粘性流体对结构固有频率及阻尼的影响

从结构与液体的耦联振动方面分析了粘性流体对结构固有频率和阻尼的影响：利用水中柱体的振动方程分析结构固有频率下降的原因，利用结构损耗因子及粘性流体的粘性系数说明阻尼增大的原因，同时对自由梁及簿壁圆柱体结构水下和空气中试验模态分析得到的模态参数作了叙述．     

基于MATLAB软件的电磁振动给料系统仿真

分析了电磁振动给料系统的力学模型,通过幅频特性曲线分析了固有频率和激振频率的关系,确定了电磁振动给料系统的工作状态,利用MATLAB软件对系统在有阻尼和无阻尼两种不同的情况下进行振动仿真分析,对在有阻尼振动系统中其动力放大系数、调谐值和阻尼比之间的密切关系进行了研究,确定电磁振动给料系统应设计成在低临界近共振的状态下工作。调谐值的理想取值为Z=0．8～0．95。     

不同长度的三维四向编织复合材料T型梁模态性能研究

文章分别用实验和有限元模拟方法研究了碳纤维三维四向编织复合材料T型梁的模态性能,分析了悬臂梁自由端长度的变化对其固有频率的影响以及固有频率与阻尼性能的关系,并对比了实验和有限元模拟固有频率。结果表明:三维四向编织复合材料T型梁的固有频率随着其自由端长度的增加而降低;其固有频率和其阻尼性能没有明显的相关关系;模拟结果与实验结果相一致。     

化学浆未蒸解份处理措施的改进

我公司硫酸盐马尾松制浆未蒸解份（以下简称节子）处理经历了节子与木片混合制浆，节子机械处理成浆至节子单独制浆三种方式。本文通过分析对比得出节子回煮是一种比较好的处理方式。不但节约了木材纤维资源，还创造了可观的经济效益。     

1511织机侧板打棒经阻尼处理后的降噪效果

1511织机是纺织厂中噪声最严重的机台。其中投梭机构发出的撞击噪声是织机中最主要的噪声源之一。我们就织机击梭机构中由不同材料组成的几种打棒和侧板作了振动与噪声测试分析。并对各阶模态的阻尼率与噪声dB(A)值之间的关系进行了分析研究,提出了阻尼率ξ这个衡量指标。测试分析取得了与理论定性分析较为接近的结果。最后得出了具有实际意义的结论。     

PE基木塑复合材料动态阻尼比特性的测量分析

本文采用"悬臂梁"自由端的方式,测定了PE基木塑试材的阻尼比值,可为提高木塑复合材料阻尼特性的主动控制等技术发展提供借鉴。结果表明,本PE基木塑复合材料的阻尼能力明显高于钢铁等弹性材质,但低于木材及木质复合材料;振动法测量木塑复合材料的动态阻尼比具有快速、简便、重复性好等优点。     

基于ANSYS的薄膜干燥器转子系统振动特性分析

运用ANSYS软件对A=0.4 m2薄膜干燥器转子系统三维模型进行模态分析,提取转子的前4阶固有频率和振型,求得相应的临界转速;在模态分析的基础上,用Full法对转子进行谐响应振动分析,研究偏心(c=2.5 mm)与质量不平衡(m=0.1,0.2kg)对转子振动的影响,并分析不同轴承刚度对转子系统的固有频率和振动特性的影响。结果表明:偏心、不平衡质量等对转子振动响应影响较大;轴承刚度对转子固有频率和振动特性有一定影响。     

基于有限元分析法的某汽车转向节的模态研究

在某汽车中心孔类转向节的优化设计过程中,为了克服汽车行驶过程中的振动和冲击,需要进行必要的模态分析。本文通过动力学原理用有限元分析法对转向节提取前十阶频率与振型进行模态分析,从而得出转向节的动态特性,了解它各阶固有频率以及振型特点,为优化设计中的分析研究结构动力特性提供依据。     

经编机梳栉的横移振动分析

针对经编机高速运行时导纱梳栉因机械振动而产生擦针或擦伤纱线,并导致控制系统运行失效的问题,提出应用模态分析技术对经编导纱梳栉进行动力学分析.通过搭建声学振动测试平台,实测经编梳栉的关键动力学参数:阻尼比、固有频率和固有振型,获取反映梳栉单位激振力振幅响应的幅频特性.在此基础上提出经编梳栉高速运行时的减振措施,并经对比实验证明该措施有效,且梳栉振动分析方法正确可行.     

双电机驱动自同步振动磨动力响应研究

以双电机驱动自同步振动磨为研究对象,对其进行简化处理,建立力学模型。通过对其施加X,Y方向的简谐激振力,建立无阻尼系统强迫振动和有阻尼系统强迫振动的数学模型,在此基础上,确定Rayleigh阻尼的常数α和β,利用有限元ANSYS软件进行瞬态动力学分析,研究其在动载荷作用下的变形情况。研究结果表明:在工况频率16 Hz下,节点运动轨迹更接近圆形,距理想粉碎效果的圆形轨迹较近,粉碎效果较好,进入稳定期的时间较快;在共振频率42 Hz下,节点轨迹振幅更大,进入稳定期较慢,阻尼对振动的衰减作用较强;不同阻尼比对振动的衰减作用成正比关系。因此,可以通过控制阻尼比等参数获得更有利的研磨效果,提高研磨效率。     

Die Rohdichteverteilung und deren feuchteabhängige Änderung bei astiger fichte im hinblick auf die maschinelle Holzsortierung

For the machine grading of timber by isotopic-radiation the difference in density between sections with and without knots is being used as a criterion to evaluate the knot ratio. The precondition for the general use of this method, even for wood with varying moisture contents would be to prove that there are no changes in density relationship at least up to fiber saturation. For determining the relationship and density changes in spruce, with and without knots, comprehensive investigation on 230 test groups (1600 specimens) were carried out. Evaluation of results led to the conclusion that a reliable differentiation regarding the criterion “knot ratio” is possible for moisture content up to 30%, independent of the initial data of the timber to be graded.     

Matching of wood for testing purpose

Different methods of pair matching both small “clean” wood specimens and structural timber are described. Matching of structural timber according to modulus of elasticity and knots results in pairs that differ in strength by an average of about 15 percent with extremes up to 40 percent. These differences are due to other parameters such as fibre deviation, number of cut fibres, tension strength perpendicular to grain, whose influence is not sufficiently reflected in the deviation of local stiffness.     

Distortion of Norway spruce timber Part 1. Variation of relevant wood properties Part 1. Variation of relevant wood properties

) trees from one fast-grown and one slow-grown stand in southern Sweden. From the trees 240 studs (45 × 70 × 2500 mm) were taken for measurement of distortion. Wood properties were measured on small specimens (13 × 13 × 200 mm) cut from the studs. Spiral grain angle was found to vary from approximately +3° (left-handed) close to pith to zero 150 mm from pith with a strong individual variation. The material from the fast-grown stand had a larger spiral grain angle compared with the slow-grown material. Spiral grain was poorly correlated to other parameters. Presence of knots had a substantial influence on longitudinal shrinkage (α_l) measurements. Specimens with large knots (KAR > 33%) had almost 100% higher longitudinal shrinkage than specimens without knots. It should be pointed out, however, that measuring shrinkage in small specimens containing even small knots can create a problem with regards to the obtained results, especially results of α_l. It was found that presence of compression wood in several growth rings more than doubled the longitudinal shrinkage. For the radial and tangential direction the presence of compression wood decreased shrinkage with about 30%. The ratio between tangential and longitudinal shrinkage was 49 for normal wood whereas for compression wood the ratio was 13. These results confirm the theory that the microfibril angle governs shrinkage. Longitudinal shrinkage decreased slightly with increased distance from pith whereas radial and tangential shrinkage did not display any substantial radial variation. The fast-grown material had generally a higher longitudinal shrinkage and lower transverse shrinkage than the material from the slow-grown stand. About 50% of the variation in longitudinal shrinkage was explained by radial position, density and ring width. Density and ring width did explain 60% of the variation in radial shrinkage but only 30% of the variation in tangential shrinkage.     

Experimental verification of an orthotropic finite element model for numerical simulations of ultrasonic testing of wood poles

Ultrasonic testing is a non-destructive and non-invasive method which has been used for internal condition assessment of wood poles in electric transmission and distribution lines. The reliability of this method of evaluation relies on a good understanding of propagation of ultrasonic waves in wood. However, a full-waveform analysis in ultrasonic testing is rarely performed in practice because of difficulties in establishing realistic values for the elastic parameters, modeling the material damping and characterizing the dynamic response of an ultrasonic transmitter. In this paper, a calibrated orthotropic finite element model for numerical simulations of ultrasonic testing of a sound red pine pole is presented. In the calibrated model, the dynamic modulus of elasticity in the radial and tangential directions, Poisson’s ratio and damping ratios are estimated from ultrasonic testing; whereas the dynamic modulus of elasticity in the longitudinal direction is estimated from transverse-vibration testing. The measured response of an ultrasonic transmitter to a one-cycle sinusoidal pulse of 50 kHz is used as dynamic excitation and introduced in the numerical model as displacement-time history. Results of the first arrival of compression waves and the frequency response magnitude computed at three receiver locations are in good agreement with the obtained ones from ultrasonic testing. The calibrated orthotropic finite element model will be used for a better understanding of propagation of surface and compression waves in ultrasonic testing for the detection of early stages of decay in wood poles.     

原木节子及内部孔洞缺陷的数学模型描述及模拟研究

为了探讨原木节子及内部缺陷对其力学性能的影响,本文借助应力波理论方法和原木内部节子及孔洞模型及理论分析,并以松木原木为试验材料,应用应力波法和频率法测出含不同节子及内部孔洞的原木波速和动态弹性模量。通过建立原木缺陷的数学模型说明节子及孔洞的影响机理和试验分析表明,木材节子及空洞的存在影响了应力波波速及弹性模量等力学性能水平,应力波等技术对原木内部缺陷检测具有较高的理论意义和应用价值。     

Effect of heat treatment intensity on wood chemical composition and decay durability of Pinus patula

Heat treatment is an effective method to improve biological resistance of low natural durability wood species. The aim of this study was to enhance the decay resistance of Pinus patula, an African low natural durability softwood species, via wood thermal modification technique. Heat treatment was performed on wood specimens under inert conditions at different heat treatment intensities to reach mass losses of 5, 10 and 15%. Heat treated specimens were exposed to fungal decay using the brown rot fungus Poria placenta. The wood chemical and elemental composition was determined as well as extractives toxicity before and after wood thermal modification to understand the reasons of durability improvement. The treated specimens exhibited a significant increase in their durability against wood decay in line with the severity of the treatment. Wood holocellulose was found to be distinctly more sensitive to the heating process than the lignin constituent. In addition, obvious correlations were observed between weight losses recorded after fungal exposure and both holocellulose decrease and lignin ratio increase. The same correlations were observed with the elemental composition changes allowing using the observed differences for predicting of wood durability conferred by heat treatment. Furthermore, no significant differences were observed between the toxicity of Pinus patula wood extractives before and after its thermal modification.     

Microfoaming of flax and wood fibre reinforced polypropylene composites

Natural and wood fibre reinforced plastics as a relatively new group of environmental friendly materials have been extensively applied in interior, building applications and in the automobile industry. Among others, natural and wood fibre reinforced foamed polymer materials are of high significance because of the possibility of their reducing the density of automotive components which have a cellular structure. However, the properties of these materials have not been fully investigated and described. Microcellular composites of polypropylene containing natural and wood fibre was prepared using an injection moulding process. In the present work, the manufacturing technology of natural and wood fibre reinforced polymer microfoams was developed and the influence of fibre and microvoid content on its property spectrum was systematically investigated. The forming of microvoids and the degree of foaming related to the variation of the processing parameters in connection with manufacture technology was characterised. Measurement of density, cell size, tensile and flexural properties of the prepared composites was carried out. The cell structure is dependent on flow direction of foaming. The density of microfoamed wood fibre-PP composites was reduced by about 24% and decreased by as much as 0.77 g/cm³.Light microscopy showed that the cells are circular and it was also observed that the maximum cell sizes are between 10–50 μm. Water absorption and scanning electron microscopy of the composites were also investigated. Considering the experimental results, it can be deduced that the microcellular composites exhibit a possible combination of relatively good engineering properties and lower density for different technical applications.     

Small resistive wood moisture sensors: a method for moisture content determination in wood structures

The wood moisture content influences the service life of wood structures since wood is susceptible to decay by rot fungi if it is exposed to high moisture contents during long periods of time. In rain exposed structures, the moisture content close to end grain surfaces and joints can be significantly higher than the average moisture content, but moisture content determinations at such locations require small moisture content sensors. This paper presents small resistive moisture content sensors fastened by electrically conductive adhesive. The relationship between moisture content and electrical resistance was determined for Norway spruce (Picea abies (L.) Karst.) for a wide range of moisture conditions achieved both by equilibrating specimens over saturated salt solutions and by the pressure plate method. The error, i.e. the difference between the gravimetric moisture content and the moisture content from the regression equation, increased with increasing moisture content. Neither the wood type (heartwood/sapwood) nor the growth rate (southern or northern Sweden) influenced the resistance-moisture content relationship.