全文获取类型
收费全文 | 1461篇 |
免费 | 203篇 |
国内免费 | 321篇 |
专业分类
测绘学 | 18篇 |
大气科学 | 126篇 |
地球物理 | 612篇 |
地质学 | 679篇 |
海洋学 | 323篇 |
天文学 | 2篇 |
综合类 | 58篇 |
自然地理 | 167篇 |
出版年
2024年 | 5篇 |
2023年 | 17篇 |
2022年 | 46篇 |
2021年 | 45篇 |
2020年 | 50篇 |
2019年 | 73篇 |
2018年 | 63篇 |
2017年 | 62篇 |
2016年 | 50篇 |
2015年 | 43篇 |
2014年 | 94篇 |
2013年 | 101篇 |
2012年 | 77篇 |
2011年 | 108篇 |
2010年 | 72篇 |
2009年 | 106篇 |
2008年 | 96篇 |
2007年 | 102篇 |
2006年 | 108篇 |
2005年 | 70篇 |
2004年 | 83篇 |
2003年 | 70篇 |
2002年 | 38篇 |
2001年 | 39篇 |
2000年 | 55篇 |
1999年 | 46篇 |
1998年 | 52篇 |
1997年 | 38篇 |
1996年 | 27篇 |
1995年 | 28篇 |
1994年 | 22篇 |
1993年 | 17篇 |
1992年 | 16篇 |
1991年 | 15篇 |
1990年 | 12篇 |
1989年 | 6篇 |
1988年 | 10篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 5篇 |
1977年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有1985条查询结果,搜索用时 17 毫秒
1.
IwrRODUcrlONStainlessstalhasbeenusedwidelyasbuildingrnateria1,especiallyasdecoratingrnate-rialinbuildings.Ordinarystainlesssteehasonlyonemonot0noussilverywhitCcolourwhichsomeimesdoesnotrnatchwithitSsurmundingenvironment.lnordert0improveitSdereratingperfonnaneeandexpanditsuse,muchresearchhasbeenconductedsincethe4O'st0trytoformabright,uniform,highcorrosi0nandwearresistantco1ourfilmonastainlesssteesurface.Duringtheeariystage,anirnmersionedhodwasused,i.e.inimrs-ingthestainlesssteelinahots0luti… 相似文献
2.
A discrete element modelling of bonded granulates and investigation on the bond effect on their behaviour are very important to geomechanics. This paper presents a two‐dimensional (2‐D) discrete element theory for bonded granulates with bond rolling resistance and provides a numerical investigation into the effect of bond rolling resistance on the yielding of bonded granulates. The model consists of mechanical contact models and equations governing the motion of bonded particles. The key point of the theory is that the assumption in the original bond contact model previously proposed by the authors (55th CSCE‐ASCE Conference, Hamilton, Ont., Canada, 2002; 313–320; J. Eng. Mech. (ASCE) 2005; 131 (11):1209–1213) that bonded particles are in contact at discrete points, is here replaced by a more reliable assumption that bonded particles are in contact over a width. By making the idealization that the bond contact width is continuously distributed with the normal/tangential basic elements (BE) (each BE is composed of spring, dashpot, bond, slider or divider), we establish a bond rolling contact model together with bond normal/tangential contact models, and also relate the governing equations to local equilibrium. Only one physical parameter β needs to be introduced in the theory in comparison to the original bond discrete element model. The model has been implemented into a 2‐D distinct element method code, NS2D. Using the NS2D, a total of 86 1‐D, constant stress ratio, and biaxial compressions tests have been carried out on the bonded granular samples of different densities, bonding strengths and rolling resistances. The numerical results show that: (i) the new theory predicts a larger internal friction angle, a larger yielding stress, more brittle behaviour and larger final broken contact ratio than the original bond model; (ii) the yielding stress increases nonlinearly with the increasing value of β, and (iii) the first‐yield curve (initiation of bond breakage), which define a zone of none bond breakage and which shape and size are affected by the material density, is amplified by the bond rolling resistance in analogous to that predicted by the original bond model. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
3.
刘满全 《地球科学与环境学报》1994,(2)
本文介绍紫阳骨坡治理工程中有关抗滑桩的设计与施工。文中详细地论述了设计中设计人员应根据工程的具体情况去灵活处理的一些技术问题,如确定有关参数的选择原则,计算上的条件简化和一些具体技术细节的处理方法等. 相似文献
4.
Soil nails have been widely used to stabilize slopes and earth retaining structures in many countries and regions, especially, in Hong Kong. The analysis of the interaction between a soil nail and the surrounding soil is of great interests to both design engineers and researchers. In this paper, authors present a simple mathematical model for the interaction analysis of a soil nail and the surrounding soil considering a few key factors which are soil dilation, bending of the soil nail, vertical pressure, and non-linear subgrade reaction stiffness. The lateral subgrade reaction between the soil and the soil nail is assumed to obey a hyperbolic relation. Reported test data in the literature are used to verify the present model. The contributions of the soil-nail bending on the pull-out resistance are evaluated in two case studies. 相似文献
5.
A Triassic carbonate unit has been intensively drained by zinc and lead ore mines and numerous borehole fields since the nineteenth
century. Its groundwater recharge has increased due to: pumping of water from boreholes, mining activity, and urbanization.
An approach to determine the amounts of the recharge at a variety of spatial scales is presented in the paper. Different methods
were used to identify and quantify recharge components on a regional and local scale: mathematical modelling was performed
for four aquifers included in an aquifer system, an analytical estimation based on the assumption that an average recharge
is equal to the average discharge of the hydrogeological system—for six man-made drainage centres, and the method of water
level fluctuation (WLF) was applied in one observation borehole. Results of modelling have been supplemented by observation
of environmental tracers (δ18O, δ2H, 3H), noble gases temperatures, and 4Heexc in groundwater. The regional aquifer’s current recharge according to estimations performed by means of modelling varies from
39 to 101 mm/year on average. Depending on the aquifer site the average precipitation ranges from 779 to 864 mm/year. In the
confined part of the aquifer average recharge ranges from 26 to 61 mm/year. Within outcrops average recharge varies from 96
to 370 mm/year. Current recharge estimated by the analytical method for man-made drainage centres varies from 158 up to 440 mm/year.
High values are caused by different recharge sources like precipitation, induced leakage from shallow aquifers, and water
losses from streams, water mains and sewer systems. Pumping of water, mining and municipal activities constitute additional
factors accounting for the intensified recharge. 相似文献
6.
7.
筒型基础沉贯阻力估算方法 总被引:1,自引:0,他引:1
系统地总结国外有代表性工程实践对筒型基础沉贯阻力的研究 ,并将各种方法综合起来 ,提出了加权综合估算法 ,供今后的研究和工程实践参考 相似文献
8.
平台在海上受风、浪、流、冰等水平载荷作用产生倾斜 ,而土壤粘接力、摩擦力和土抗力是平台的抗拔和抗倾力 ,桶基平台应满足抗拔、抗倾稳定的要求 ,以保证平台的整体稳定性。讨论了桶基平台受水平力作用时 ,桶形基础的破坏模式及其计算方法 ,阐述了抗倾稳定计算方法 相似文献
9.
A surface panel method treating a boundary-value problem of the Dirichlet type with the free surface is presented to design a three-dimensional body corresponding to a prescribed pressure distribution. The free surface boundary condition is linearized with respect to the oncoming flow, and computed by four-point finite difference scheme. Sample designs for submerged spheroids and Wigley hull are carried out to demonstrate the stable convergence, the effectiveness and the robustness of the method. The design of a 5500TEU container carrier is performed with respect to reduction of the wave resistance. To reduce the wave resistance, calculated pressure on the hull surface is modified to have the lower fluctuation, and is applied as a Dirichlet type dynamic boundary condition on the hull surface. The designed hull form is verified to have the lower wave resistance than the initial one not only by computation but also by experiment. 相似文献
10.
雄性三倍体长牡蛎繁殖潜力的初步研究 总被引:1,自引:0,他引:1
国内外很多人做过三倍体长牡砺的诱导工作,但在三倍体性腺发育及配子活力方面的研究,目前仅见Allen等1986年和1990年对三倍体的海螂和长蛎异常配子的发生报道。Cox1996年研究了悉尼岩牡蛎三倍体性腺发育情况,姜卫国等对三倍体合浦珠母贝的生殖腺进行了观察,李霞[1]对三倍体长牡蛎的性腺发育做了较系统的分析和总结。作者用流式细胞仪对长牡蛎Crassostrea gigas二、三倍体的性腺细胞进行了初步分析,回答了三倍体长牡蛎生殖潜力及性比方面的一些问题,为进一步研究三倍体长牡蛎的生长、繁殖、杂… 相似文献