黏弹性材料的动态力学特性及其对覆盖层吸声性能的影响

本文利用标准化动态力学测量手段获得了某种高分子聚合物的动态杨氏模量,并根据时温等效原理对动态杨氏模量与声学测量在频段上的差异加以分析和转换,得到了500—7500 Hz频率范围内该黏弹性材料杨氏模量随频率变化的特性.基于所测得动态杨氏模量,采用有限元方法分析了均匀黏弹材料的吸声性能,并将仿真结果与样品声管实验数据进行对比,验证了测试所得参数的准确性.进一步仿真分析了含有局域共振结构的声学覆盖层吸声性能,并讨论了黏弹性材料的动态特性对其吸声性能的影响,提出了改进水声覆盖层低频宽带吸声特性的建议.     

Mechanical spectroscopy of decagonal Al–Cu–Fe–Cr quasicrystalline coatings

Mechanical spectroscopy measurements were performed on decagonal quasicrystalline Al–Cu–Fe–Cr coatings of three different thicknesses deposited on a mild steel substrate. The mechanical loss spectra indicate that the internal friction is mostly caused by the quasicrystalline coating and that the contributions of both the steel substrate and the interface are small. The shear modulus measured in torsion increases with temperature, while the Young’s modulus measured in flexion behaves normally. This shear modulus anomaly is interpreted as being due to solid friction between cracked segments of the quasicrystalline coating. This phenomenon also explains the broad athermal maximum found to occur in isochronal internal friction measurements. A quantitative model successfully reproducing the observed behaviour has been developed. Finally, the reversible high-temperature exponential background was interpreted as being due to the onset of the brittle-to-ductile transition in the quasicrystalline coating. The measured activation enthalpy is similar to the value that was deduced from compression tests performed at high temperatures on icosahedral Al–Cu–Fe bulk material.     

Mechanical properties of novel forms of graphyne under strain: A density functional theory study

The mechanical properties of two forms of graphyne sheets named α-graphyne and α2-graphyne under uniaxial and biaxial strains were studied. In-plane stiffness, bulk modulus, and shear modulus were calculated based on density functional theory. The in-plane stiffness, bulk modulus, and shear modulus of α2-graphyne were found to be larger than that of α-graphyne. The maximum values of supported uniaxial and biaxial strains before failure were determined. The α-graphyne was entered into the plastic region with the higher magnitude of tension in comparison to α2-graphyne. The mechanical properties of α-graphyne family revealed that these forms of graphyne are proper materials for use in nanomechanical applications.     

Nanomechanics of microtubules

We have determined the mechanical anisotropy of a single microtubule by simultaneously measuring the Young's and the shear moduli in vitro. This was achieved by elastically deforming the microtubule deposited on a substrate tailored by electron-beam lithography with a tip of an atomic force microscope. The shear modulus is 2 orders of magnitude lower than the Young's, giving rise to a length-dependent flexural rigidity of microtubules. The temperature dependence of the microtubule's bending stiffness in the (5-40) degrees C range shows a strong variation upon cooling coming from the increasing interaction between the protofilaments.     

Shear elasticity of fluids at low-frequent shear influence

The visco-elastic properties of liquids have been investigated using acoustical resonance method. Piezoquatrz performed tangential oscillations on the main resonance frequency of 74 kHz contacts by the one end of horizontal surface with the studied liquid layer covered by quartz cover-plate. So the stagnant shear waves are installed in layer. The solution of interaction of piezoquartz-liquid layer-cover-plate gives three methods of determination of the real shear modulus (G) and the tangent of mechanical loss angle (tan theta) of liquid. The first method is realized at smaller thickness of liquid layer then the length of shear wave. Liquids of different classes have been studied using this method: polymer liquids, oils, glycols and alcohols. The second method is connected with the propagation of shear wave in liquid layer, parameters of which are determined the G and tan theta. And the third method is based on the determination of limit shift of resonance frequencies at completes damping of shear wave in thick layer of liquid. All these three methods give satisfactory agreement of results.     

Influence of static compression on mechanical parameters of acoustic foams

The modification of elastic properties of compressed acoustic foams is investigated. The porous sample is first submitted to a static compression and then to a dynamic excitation of smaller amplitude, corresponding to acoustical applications. The static compression induces the modification of the dynamic elastic parameters of the material. This work focuses on Young's modulus. The variation is measured with two different experimental methods: The classical rigidimeter and an absorption measurement. The effective Young's modulus is directly measured with the first method and is indirectly determined through the quarter-wave length resonance of the frame with the second one. The results of the two measurements are compared and give similar tendencies. The variation of the dynamic Young's modulus as a function of the degree of compression of the sample is shown to be separated in several zones. In the zones associated with weak compression (those usually zones encountered in practice), the variation of the effective Young's modulus can be approximated by a simple affine function. The results are compared for different foams. A simple model of the dependency of the Young's modulus with respect to the static degree of compression is finally proposed for weak compressions.     

Ultrasonic characterization of orthotropic elastic bovine bones

The aim of the present study was to determine the mechanical properties of bovine bones. An ultrasonic method was used to determine acoustical parameters such as the longitudinal and transverse velocities in the longitudinal and two radial directions of compact bone, i.e., in all directions of the plane. Waves propagating through bovine femoral bones were studied using an ultrasonic scanner for linear and sectorial scanning. The mechanical parameters of compact bone, such the Young's modulus and Poisson's ratio in the orthotropic case, were then determined from the measured velocities. The results are in line with those in the literature.     

The use of ultrasound-stimulated acoustic emission in the monitoring of modulus changes with temperature

It has been previously shown that the amplitude of the ultrasound-stimulated acoustic emission (USAE) signal is sensitive to tissue temperature and, therefore, can help detect it. Its amplitude, however, is sensitive to both acoustical and mechanical parameters, that at most frequencies have opposite effects due to temperature. In this paper, we explore the feasibility of using a frequency shift of the resonant peaks of the USAE signal for monitoring the tissue stiffness variation with temperature. In a numerical simulation, the variation of the frequency shift at different temperatures is shown. Then, in a series of experiments involving a gel phantom and porcine muscle tissue, the frequency shift variation is shown to follow the known stiffness changes due to temperature. It is also shown that this shift indicates reversible changes as well as the onset of thermal coagulative necrosis. The necrosis is marked by a monotonically increasing positive frequency shift. It was thus shown that the USAE spectrum peaks undergo a negative shift (or, downshift) when the stiffness decreases and a positive shift (or, upshift) when the stiffness increases. The experimental frequency shifted around a peak at 22.1-22.5 kHz within a range of -250 to 80 Hz and -200 to 250 Hz for the gel and muscle tissue for the temperatures of 25-70 and 30-70 degrees C, respectively. Simulation and ex vivo experimental results indicate that the USAE frequency shift method can help decouple the mechanical from the acoustical parameter dependence as well as detect the onset of thermal coagulative necrosis.     

Bulk Mechanical Behavior of Rootzone Sand Mixtures as Influenced by Particle Shape,Moisture and Peat

Mechanical properties such as bulk modulus, shear modulus, and failure stress of rootzone sand mixtures are some of the key parameters in understanding the load‐response of sands used in professional golf courses. According to the United States Golf Association (USGA) specifications, appropriate particle size distributions and their shape are important for preparing putting greens and bunker sands. Despite being an important parameter, the influence of sand particle shape on the bulk mechanical properties of the rootzone mixtures has not been studied systematically using a fundamental tester. Toward this end, bulk mechanical properties were measured using a low‐pressure cubical (true) triaxial tester. In this study, four of the commonly used basic shapes, i.e., round, subround, subangular, and angular sand particles comprising rootzone mixtures with sphagnum peat (organics) were tested at two different moisture levels, air‐dried and 30 cm moisture tension conditions. For all rootzone sand mixtures, an increase in bulk modulus was observed with increasing isotropic pressure. The failure stress values increased with the increase in mean pressure for air dried samples. In general, moisture increased compressibility of sands and decreased failure strength and shear modulus values. Peat had a dominant influence on the mechanical response of all four sand shapes. When peat and moisture were added, the rootzone mixture became most compressible and easier to fail, with noticeable changes in bulk mechanical properties.     

Shock-wave compression of x-cut quartz as determined by electrical response measurements

The mechanical response of x-cut quartz in the vicinity of the Hugoniot elastic limit is determined from measurements of the piezoelectric current from samples impact loaded from 26 to 130 kbar. The Hugoniot elastic limit is determined to be 60_?1·5⁺³ kbar at a compression of 0·066_?0·002^+0·004 This Hugoniot elastic limit corresponds to a shear strength of 5·5 per cent of the C₄₄ shear modulus. For stresses well above the Hugoniot elastic limit the electrical current measurements show that the material exhibits a substantial reduction of shear strength. The pressure derivative of the bulk modulus is determined to be 4·5, substantially less than the ultrasonic value. The experimental records show evidence for a time delay for reduction of shear strength which varies from about 10^?7 sec immediately above the 60 kbar Hugoniot elastic limit to about 10^?8 sec for stresses well above the Hugoniot elastic limit. The measurements also show stress relaxation below the Hugoniot elastic limit between 40 and 60 kbar.     

Phase velocity and attenuation of 9 GHz hypersound in polycrystalline nickel films as a function of quartz-substrate temperature

Using a movable narrow beam of 9 GHz hypersound and wedge-shaped evaporated Ni films, the phase velocity and attenuation of longitudinal as well as of transverse hypersound was determined at a temperature of 2 K by multiple-beam interferometry in these Ni films. They were evaporated onto a quartz substrate at substrate temperatures ranging from 50°C to 500°C. The macroscopic moduliK (bulk modulus) andG (shear modulus) can be obtained from the measured phase velocities. WhereasK appears independent on substrate temperature and agrees within experimental error with theoretical prediction deduced from known elastic constants of the crystallites, the observed shear modulusG possesses at a substrate temperature of 50°C only 2/3 of the predicted value and increases with substrate temperature by reaching the theoretical value at about 300°C.     

Anomalous phonon behavior: blueshift of the surface boson peak in silica glass with increasing temperature

We present helium atom scattering measurements of the boson peak at the surface of vitreous silica between 127.0 and 368.5 K. The most probable energy shows a strong temperature dependence and increases linearly with temperature in the measured range. The observed blueshift of the surface boson peak (shift rate 0.008+/-0.002 meV/K) is a factor of 4 to 10 times stronger than shift rates measured in the bulk by inelastic neutron and Raman scattering. We suggest that the anomalous shift direction of the boson peak to higher energies with increasing temperature has the same origin as the unusual temperature dependence of the bulk modulus of silica glass.     

Interphase sizing temperature effect of LaRC PETI-5 on the dynamic mechanical thermal properties of carbon fiber/BMI composites

The dynamic mechanical thermal properties of carbon fiber-reinforced bismaleimide (BMI) composites processed using polyacrylonitrile(PAN)-based carbon fibers unsized and sized with LaRC PETI-5 amic acid oligomer as interphase material at 150°C, 250°C, and 350°C were investigated by means of dynamic mechanical thermal analysis. It was found that the storage modulus, loss modulus, tan δ and the peak temperature significantly depend on the sizing temperature as well as on the presence and absence of LaRC PETI-5 sizing interphase. The result showed that the carbon fiber/BMI composite sized at 150°C had the highest storage modulus at a measuring temperature of 250°C. The storage modulus decreased with increasing sizing temperature from 150°C to 350°C, being influenced by interdiffusion and co-reaction between the LaRC PETI-5 interphase and the BMI matrix resin. The present result is quite consistent with the interfacial result reported earlier in term of interfacial shear strength and interlaminar shear strength of carbon fiber/BMI composites. It is addressed that in the present composite system the sizing temperature of LaRC PETI-5 interphase critically influences not only the interfacial properties but also the dynamic mechanical thermal properties and its control is also important.     

Viscoelastic phase separation in polymer blends

In this paper, the dynamics and morphology of viscoelastic phase separation in polymer blends is investigated based on the two-fluid model in two dimensions. At critical composition, we have carefully checked the role of shear modulus, without taking account of bulk modulus. The results show that the higher shear modulus component tends to form a dispersed phase in the intermediate stage of phase separation, if the difference between the shear moduli of the components is large enough. This is opposite to the role of bulk modulus, that the higher bulk modulus component forms a networklike pattern without taking account of the shear modulus even if it is the minority phase. The morphological formation is determined by the competition of opposite effects of shear modulus and bulk modulus. For polymer blends at critical composition, the bulk modulus difference leads to a networklike pattern formed by the higher modulus component in the intermediate stage of phase separation. But if the difference between the shear moduli of the components is large enough, a co-continuous structure is observed, resulting from the competition between shear and bulk moduli. For off-critical composition, difference in bulk modulus also leads to a networklike pattern of the component with higher bulk modulus in the intermediate stage of phase separation, but phase inversion is observed rapidly. A small difference between the shear moduli of the components can support the networklike pattern to continue for longer time. But the networklike pattern does not occur for large difference between shear moduli.Received: 9 September 2004, Published online: 10 November 2004PACS: 64.75. + g Solubility, segregation, and mixing; phase separation - 83.80.Tc Polymer blends     

Ultrasonic measurement of viscoelastic shear modulus development in hydrating cement paste

A test procedure for measuring changes in amplitude and phase of SH ultrasonic waves from the interface between fused-quartz and cement paste samples is presented. The phase change is determined from the temporal shift in the reflected signal relative to the incident signal. The sensitivity of the measured parameters to changes in acoustic impedance of the materials in contact with fused-quartz is evaluated for different angles of incidence. It is shown that a reflection measurement at normal incidence at nano-second temporal resolution does not provide sufficient sensitivity to measure the viscous component of shear modulus of low viscosity fluids and cannot be applied to cement paste while it is in a fluid state. Monitoring the measured amplitude and phase at oblique angle of incidence allows for measuring fluids with acoustic impedance comparable to cement paste. The reflection measurements are used to determine the evolution of elastic and viscous components of shear modulus cement paste with time. Influence of sampling rate and temperature effects on the phase measurements are evaluated and shown to be significant. It is shown that the initial loss of workability of cement paste through setting process is associated with a larger relative increase in the viscous component of shear modulus. Following the initial rapid rise of the viscous component of shear modulus, there is a larger relative increase in the elastic component, which can be related to the emergence of a solid structure capable of retaining an imprint.     

Evidence of distributed interstitialcy-like relaxation of the shear modulus due to structural relaxation of metallic glasses

The interstitialcy theory is used to calculate the kinetics of shear modulus relaxation induced by structural relaxation of metallic glasses. A continuous distribution of activation energies is shown to be a salient feature of the relaxation. High precision in situ contactless electromagnetic acoustic-transformation shear modulus (600- kHz) measurements performed on a Zr-based bulk metallic glass are found to strongly support the approach under consideration. It is revealed that the activation energy spectra derived from isothermal and isochronal shear modulus measurements are in good agreement with each other. It is concluded that the increase of the shear modulus during structural relaxation can be understood as a decrease of the concentration of structural defects similar to dumbbell interstitials in simple crystalline metals.     

新型石墨插层化合物HfC2结构与性质的第一性原理研究

在高压下,预测一种新型石墨插层化合物HfC₂.采用第一性原理方法对其在0 GPa下的结构和性质进行研究,分别采用GGA-PBESOL、GGA-PW91和LDA方法进行结构优化,得到的晶体学数据基本相同.弹性常数和声子谱计算证实其力学和晶格动力学稳定性,表明HfC₂在0 GPa下能够稳定存在.采用GGA-PBESOL方法计算得到HfC₂的体模量和剪切模量达到265 GPa和118 GPa,Pugh比k<0.57,是一种具有高体模量的韧性材料.HfC₂存在C-C、Hf-C共价作用,且具有金属特性和特殊层状结构,是其具有高体模量和韧性的原因.最后,对HfC₂在0~500 GPa内的键长、体模量、剪切模量、k值等进行研究,探索其力学性质随压力变化的规律.     

Dynamic mechanical relaxation and loss in the incommensurate phase of quartz

The dynamic mechanical properties of quartz have been studied as a function of temperature across the α-β phase transition and in the vicinity of the incommensurate (IC) phase on cooling from the β phase. The mechanical response of the IC phase shows strong anelasticity for measurement of Young's modulus (closely related to C(11) in our geometry) with modulated stress driven at 1 Hz. The dynamic shear modulus does not show similar strong effects in its imaginary component, although a very weak anomaly is barely detectable in the real part of the modulus. Our results indicate that the incommensurate microstructures within the quartz transition interval are susceptible to dilatational stress with relaxation times around 1 s.     

Influence of composition of carbonyl iron particles on dynamic mechanical properties of magnetorheological elastomers

Magnetorheological elastomers (MRE) are known as smart materials. However, the magnetorheological (MR) effect of MRE is not high enough at present, which limits its engineering applications. Prior studies have shown that magneto-induced shear storage modulus and MR effect were mainly determined by the performance of the ferromagnetic particles. In this paper, MRE samples were prepared by carbonyl iron particles (CIP) of different compositions based on silicon rubber under external magnetic field. Their microstructures were observed using an optical digital microscope and a scanning electron microscope. The dynamic mechanical properties of MRE samples were measured using a modified dynamic mechanical analyzer under varying magnetic field strength and frequency. The results show that the carbon content of CIP have a greater impact on the dynamic mechanical properties of MRE. The magneto-induced shear storage modulus and MR effect can be increased by selecting CIP of low carbon content. In addition, the damping property is also significantly influenced by the carbon content of the CIP. This study is expected to provide guidance for fabrication of high performance MRE.     

Influence of Tester Geometry,Speed and Procedure on the Results from a Ring Shear Tester

Automated ring shear testers have gained widespread acceptance in the chemical industry. In this paper, comparative measurements with different shear testers are described and possible factors influencing the measurement of flowability, are discussed. The measurement results presented show a dependency on the bulk solid investigated. No significant influence of the shear tester was measured for easy flowing bulk solids. An influence of the shear tester is observed for poor flowing, compressible bulk solids. Experiments have shown that test procedure and shear cell design (tester geometry) affect the measurements considerably.