全文获取类型
收费全文 | 7456篇 |
免费 | 835篇 |
国内免费 | 1518篇 |
专业分类
化学 | 3339篇 |
晶体学 | 150篇 |
力学 | 3220篇 |
综合类 | 89篇 |
数学 | 807篇 |
物理学 | 2204篇 |
出版年
2024年 | 25篇 |
2023年 | 96篇 |
2022年 | 202篇 |
2021年 | 206篇 |
2020年 | 307篇 |
2019年 | 258篇 |
2018年 | 229篇 |
2017年 | 361篇 |
2016年 | 375篇 |
2015年 | 271篇 |
2014年 | 399篇 |
2013年 | 580篇 |
2012年 | 390篇 |
2011年 | 498篇 |
2010年 | 346篇 |
2009年 | 371篇 |
2008年 | 427篇 |
2007年 | 472篇 |
2006年 | 462篇 |
2005年 | 465篇 |
2004年 | 425篇 |
2003年 | 385篇 |
2002年 | 281篇 |
2001年 | 294篇 |
2000年 | 240篇 |
1999年 | 223篇 |
1998年 | 215篇 |
1997年 | 161篇 |
1996年 | 137篇 |
1995年 | 114篇 |
1994年 | 114篇 |
1993年 | 91篇 |
1992年 | 70篇 |
1991年 | 74篇 |
1990年 | 55篇 |
1989年 | 43篇 |
1988年 | 31篇 |
1987年 | 21篇 |
1986年 | 19篇 |
1985年 | 14篇 |
1984年 | 11篇 |
1983年 | 2篇 |
1982年 | 11篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1979年 | 11篇 |
1977年 | 4篇 |
1975年 | 1篇 |
1971年 | 3篇 |
1957年 | 9篇 |
排序方式: 共有9809条查询结果,搜索用时 31 毫秒
31.
An experimentally validated finite element method is used to model the sound level in rooms at low frequencies. It is demonstrated that the dimensions of rectangular rooms strongly influence the sound pressure level difference. Additional factors were investigated which are not normally considered in the frequency range where diffuse sound field conditions can be assumed. Three effects were investigated: room damping due to wall vibrations, furniture, the effect of small deviations from simple rectangular shapes. It is confirmed by field measurements that the vibrations of masonry walls and floors introduce less damping than surfaces of lightweight construction. Assigning to the FE model a damping equivalent to a surface absorption of 0.02 reproduces the effect of walls of heavyweight construction. Damping equivalent to a surface absorption of 0.15 reproduces the effects of plastered timber-frame walls, floors and ceilings. The work was briefly extended to a room pair built with heavyweight and lightweight material of construction. The modification of the shape of the room frequency response highlights well the effect of material of construction. In-situ and laboratory measurements show that furniture has little effect on steady-state room response below 100 Hz. Modelling a wall recess smaller than 0.5 m improved the agreement between prediction and measurements but the assumption of a simple rectangular room remains appropriate. 相似文献
32.
M.B. Xu 《Applied Acoustics》2003,64(7):731-752
The forced vibration of an infinite elastic circular cylindrical shell filled with fluid is studied. Three methods are employed to analyze the forced vibration problem of this shell-fluid coupled system, that is, wave propagation approach (wave mode superposition), theorem of residues and a numerical integral method. In order to explain these methods more explicitly, before being used to investigate the vibration of an infinite fluid-filled elastic circular cylindrical shell, all these three methods are employed firstly to analyze the forced vibration problem of an infinite beam and an infinite elastic circular cylindrical shell in vacuo. Advantage and disadvantage of these three methods are discussed and their interesting relationship is revealed. That is, to any circumferential wavenumber and frequency of the external force, there is an unchangeable relationship between the general coordinates of various waves in the wave propagation approach and the residuals in the theorem of residues. 相似文献
33.
35.
The load-carrying capacity of circular plates made of a fiber-reinforced composite and loaded with axisymmetric transverse forces under axisymmetric boundary conditions is investigated. The plates can be freely supported or clamped along their contour. To the known yield condition used, in the plane of main bending moments, there correspond irregular hexagons, which depend on the mechanical properties of composite constituents and some characteristic geometrical parameters. It is shown that, depending on the ratio of ultimate radial and circumferential bending moments, in the plastic state, the circular plates (the central part of the clamped plates) take the form of a cone or a frustum of cone. For each of the cases, the limit load is found, which is a function of the above-mentioned mechanical parameters. The statically allowable fields of bending moments and the corresponding allowable fields of deflection rates are determined. The solutions found are exact. Special cases for the freely supported plates are analyzed, and the results obtained are presented in the form of graphs.__________Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 2, pp. 177–192, March–April, 2005. 相似文献
36.
李辉 《宁波大学学报(理工版)》1995,(1)
本文对低叠合面钢筋破简支在合梁二阶段受力的斜截面抗剪性能进行了试验研究,并运用非线性有限元分析和试验实测数据,得出了一个适合于集中荷载下高低叠合面叠合梁的抗剪强度计算公式. 相似文献
37.
A new method of using time-averaged digital speckle pattern interferometry for the quantitative measurement of vibration amplitude was developed. Signal processing techniques especially the Hilbert transformation for quantitative evaluation of the Bessel fringes obtained in time-averaged digital speckle pattern interferometry were explored. The quadrature signal after Hilbert transformation is equivalent to a 90° phase-shifted interferogram for a monotonically increasing or decreasing phase function. An algorithm was developed for Bessel fringe contrast enhancement and phase extraction. The techniques were tested numerically and experimentally. Sub-fringe quantification of the time-averaged vibration fringes is realised with the proposed method. Compared with the commonly used phase shift method which requires a minimum of two images for image processing, this method requires only one fringe pattern for data extraction. 相似文献
38.
The mucosal upheaval (MU), where the mucosal wave starts and propagates upward, appears only when the vocal fold vibrates. The location of the MU histologically and the effect of changes in mean air flow rate (MFR) and vocal fold length on occurrence of the MU were studied in twelve excised canine larynges. The lower surface of the vocal fold was marked to serve as a landmark for subsequent study. Cricothyroid approximation was performed to lengthen the vocal fold. After taking high-speed pictures or recording stroboscopic images from the tracheal side, a small cut wound was made at the mark. This wound served to compare the position of the MU with the histologically identified location of the mark. The larynx was then sectioned in the frontal plane. Before lengthening the vocal fold, the MU occurred on the area where the lamina propria became thinner and where the muscular layer neared the epithelial layer. After lengthening the vocal fold, the MU actually shifted medially compared with its original position. The subglottic area surrounded by the bilateral MUs became longer and thinner. Whether or not complete glottal closure during a vibratory cycle was achieved did not alter these findings. In contrast, with a fixed vocal fold length the MU appeared more laterally as MFR increased, but, based on the relation with the mark, its location on the vocal fold did not change from its original position before increase of MFR. 相似文献
39.
Enrique J. Baran 《Monatshefte für Chemie / Chemical Monthly》1991,122(6-7):479-481
Summary Mean amplitudes of vibration for OTeF
5
–
have been calculated from known spectroscopic and structural data in a wide temperature range. The results are briefly discussed in comparison with those of related species. 相似文献
40.
Summary If hydrogen is diffused electrolytically in a metal, it subsequently escapes out if the metal is left to itself. However this
hydrogen-treated metal does not totally regain the original magnitudes of its various physical properties,e. g., thermo e.m.f., electrical resistivity, elastic modulii etc. This effect has also been observed in the second-order elastic
effect (the Poynting effect) in nickel and two varieties of steel. Measurements were carried out first before hydrogen diffusion.
The samples were electrolytically diffused with hydrogen and the left to themselves for a few days and the measurements were
repeated. The changed values of the coefficient of Poynting effect are discussed in terms of the dislocations created by in-and
out-going hydrogen.
Now retired 相似文献