全文获取类型
收费全文 | 3676篇 |
免费 | 844篇 |
国内免费 | 1699篇 |
专业分类
测绘学 | 27篇 |
大气科学 | 594篇 |
地球物理 | 1494篇 |
地质学 | 3226篇 |
海洋学 | 431篇 |
天文学 | 36篇 |
综合类 | 160篇 |
自然地理 | 251篇 |
出版年
2024年 | 12篇 |
2023年 | 61篇 |
2022年 | 116篇 |
2021年 | 121篇 |
2020年 | 167篇 |
2019年 | 198篇 |
2018年 | 192篇 |
2017年 | 134篇 |
2016年 | 210篇 |
2015年 | 210篇 |
2014年 | 247篇 |
2013年 | 314篇 |
2012年 | 267篇 |
2011年 | 297篇 |
2010年 | 261篇 |
2009年 | 291篇 |
2008年 | 261篇 |
2007年 | 342篇 |
2006年 | 315篇 |
2005年 | 245篇 |
2004年 | 227篇 |
2003年 | 196篇 |
2002年 | 160篇 |
2001年 | 156篇 |
2000年 | 161篇 |
1999年 | 141篇 |
1998年 | 124篇 |
1997年 | 124篇 |
1996年 | 124篇 |
1995年 | 97篇 |
1994年 | 93篇 |
1993年 | 79篇 |
1992年 | 61篇 |
1991年 | 43篇 |
1990年 | 47篇 |
1989年 | 33篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1986年 | 14篇 |
1985年 | 6篇 |
1984年 | 10篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1974年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有6219条查询结果,搜索用时 46 毫秒
1.
Difficulties are involved in discrete element method (DEM) modelling of the flexible boundary, that is, the membranes covering the soil sample, which can be commonly found in contemporary laboratory soil tests. In this paper, a novel method is proposed wherein the finite difference method (FDM) and DEM are coupled to simulate the rubber membrane and soil body, respectively. Numerical plane strain and triaxial tests, served by the flexible membrane, are implemented and analysed later. The effect of the membrane modulus on the measurement accuracy is considered, with analytical formulae derived to judge the significance of this effect. Based on an analysis of stress-strain responses and the grain rotation field, the mechanical performances produced by the flexible and rigid lateral boundaries are compared for the plane strain test. The results show that (1) the effect of the membrane on the test result becomes more significant at larger strain level because the membrane applies additional lateral confining pressure to the soil body; (2) the tested models reproduce typical stress and volumetric paths for specimens with shear bands; (3) for the plane strain test, the rigid lateral boundary derives a much higher peak strength and larger bulk dilatation, but a similar residual strength, compared with the flexible boundary. The latter produces a more uniform (or ‘diffuse') rotation field and more mobilised local kinematics than does the former. All simulations show that the proposed FDM-DEM coupling method is able to simulate laboratory tests with a flexible boundary membrane. 相似文献
2.
A formula for the thickness of a shear band formed in saturated soils under a simple shear or a combined stress state has been proposed. It is shown that the shear band thickness is dependent on the pore pressure properties of the material and the dilatancy rate, but is independent of the details of the combined stress state. This is in accordance with some separate experimental observations. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
3.
Experimental results are presented from the extensive program of drained plane strain compression tests on sand carried out in Grenoble over the last two decades. Systematic analysis of photographs of the deforming specimen allowed for measuring deformations and determining strain fields throughout the test, that is: prior to, at, and after the onset of strain localization. The principles, details and accuracy of the procedure are described, as well as its suitability to properly depict the patterns of deformation. Findings concerning the occurrence and progression of strain localization are discussed. The issues of shear band orientation and thickness are addressed, as well as temporary and persistent complex localization patterns, and the volumetric behaviour inside a band after its formation. The influence of such variables as initial state of the sand (effective stress and relative density), specimen size and slenderness, as well as grain size, is discussed. Copyright © 2004 John Wiley & Sons, Ltd 相似文献
4.
Longitudinal and transverse dispersion in estuaries may be determined in principle by measuring the change in time of the concentration field of a water soluble dye. In practice, measurements may take the form of several sampling runs crossing the dye patch. The coefficients of dispersion are usually estimated from the second moments of the concentration field; the use of second moments, in particular, introduces errors because measurements at long times or distance are unduly emphasized.A self-consistent method, free of the disadvantages inherent in the use of higher moments, is described in which the distribution of concentration in space is related to the zeroth spatial moment of concentration. In this formulation, based upon sampling runs across the dye patch, the concentration field in the estuary may be directly and simply related to the dispersion coefficients in both the intercept and gradient of a linear graph.The methods are illustrated by the characterization of a series of experiments in the Inner Estuary of Milford Haven. 相似文献
5.
IntroductionThe area of eastern Liaoning is an importantmetal and nonmetal metallogenetic district in China,and the Liaohe group is one of the most importantstrata that hosts Pb, Zn, Au, B and Mg etcstratabound deposits. Up to now many geo1ogistssuch as Z… 相似文献
6.
7.
Wave propagation in anisotropic linear viscoelastic media: theory and simulated wavefields 总被引:6,自引:0,他引:6
J. M. Carcione 《Geophysical Journal International》1990,101(3):739-750
8.
Randall D. Campbell 《Experimental Astronomy》2002,14(1):57-60
Keck's Long Wavelength Spectrometer (LWS), is the facility instrument used for imaging and spectroscopy in the wavelength
range of 3–28 μm at the Keck Observatory. LWS uses an 128 × 128 Si:As blocked impurity band (BIB) array manufactured by the
Boeing Corporation. This paper discusses the method used for optimizing the detector's operating parameters at a temperature
of 8.5 K and bias voltage of 1.2 V. A process for characterizing detective quantum efficiency of BIB detectors is also presented.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
In this paper the second order characteristic (discontinuous bifurcation) condition is derived for the granular flow (fully plastic) equations. This second order bifurcation equation is shown to be formally identical to the first order localization requirement during steady elastoplastic deformation provided the elastic compliance tensor is substituted for the product of the plastic multiplier with the flow Hessian. For isotropic yield and flow functions the invariant form of the characteristic condition is given in detail, as well as an alternative expression in adapted co‐ordinates. The characteristic condition can be regarded as defining a hardening function which is maximized to identify the critical angles. When the method is applied to 3D Coulomb flow, Mohr's 3D fracture plane conditions are obtained uniquely. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
10.