全文获取类型
收费全文 | 7416篇 |
免费 | 1559篇 |
国内免费 | 3733篇 |
专业分类
测绘学 | 333篇 |
大气科学 | 1077篇 |
地球物理 | 2691篇 |
地质学 | 6723篇 |
海洋学 | 930篇 |
天文学 | 35篇 |
综合类 | 368篇 |
自然地理 | 551篇 |
出版年
2024年 | 40篇 |
2023年 | 177篇 |
2022年 | 276篇 |
2021年 | 268篇 |
2020年 | 421篇 |
2019年 | 529篇 |
2018年 | 455篇 |
2017年 | 327篇 |
2016年 | 539篇 |
2015年 | 514篇 |
2014年 | 613篇 |
2013年 | 699篇 |
2012年 | 652篇 |
2011年 | 631篇 |
2010年 | 562篇 |
2009年 | 635篇 |
2008年 | 568篇 |
2007年 | 582篇 |
2006年 | 615篇 |
2005年 | 479篇 |
2004年 | 425篇 |
2003年 | 368篇 |
2002年 | 304篇 |
2001年 | 244篇 |
2000年 | 239篇 |
1999年 | 228篇 |
1998年 | 198篇 |
1997年 | 198篇 |
1996年 | 186篇 |
1995年 | 151篇 |
1994年 | 127篇 |
1993年 | 114篇 |
1992年 | 86篇 |
1991年 | 52篇 |
1990年 | 58篇 |
1989年 | 38篇 |
1988年 | 35篇 |
1987年 | 20篇 |
1986年 | 15篇 |
1985年 | 7篇 |
1984年 | 17篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1974年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 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.
This study evaluated the spatial variability of streambed vertical hydraulic conductivity (Kv) in different stream morphologies in the Frenchman Creek Watershed, Western Nebraska, using different variogram models. Streambed Kv values were determined in situ using permeameter tests at 10 sites in Frenchman, Stinking Water and Spring Creeks during the dry season at baseflow conditions. Measurements were taken both in straight and meandering stream channels during a 5 day period at similar flow conditions. Each test site comprised of at least three transects and each transect comprised of at least three Kv measurements. Linear, Gaussian, exponential and spherical variogram models were used with Kriging gridding method for the 10 sites. As a goodness-of-fit statistic for the variogram models, cross-validation results showed differences in the median absolute deviation and the standard deviation of the cross-validation residuals. Results show that using the geometric means of the 10 sites for gridding performs better than using either all the Kv values from the 93 permeameter tests or 10 Kv values from the middle transects and centre permeameters. Incorporating both the spatial variability and the uncertainty involved in the measurement at a reach segment can yield more accurate grid results that can be useful in calibrating Kv at watershed or sub-watershed scales in distributed hydrological models. 相似文献
3.
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
4.
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
5.
6.
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
7.
8.
9.
寇新建 《大地测量与地球动力学》1994,(3)
由于观测条件的变化,观测值方差将产生随机波动。本文根据Bayes估计理论,推导了线性模型待估参数的分布函数及观测值方差估计公式,并将其用于位移检验。计算结果表明,在同样的置信度下,本方法更符合实际。文中还对参数估值的统计性质进行了讨论。 相似文献
10.
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