全文获取类型
收费全文 | 212314篇 |
免费 | 22193篇 |
国内免费 | 12848篇 |
专业分类
电工技术 | 14461篇 |
技术理论 | 6篇 |
综合类 | 24504篇 |
化学工业 | 23026篇 |
金属工艺 | 11298篇 |
机械仪表 | 18193篇 |
建筑科学 | 25790篇 |
矿业工程 | 9107篇 |
能源动力 | 6564篇 |
轻工业 | 9779篇 |
水利工程 | 8187篇 |
石油天然气 | 8310篇 |
武器工业 | 2700篇 |
无线电 | 15894篇 |
一般工业技术 | 32482篇 |
冶金工业 | 7532篇 |
原子能技术 | 3014篇 |
自动化技术 | 26508篇 |
出版年
2024年 | 901篇 |
2023年 | 3040篇 |
2022年 | 5107篇 |
2021年 | 6265篇 |
2020年 | 6868篇 |
2019年 | 6053篇 |
2018年 | 5770篇 |
2017年 | 7106篇 |
2016年 | 7908篇 |
2015年 | 8453篇 |
2014年 | 13066篇 |
2013年 | 12485篇 |
2012年 | 15873篇 |
2011年 | 16777篇 |
2010年 | 12914篇 |
2009年 | 13401篇 |
2008年 | 12088篇 |
2007年 | 14667篇 |
2006年 | 12709篇 |
2005年 | 10959篇 |
2004年 | 9127篇 |
2003年 | 7974篇 |
2002年 | 6743篇 |
2001年 | 5176篇 |
2000年 | 4624篇 |
1999年 | 3932篇 |
1998年 | 3042篇 |
1997年 | 2687篇 |
1996年 | 2309篇 |
1995年 | 1977篇 |
1994年 | 1623篇 |
1993年 | 1191篇 |
1992年 | 1035篇 |
1991年 | 739篇 |
1990年 | 618篇 |
1989年 | 587篇 |
1988年 | 413篇 |
1987年 | 248篇 |
1986年 | 205篇 |
1985年 | 104篇 |
1984年 | 99篇 |
1983年 | 52篇 |
1982年 | 67篇 |
1981年 | 79篇 |
1980年 | 53篇 |
1979年 | 49篇 |
1978年 | 22篇 |
1975年 | 18篇 |
1973年 | 18篇 |
1959年 | 33篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
Detection of multiple cracks using frequency measurements 总被引:2,自引:0,他引:2
D.P. Patil 《Engineering Fracture Mechanics》2003,70(12):1553-1572
A method for detection of multiple open cracks in a slender Euler-Bernoulli beams is presented based on frequency measurements. The method is based on the approach given by Hu and Liang [J. Franklin Inst. 330 (5) (1993) 841], transverse vibration modelling through transfer matrix method and representation of a crack by rotational spring. The beam is virtually divided into a number of segments, which can be decided by the analyst, and each of them is considered to be associated with a damage parameter. The procedure gives a linear relationship explicitly between the changes in natural frequencies of the beam and the damage parameters. These parameters are determined from the knowledge of changes in the natural frequencies. After obtaining them, each is treated in turn to exactly pinpoint the crack location in the segment and determine its size. The forward, or natural frequency determination, problems are examined in the passing. The method is approximate, but it can handle segmented beams, any boundary conditions, intermediate spring or rigid supports, etc. It eliminates the need for any symbolic computation which is envisaged by Hu and Liang [J. Franklin Inst. 330 (5) (1993) 841] to obtain mode shapes of the corresponding uncracked beams. The proposed method gives a clear insight into the whole analysis. Case studies (numerical) are presented to demonstrate the method effectiveness for two simultaneous cracks of size 10% and more of section depth. The differences between the actual and predicted crack locations and sizes are less than 10% and 15% respectively. The numbers of segments into which the beam is virtually divided limits the maximum number of cracks that can be handled. The difference in the forward problem is less than 5%. 相似文献
922.
The main problems in estimating the uncertainty due to the presence of impurities in the material: the absence in the documents of a unique definition of the quantity being measured – the fixed point temperature, the lack of agreed methods of realizing the fixed points and the insufficient justification of the equations used for the estimate, are analyzed. The results of a measurement of the melting point of gallium are presented, which differ in the conditions in which they were realized – with one and two liquid-solid interfaces and different solidification conditions, are presented. The reasons for the considerable difference between the values of the temperatures obtained in these experiments are discussed. 相似文献
923.
924.
本文主要介绍以单片机80c196kc为CPU对模拟信号进行等间隔采样以及实现对采样值回显的问题。详细介绍了测频软件及电路的实现,对等间隔采样软件实现进行了详细论述:详尽讨论了对于MG-12864-2液晶显示器的电路连接与软件控制方法;对MG-12864-2液晶显示器应用的独特之处做了细致的分析。 相似文献
925.
用热失重法确定水合粘土水分含量及存在形式 总被引:1,自引:1,他引:0
文章以蒙脱土为例,采用热失重法定性定量确定了水合粘土中结合水类型、界限及其相对含量,并与等温吸附法结果进行了对比。结果表明,用该方法得到的水合蒙脱土中各种水分的相对含量及总含水量与等温吸附实验结果相一致。热失重(25~250 ℃)分析结果表明,水合蒙脱土中水分子主要存在自由水、松散吸附结合水、紧密吸附水(强吸附结合水)3种状态,分别对应于25~75 ℃、75~120 ℃、120~230 ℃区间里的失水。从热失重(25~250 ℃)规律来看,仍存在两个明显的转折点p/ps=0.9、0.98,它们为粘土-水体系的两个特征湿度,前者为粘土表面强结合水与松散结合水(渗透结合水)之间的界限,后者为松散结合水与自由水之间的界限,这验证了等温吸附法确定的两个特征湿度及对粘土表面结合水界限划分的正确性。 相似文献
926.
927.
A new interaction integral formulation is developed for evaluating the elastic T-stress for mixed-mode crack problems with
arbitrarily oriented straight or curved cracks in orthotropic nonhomogeneous materials. The development includes both the
Lekhnitskii and Stroh formalisms. The former is physical and relatively simple, and the latter is mathematically elegant.
The gradation of orthotropic material properties is integrated into the element stiffness matrix using a “generalized isoparametric
formulation” and (special) graded elements. The specific types of material gradation considered include exponential and hyperbolic-tangent
functions, but micromechanics models can also be considered within the scope of the present formulation. This paper investigates
several fracture problems to validate the proposed method and also provides numerical solutions, which can be used as benchmark
results (e.g. investigation of fracture specimens). The accuracy of results is verified by comparison with analytical solutions. 相似文献
928.
利用双套环法对长江中上游水土流失严重区的50个县(市)进行原他测试,对其典型土壤的稳渗速率及其影响因素进行了分析研究。结果表明:土壤稳渗速率在长江中上游地区呈现规律性的地域分异,四川盆地以及毕节地区是稳渗速率高值区,陕南和陇南地区、三峡库区以及丹江口库区足稳渗速率一般区,金沙江下游地区以及湘东赣南红壤丘陵区是稳渗速率低值区;上壤稳渗速率随土类发生有规律变化,黄壤系列土类最大,然后是石灰土、紫色土,红壤系列土类稳渗速率最小;土壤稳渗速率与质量密度呈负相关关系,与孔隙度、大于0.25mm风干土水稳性团粒含量、有机质含量以及粉/粘呈正相关关系。 相似文献
929.
No authorship indicated 《Canadian Metallurgical Quarterly》2006,61(8):811
Presents the citation for Anthony D. Wagner, who received the Award for Distinguished Scientific Early Career Contributions to Psychology (cognition and human learning) "for outstanding and innovative research on the neural basis of memory and executive control." A brief profile and a selected bibliography accompany the citation. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
930.
In general, this paper deals with general nonlinear oscillations of a nonconservative and single degree-of-freedom system with odd nonlinearity and, in particular, it presents accurate higher-order analytical approximate solutions to van der Pol damped nonlinear oscillators having odd nonlinearity and the Rayleigh equation. By combining the linearization of the governing equation with harmonic balancing and the method of averaging, we establish accurate analytical approximate solutions for the general weakly damped nonlinear systems. Unlike the classical harmonic balance method, simple linear algebraic equations instead of nonlinear algebraic equations are obtained upon linearization prior to harmonic balancing. The combination of these two methods results in very accurate transient response of the periodic solution. In addition and for the first time, this paper also presents a method for deducing fourth-, fifth- and higher-order linearized governing equations from the lower-order equations without the requirement of formulating the problem from the first principle. Three examples including the van der Pol damped nonlinear oscillator are presented to illustrate the excellent agreement with approximate solution using the exact frequency. 相似文献