全文获取类型
收费全文 | 8004篇 |
免费 | 107篇 |
专业分类
公路运输 | 1798篇 |
综合类 | 589篇 |
水路运输 | 2546篇 |
铁路运输 | 770篇 |
综合运输 | 2408篇 |
出版年
2022年 | 112篇 |
2021年 | 59篇 |
2020年 | 47篇 |
2019年 | 80篇 |
2018年 | 193篇 |
2017年 | 116篇 |
2016年 | 159篇 |
2015年 | 93篇 |
2014年 | 244篇 |
2013年 | 1222篇 |
2012年 | 329篇 |
2011年 | 408篇 |
2010年 | 260篇 |
2009年 | 370篇 |
2008年 | 323篇 |
2007年 | 271篇 |
2006年 | 230篇 |
2005年 | 270篇 |
2004年 | 294篇 |
2003年 | 183篇 |
2002年 | 147篇 |
2001年 | 132篇 |
2000年 | 138篇 |
1999年 | 99篇 |
1998年 | 130篇 |
1997年 | 111篇 |
1996年 | 138篇 |
1995年 | 149篇 |
1994年 | 84篇 |
1993年 | 189篇 |
1992年 | 160篇 |
1991年 | 77篇 |
1990年 | 84篇 |
1989年 | 57篇 |
1988年 | 74篇 |
1987年 | 65篇 |
1986年 | 59篇 |
1985年 | 77篇 |
1984年 | 73篇 |
1983年 | 73篇 |
1982年 | 73篇 |
1981年 | 95篇 |
1980年 | 68篇 |
1979年 | 91篇 |
1978年 | 56篇 |
1977年 | 69篇 |
1976年 | 53篇 |
1975年 | 65篇 |
1974年 | 50篇 |
1973年 | 45篇 |
排序方式: 共有8111条查询结果,搜索用时 15 毫秒
991.
992.
E. C. Mikulcik 《Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility》1980,9(5):281-289
The characteristic equation for a simple automobile-trailer combination is analyzed, revealing the parameter groups which are important in determining the stability characteristics. Application of Routh's method results in separate criteria for oscillatory and non-oscillatory criteria which can be evaluated algebraically, and which can also be displayed graphically showing a region of stability on a two-dimensional plot. The stability region is bounded by limits of oscillatory and non-oscillatory stability, and the evaluation of a specific case corresponds to the location of a point relative to the boundaries. 相似文献
993.
994.
E. Esmailzadeh 《Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility》1978,7(2):83-96
The transmission of road-generated vibrations into a vehicle body is treated as a source-path-receiver problem. The suspension system acts as the path, and improved isolation can be achieved by having a single compliant bushing at the connecting point of the shock absorber to the body with none at the other end. A mathematical model is derived for such a system which would enable detailed parameter investigation to be undertaken using the gradient method of optimization. An expression for the absolute displacement transmissibility of the body is derived and the optimization procedure is applied in order to evaluate the optimum values of the non-dimensional variables involved. This minimizes the maximum motion transmitted to the body from the road surface over a broad frequency range. Design data which are presented non-dimensionally for parameter variations show the influence of three variables: the bushing stiffness, the resonant frequency ratio and the damping coefficient upon the transmissibility. 相似文献
995.
用克山病区的粮食饲养大鼠,复制低硒动物模型、用浮置玻璃微电极技术,研究低硒大鼠在体心室肌细胞动作电位的变化,并与对照组进行了对比观察。结果发现低硒大鼠动作电位振幅明显降低(P<0.05),动作电位除极时限显著延长(P<0.01),而动作电位复极50%及90%的持续时间变化不显著(P>0.05)。实验结果说明低硒可明显影响大鼠心室肌细胞动作电位变化。为克山病心肌损害的病因学研究提供了重要的实验依据。 相似文献
996.
Benjamin G. Heydecker 《Transportation Research Part B: Methodological》1983,17(5):359-364
The values of the dual variables in a constrained optimisation problem can be used to estimate the sensitivity of the optimal value of the objective function to changes in the constraints. Allsop (1972) used standard methods of linear programming to derive expressions for the sensitivity of the reserve capacity at a signal-controlled road junction to various changes in the traffic engineering constraints. That analysis used the assumption that the maximum reserve capacity would be achieved when the cycle time used is maximal. While this normally occurs, some junctions have come to light where a reduction in the cycle time increases the capacity. Allsop's analysis is extended here to account for this possibility. 相似文献
997.
Okechukwu C. Iheduru 《Maritime Policy and Management》1993,20(1):31-49
While recognizing the need for increased participation of the private sector in the maritime industry of developing countries, there is a need to go beyond the call for privatization in terms of mere change of ownership from state to private sector as the only solution to the inefficiency of the maritime industry in developing countries. This limited understanding of the maritime problems of these countries abounds in the literature on maritime privatization. This study departs from this approach and focuses on the political economy of the maritime industry, and suggests that maritime privatization will not necessarily result in increased efficiency of the divested maritime enterprises. Rather, it argues that maritime problems in these countries have less to do with ownership than with the structural constraints in world shipping together with internal management and local ~olitics. The maritime industrv in West and Central Africa is used as a case studv. and recommendations such as selective privatization, 'programme contract', and most importantly, corporatization, are suggested as alternative strategies that could enhance maritime privatization and the development of the maritime industry in developing countries. 相似文献
998.
房志宇 《西安交通大学学报(英文版)》1995,(2)
INVITROGLYCATIONOFHEMOGLOBIN(HB):CHOOSINGTHEAPPROPRIATEMODELFangZhiyu;R.R.Little,H.M.Wiedmeyer,D.E.Goldstein(DepartmentofChil... 相似文献
999.
Hartmut H. Topp 《Transportation》1988,15(4):279-295
West Germany is densely populated, averaging 245 inhabitants/km2, but varying widely between urban agglomerations and rural areas. Transport volume has increased by 40% since 1970, with virtually all growth due to private automobiles. Since 1981 public transit has been suffering from decreasing demand.A 1964 Expert's Report to the German federal government was the stimulus for initiating an effective funding mechanism for new public transit construction. In 1965 Germany's first federated transit authority was founded for the region of Hamburg.Principal among the goals of any cooperative agreement among transit companies are improvements for the passengers and improvement of revenues for the companies. To attain these ends, two distinct forms of transit aggrements have been developed in Germany: transit cooperative (Verkehrsgemeinschaft) and transit federation (Verkehrsverbund). The former is more suitable for smaller to medium-sized towns, while the latter is more suitable for larger cities. The two types are described in this article.German transit federations during the 1970s succeeded in significantly increasing ridership, while during the 1980s patronage has either remained steady or has declined. Yet transit federations showed much better perfomance than did public transit in general. In terms of costs and revenues, no public transit organization in Germany is able to break even; deficits vary between 42% and 55%. The author concludes, however, that hidden subsidies for automobile traffic are far higher, because of environmental damage and the high social cost of traffic accidents. 相似文献
1000.
This paper investigates the influence that women's fears and apprehension about attack and harassment have on use of transport facilities. After initial consideration of the factors that most affect perceived risk, the personal security aspects of different modes are reviewed by reference to a survey conducted in Southampton in 1986. High levels of perceived insecurity were found, particularly for walking at night, in parks and subways and when waiting for public transport services in isolated areas. The conclusions argue that personal security is an important but neglected issue, deserving of greater attention by transport planners. 相似文献