全文获取类型
收费全文 | 7026篇 |
免费 | 1506篇 |
国内免费 | 510篇 |
专业分类
化学 | 909篇 |
晶体学 | 69篇 |
力学 | 2507篇 |
综合类 | 112篇 |
数学 | 862篇 |
物理学 | 4583篇 |
出版年
2024年 | 20篇 |
2023年 | 64篇 |
2022年 | 171篇 |
2021年 | 161篇 |
2020年 | 219篇 |
2019年 | 185篇 |
2018年 | 187篇 |
2017年 | 279篇 |
2016年 | 307篇 |
2015年 | 268篇 |
2014年 | 494篇 |
2013年 | 450篇 |
2012年 | 406篇 |
2011年 | 483篇 |
2010年 | 358篇 |
2009年 | 393篇 |
2008年 | 424篇 |
2007年 | 449篇 |
2006年 | 442篇 |
2005年 | 388篇 |
2004年 | 352篇 |
2003年 | 316篇 |
2002年 | 269篇 |
2001年 | 273篇 |
2000年 | 221篇 |
1999年 | 206篇 |
1998年 | 204篇 |
1997年 | 143篇 |
1996年 | 142篇 |
1995年 | 111篇 |
1994年 | 107篇 |
1993年 | 73篇 |
1992年 | 75篇 |
1991年 | 87篇 |
1990年 | 59篇 |
1989年 | 47篇 |
1988年 | 39篇 |
1987年 | 26篇 |
1986年 | 20篇 |
1985年 | 19篇 |
1984年 | 14篇 |
1983年 | 9篇 |
1982年 | 17篇 |
1981年 | 16篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1971年 | 5篇 |
1957年 | 8篇 |
排序方式: 共有9042条查询结果,搜索用时 171 毫秒
101.
102.
重庆“朝天扬帆”钢结构工程风振研究 总被引:1,自引:0,他引:1
“朝天扬帆”是位于重庆朝天间广场的钢结构雕塑工程,该工程建筑型体复杂、结构布局不规则,通过该雕塑工程缩尺模型的几洞试验,确定其风载计算的体型系数;用时程法计算结构的风振响应,且在模拟作用于结构上的脉动风时程曲线时考虑结构的高度相关性,应用ALGOR93软件对结构进行风振时程分析,并自编程序加以前、后处理。通过对比该结构在脉动风和平均风作用下的杆件内力、节点位移响应,约定结构风振系数,为该雕塑工程抗风设计参数的确定提供理论依据。 相似文献
103.
Liu Rongmei 《力学与实践》1979,42(4):442
建立了扁担挑物的动力学模型和动力学方程,并进行了数值求解。扁担挑物时存在三种频率,即重物类似于单摆的振动频率、挑运者的步行频率和扁担-重物系统铅垂方向的振动频率。计算结果表明,为了获得较好的挑担体验,三种频率应该相互协调。当人挑担行走的频率为扁担-重物系统铅垂方向振动频率的70%,$\sim$,80%时,肩部铅垂附加动反力为重物重力的30%左右且与肩部铅垂运动同相,当重物类似于单摆的振动频率为人步行频率的25%,$\sim$,30%时,重物的摆角和肩部受到的水平附加动反力均较小,从而有利于人挑担行走。 相似文献
104.
105.
以子结构模态综合分析为基础,提出一种求解大型结构特征值问题的并行解法.采用子结构模态综合算法,结构特征模态采用子空间迭代方式并行求解.这种子空间迭代法的子结构并行计算的实施是利用子结构的刚度阵和质量阵而不必完全组集系统刚度阵和质量阵求解综合系统的特征值问题.数值结果表明这种求解大型结构特征值问题的并行算法是可行有效的. 相似文献
106.
P. Z. Lugovoi V. F. Meish B. P. Rybakin G. V. Sekrieru 《International Applied Mechanics》2006,42(5):536-540
Dynamic problems for cylindrical shells reinforced with discrete ribs are examined. A numerical algorithm based on Richardson
extrapolation is developed. Specific problems are solved, and the results are analyzed
__________
Translated from Prikladnaya Mekhanika, Vol. 42, No. 5, pp. 50–56, May 2006. 相似文献
107.
讨论螺旋细杆的特殊形式扭转振动,即均匀扭转振动.以非圆截面杆和有原始曲率的圆截面杆为研究对象.杆作均匀扭转振动时各截面有相同的扭角变化规律,且杆中心线的几何形状不受振动过程的影响.研究表明,扭振来源于杆截面的非对称性及杆的原始曲率.杆的扭振规律与单摆运动相似,其动力学方程存在精确解.圆环杆的均匀扭振为螺旋杆的倾角为零时的特例. 相似文献
108.
Nonlinear Longitudinal Vibrations of Transversally Polarized Piezoceramics: Experiments and Modeling 总被引:1,自引:0,他引:1
Nonlinear behavior of piezoceramics at strong electric fields is a well-known phenomenon and is described by various hysteresis curves. On the other hand, nonlinear vibration behavior of piezoceramics at weak electric fields has recently been attracting considerable attention. Ultrasonic motors (USM) utilize the piezoceramics at relatively weak electric fields near the resonance. The consistent efforts to improve the performance of these motors has led to a detailed investigation of their nonlinear behavior. Typical nonlinear dynamic effects can be observed, even if only the stator is experimentally investigated. At weak electric fields, the vibration behavior of piezoceramics is usually described by constitutive relations linearized around an operating point. However, in experiments at weak electric fields with longitudinal vibrations of piezoceramic rods, a typical nonlinear vibration behavior similar to that of the USM-stator is observed at near-resonance frequency excitations. The observed behavior is that of a softening Duffing-oscillator, including jump phenomena and multiple stable amplitude responses at the same excitation frequency and voltage. Other observed phenomena are the decrease of normalized amplitude responses with increasing excitation voltage and the presence of superharmonics in spectra. In this paper, we have attempted to model the nonlinear behavior using higher order (quadratic and cubic) conservative and dissipative terms in the constitutive equations. Hamilton's principle and the Ritz method is used to obtain the equation of motion that is solved using perturbation techniques. Using this solution, nonlinear parameters can be fitted from the experimental data. As an alternative approach, the partial differential equation is directly solved using perturbation techniques. The results of these two different approaches are compared. 相似文献
109.
J. Náprstek 《Meccanica》1998,33(3):267-277
Abstract. The paper presents the solution to the properties of stochastic response of a system with random parametric noises, which is prone to the loss of aerodynamical stability. The system is described by an equation of van der Pol type with the negative linear, and with the positive cubic dampings. The coefficients of the linear damping and of the stiffness include the multiplicative random perturbations, the external excitation being given as a sum of a deterministic function and of an additive perturbation. All three input random processes are supposed to be Gaussian and centered, with the non-zero mutual stochastic parameters, as it corresponds to the properties of real systems. The solution has been based on the method of stochastic linearisation and of the subsequent solution of the Fokker–Planck–Kolmogorov equation in the sense of the first and second stochastic moments for the transient and stationary states. There have been demonstrated several effects, which are typical for systems with parametric noises, differentiating them from the systems with constant coefficients. The principal attention has been devoted to the properties of the spectral density of the response, the character of which changes abruptly with the degree of non-linearity of the damping and of the level of random perturbations.Sommario. La presente memoria studia le proprietà della risposta stocastica di un sistema con eccitazione casuale parametrica, che tende alla perdita della stabilità aerodinamica. Il sistema è descritto mediante un'equazione del tipo di van der Pole con il termine lineare dello smorzamento negativo e il termine cubico positivo. Poichá l'eccitazione esterna è la somma di una funzione deterministica e di una perturbazione additiva, i coefficienti dello smorzamento lineare e della rigidezza comprendono le perturbazioni casuali moltiplicative. I tre processi stocastici di eccitazione sono assunti gaussiani e a media nulla con parametri stocastici incrociati diversi da zero, come si verifica per le proprietà dei sistemi reali. La soluzione è basata sul metodo della linearizzazione stocastica e della successiva soluzione dell'equazione di Fokker-Planck-Kolmogorov studiando i primi e i secondi momenti statistici per gli stati transitori e stazionari. Vengono mostrati diversi effetti, tipici dei sistemi con eccitazione parametrica, differenziandoli dai sistemi a coefficienti costanti. Particolare attenzione è rivolta alle proprietà della densità spettrale della risposta le cui caratteristiche cambiano bruscamente con il grado di non linearità dello smorzamento e del livello di casualità delle perturbazioni. 相似文献
110.
大跨度斜拉桥动力特性分析 总被引:17,自引:2,他引:17
本文提出一种计算大距度钢桁梁斜拉桥动力特性的方法。文中分别采用桁段有限单元、空间梁元、空间杆元计算斜拉桥中桁架,桥塔、拉索的刚度矩阵与质量矩阵,采用子空间迭代法求解特征方程,所得结果可供设计参考。 相似文献