全文获取类型
| 收费全文 | 1508篇 |
| 免费 | 269篇 |
| 国内免费 | 439篇 |
专业分类
| 测绘学 | 44篇 |
| 大气科学 | 108篇 |
| 地球物理 | 369篇 |
| 地质学 | 1021篇 |
| 海洋学 | 307篇 |
| 天文学 | 46篇 |
| 综合类 | 63篇 |
| 自然地理 | 258篇 |
出版年
| 2024年 | 4篇 |
| 2023年 | 25篇 |
| 2022年 | 57篇 |
| 2021年 | 51篇 |
| 2020年 | 58篇 |
| 2019年 | 87篇 |
| 2018年 | 66篇 |
| 2017年 | 57篇 |
| 2016年 | 71篇 |
| 2015年 | 80篇 |
| 2014年 | 110篇 |
| 2013年 | 128篇 |
| 2012年 | 83篇 |
| 2011年 | 116篇 |
| 2010年 | 87篇 |
| 2009年 | 103篇 |
| 2008年 | 108篇 |
| 2007年 | 104篇 |
| 2006年 | 110篇 |
| 2005年 | 94篇 |
| 2004年 | 82篇 |
| 2003年 | 63篇 |
| 2002年 | 72篇 |
| 2001年 | 68篇 |
| 2000年 | 39篇 |
| 1999年 | 44篇 |
| 1998年 | 38篇 |
| 1997年 | 30篇 |
| 1996年 | 29篇 |
| 1995年 | 31篇 |
| 1994年 | 32篇 |
| 1993年 | 17篇 |
| 1992年 | 9篇 |
| 1991年 | 10篇 |
| 1990年 | 12篇 |
| 1989年 | 5篇 |
| 1988年 | 2篇 |
| 1987年 | 8篇 |
| 1986年 | 7篇 |
| 1985年 | 4篇 |
| 1984年 | 2篇 |
| 1983年 | 1篇 |
| 1982年 | 2篇 |
| 1981年 | 1篇 |
| 1980年 | 2篇 |
| 1979年 | 2篇 |
| 1978年 | 1篇 |
| 1977年 | 1篇 |
| 1954年 | 3篇 |
排序方式： 共有2216条查询结果，搜索用时 15 毫秒
31.
32.
33.
34.
35.
In order to improve the engineering stability of saline soil of high chloride content in the Chaerhan salt lake region, six typical characteristics saline soil samples were selected, and tests on their... 相似文献
36.
This paper aims to determine the optimal fines content of coarse-grained soil required to simultaneously achieve weaker frost susceptibility and better bearing capacity. We studied the frost susceptibility and strength properties of coarse-grained soil by means of frost heaving tests and static triaxial tests, and the results are as follows:(1) the freezing temperature of coarse-grained soil decreased gradually and then leveled off with incremental increases in the percent content of fines; (2) the fines content proved to be an important factor influencing the frost heave susceptibility and strength properties of coarse-grained soil. With incremental increases in the percent content of fines, the frost heave ratio increased gradually and the cohesion function of fines effectively enhanced the shear strength of coarse-grained soil before freeze-thaw, but the frost susceptibility of fines weakened the shear strength of coarse-grained soil after freeze-thaw; (3) with increasing numbers of freeze-thaw cycles,the shear strength of coarse-grained soil decreased and then stabilized after the ninth freeze-thaw cycle, and therefore the mechanical indexes of the ninth freeze-thaw cycle are recommended for the engineering design values; and (4) considering frost susceptibility and strength properties as a whole, the optimal fines content of 5% is recommended for railway subgrade coarse-grained soil fillings in frozen regions. 相似文献
37.
The stress produced by repeated train loads decreases with increasing railway subgrade bed depth, and slightly weathered coarse particles of subgrade bed fillings can be broken at different levels under continuous load. Thus, the mass of fine soil, with a diameter of not more than 0.075 mm, is different at different depths. Fine soil is also sensitive to frost heave and thaw settlement. In order to study the effects of non-uniformly distributed fine soil on the mechanical properties of coarse-grained soil of the Shenyang-Dandong Railway, triaxial tests were conducted with three types of specimens, undergoing six freeze-thaw cycle numbers(0, 1, 3, 7, 9, 12) and three confining pressures(100, 200, 300 k Pa). The freezing temperature is-5 °C and the thawing temperature is +15 °C. The stress-strain behavior, static strength, resilient modulus, cohesive force and the angle of internal friction were measured for different tested specimens. As a result, the law of static strength and resilient modulus of different specimens following the increase of freeze-thaw cycles under three confining pressures is obtained. The changing law of cohesive force and friction angle of three specimens following the increase of freeze-thaw cycles is also calculated, and the different results of different specimens are also compared. 相似文献
38.
Spectroscopic techniques have become attractive to assess soil properties because they are fast, require little labor and may reduce the amount of laboratory waste produced when compared to conventional methods. Imaging spectroscopy (IS) can have further advantages compared to laboratory or field proximal spectroscopic approaches such as providing spatially continuous information with a high density. However, the accuracy of IS derived predictions decreases when the spectral mixture of soil with other targets occurs. This paper evaluates the use of spectral data obtained by an airborne hyperspectral sensor (ProSpecTIR-VS – Aisa dual sensor) for prediction of physical and chemical properties of Brazilian highly weathered soils (i.e., Oxisols). A methodology to assess the soil spectral mixture is adapted and a progressive spectral dataset selection procedure, based on bare soil fractional cover, is proposed and tested. Satisfactory performances are obtained specially for the quantification of clay, sand and CEC using airborne sensor data (R2 of 0.77, 0.79 and 0.54; RPD of 2.14, 2.22 and 1.50, respectively), after spectral data selection is performed; although results obtained for laboratory data are more accurate (R2 of 0.92, 0.85 and 0.75; RPD of 3.52, 2.62 and 2.04, for clay, sand and CEC, respectively). Most importantly, predictions based on airborne-derived spectra for which the bare soil fractional cover is not taken into account show considerable lower accuracy, for example for clay, sand and CEC (RPD of 1.52, 1.64 and 1.16, respectively). Therefore, hyperspectral remotely sensed data can be used to predict topsoil properties of highly weathered soils, although spectral mixture of bare soil with vegetation must be considered in order to achieve an improved prediction accuracy. 相似文献
39.
40.
Sandwich pipes (SP), a composite structure consisting of two concentric steel tubes and a polymeric or cement-based core has been proposed to be well-insulated and withstand high installation and operational loads in deepwater oil and gas field. In the paper, the post-buckling responses and pressure capacity of sandwich pipes filled with the solid polypropylene annular were investigated. The degree of the inter-layer adhesion condition between the core layer and the surrounding pipes was modeled by the contact surfaces adopting different maximum shear strength values to allow the relative displacement between the layers. The effects such as inter-layer adhesion interactions, thickness-to-radius ratios, the core thickness, the material parameters, the relative initial ovality directions and the inelastic anisotropy on the collapse pressure of SPs were discussed. More than 2000 finite element (FE) models of the sandwich pipes with practical configurations were constructed and analyzed using the programming language Python within the FE software package ABAQUS. Based on the FE results, a simplified equation was developed and recommended for the collapse pressure calculation of sandwich pipes with the polypropylene annular. 相似文献