全文获取类型
收费全文 | 3450篇 |
免费 | 35篇 |
专业分类
公路运输 | 985篇 |
综合类 | 173篇 |
水路运输 | 1146篇 |
铁路运输 | 139篇 |
综合运输 | 1042篇 |
出版年
2022年 | 53篇 |
2021年 | 27篇 |
2020年 | 28篇 |
2019年 | 39篇 |
2018年 | 80篇 |
2017年 | 98篇 |
2016年 | 126篇 |
2015年 | 48篇 |
2014年 | 118篇 |
2013年 | 543篇 |
2012年 | 172篇 |
2011年 | 190篇 |
2010年 | 155篇 |
2009年 | 171篇 |
2008年 | 158篇 |
2007年 | 112篇 |
2006年 | 83篇 |
2005年 | 89篇 |
2004年 | 55篇 |
2003年 | 62篇 |
2002年 | 54篇 |
2001年 | 53篇 |
2000年 | 57篇 |
1999年 | 40篇 |
1998年 | 54篇 |
1997年 | 56篇 |
1996年 | 65篇 |
1995年 | 74篇 |
1994年 | 22篇 |
1993年 | 38篇 |
1992年 | 38篇 |
1991年 | 34篇 |
1990年 | 30篇 |
1989年 | 20篇 |
1988年 | 25篇 |
1987年 | 26篇 |
1986年 | 22篇 |
1985年 | 27篇 |
1984年 | 29篇 |
1983年 | 25篇 |
1982年 | 23篇 |
1981年 | 35篇 |
1980年 | 33篇 |
1979年 | 34篇 |
1978年 | 32篇 |
1977年 | 26篇 |
1976年 | 19篇 |
1975年 | 29篇 |
1974年 | 19篇 |
1973年 | 18篇 |
排序方式: 共有3485条查询结果,搜索用时 15 毫秒
611.
Reliability-based sensitivity analysis of vehicle components with non-normal distribution parameters
Techniques from the perturbation method, fourth moment method, reliability-based design theory, and sensitivity analysis approach
are employed to present a practical and efficient method for testing the reliability sensitivity of vehicle components with
non-normal distribution parameters. With the condition that the first four moments of original random variables are known,
the reliability sensitivity theory and cases are researched using the presented numerical method. The variation regularities
of reliability sensitivity are obtained and the effects of design parameters on reliability of the vehicle components are
studied. The sophisticated formulation provided in this paper is easily amenable to computational procedures. The respective
program can be used to obtain the reliability sensitivity of vehicle components with non-normal distribution parameters accurately
and quickly. The results obtained are perfect and the solutions compared very well with those from Monte Carlo simulation.
The method presents a theoretic basis for the reliability design of the vehicle components. 相似文献
612.
Model-based control system design in a urea-SCR aftertreatment system based on NH3 sensor feedback 总被引:1,自引:0,他引:1
M. Devarakonda G. Parker J. H. Johnson V. Strots 《International Journal of Automotive Technology》2009,10(6):653-662
This paper presents preliminary control system simulation results in a urea-selective catalytic reduction (SCR) aftertreatment
system based on NH3 sensor feedback. A four-state control-oriented lumped parameter model is used to analyze the controllability and observability
properties of the urea-SCR plant. A model-based estimator is designed via simulation and a control system is developed with
design based on a sliding mode control framework. The control system based on NH3 sensor feedback is analyzed via simulation by comparing it to a control system developed based on NOx sensor feedback. Simulation results show that the NH3 sensor-based strategy performs very similarly in comparison to a NOx sensor-based strategy. The control system performance metrics for NOx index, urea index, urea usage, and NH3 slip suggest that the NOx sensor can be a potential alternative to a NOx sensor for urea-SCR control applications. 相似文献
613.
美国桥梁技术近期发展概况 总被引:1,自引:1,他引:0
美国是公认的世界上交通运输网最广泛的国家.随着74 674 km国道系统的建成,并伴随着人们对于旅游和货物运输要求的持续增长,交通便利性已经成为了一种权利.全国统计在册的600 000座桥梁的平均桥龄为45年,这就为推进及推广应用桥梁技术的研究提供了巨大的机会,通过这些技术,可使公路基础设施更新中的桥梁更耐久、更经济,可采用更快速的施工技术及修建更可靠、更便于维护的结构.介绍美国桥梁技术近期发展概况. 相似文献
614.
Paula C. Pardo Marcos Vázquez-Rodríguez Fiz F. Pérez Aida F. Rios 《Journal of Marine Systems》2011,84(3-4):67-77
This work estimates new regionalized empirical parameterizations for preformed alkalinity (ATo) and the CO2 air–sea disequilibrium (?Cdis). Both are key terms for the computation of anthropogenic CO2 in the back-calculation methods. Data from the subsurface layer (75–180 m depth range) covering an area from North to South and from 19°E to 67.5°W (Pacific and Indian oceans) were taken from GLODAP (The Global Ocean Data Analysis Project) database. The subsurface layer is proved as a reliable reference for representing the main characteristics of the different water masses of the oceans. Besides, handing data from the two ocean basins altogether makes the new parameterizations of ATo and ?Cdis to be more globally consistent. Nevertheless, each ocean basin, at least in some regions, has different oceanographic characteristics based on its proper dynamical processes and water masses formation. In order to maintain each ocean basin ‘identity’ the whole domain was divided in six different regions (two of them sharing waters from Pacific and Indian oceans) and parameterizations in each region for both terms were obtained. Previously, data were transformed into a grid of 4°lat. × 5°lon. and the results obtained from the parameterizations were visualized and compare with pCO2 climatologies. From the comparisons with previous ?Cdis estimations good results are obtained showing the reliability and robustness of the new regionalized empiric parameterizations. 相似文献
615.
D. B. Sonawane K. Narayan V. S. Rao S. C. Subramanian 《International Journal of Automotive Technology》2011,12(5):697-704
Most commercial vehicles such as buses and trucks use an air brake system, often equipped with an S-cam drum brake, to reduce their speed and/or to stop. With a drum brake system, the clearance between the brake shoe/pad and the brake drum may increase because of various reasons such as wearing of the brake shoe and/or brake drum and drum expansion caused by high heat generation during the braking process. Hence, to ensure proper functioning of the brake system, it is essential that the clearance between the brake shoe and the brake drum is monitored. In this paper, we present a mathematical model for the mechanical subsystem of the air brake system that can be used to monitor this clearance. This mathematical model correlates the push rod stroke transients and the brake chamber pressure transients. A kinematic analysis and a dynamic analysis of the mechanical subsystem of the air brake system were performed, and the results are corroborated with experimental data. 相似文献
616.
A wheel tracking test was modelled to gain better understanding of the deflection and stress-strain distribution in an overlaid cracked pavements with and without membrane interlayer (SAMI). For this purpose, commercial finite element software Abaqus 6.7-1 was used. Two different models were considered, one incorporating stress absorbing membrane interlayers (SAMIs) and the other without SAMI. In the study, full bond condition was assumed for the boundaries between the layers, and a linear elastic model was used for the analysis. The results show that introduction of SAMI caused greater deflection of the pavement. It is found that although with SAMIs, low stiffness is required, a very low stiffness may yield undesirable results. The results show that the introduction of SAMIs results in high strain concentration around the crack region, whilst the strain in the overlay is smaller than the values predicted in the models without SAMIs. 相似文献
617.
High resolution structural studies of DNA and DNA binding proteins by atomic force microscopy (AFM) require well-bound samples on suitably flat substrates. Adsorbing the DNA onto a positively charged supported lipid bilayer has previously been shown to be a potentially effective strategy for structural studies with AFM. Here, using our home-built frequency-modulation AFM (FM-AFM), we show that these bilayer substrates are only maximally effective for high resolution AFM when the samples are short, linear DNA, compared with circular plasmid DNA. We find that, with the former sample, the measured width of the DNA is about 2 nm, the known DNA diameter, and there is a clear height modulation along the length of the DNA with a periodicity of about 3.4 nm, in excellent agreement with the known pitch of the double helix. This sample preparation strategy is expected to enable higher resolution studies of DNA and DNA binding proteins with FM-AFM than that can presently be achieved. 相似文献
618.
Bacterial pore-forming toxins (PFTs) are essential virulence factors of many human pathogens. Knowl- edge of their structure within the membrane is critical for an understanding of their function in pathogenesis and for the development of useful therapy. Atomic force microscopy (AFM) has often been employed to structurally interrogate many membrane proteins, including PFTs, owing to its ability to produce sub-nanometer resolution images of samples under aqueous solution. However, an absolute prerequisite for AFM studies is that the samples are single-layered and closely-packed, which is frequently challenging with PFTs. Here, using the prototypical member of the cholesterol-dependent cytolysin family of PFTs, perfringolysin O (PFO), as a test sample, we have developed a simple, highly robust method that routinely produces clean, closely-packed samples across the entire specimen surface. In this approach, we first use a small Teflon well to prepare the supported lipid bilayer, remove the sample from the well, and then directly apply the proteins to the bilayer. For reasons that are not clear, bilayer preparation in the Teflon well is essential. We anticipate that this simple method will prove widely useful for the preparation of similar samples, and thereby enable AFM imaging of the greatest range of bacterial PFTs to the highest possible resolution. 相似文献
619.
王志恩 《西安交通大学学报(英文版)》1995,(2)
T_H1ANDT_H2CELLANTIGENRECEPTORSINEXPERIMENTALLEISHMANIASISStevenL.Reiner,WangZhi'en;FarahHatam,PhillipScott,RichardM.Locksley(... 相似文献
620.
郑时春 《西安交通大学学报(英文版)》1995,(2)
LEISHMANIAPROMASTIGOTESEVADEINTERLEUKIN12INDUCTIONBYMACROPHAGESANDSTIMULATEABROAD RANGEOFCYTOKINESFROM CD_(4)~(+)TCELLSDURINGI... 相似文献