全文获取类型
收费全文 | 17414篇 |
免费 | 388篇 |
专业分类
公路运输 | 4595篇 |
综合类 | 4984篇 |
水路运输 | 4165篇 |
铁路运输 | 3530篇 |
综合运输 | 528篇 |
出版年
2024年 | 7篇 |
2023年 | 40篇 |
2022年 | 292篇 |
2021年 | 533篇 |
2020年 | 377篇 |
2019年 | 135篇 |
2018年 | 158篇 |
2017年 | 195篇 |
2016年 | 223篇 |
2015年 | 450篇 |
2014年 | 690篇 |
2013年 | 857篇 |
2012年 | 1199篇 |
2011年 | 1312篇 |
2010年 | 1346篇 |
2009年 | 1495篇 |
2008年 | 1391篇 |
2007年 | 1744篇 |
2006年 | 1628篇 |
2005年 | 1188篇 |
2004年 | 363篇 |
2003年 | 310篇 |
2002年 | 278篇 |
2001年 | 361篇 |
2000年 | 310篇 |
1999年 | 174篇 |
1998年 | 110篇 |
1997年 | 89篇 |
1996年 | 107篇 |
1995年 | 80篇 |
1994年 | 77篇 |
1993年 | 65篇 |
1992年 | 54篇 |
1991年 | 45篇 |
1990年 | 35篇 |
1989年 | 38篇 |
1988年 | 17篇 |
1987年 | 11篇 |
1986年 | 9篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
321.
322.
323.
324.
325.
326.
327.
V形刚构桥施工中倒三角区域支架的拆除时机分析 总被引:1,自引:0,他引:1
采用桥梁博士软件模拟一座典型的V形墩连续刚构桥的施工过程,为找出支架拆除对传力途径的影响,分别单独计算结构自重、预应力以及收缩徐变内力,然后对单因素分析的结果进行组合。分析结果表明倒三角区域支架的拆除时机对桥梁在施工阶段及成桥后的内力均有显著影响,可达50%以上。建议优先采用早拆支架方案,以使结构受力明确,且对V形墩区域受力有利。 相似文献
328.
With the development of vehicle gearbox to high-power-density and high-speed, how to predict and optimize the dynamic characteristics of vehicle gearbox becomes increasingly prominent. Aiming at the vehicle gearbox, this paper comprehensively and deeply studies the dynamic characteristics under the multi-boundary conditions. The generation mechanism of the multi-source excitations triggering the gearbox vibration is analyzed firstly. The vibration transfer path of the gearbox is explored. Secondly, the engine excitation, the gear meshing excitation and the bearing support load are numerically calculated. According to the finite element method, a fluid-solid coupling finite element model of the gearbox body is established to predict the gearbox dynamic responses based on the Galerkin method and the Hamiltonian variational principle. Finally, the effects of the excitation condition, oil height and reinforcement forms on the vibration responses of the gearbox body are thoroughly studied by simulation. The analysis indicates that it not only helps to modify and improve the method of forecasting the gearbox dynamic response, and also provides the theoretical and technical guidance for the gearbox design and optimization. 相似文献
329.
330.
Project portfolio management (PPM) is the centralized management method, process and technology in multiple projects. When multiple projects in the space industry are implemented, it provides an effective methodology to resolve the problems at the same time such as conflicts among models, decrease in design efficiency, and increase in target deviation. Hence, a PPM dedicated to multiple projects management in space enterprise is presented in this paper. Firstly, an analysis of features and contents in space enterprise portfolio management mode is performed by using PPM based on its specific strategic characteristics. Then, the principle and selection methods of PPM are provided as a reference for the future development of an enterprise. Finally, a multiple-level organization architecture including decision making layers, function management layers and project execution layers is proposed so as to adapt to possible changes in the multiple projects and correspond to the strategic development. As a consequence, a perfect matching mechanism to fit the changes in PPM modes is reached. In addition, the flow chart of PPM is designed and optimized by analyzing the implementation procedure of strategic target and project portfolio life-cycle, which is expected to realize the purpose of improving space enterprise management efficiency, project management capacity, innovation development and economic benefits. 相似文献