首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   586篇
  免费   6篇
  国内免费   5篇
电工技术   8篇
综合类   18篇
化学工业   64篇
金属工艺   27篇
机械仪表   50篇
建筑科学   45篇
矿业工程   3篇
能源动力   35篇
轻工业   34篇
水利工程   3篇
石油天然气   12篇
武器工业   1篇
无线电   7篇
一般工业技术   184篇
冶金工业   79篇
原子能技术   2篇
自动化技术   25篇
  2022年   2篇
  2021年   5篇
  2020年   4篇
  2019年   9篇
  2018年   5篇
  2017年   5篇
  2016年   11篇
  2015年   34篇
  2014年   22篇
  2013年   39篇
  2012年   18篇
  2011年   46篇
  2010年   36篇
  2009年   44篇
  2008年   32篇
  2007年   40篇
  2006年   27篇
  2005年   23篇
  2004年   33篇
  2003年   20篇
  2002年   30篇
  2001年   12篇
  2000年   23篇
  1999年   12篇
  1998年   17篇
  1997年   9篇
  1996年   4篇
  1995年   6篇
  1994年   2篇
  1993年   4篇
  1991年   5篇
  1990年   2篇
  1989年   4篇
  1988年   5篇
  1987年   3篇
  1986年   2篇
  1984年   2篇
排序方式: 共有597条查询结果,搜索用时 15 毫秒
1.
The substantial subsidizing of energy prices over the years has led to high energy consumption, inefficiencies, fiscal pressures, and environmental problems in Iran. To address the increasing socio-economic problems associated with the energy subsidies, the government embarked on an aggressive energy price reform through which energy subsidies were removed and cash handouts were given to all households in 2010. In this paper, I analyze the effectiveness of the energy price reform in Iran by estimating energy demand elasticities for households in different income groups. I apply a two-stage consumer optimization model and estimate the system of energy expenditures shares using the household budget survey data for the period 2001–2008. The results show that the overall price elasticities of demand are small, but income elasticities are close to one. The results also indicate heterogeneous responses to energy price and income changes in different income groups. Specifically, the urban households show stronger response to price changes, but rural households, particularly mid-income households, to income changes. These findings suggest that the current policy of price increases would not solely be able to reduce energy consumption and, therefore, it should be geared towards increasing energy efficiency through a series of price and non-price measures.  相似文献   
2.
3.
The objective of this work was to identify the effects that milk phospholipids (PL) have on crystallization of anhydrous milk fat (AMF). Three mixtures were prepared by adding 0%, 0.01%, and 0.1% PL to AMF. Each mixture was crystallized for 90 min at 24, 26, and 28 °C. The solid fat content was measured as a function of time and fitted to the Avrami equation. Melting point, thermal behavior, viscoelastic properties, and crystal morphology were all measured at 90 min. All assays were repeated, as well as hardness, after being stored at 5 °C for 48 hours. Samples containing PL showed slower crystallization as concentration increased especially at higher temperatures (26 and 28 °C). The addition of PL caused a difference in crystal morphology resulting in visibly larger crystals at 90 min. The elasticity and hardness at 90 min were influenced by the addition of PL at 24 °C with lower values obtained in samples with PL compared to the AMF alone. No differences in hardness nor in elasticity was observed for samples crystallized at 26 and 28 °C. A decrease in melting enthalpy was observed in samples with PL indicating a reduction in crystallization at all temperatures, which was supported by crystal morphology.  相似文献   
4.
According to the exact three-dimensional (3D) thermoelasticity theory, the elasticity solution of the simply-supported layered rectangular plates subjected to steady temperature loads was studied. An analytical method was developed to solve the temperature, stress and displacement fields in the plate. Firstly, the general solutions of the temperature, displacements and stresses in a simply-supported isotropic layer were obtained by solving the 3-D heat conduction equation and the 3-D equations of elasticity respectively, which were expressed in the form of double Fourier series. Then, the temperature, displacement and stress relationships between the upper surface and the lower surface of the isotropic layer were derived. Based on the continuity of the temperature, the heat flux, the displacements and the stresses on the interface of two adjacent layers with different material properties, the recursive formulae of temperature, displacements and stresses between the bottom layer and the top layer of the layered plate were obtained by using the transfer matrix method. The unknown coefficients in the solutions for every layer were uniquely determined by the upper surface and lower surface conditions of the plate. The distributions of the temperature, displacements and stresses in the plate were given by substituting the unknown coefficients obtained back to the recurrence formulae and the solutions. The convergence of the solutions was checked with respect to the number of the terms of series. Comparing the results with those obtained from the finite element method, the correctness of the present method was verified. Finally, the effects of surface temperatures, plate thickness, layer number and material properties of each layer on the distributions of the temperature, displacements and stresses in the plates were discussed in detail.  相似文献   
5.
This paper addresses the effective use of carbon fiber reinforced polymeric (CFRP) materials in the cable system. As the span length of cable-stayed bridges increases, several technical challenges become more dominant with traditional material. This paper mainly focuses on improving the aerodynamic performance through implementing CFRP composites in the cable system in combination with steel. In order to maximize the improvement, a genetic algorithm (GA)-based optimization procedure is developed to optimize the distribution of CFRP and steel. A numerical example is presented and the results suggest the typical composition of an optimized CFRP-steel cable system for long-span cable-stayed bridges.  相似文献   
6.
Weldments geometry with failures occurring at the weld toe or at the weld root cannot, by its nature, be precisely defined. Parameters such as bead shape and toe or root radius vary from joint to joint even in well-controlled manufacturing operations. The worst case configuration can be achieved by modelling as a sharp, zero radius, notch both the toe and the weld root. The intensity of asymptotic stress distributions obeying Williams’ solution is quantified by means of the Notch Stress Intensity Factors (NSIFs). For steel welded joints with failures originated from the weld roots, where the lack of penetration zone is treated as a crack-like notch, units for NSIFs are the same as conventional SIFs used in LEFM. The different dimensionality of NSIFs for different notch opening angles does not allow a direct comparison of failures occurring at the weld toe or at the weld root. In order to overcome the problem related to the variability of the V-notch opening angle, a simple scalar quantity, i.e. the value of the strain energy density (SED) averaged in the structural volume surrounding the notch tip, has been introduced. This energy is given in closed form on the basis of the relevant NSIFs for modes I, II and III. The radius Rc of the averaging zone is carefully identified with reference to conventional arc welding processes being equal to 0.28 mm for welded joints made of steel.The local-energy based criterion is applied here to steel welded rollers produced by Rulmeca and subjected to prevailing mode I (with failures at the weld root). The aim of the paper is firstly to describe the employed methodology for the fatigue assessment and secondly to show the first synthesis of fatigue data by means of local SED for a specific geometry.  相似文献   
7.
The non-linear stress–strain behaviour of plant fibre composites is well-known in the scientific community. Yet, the important consequences of this, in terms of the evolution of stiffness as a function of applied strain and cycles to failure, are not well-studied in literature. This is despite the fact that stiffness degradation is a well-accepted indicator of damage in a composite material, and is regularly used as a component failure criterion. This article systematically explores the evolution of stiffness of various aligned plant fibre composites, subjected to (i) monotonic loading, (ii) low-cycle, repeated progressive loading, and (iii) fatigue loading. The evolution in stiffness in plant fibre composites is found to be complex: structural changes in the elementary fibre cell wall and damage development in the composite have often competing effects on stress–strain behaviour. Indeed, the evolution in stiffness of plant fibre composites is found to be unlike that typically observed in traditional composites, and therefore needs to be taken into account in the design of structural components.  相似文献   
8.
A solution to the problem of a rigid cylindrical inclusion pressed between two elastic half spaces is obtained using the distributed dislocation technique. The solution is compared with previously published analytical and numerical results for a rigid cylindrical inclusion bounded by two parabolic arcs with rounded corners. A simplified solution to the problem based on the classical contact theory and well-known results for crack problems is also suggested and validated. The simplified solution agrees well with analytical results in the case when the length of the opening around inclusion is much larger than the length of the contact zone.  相似文献   
9.
油环弹力对发动机机油消耗量有着十分显著的影响,人们通常采用单纯调整油环弹力的办法来控制机油消耗量。这种单因素的变化,在控制机油消耗量的同时,会给发动机带来什么不利影响?本文通过对活塞环的工作过程动态模拟计算来说明这个问题。对某主机厂新研发的六缸柴油机活塞环工作过程动态模拟计算结果表明,油环弹力的大小对环组摩擦功率损失和机油消耗量的影响较大.对漏气量影响微弱。通过单纯的增加油环弹力来降低机油消耗量的办法,除了需要付出增大环组摩擦功率损失的代价外,不一定就能取得降低机油消耗量的满意效果。  相似文献   
10.
《Ergonomics》2012,55(10):1361-1371
The coefficient of rolling resistance (C r) for pneumatic tyres is dependent on hysteresis loss from tyre deformation which is affected by the vertical force applied to the tyres (F v) and the tyre inflation pressure (P r). The purpose of this paper was to determine the relative influence of five different levels of P r and four different levels of F v on C r and to examine the relationships of C r with P r and F v during cycling locomotion. F v was modified through carriage of additional mass by the subject. C r was determined with the coasting deceleration method from measurements performed in a level hallway. Iterations minimizing the sum of the squared difference between the actual deceleration distance and a predicted deceleration distance were used to determine C r. This latter distance was computed from a derivation based on Newton's second law applied to the forces opposing motion. C r was described by a hyperbolic function of P r (C r = 0.1071 P r ?0.477, r 2 = 0.99, p < 0.05), decreasing 62.4% from 150 kPa (Cr= 0.0101) to 1200 kPa (Cr = 0.0038). F v was related to C r by a polynomial function (C r = 1.92.10?8 F v 2 ?2.86.10?5 F v + 0.0142, r 2 = 0.99, p = 0.084), with an added mass of 15 kg (C r = 0.0040) resulting in an 11.4% increase in C r compared with no added mass (C r = 0.0035). From this study, it is concluded that the relationships of P r and F v with C r for cycling are non-linear. Furthermore, a simulation model shows that changes in P r and F v of the magnitude examined here have an important effect on competitive cycling performance.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号