全文获取类型
收费全文 | 12598篇 |
免费 | 1384篇 |
国内免费 | 2347篇 |
专业分类
测绘学 | 575篇 |
大气科学 | 660篇 |
地球物理 | 2014篇 |
地质学 | 5556篇 |
海洋学 | 1260篇 |
天文学 | 5085篇 |
综合类 | 480篇 |
自然地理 | 699篇 |
出版年
2024年 | 32篇 |
2023年 | 121篇 |
2022年 | 283篇 |
2021年 | 310篇 |
2020年 | 356篇 |
2019年 | 463篇 |
2018年 | 371篇 |
2017年 | 360篇 |
2016年 | 456篇 |
2015年 | 459篇 |
2014年 | 758篇 |
2013年 | 728篇 |
2012年 | 727篇 |
2011年 | 842篇 |
2010年 | 893篇 |
2009年 | 1194篇 |
2008年 | 1109篇 |
2007年 | 1019篇 |
2006年 | 946篇 |
2005年 | 818篇 |
2004年 | 679篇 |
2003年 | 609篇 |
2002年 | 500篇 |
2001年 | 430篇 |
2000年 | 369篇 |
1999年 | 338篇 |
1998年 | 255篇 |
1997年 | 147篇 |
1996年 | 119篇 |
1995年 | 89篇 |
1994年 | 82篇 |
1993年 | 91篇 |
1992年 | 44篇 |
1991年 | 49篇 |
1990年 | 45篇 |
1989年 | 39篇 |
1988年 | 31篇 |
1987年 | 7篇 |
1986年 | 22篇 |
1985年 | 24篇 |
1984年 | 27篇 |
1983年 | 20篇 |
1982年 | 18篇 |
1981年 | 7篇 |
1980年 | 12篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 15篇 |
1877年 | 1篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 781 毫秒
1.
A FORTRAN program, consistent with the commercially available finite element (FE) code ABAQUS, is developed based on a three-dimensional (3D) linear elastic brittle damage constitutive model with two damage criteria. To consider the heterogeneity of rock, the developed FORTRAN program is used to set the stiffness and strength properties of each element of the FE model following a Weibull distribution function. The reliability of the program is assessed against available experimental results for granite cylindrical specimens with a throughgoing, flat and inclined fissure. The calibration procedure of the material parameters is explained in detail, and it is shown that the compressive to tensile strength ratio can have a substantial influence on the failure response of the specimens. Numerical simulations are conducted for models with different levels of heterogeneity. The results show a smaller load bearing capacity for models with less homogeneity, representing gradual coalescence of fully damaged elements forming throughout the models during loading. The maximum load bearing capacity is studied for various combinations of inclination angles of two centrally aligned, throughgoing and flat fissures of equal length embedded in cylindrical models under uniaxial and multiaxial loading conditions. The key role of the compressive to tensile strength ratio is highlighted by repeating certain simulations with a lower compressive to tensile strength ratio. It is proven that the peak loads of the rock models with sufficiently small compressive to tensile strength ratios containing two throughgoing fissures of equal length are similar, provided that the minimum inclination angles of the models are the same. The results are presented and discussed with respect to the existing experimental findings in the literature, suggesting that the numerical model applied in this study can provide useful insight into the failure behaviour of rock-like materials. 相似文献
2.
Implementation of a coupled plastic damage distinct lattice spring model for dynamic crack propagation in geomaterials 下载免费PDF全文
This paper studies dynamic crack propagation by employing the distinct lattice spring model (DLSM) and 3‐dimensional (3D) printing technique. A damage‐plasticity model was developed and implemented in a 2D DLSM. Applicability of the damage‐plasticity DLSM was verified against analytical elastic solutions and experimental results for crack propagation. As a physical analogy, dynamic fracturing tests were conducted on 3D printed specimens using the split Hopkinson pressure bar. The dynamic stress intensity factors were recorded, and crack paths were captured by a high‐speed camera. A parametric study was conducted to find the influences of the parameters on cracking behaviors, including initial and peak fracture toughness, crack speed, and crack patterns. Finally, selection of parameters for the damage‐plasticity model was determined through the comparison of numerical predictions and the experimentally observed cracking features. 相似文献
3.
利用鄂尔多斯地块及其周缘1970~2014年的垂直形变速率场资料,借助负位错反演研究该区域长期应变积累。结果表明,地块东北缘山西断陷带中北段年均能量积累增量、剪应力强度都较高,西南缘六盘山断裂与渭河断裂西段次之;山西断陷带中南段至晋陕交界处年均剪应力强度较高且显示一定程度的能量积累;西秦岭构造区尤其西秦岭北缘断裂西段、晋冀蒙交界区也反映一定程度的能量积累特性。 相似文献
4.
5.
采用香港11个GPS测站的观测资料进行1 h、2 h、3 h和4h静态PPP解算,获得4组PPP坐标序列,利用调和分析求取11个测站处8个主要分潮的负荷位移参数(振幅和相位),将其与海潮模型计算的负荷位移参数进行对比,并比较分析PPP反演值与海潮模型值改正海潮负荷信号的效果。结果表明,垂直和水平方向上,不同PPP结果反演8个分潮的负荷位移分别具有约5 mm和7 mm的差异;PPP反演8个分潮垂向负荷位移优于全球海潮模型,但水平方向上的反演效果稍弱。 相似文献
6.
Micromechanical approach to effective viscoelastic properties of micro‐fractured geomaterials 下载免费PDF全文
The aim of this paper is to formulate a micromechanics‐based approach to non‐aging viscoelastic behavior of materials with randomly distributed micro‐fractures. Unlike cracks, fractures are discontinuities that are able to transfer stresses and can therefore be regarded from a mechanical viewpoint as interfaces endowed with a specific behavior under normal and shear loading. Making use of the elastic‐viscoelastic correspondence principle together with a Mori‐Tanka homogenization scheme, the effective viscoelastic behavior is assessed from properties of the material constituents and damage parameters related to density and size of fractures. It is notably shown that the homogenized behavior thus formulated can be described in most cases by means of a generalized Maxwell rheological model. For practical implementation in structural analyses, an approximate model for the isotropic homogenized fractured medium is formulated within the class of Burger models. Although the approximation is basically developed for short‐term and long‐term behaviors, numerical applications indicate that the approximate Burger model accurately reproduce the homogenized viscoelastic behavior also in the transient conditions. 相似文献
7.
《Astroparticle Physics》2002,16(4):183-386
Frequency distributions of local muon densities in high-energy extensive air showers (EAS) are presented as signature of the primary cosmic ray energy spectrum in the knee region. Together with the gross shower variables like shower core position, angle of incidence, and the shower sizes, the KASCADE experiment is able to measure local muon densities for two different muon energy thresholds. The spectra have been reconstructed for various core distances, as well as for particular subsamples, classified on the basis of the shower size ratio Nμ/Ne. The measured density spectra of the total sample exhibit clear kinks reflecting the knee of the primary energy spectrum. While relatively sharp changes of the slopes are observed in the spectrum of EAS with small values of the shower size ratio, no such feature is detected at EAS of large Nμ/Ne ratio in the energy range of 1–10 PeV. Comparing the spectra for various thresholds and core distances with detailed Monte Carlo simulations the validity of EAS simulations is discussed. 相似文献
8.
9.
10.