全文获取类型
收费全文 | 4277篇 |
免费 | 686篇 |
国内免费 | 1098篇 |
专业分类
测绘学 | 496篇 |
大气科学 | 707篇 |
地球物理 | 1980篇 |
地质学 | 1544篇 |
海洋学 | 488篇 |
天文学 | 135篇 |
综合类 | 556篇 |
自然地理 | 155篇 |
出版年
2024年 | 19篇 |
2023年 | 59篇 |
2022年 | 127篇 |
2021年 | 152篇 |
2020年 | 208篇 |
2019年 | 226篇 |
2018年 | 201篇 |
2017年 | 189篇 |
2016年 | 215篇 |
2015年 | 264篇 |
2014年 | 300篇 |
2013年 | 315篇 |
2012年 | 292篇 |
2011年 | 328篇 |
2010年 | 279篇 |
2009年 | 314篇 |
2008年 | 262篇 |
2007年 | 281篇 |
2006年 | 251篇 |
2005年 | 253篇 |
2004年 | 212篇 |
2003年 | 186篇 |
2002年 | 157篇 |
2001年 | 127篇 |
2000年 | 119篇 |
1999年 | 94篇 |
1998年 | 96篇 |
1997年 | 93篇 |
1996年 | 81篇 |
1995年 | 70篇 |
1994年 | 72篇 |
1993年 | 55篇 |
1992年 | 54篇 |
1991年 | 35篇 |
1990年 | 20篇 |
1989年 | 15篇 |
1988年 | 13篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1980年 | 1篇 |
1979年 | 4篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1954年 | 1篇 |
排序方式: 共有6061条查询结果,搜索用时 500 毫秒
21.
Effect of Cyclic Loading Frequency on Undrained Behaviors of Undisturbed Marine Clay 总被引:2,自引:0,他引:2
Based on a series of cyclic triaxial tests, the effect of cyclic frequency on the undrained behaviors of undisturbed marine clay is investigated. For a given dynamic stress ratio, the accumulated pore water pressure and dynamic strain increase with the number of cycles. There exists a threshold value for beth the accumulated pore water pressure and dynamic strain, below which the effect of cyclic frequency is very small, but above which the accumulated pore water pressure and dynamic strain increase intensely with the decrease of cyclic frequency for a given number of cycles. The dynamic strength increases with the increase of cyclic frequency, whereas the effect of cyclic frequency on it gradually diminishes to zero when the number of cycles is large enough, and the dynamic strengths at different frequencies tend to the same limiting minimum dynamic strength. The test results demonstrate that the reasons for the frequency effect on the undrained soil behaviors are beth the creep effect induced by the loading rate and the decrease of sample effective confining pressure caused by the accumulated pore water pressure. 相似文献
22.
本文以南麂海洋站1983~1990年风、浪的实测资料为依据,建立了南麂海城春、夏、秋、冬季定常波风浪波高与风速的经验关系式。检验结果表明,曲线回归显著,计算值与实测值吻合良好。文中还对偏NNE向和偏SSW向计算波高随风速增大的快慢,同一方向在同一风速作用下计算波高的季节变化及其机理作了初浅的讨论。 相似文献
23.
随着Linux技术逐步走向成熟,嵌入式Linux技术在海洋自动观测系统中的应用将全面提高海洋观测系统整体性能,降低系统开发成本,进一步推进我国海洋自动观测系统产权自主化的建设。文章就如何研制适用于海洋自动观测系统的嵌入式Linux系统进行了论述。 相似文献
24.
Noriaki Kimura 《Journal of Oceanography》2007,63(4):685-694
This paper examines the mechanism controlling the short time-scale variation of sea ice cover over the Southern Ocean. Sea
ice concentration and ice velocity datasets derived from images of the Defense Meteorological Satellite Program (DMSP) Special
Sensor Microwave Imager (SSM/I) are employed to reveal this mechanism. The contribution of both dynamic and thermodynamic
processes to the change in ice edge location is examined by comparing the meridional velocity of ice edge displacement and
sea ice drift. In the winter expansion phase, the thermodynamic process of new ice production off the ice edge plays an important
role in daily advances of ice cover, whereas daily retreats are mostly due to southward ice drift. On the other hand, both
advance and retreat of ice edges in the spring contraction phase are mostly caused by the dynamic process of the ice drift.
Based on the above mechanism and the linear relation between the degree of ice production at the ice edge and northward wind
speed, the seasonal advance of ice cover can be roughly reproduced using the meridional velocity of ice drift at the ice edge. 相似文献
25.
1990年7~8月,舟山地区的对虾养殖场暴发了流行性对虾黑鳃褐斑综合症,造成对虾大量死亡。作者根据这一病情,对病虾的主要器官组织进行了电镜观察。结果发现,病虾鳃丝细胞中有支原体感染。寄生部位为鳃呼吸上皮细胞的细胞质和核周腔。支原体多形态,呈近圆形、细长条形不等,无细胞壁,仅有单一膜包围,中心为一电子密度较低的区域。 相似文献
26.
Chen Tieyun Zhu Zhenghong
Professor Dept. of Naval Architecture Ocean Engineering Shanghai Jiao Tong University Shanghai Lecturer 《中国海洋工程》1991,(1)
The plastic node method is reformulated by the variational principle and is applied to elasto-plastic finite element analysis of tubular joints, eventually including the effect of internal and external gussets, stiffener rings, etc., if necessary. Four different joints are studied here in detail for the elasto-plastic behavior, the strain at the hot spot, the strain concentration factor around the intersection line, and the propagation of the plastic region with loading up to collapse in order to determine the ultimate strength, safety factor, and development of the plastic field. The present results are in good agreement with the experimental results. 相似文献
27.
28.
天然感潮河道水流紊动特性分析 总被引:5,自引:0,他引:5
采用多谱勒三相流速仪,在长江口徐六泾水文观测断面分别进行了涨潮、落潮时中泓与近岸垂线的流速观测,根据这些观测资料,对天然感潮河段潮流紊动的周期、频率、概率密度函数等进行了定量的数学描述,并对时均流速、紊动强度、雷诺应力等沿垂线分布进行了分析计算。 相似文献
29.
Plate anchors are extensively used in civil engineering constructions as they provide an economical alternative to gravity and other embedded anchors. The rate of loading is one of the important factors that affects the magnitude of soil resistance as well as soil suction force. This article outlines the effect of pullout rate on uplift behavior of plate anchors (70 mm diameter) buried in soft saturated clay by varying the pullout rate from 1.4 mm/min to 21.0 mm/min. The variation of breakout force and suction force with embedment depth and rate of pull are presented. A correlation between the rate of increase of undrained strength of clay and anchor capacity with rate of strain has been established. Finally an empirical equation has been proposed that includes the rate of pull in the estimation of breakout capacity of anchors. 相似文献
30.
AbstractHelical piles have emerged as an attractive foundation system for offshore applications with renewed interest from the offshore community. Significant research gap currently exists in transferring this technology offshore and this paper discusses how existing and emerging knowledge can be successfully used to bridge some of the gaps. We focus on the Coupled Eulerian Lagrangian (CEL) large deformation finite element (LDFE) modelling technique that is commercially available and can be used to model the three-dimensional installation process with consideration of strain rate and softening effects in soft offshore clays. A helical pile of L?=?7.5?m long is modelled with one or two large-diameter helices (D?=?2?m) attached to a central shaft of d?=?0.5?m in diameter.The net effect of strain rate and softening is to increase the installation torque. The measured torque is within the range of 200–400?kN.m for the offshore clay and the pile geometry studied. Additional helices increase the uplift force but to a lesser degree than that of the measured torque. Remoulding induced strength reduction is found to be within the range of 25–33% of the intact clay strength. Issues of extracting and reusing offshore helical pile foundations are discussed. 相似文献