Young’s modulus obtained by flexural vibration test of a wooden beam with inhomogeneity of density

The object of this study was to investigate the inhomogeneity of density within a beam from a relationship between the dynamic Young’s moduli from the Euler-Bernoulli elementary theory of bending (E _n) and resonance mode numbers (n), which is plotted as the “E-n” diagram in this article. Rectangular beams with dimensions of 300 (L) × 25 (R) × 5mm (T) of Sakhalin spruce (Picea glehnii Mast.), Sitka spruce (Picea sitchensis Carr.), Japanese red pine (Pinus densiflora Zieb. et Zucc.) and white oak (Cyclobalanopsis myrsinaefolia Oerst.) were used for specimens. Small parts of beams were replaced with a small portion of another species to examine the influence of the inhomogeneity of density on E _n. A free-free flexural vibration test was undertaken and E _n was calculated by the Euler-Bernoulli theory. The resonance frequency of a specimen with inhomogeneity of density was simulated by modal analysis. The density distribution in the longitudinal direction of the specimen for which E _n did not decrease monotonically with n was obtained. From the modal analysis, the inhomogeneity of density was equivalent to a concentrated mass attached to a uniform beam. The pattern of the E-n diagram was changed by replacing a part of the specimen with another species. Specimens for which E _n did not decrease monotonically with n had a high density part because of indented rings, knots, or resin.     

Characterization of major,unused, and unvalued Indonesian wood species I. Dependencies of mechanical properties in transverse direction on the changes of moisture content and/or temperature

Mechanical property changes due to the moisture content (MC) and/or temperature changes were examined for 15 Indonesian wood species. A static bending test was carried out at 20°C, 65% relative humidity (air-dry), and water-saturated at 20°C (wet-20) and 80°C (wet-80). For individual test conditions, modulus of elasticity (MOE) and modulus of rupture (MOR) increased linearly with specific gravity regardless of wood species; however, maximum deflection did not correlate with specific gravity for any MC or temperature conditions. The relative values of MOE and MOR measured in wet-20 to air-dry conditions were variously affected from slightly to strongly depending on the wood species. However, the relative values always decreased markedly when saturated in water at 80°C, regardless of wood species. The relative MOE, MOR, and maximum deflection values due to the change in MC or MC and temperature combined were independent of specific gravity but may be dependent on wood type: softwood or hardwood.     

在实木家具结构中圆棒榫的强度分析

分析圆棒榫在实木家具结构中的主要力学指标,通过圆棒榫的抗拔力、抗弯强度与抗剪强度等指标的计算与比较,总结圆棒榫在实际应用中应注意的主要问题,以期对相关的设计与生产提供参考依据.     

CYLINDRICAL BENDING PROBLEM OF RECTANGULAR PLATES STIFFENED ON ONE SIDE ALONG ONE DIRECTION WITH ELASTIC SUPPORTS AND TENSILE FORCES

On the basis of the calculated results of accurate analytic solution of stiffened rectangular plates,the general equations of cylindrical bending of the plates with tensile forces are established for engineering practice.Its solution combining with engineering problem is given for the special defor-mation conditions of elastic supports.In the paper,the numerical example is given and compared with references,it shows that the results are available.     

PDMS/PTFE限制层材料软体机械手设计与性能试验

为抑制软体驱动器弯曲过程中的不必要变形,提高软体机械手抓取能力,提出一种限制层材料为PDMS和PTFE混合制备的软体机械手。通过调整PDMS和PTFE之间的质量比来改变限制层刚度,进而改变软体机械手的弯曲角度。考虑到软体机械手工作时,应变层变形远大于限制层,采用Yeoh和Neo-Hookean形式的应变能方程表述应变层和限制层材料力学行为。基于模型和力矩平衡原理建立软体驱动器弯曲角度数学模型,进而研究限制层材料和结构参数对软体机械手弯曲性能的影响。利用单轴拉伸试验获取不同质量比的PDMS和PTFE混合制备的限制层样品应力应变曲线,并拟合获取Yeoh模型材料参数,进而进行有限元仿真分析,确定限制层材料的最佳比例。对不同限制层材料制成的驱动器进行弯曲角度测试,试验结果与理论分析偏差在5%以内,验证了数学模型的准确性。同时,驱动器的末端力测试结果表明,限制层PDMS/PTFE质量比为8∶1的驱动器明显优于纯硅胶软体驱动器,最大末端力可达2.45 N。使用软体机械手对多种物品进行抓取试验,其最大抓取质量达420 g。     

顶驱装置导轨弯曲问题的理论分析

针对顶驱装置导轨在长期使用后出现的变形问题,利用材料力学的基本原理,在导轨截面一致且无导轨接头影响的前提下,忽略导轨所承受的扭转力,从静载抗弯角度分析验证导轨强度,计算顶驱装置位于导轨不同位置时的导轨变形趋势与变形量。分析结果证实了导轨的大位移变形是因导轨接头的使用磨损,进而出现的导轨间相对转角位移过大造成的。顶驱装置导轨的改进在提高导轨本身弯曲强度与弯曲刚度的同时,需加强导轨接头的局部结构。     

红松木倾斜角度对其木材形成组织的影响

为研究树干倾斜角度对应压木木材细胞壁形成过程的影响,以7年生红松苗木为研究对象,对其茎干进行不同角度的倾斜处理.先后测定苗木的树高和胸径、木材形成组织中的木质素和纤维素含量、傅里叶红外变化光谱和极性代谢产物.结果表明:倾斜角度对高生长的抑制作用非常显著,对直径生长无显著影响.倾斜后木质素含量增加,纤维素含量降低;倾斜角度对木质素和纤维素含量均有显著影响.不同倾斜角度FTIR吸收峰强度也有明显差异.糖类、脂类、氨基酸、含氮化合物、有机酸等代谢物随倾斜角度呈现不同的变化规律.因此,红松苗木50°倾斜处理形成典型的应压木结构.代谢物相对含量的变化规律与形成的木材宏观性质相吻合.代谢物的变化也反映了树木对应力刺激的响应机制.     

Meso-experimental analysis on fracture characteristics ofJinping bedded marble

The project problem because of wall rock’s deformation fracture in deep-buried and high geostress conditions is very prominent at Jinping hydropower Ⅱ station underground engineering. According to the characteristics of the bedded marble collected from Jinpiing Ⅱ hydropower station auxiliary traffic tunnel in deep-buried area, three-point bending experiment on different bedded direction specimens with SEM high temperature testing system is carried out to test the stress of bedded marble in different directions and the failure mechanism and deformation features of homogeneous marble. In the test, there are 3 groups of specimens, include parallel bedded specimen, vertical bedded specimen and homogeneous marble specimen. The results show that: bedded marble’s bedded feature is because of the banded structure feature between dolomite mineral particles and calcite mineral particles; the fracture mechanism and strength parameter of bedded marble depend on different force directions and mineral composition. Because of the cement capacity between dolomite minerals and particles size, homogeneous marble specimen has the lowest strength. The marble specimens’ deformation shows 3 stages, and because of different fracture mechanisms, the plastic deformation of parallel bedded specimens is more obvious than that of vertical bedded specimen. While the deformation of homogeneous marble can be divided into 2 stages, i.e. before and after peak stage, and the after peak stage shows the complete deformation development process.     

钢结构节点设计中的弯扭识别

梁柱刚接是钢框架结构中常用的连接形式。采用刚接可以减少梁跨中的弯矩,但设计、制作、施工较复杂,节点处连接件受力情况多样。梁(牛腿)与柱刚接时,相互的位置关系不同,可能使节点受弯矩拉力或扭矩剪力的作用,本文以形象直观的形式介绍梁与柱"顶"接、"搭"接等弯扭识别的简单方法。     

农机螺旋钻弯振值影响因素的模态分析

通过理论分析和计算，找出了影响农机螺旋钻头弯曲振动的主要因素，对样机进行了改进设计，并借助计算机对其进行了动态优化设计，使其抗振性大为提高