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   314篇
  免费   20篇
  国内免费   11篇
耳鼻咽喉   2篇
儿科学   1篇
基础医学   36篇
口腔科学   91篇
临床医学   21篇
内科学   58篇
皮肤病学   2篇
神经病学   5篇
特种医学   9篇
外科学   50篇
综合类   38篇
预防医学   7篇
眼科学   1篇
药学   8篇
  1篇
中国医学   13篇
肿瘤学   2篇
  2024年   1篇
  2023年   9篇
  2022年   5篇
  2021年   31篇
  2020年   25篇
  2019年   5篇
  2018年   4篇
  2017年   7篇
  2016年   8篇
  2015年   15篇
  2014年   21篇
  2013年   24篇
  2012年   14篇
  2011年   25篇
  2010年   13篇
  2009年   18篇
  2008年   8篇
  2007年   10篇
  2006年   11篇
  2005年   11篇
  2004年   10篇
  2003年   13篇
  2002年   5篇
  2001年   7篇
  2000年   1篇
  1999年   9篇
  1998年   1篇
  1997年   3篇
  1996年   5篇
  1995年   6篇
  1994年   2篇
  1993年   3篇
  1992年   1篇
  1991年   3篇
  1990年   1篇
  1988年   1篇
  1987年   2篇
  1986年   1篇
  1985年   1篇
  1984年   2篇
  1982年   1篇
  1978年   1篇
  1977年   1篇
排序方式: 共有345条查询结果,搜索用时 15 毫秒
1.
To study the effect of the surface properties on the bending fatigue performance of heavy-duty gear steel, the authors of this paper used the ultrasonic surface rolling process (USRP) to strengthen 20Cr2Ni4A carburized gear steel. USRP is a novel technique in which the ultrasonic technology is incorporated into the concept of conventional deep rolling. In this study, we illustrated how the surface properties and cross-section mechanical property influence the three-point bending fatigue life of the samples before and after USRP treatment. At the same time, the predicted failure probability-stress-number of cycles (P-S-N) curve was drawn, and the fatigue fracture was analysed. The results show that the fatigue limit increased from 651.36 MPa to 918.88 MPa after USRP treatment. The fatigue source is mainly from the sample interior or surface scratches, and the fatigue performance is positively correlated with the results of the material surface roughness, surface residual stress and surface hardness. At the same time, combined with the change in the phase structure, dislocation structure, residual stress and hardness of the cross section of the material, it is found that the USRP process turns the steel into a gradient material with five layers. Finally, the coupling mechanism between the ultrasonic surface strengthening deformation layer and the carburized layer of 20Cr2Ni4A carburized gear steel is presented, and the grain structure distribution diagram of the section of the 20Cr2Ni4A model after surface strengthening treatment was simulated. The mechanism that influenced the fatigue performance after USRP treatment is explained from the perspectives of the surface and cross section of the samples.  相似文献   
2.
ObjectiveOptimizing monolithic zirconia (ZrO2) aesthetically without affecting the unique mechanical properties remains a major ongoing interest. The purpose of this review is to evaluate the improvement of recent monolithic ZrO2 generations to meet aesthetic optimization qualities. Additionally, for how the extent of the former modifications negatively affected their mechanical properties and the impact on their clinical indications.MethodsThe current literature examines in-vitro studies evaluating both monolithic ZrO2 translucency and mechanical properties. The electronic search was done within these databases: Web of Science, ScienceDirect, Scopus, and PubMed within the period between 2009/10/01 and 2019/10/31. Search results that met eligibility criteria were classified into four groups; one for translucency parameter, two for uniaxial flexural strength and one for biaxial strength.ResultsArticles that remained for comprehensive analysis were fifty-three. They reported a significant impact of composition, microstructure and surface treatment on translucency and flexural strength assessment. Aging was found to be of no concern for zirconia structures with high yttria content. Smooth surface polish was found to significantly enhance strength while coarse grinding and mechanical fatigue was found to do the reverse.SignificanceBased on analyzing the previous in-vitro studies the following was found: Although significant improvement has occurred with recent monolithic zirconia types with higher yttria content having higher cubic/tetragonal ratio, they are still inferior to glass-ceramics’ unique translucency. With improving zirconia aesthetically, some of mechanical performance was sacrificed. Care must be taken when dealing with thin sections of aesthetic zirconia structures especially when used in high bearing stress areas.  相似文献   
3.
目的 通过有限元分析方法,观察提拉旋转斜扳法操作时椎间盘应力及应变发生的变化规律,探寻手法治疗腰椎疾患的科学性和安全性。 方法 在构建的L4~5节段腰椎模型上模拟加载提拉旋转斜扳手法,观察手法作用过程中椎间盘应力的分布及变化,髓核及纤维环的位移及应变。 结果 椎间盘应力变化从右后方开始出现,以弧形向周围传递扩散,应力的变化呈递减分布。椎间盘应变最小的位置在髓核偏后,以此为中心呈圆弧状向周围递增。应变最大的位置主要发生于纤维环,特别是椎间盘右侧外缘。 结论 手法操作中应力主要集中于后侧关节突关节,椎间盘的应力变化相对较小,提示手法治疗是安全的。纤维环后外侧在操作中有较明显的应变,局部的位移变化可能是手法疗效的机制之一。  相似文献   
4.
Transcatheter aortic valve (TAV) replacement has become a viable alternative to surgery for high and intermediate risk patients with severe aortic stenosis. This technology may extend to the younger and lower risk patients. In this population, long-term durability of the TAV is key. Increased leaflet mechanical stress is one of the main determinants of valve structural deterioration. This in vitro study aims at evaluating leaflet bending stress (LBS) in the self-expanding TAV for different valve sizes, stroke volumes (SV), and degrees of valve oversizing (OS). Three different sizes (23, 26, and 29 mm) of CoreValve (CV) were tested on a pulse duplicator in annulus size ranging from 17 to 26 mm. Leaflet bending stress and bending of the leaflet coaptation line in diastole pinwheeling index (PI) were measured using high-speed camera imaging (1000 images/s). For each given CV and annulus size, geometric orifice area (GOA) increased significantly with OS (P < .001) and SV (P = .001). LBS decreased with increasing prosthesis size and aortic annulus (AA) size while increasing with SV (P < .03). The largest value of peak LBS (3.79 MPa) was obtained with the CV 23 mm in AA of 17 mm (%OS = 35%), SV 90 mL and the smallest value (0.99 MPa) for the CV 29 mm in AA of 26 mm (%OS = 12%), SV 30 mL. On multivariable analysis, LBS increased independently with larger OS, smaller AA size and higher SV. The PI increased with decreasing AA size and increasing OS. Moderate valve OS, such as generally used for transcatheter aortic valve implantation, is associated with increased LBS during valve opening and closing, especially in small annuli. Hence, TAV OS may negatively impact long-term valve durability.  相似文献   
5.
背景:随着中国航天事业的发展,飞行员面临承受高G力学环境,这种环境会对飞行员骨骼造成严重影响。而胫骨作为最容易发生骨折的骨骼之一,目前对极端力学环境下胫骨生物力学研究较少。目的:通过高G离心加载装置制作动物模型,探究不同高G力学环境对大鼠生长发育和胫骨力学性能的影响。方法:取解放军军事医学科学院实验动物中心提供的雄性Wistar大鼠,通过高G离心加载装置设置悬臂以不同的转速和加速度运行模拟高G环境,并制作动物模型。每周称量大鼠体质量。取大鼠左侧胫骨进行三点弯曲实验,计算胫骨挠度、弹性模量、极限载荷;右侧胫骨进行蠕变实验,在胫骨中段皮质骨表面施加恒定应力并保持3600 s,观察其蠕变应变变化。实验已由天津理工大学动物伦理委员会批准。结果与结论:高G环境会影响大鼠正常生长发育,抑制体质量增长并降低了大鼠胫骨的力学性能,使胫骨的极限挠度分别下降了8.1%,12.2%,37.8%,51.4%;极限载荷分别下降了16%,9%,25.2%,29%。说明极端高G环境会对大鼠产生严重的负面作用。  相似文献   
6.
7.
《Dental materials》2022,38(9):1492-1506
ObjectivesThis study investigated the effect of ageing in three food-simulating liquids (FSLs) on mechanical properties of three prosthodontic CAD/CAM polymer composites intended for construction of implant-supported frameworks.MethodsMaterials investigated were: (i) a carbon fibre-reinforced composite (CarboCAD 3D dream frame; CC), (ii) a glass fibre-reinforced composite (TRINIA; TR), and (iii) a reinforced PEEK (DentoKeep; PK). Filler contents and microstructural arrangements were determined by thermo-gravimetry and tomography (µ-CT), respectively. Flexural properties (FS and Ef) were measured by 3-point bending (3PB) of 1 mm and 2 mm thick beam specimens. Fracture toughness (KIC) was measured by single-edge-notched-bending (SENB). All measurements were made at baseline (dry) and after 1-day and 7-day storage at 37 ℃ in either water, 70 % ethanol/water (70 % E/W) or methyl ethyl ketone (MEK). Failed specimens were examined microscopically. Statistical analyses included four-way ANOVA, two-way ANOVA and multiple Tukey comparison tests (α = 0.05). Multiple independent t-tests were performed regarding thickness effects on FS and Ef (α = 0.05).ResultsAt baseline, the mechanical properties increased in the sequence: PK < TR < CC (p < 0.001). FS ranged from 192.9 to 501.5 MPa; Ef from 4.2 to 18.1 GPa; and KIC from 4.9–12.4 MPa.m0.5. Fibre-reinforced composites (CC and TR) were significantly stronger than PK. However, all properties of CC and TR reduced after 1 d storage in 70 % E/W and MEK with FS ranging from 58.6 to 408 MPa; Ef from 1 to 15.4 GPa; KIC from 6.87 to 10.17 MPa.m0.5. Greater reductions occurred after 7 d storage. MEK was more detrimental than 70 % E/W and water on fibre-reinforced composites.SignificanceMechanical properties of each CAD/CAM composite were strongly dependent upon media and ageing. Although the mechanical properties of PK were initially inferior, it was relatively stable in all FSLs. All three materials exhibited sufficient mechanical properties at 1 mm thickness, but thicker specimens were more tolerant to ageing.  相似文献   
8.
The modification of wood and its treatment with various preservatives may affect its mechanical properties, hence the knowledge of the character changes in wood caused by impregnation is of great importance. Therefore, the aim of the research was to determine the effect of impregnation, with the propolis-silane preparation (EEP-MPTMOS/TEOS) consisting of the propolis extract (EEP) and silicon compounds: 3-(trimethoxysilyl)propyl methacrylate (MPTMOS) and tetraethoxysilane (TEOS), on the bending strength of treated wood. Moreover, in the study wood treated with components of the propolis-silane formulation was used, namely 70% ethanol, the propolis extract, and silanes (MPTMOS/TEOS). In order to determine whether the impregnation of wood affects its long-term bending, creep tests were performed depending on the humidity. The impregnation of wood with the propolis extract and the propolis-silane preparation (EEP-MPTMOS/TEOS) contributed to the increase in modulus of rapture and work to maximum load values compared to the untreated wood. In dry wood condition, the wood treated with EEP and EEP-MPTMOS/TEOS was characterized by lower modulus of elasticity values than the control samples. In turn, in wet wood condition, wood treated with the propolis-silane preparation showed an increase in the MOE value. Moreover, the impregnation of wood had an influence on the wood creep process under bending loads. The treated wood was characterized by higher relative creep compliance than the untreated wood. The exception was the wood impregnated with EEP-MPTMOS/TEOS, which showed comparable relative creep compliance to the control samples. The presented results indicate that wood treated with a bio-friendly preparation based on propolis and silicon compounds can be used in various application and also in variable humidity conditions.  相似文献   
9.
《Dental materials》2020,36(1):60-67
ObjectivesThe rapidly increasing use of zirconia-based CAD/CAM multi-layer structures in dentistry calls for a thorough evaluation of their mechanical integrity. This work examines the effect of the multi-layering architecture as well as variations in composition and inclusion of pigments among the layers on the flexural strength of multi-layer zirconias.MethodsA modified 4-point bending test, aided by a Finite Element Analysis (FEA), was used to probe the interfacial strength of 3 classes of yttria-partially-stabilized zirconia: Ultra Translucent Multi-Layer (UTML—5Y-PSZ), Super Translucent Multi-Layer (STML—4Y-PSZ), Multi-Layer (ML—3Y-PSZ). In accord with the size limitation (22-mm height) of CAD/CAM pucks, test samples were prepared in the form of “long” (25 × 2 × 3 mm) and “short” (17.8 × 1.5 × 2 mm) beams. Homogeneous beams (both long and short) were produced from either the Enamel (the lightest shade) or Dentin (the darkest shade) layer, whereas multi-layer beams (short beam only) were obtained by cutting the pucks along their thickness direction, where the material components of various shades were stacked.ResultsThe Enamel and Dentin layers exhibited similar flexural strength for a given material class, with ML amassing the highest strength (800–900 MPa) followed by STML (560–650 MPa) and UTML (470–500 MPa). The 3 classes of multi-layer zirconia showed a trade-off between strength and translucency, reflecting different yttria contents in these materials. The failure stress of the cross-sectional multi-layer beams was, however, ∼30% lower than that of their Enamel or Dentin layer counterparts, regardless of material tested.SignificanceThe weakness of interfaces is a drawback in these materials. Additionally, when measuring strength using short beam flexure, friction between the specimen and supporting pins and accuracy in determining loading span distances may lead to major errors.  相似文献   
10.
This article presents experimental results from the bending of technical-scale models of beams reinforced in the tension zone with CFRP (Carbon Fiber Reinforced Polymers) materials, with a focus on the benefits resulting from the increased ductility in the tension zone of these beams. In experimental tests, the mechanical properties of reinforced beams were compared with unreinforced beams in terms of the maximum load, deflection, images of damage, stiffness, and distribution of deformation. The results showed that the proposed reinforcement solution was advantageous due to its strength and stiffness, and the safety of the structure. Based on this analysis, it was concluded that the reinforcement of wood with CFRP materials has a positive effect on the behavior and safety of structures. Also, a method of analytical checking of strengthened beams with small cross-sections was presented in the article.  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