低频内耗测量时标准滞弹性固体的内耗行为

用三参量力学模型的恒应力和恒应变弛豫时间描述了标准滞弹性固体的内耗行为.在低频内耗测量时,振动系统的惯量不可避免地影响测量的内耗值,当材料的内耗较大和测量频率与振动系统的共振频率可以比较时,惯量对内耗测量的影响较大,这时惯量的影响不可忽略. 关键词：     

用倒扭摆测量金属玻璃内耗应注意的问题

本文指出，在用倒扭摆测量金属玻璃的内耗时应特别注意到悬丝的影响。文中根据倒扭摆的自由衰减振动方程得出了试样的内耗与系统和等效悬丝的内耗之间的相互关系，同时给出了试术的切变模量的表达式，并从能量损耗的关系出发对所涉及的问题作了进一步的讨论。     

液晶苯酚酯相变内耗峰与振动频率的相关性

采用低频内耗方法，在强迫振动模式下研究了液晶苯酚酯材料相变过程内耗峰与振动频率之间的相关性.由一个简单的粘弹性的Maxwell模型出发，推导了在强迫振动模式下内耗峰的峰高与频率的关系式.从理论上得出了内耗峰高与振动频率之间的非线性关系.实验数据拟合的结果很好地符合这一关系式.这表明内耗峰随频率的变化规律是由振动系统本身的特征决定. 关键词：     

晶界内耗的研究

晶界内耗是中国科学家葛庭燧开创的一个研究领域．以往晶界内耗的研究主要是用多晶试样进行的．最近作者用不同取向差的双晶试样研究了单个晶界的内耗，取得了一些新的结果．文章综述了研究晶界内耗的意义以及新近的进展．     

用扭摆测量获取材料非线性内耗参数的计算方法

在小内耗并且内耗参数只与切变振幅相关的前提下,采用分析力学方法求出由单自由度自由衰减振动扭摆位移采样值,获取材料非线性内耗和切变模量参数的计算方法。 关键词：     

晶界内耗研究的进展

固体在机械振动过程中由于材料内部原因引起的能量损耗称为内耗。晶界内耗峰是我国科 学家葛庭燧于1947 年用他发明的“葛氏扭摆” 在多晶纯铝中发现的。晶界内耗峰和相关的滞弹 性效应可以用滞弹性理论和粘滞性滑动模型给予合理的解释。这个内耗峰已被广泛地用来研究晶 界的动力学行为,杂质在晶界的偏聚,以及材料科学中相关的问题。 以往晶界内耗的研究大多数是用多晶试样进行的,其中包含了不同类型晶界的贡献。由于不 同类型晶界的结构和性质不同,因而多晶试样中的晶界内耗只能反映不同晶界的“平均效应”, 它的具体机制也难以解释清楚。 二十一世纪以来,人们对双晶试样（其中只包含单一晶界）中的晶界内耗进行了比较细致的 研究。实验结果表明,晶界内耗可以反映不同类型晶界的“个性”,因而可以应用于“晶界的设 计和控制”（或称“晶界工程”）。此外,新近还发现了晶界内耗中的“耦合效应” 和“补偿效 应”。这些发现加深了对晶界内耗机制的认识。 本文首先对以往多晶试样中的晶界内耗研究做一个简要的概述,然后介绍近年来双晶试样中 晶界内耗研究的新进展,并对晶界内耗的微观机制和应用前景进行分析和展望。     

马氏体式相变过程中所引起的内耗变化

用扭摆测量两种跌锰合金(Mn17.5％和12.8％)和一种铜铝合金(Al13％)的内耗,在发生正和反的马氏体相变的温度范围内各出现一个内耗峰。这种内耗峰出现的条件是必须伴随着马氏体式相变过程的进行。用含Mn17.5％的铁锰合金作了系统实验,观察到内耗峰的高度随升温(或降温)速度和应力的增加而增高,随振动频率和含碳量的增加而减低。可以用振动一周内试样中转变量愈多内耗也愈大的关系得到统一的解释。讨论了关于产生内耗峰的机构。认为主要由于马氏体相变是突然间完成,此时扭转的外力可视为常数,从应力和不均匀物质交互作用能的计算,可以证明外力所做的功必须损失一半,因而引起内耗。此外应力感生相变也可以引起很小一部分内耗。     

碳在面心立方系合金钢中扩散内耗峰的机构

在三种面心立方系的铁锰合金(Mn18.5％,25.4％,36.0％)和铁锰镍铬合金(Mn9.5％,Ni8％,Cr3％)中观察到因含碳而引起的内耗峰,当振动频率约为2周/秒时,内耗峰出现在250℃左右。实验结果指出内耗峰高度与固溶体中的含碳量成直线关系。 提出了一个产生内耗峰的初步模型。假定碳在面心立方系晶体点阵中占据八面体的间隙位置,点阵中的异类原子(合金元素的原子或空穴)与这碳原子构成一个在各方向引起不同畸变的原子对,由于这个原子对在应力作用下的转动而产生内耗。由初步模型所推导出的式子可以解释内耗峰高度与固溶体中的含碳量成直线关系。根据粗略推算所得的内耗峰高度值与实验值相近。 关于晶体中空穴浓度的推算指出,对于碳在面心立方系合金钢中微扩散所引起的内耗峰而言,空穴的贡献少于合金元素的贡献的十分之一。     

泡沫铝的低频内耗特征研究

采用粉末冶金发泡法制备的泡沫铝试样，研究了室温下低频范围（0.04—4Hz）泡沫铝的内耗与孔结构（孔隙率和孔径）、频率、振幅之间的关系．结果表明泡沫铝的内耗具有以下特征：孔径一定时，内耗随孔隙率的增加而增大；孔隙率一定时，内耗随孔径的减小而增大；在测量频率范围内，内耗与频率近似无关；小应变振幅时内耗随振幅的增大而增大，具有正常振幅效应．提出了泡沫铝的内耗产生机制，根据等效夹杂方法和钉扎位错内耗特点进行了理论分析，较理想地解释了实验结果 关键词：     

镍在交变磁场中的一个磁内耗峰

在铁磁性物质中,弹性现象与磁性现象之间的耦合每每引起内耗。过去作磁弹性内耗研究所用的磁场大都是静磁场。我们令镍试样在交变磁场中作扭转振动,测量内耗。把内耗表示为磁场强度的函数时,得到一个显著的内耗峰。这个内耗峰在静磁场下不出现。把镍试样经过冷加工处理或在试样中加碳,都足以把内耗峰的位置移向高磁场强度的一边,而保温退火则把内耗峰移向低磁场强度的一边。此外,这个内耗峰的高度由于所用扭转频率的增加而显著地增加。根据镍的磁化曲线上各部分所相当的可逆及不可逆过程,可以定性地解释上述内耗峰的来源及其所受各种因素的影响。     

EVALUATING THE VIBRATION ISOLATION OF SOFT SEAT CUSHIONS USING AN ACTIVE ANTHROPODYNAMIC DUMMY

Seat test standards require human subjects to be used for measuring the vibration isolation of vehicle seats. Anthropodynamic dummies, based on passive mass-spring- damper systems, have been developed for testing seats but their performance has been limited at low excitation magnitudes by non-linear phenomena, such as friction in the mechanical components that provide damping. The use of an electrodynamic actuator to generate damping forces, controlled by feedback from acceleration and force transducers, may help to overcome these limitations and provide additional benefits. The transmissibilities of five foam cushions have been measured using an actively controlled anthropodynamic dummy, in which damping and spring forces were supplied by an electrodynamic actuator. The dummy could be set up to approximate alternative single-degree-of-freedom and two-degree-of-freedom apparent mass models of the seated human body by varying motion feedback parameters. Cushion transmissibilities were also measured with nine human subjects, having an average seated weight similar to the dummy. At frequencies greater than 4 Hz, mean cushion transmissibilities measured with subjects were in closer agreement with the transmissibilities obtained with a two degree-of-freedom dummy than with a single degree-of-freedom dummy. However, at frequencies between 2 and 4 Hz, cushion transmissibilities obtained with the two-degree-of-freedom dummy showed consistently larger differences from mean transmissibilities with subjects than single-degree-of-freedom dummies, indicating a need for further development of human apparent mass models to account for the effects of magnitude and spectral content of the input motion. Vertical vibration isolation efficiencies (SEAT values) of the five foams were measured with four input motions, including three motions measured in a car. The SEAT values obtained using the active dummy were highly correlated with the median SEAT values obtained with the nine human subjects, with the two-degree-of-freedom apparent mass dummy giving the highest agreement.     

A COMPARISON OF BIODYNAMIC MODELS OF THE HUMAN HAND-ARM SYSTEM FOR APPLICATIONS TO HAND-HELD POWER TOOLS

The biodynamic response characteristics of various mechanical models of the human hand and arm system, reported in the literature, are evaluated in terms of their driving-point mechanical impedance modulus and phase responses. The suitability of the reported models for applications in realizing a mechanical simulator and assessment of vibration behavior of hand-held power tools is examined using three different criteria. These include the ability of the model to characterize the driving-point mechanical impedance of the human hand-arm system within the range of idealized values presented in ISO-10068 (1998); the magnitude of model deflection under a static feed force; and the vibration properties of the human hand and arm evaluated in terms of natural frequencies and damping ratios. From the relative evaluations of 12 different models, it is concluded that a vast majority of these models cannot be applied for the development of a mechanical hand-arm simulator or the assessment of dynamic behavior of the coupled hand-tool system. The higher order models, with three and four degrees of freedom, in general, yield impedance characteristics within the range of idealized values, but exhibit excessive static deflections. Moreover, these models involve very light masses (in the 1·2-4·8 g range), and exhibit either one or two vibration modes at frequencies below 10 Hz. The majority of the lower order models yield reasonable magnitudes of static deflections but relatively poor agreement with idealized values of driving-point mechanical impedance.     

碳在面心立方系合金钢与金屬中微扩散所引起的内耗峰

用扭摆作内耗测量,发现了几种面心立方系合金钢(18/8型不锈钢及高锰钢)中含碳可以引起内耗峰。当振动频率约为每秒1周时,峰的巅值温度在200-300℃之间。当钢中固溶体的碳量增多时,内耗峰升高而峰的位置移向低温,当钢中所含的碳因回火而发生沉淀时,内耗峰降低而峰的位置移向高温。把内耗方法所测得的激活能、弛豫时间和由此计算所得的扩散系数与资料上所载的碳在面心立方系的钢中宏观扩散的数据相比较,指出所观测的内耗峰确是由于碳在钢中的微扩散所引起来的。用同样的实验方法也发现了碳在镍铝合金及在纯镍中由于微扩散而引起的内耗峰。这些实验指出,碳在面心立方系晶体中微扩散而引起内耗峰这件事实,可能是一种普遍的现象。     

铁锰合金在正、反马氏体型相变温度范围内出现的位错内耗峰

用扭摆法测量了含锰18.5％的铁锰合金的内耗,在正和反马氏体型相变温度范围内都观察到一个稳定的内耗峯。在降温时,正马氏体型相变区域内的内耗峯出现在100℃左右;在重新加热时,反马氏体型相变区域内的内耗峯出现在200℃左右。对于这两个变为稳定以后的内耗峯进行了系统的研究,它们的共同特征是:1.内耗峯的位置与测量内耗所用的振动频率无关;2.稳定内耗峯的高度,随着测量内耗所用的应变振幅的增大而显著地升高;3.随着对于试样的加热和冷却循环次数的增加,内耗峯的位置离相变开始点的温度越远,而且内耗峯的高度也下降。从以上的特点可以看出,内耗峯的位置与频率无关表示产生内耗的基本过程不是受热激活所控制的。此外,内耗与振幅有关这一现象进一步指出,产生内耗的机构不是一种滞弹性效应。电子衍衬金相观测已经指出,作为扩展位错的组成部分的堆垜层错可以作为马氏体型相变的核,因此,这种内耗现象可能是由于扩展位错的应力感生加宽和收缩所引起的。这种模型可以统一地解释在铁锰合金中所观察到的各种内耗现象。     

Effects of annealing and additions on dynamic mechanical properties of SnSb quenched alloy

The elastic modulus, internal friction and stiffness values of quenched SnSb bearing alloy have been evaluated using the dynamic resonance technique. Annealing for 2 and 4 h at 120, 140 and 160 °C caused variations in the elastic modulus, internal friction and stiffness values. This is due to structural changes in the SnSb matrix during isothermal annealing such as coarsening in the phases (Sn, Sb or intermetallic compounds), recrystallization and stress relief. In addition, adding a small amount (1 wt.%) of Cu or Ag improved the bearing mechanical properties of the SnSb bearing alloy. The SnSbCu₁ alloy has the best bearing mechanical properties with thermo-mechanical stability for long time at high temperature.     

Pb-Sn合金固—液转变过程的内耗行为

在连续升温过程中,测量了Pb-Sn合金的内耗.实验发现：在共晶成分范围内,伴随着Pb-Sn 合金的固—液转变,内耗值出现突然下降.内耗值的下降幅度与Pb-Sn合金成分有关.固—液 转变过程中的内耗是由材料结构变化或弛豫时间变化引起的. 关键词： 内耗 固—液转变 Pb-Sn合金     

Optimum tuning of series and parallel LR circuits for passive vibration suppression using piezoelectric elements

Bending vibration of flexible structures can be suppressed passively using piezoelectric electromechanical transducers and optimally tuned LR circuits. Since these systems include both mechanical and electrical elements, the governing equations consist of electrically coupled equations of motion. This paper describes a new method for deriving the governing equations that describe a system’s vibration suppression based on the equilibrium of force principle and using an equivalent mechanical model of a piezoelectric element. Both series and parallel LR circuits are considered in the modeling approach. The optimum values for a mechanical vibration absorber can be formulated by using the two fixed points method. However, exact optimal values for the resistances of the LR circuits have not been formulated in the research literature thus far, and approximate values have been used. Analytical formulations are derived in this paper, and optimum values of the LR circuits are presented, not only in displacement, but also in terms of velocity and acceleration. The effects of the stiffness of the adhesive bond between the host structure and piezoelectric element, the dielectric loss in a piezoelectric element, and the internal resistance of an inductor are considered in the theoretical analysis. The effectiveness of the described analytical method is validated through simulations and experiments.     

Progress in the Investigations of Grain Boundary Relaxation

Internal friction (or damping) is a measure of energy dissipation during mechanical vibration. The internal friction peak induced by grain boundary (GB) relaxation was discovered by Kê in polycrystals in 1947. The GB internal friction and related anelastic effects have been successfully interpreted by Zener's anelastic theory and viscous sliding model. Since then, the GB internal friction peak has been widely used to study the dynamic process of GBs, impurity segregation at GBs and relevant processes in materials science.

Previously, the GB internal friction was mostly studied with polycrystalline materials, in which mixed contributions of different types of GBs are involved. Since the microstructures and behaviors for different types of GBs are different, the detailed mechanism of the GB peak in polycryatals has not been clearly clarified.

From the beginning of the 21th century, the internal friction in bicrystals (each has a single boundary) with different misorientations and rotation axes has been systematically investigated. The results indicate that the internal friction can be used to distinguish the individual behavior of different types of GBs and applied to the practice of “GB engineering.”

Moreover, the coupling effect and compensation effect involved in GB relaxation has been recently observed and explained. The coupling effect means a correlated atomic motion occurred in GB relaxation. The compensation effect indicates that the apparent activation enthalpy is linearly related to the activation entropy in GB relaxation. These findings improve the understanding of the mechanism of GB internal friction.

This article attempts to give a comprehensive review to the investigations of GB internal friction in polycrystals, bamboo-crystals, and bicrystals. The microscopic mechanisms and the further applications of GB internal friction are discussed and prospected.