全文获取类型
收费全文 | 4102篇 |
免费 | 142篇 |
专业分类
公路运输 | 1230篇 |
综合类 | 1110篇 |
水路运输 | 1093篇 |
铁路运输 | 654篇 |
综合运输 | 157篇 |
出版年
2024年 | 3篇 |
2023年 | 19篇 |
2022年 | 86篇 |
2021年 | 167篇 |
2020年 | 119篇 |
2019年 | 49篇 |
2018年 | 61篇 |
2017年 | 61篇 |
2016年 | 66篇 |
2015年 | 125篇 |
2014年 | 195篇 |
2013年 | 235篇 |
2012年 | 299篇 |
2011年 | 309篇 |
2010年 | 288篇 |
2009年 | 329篇 |
2008年 | 316篇 |
2007年 | 357篇 |
2006年 | 338篇 |
2005年 | 298篇 |
2004年 | 118篇 |
2003年 | 68篇 |
2002年 | 58篇 |
2001年 | 73篇 |
2000年 | 63篇 |
1999年 | 28篇 |
1998年 | 29篇 |
1997年 | 19篇 |
1996年 | 18篇 |
1995年 | 15篇 |
1994年 | 7篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有4244条查询结果,搜索用时 453 毫秒
681.
Weight reduction has attracted much attention among ship designers and ship owners. In the present work, based on an improved bi-directional evolutionary structural optimization (BESO) method and surrogate model method, we propose a hybrid optimization method for the structural design optimization of beam-plate structures, which covers three optimization levels: dimension optimization, topology optimization and section optimization. The objective of the proposed optimization method is to minimize the weight of design object under a group of constraints. The kernel optimization procedure (KOP) uses BESO to obtain the optimal topology from a ground structure. To deal with beam-plate structures, the traditional BESO method is improved by using cubic box as the unit cell instead of solid unit to construct periodic lattice structure. In the first optimization level, a series of ground structures are generated based on different dimensional parameter combinations, the KOP is performed to all the ground structures, the response surface model of optimal objective values and dimension parameters is created, and then the optimal dimension parameters can be obtained. In the second optimization level, the optimal topology is obtained by using the KOP according to the optimal dimension parameters. In the third optimization level, response surface method (RSM) is used to determine the section parameters. The proposed method is applied to a hatch cover structure design. The locations and shapes of all the structural members are determined from an oversized ground structure. The results show that the proposed method leads to a greater weight saving, compared with the original design and genetic algorithm (GA) based optimization results. 相似文献
682.
A new time-frequency analysis method is proposed in this study using a multi-rate signal decomposition technique for the analysis of non-stationary signals. The method uses a multi-rate filter bank for an improved non-stationary signal decomposition treatment, and uses the Wigner-Ville distribution (WVD) analysis for signal reconstruction. The method presented in this study can effectively resolves the time and frequency resolution issue for non-stationary signal analysis and the cross-term issue typically encountered in time-frequency analysis. The feasibility and accuracy of the proposed method are evaluated and verified in a numerical simulation. 相似文献
683.
684.
685.
686.
687.
688.
综合利用计算流体力学数值仿真分析与风洞试验验证相结合的方法对某量产微型客车进行整车全细节空气动力学仿真分析和研究。对比分析前保险杠、后视镜、车窗等零部件对气动阻力的影响。研究表明,采用优化前保险杠并增加导流板、修改后视镜造型、车窗一体化设计等方案,可使该车气动阻力系数减少约6%。 相似文献
689.
690.