of vehicle3-terrain interaction, based primarily on papers included in Session 2A of the 10th International Conference of ISTVS. Analytical studies of wheel-soil interaction using visco-elastic soil models and elasto-plastic and rigid-plastic finite element methods are reviewed. Results of experimental investigations of stress distributions on the wheel-soil interface as well as in the soil under various loading conditions are summarized. Various methods for predicting off-road vehicle performance are examined. 相似文献
47.
Steady-state response of a binon-linear hysteretic system 总被引:1,自引:0,他引:1
The steady-state responses are analyzed for a binon-linear hysteretic system using the method of slowly varying parameters. It is shown that the triple-valued and quintuple-valued response curves exist in this system in certain parameter ranges. The stability is studied for the steady-state responses. The results obtained show that the unstable region of response is divided into two regions in certain parameter range, and the distance between the two regions goes to infinity as the value of non-linearity parameter goes to zero. 相似文献
48.
A Sedimentological Approach to Upscaling 总被引:1,自引:0,他引:1
Optimised upscaling in reservoir simulations requires the construction of realistic petrophysical properties that are representative of the heterogeneity in the sedimentary deposits. Reservoir heterogeneities are controlled by the arrangement of various hierarchies of sedimentary facies and their internal bounding surfaces. The conventional sedimentological approach to reservoir upscaling involves subdivision and ranking of various hierarchies of architectural units and associated bounding surfaces of the reservoir sequence according to their geological significance. This global upscaling approach produces realistic scaled up models that retain both the structural and non-structural heterogeneities of the original sedimentological models. Analyses of sedimentary sequences from various depositional environments indicate that the fractional Levy model can adequately describe the heterogeneity and scaling characteristics of individual genetic sediment sequences in the clastic sedimentary system without further subdividing and ranking of the heterogeneous sequences. The heterogeneous nature of each sedimentary system can be quantified by the Levy index parameter, whereas the maximum upscaling magnitude (or upscaling index) for a particular sequence can be determined from the Levy width parameter plot. Depositional modelling mimics the sedimentary processes in a range of scales and honours hierarchies of sedimentary facies and their bounding surfaces. It can be used effectively for upgridding and upscaling in accordance with the stratigraphic framework and sedimentological models. Both the fractional Levy model and the depositional modelling provide quantitative alternatives to the conventional global sedimentological upscaling approach. 相似文献
49.
本文在作者提出的含孔洞材料下限本构方程的基础上,采用了初始缺陷带模型对微孔洞生长及分布对板材拉伸过程中变形局部影响进行了,分析着重研究了细观损演化规律对变形局部化模式及临界应变的影响,并成功预测了AISI4340钢板材拉伸试件变形局部化换稳为及失稳方向。 相似文献
50.
The finite element solutions of the full Navier-Stokes and energy equations for steady laminar flow and combined convection around square prisms with attack angles of 0° and 45° are obtained for a gas having Pr=0.7. The variations of surface shear stress, local pressure and Nusselt number are obtained over the entire prism surface including the zone beyond the point of the separation. The predicted values of drag coefficients, the location of separation, average Nusselt number and the plots of velocity flow fields and isotherms are also presented. The trend of the present numerical results seems reasonable.
Nomenclature a side length of square - C f friction drag coefficient - C p pressure drag coefficient - C D total drag coefficient - F f total friction drag force - F P total pressure drag force - Gr Groshoff number,g (T w -T )a 3/v 2 - g gravitational acceleration - h local heat transfer coefficient - K thermal conductivity - L dimensionless location of surface from the front stagna tion point,L */a - L * dimension location of prism surface - Lc location of separation - N j shape function - Nu, local and average Nusselt numbers - M l shape function - P dimensionless pressure,p */u 2 - P * pressure - p x * x-componentP * - Pe Peclet number,Re Pr - Pr Prandtl number, c/K - Ra Rayleigh number,Gr Pr - Re Reynolds number,a u /v - s direction along the sides of prism - u dimensionlessX-direction component of velocity,u */u - u * X-direction component of velocity - u free stream velocity - dimensionlessY-direction component of velocity,v*/u - * Y-direction component of velocity - X X-direction axis - x dimensionlessX-direction coordinate,x */a - x * X- direction coordinate - Y Y-direction axis - y * dimensionless 7-direction coordinate,y */a - y * Y-direction coordinate Greek symbols coefficient of volumetric thermal expansion - attack angle - dynamic viscosity - kinematic viscosity,/ - density of fluid - w dimensionless surface shear stress, w * /u 2 - w * surface shear stress - wx * x-component of w /* - dimensionless temperature, 相似文献
Finite-Elemente-Verfahren für laminare Strömung und kombinierte Naturkonvektion um ein quadratisches Prisma
Zusammenfassung Es wird über Lösungen der Navier-Stokesund der Energiegleichungen mit Hilfe der Finite-Elemente-Methode für stationäre laminare Strömung, kombiniert mit Naturkonvektion, um ein quadratisches Prisma berichtet, wobei als Anströmwinkel 0° und 45° gewählt wurden und Gasströmung mitPr=0,7 angenommen wurde. Die Rechnung ergibt den Verlauf der Wandschubspannungen, des örtlichen Druckes und der Nusselt-Zahl über die gesamte Oberfläche des Prismas, einschließlich des Bereiches hinter dem Ablösepunkt. Weiterhin werden in dem Aufsatz Angaben gemacht über die Widerstandskoeffizienten, die Lage des Ablösepunktes, der mittleren Nusselt-Zahl sowie der Geschwindigkeits- und Temperaturfelder. Die numerischen Ergebnisse erscheinen im Trend vernünftig zu sein.
Nomenclature a side length of square - C f friction drag coefficient - C p pressure drag coefficient - C D total drag coefficient - F f total friction drag force - F P total pressure drag force - Gr Groshoff number,g (T w -T )a 3/v 2 - g gravitational acceleration - h local heat transfer coefficient - K thermal conductivity - L dimensionless location of surface from the front stagna tion point,L */a - L * dimension location of prism surface - Lc location of separation - N j shape function - Nu, local and average Nusselt numbers - M l shape function - P dimensionless pressure,p */u 2 - P * pressure - p x * x-componentP * - Pe Peclet number,Re Pr - Pr Prandtl number, c/K - Ra Rayleigh number,Gr Pr - Re Reynolds number,a u /v - s direction along the sides of prism - u dimensionlessX-direction component of velocity,u */u - u * X-direction component of velocity - u free stream velocity - dimensionlessY-direction component of velocity,v*/u - * Y-direction component of velocity - X X-direction axis - x dimensionlessX-direction coordinate,x */a - x * X- direction coordinate - Y Y-direction axis - y * dimensionless 7-direction coordinate,y */a - y * Y-direction coordinate Greek symbols coefficient of volumetric thermal expansion - attack angle - dynamic viscosity - kinematic viscosity,/ - density of fluid - w dimensionless surface shear stress, w * /u 2 - w * surface shear stress - wx * x-component of w /* - dimensionless temperature, 相似文献