基于Green-Naghdi模型与三相延迟模型的耦合热弹性波传播

研究了基于G-N模型与三项延迟模型的热弹性波的传播特征.在各种广义热弹性理论中,热位移概念的引入使得G-N模型具有独特性质而引起广泛的关注和应用.基于G-N模型,最近一个三相延迟模型被提出.主要研究了这两种模型下第一和第二声波(耦合热弹性波)的色散和衰减特性,分析了温度场和位移场在两种模型下的振幅比和相位差.结果发现三相延迟模型更为通用和灵活,应得到更多的关注与应用.     

Influence of the torsional vibration of the periodically attached perpendicular beam resonator on the flexural band of a Euler-Bernoulli beam

The bending and torsional vibration of the periodic perpendicular cantilever beam-mass resonators (PCBMR) is idealized as translational and rotational oscillators attached to the main beam. In this paper, the effect of that torsional vibration of the PCBMR on the dynamics of an infinitely long Euler-Bernoulli beam is evaluated. The band-structure is explored by implementing the transfer matrix method in conjunction with Bloch-Floquet's theorem. The combination of the translational and rotational oscillator modifies the relative position of the coupling coefficient in the transfer matrix, which plays a pivotal role in the band-gap formation. The flexural band-structure is highly sensitive to the torsional vibration while the radius of gyration of the tip mass is considerably higher than the length of the PCBMR. Ill-tuning leads to split and reduction of attenuation band to 50%; whereas, around 38% elongation of the attenuation band in the low frequency regime can be achieved by proper tuning.     

切应力对牛骨蠕变变形的影响研究

骨尤其是湿骨,在恒定载荷作用下会发生蠕变变形。为了确定切应力是否影响骨的蠕变变形,采用对骨薄板试样分别施加集中载荷和均布载荷的方式,测量试样挠度实时的变化曲线。结果显示,在载荷恒定时,骨试样的挠度随时间不断增加,体现了典型的蠕变特性。集中荷载下骨的蠕变变形远大于均布荷载下骨的蠕变变形,湿骨的蠕变位移比干骨高近7倍。分析认为,对试样粘弹性性质的影响不仅有正应力的作用,也有切应力的作用;切应力产生的蠕变变形约为正应力所产生蠕变变形的0.85倍。     

Fabrication and microstructural characterization of functionally graded porous acrylonitrile butadiene styrene and the effect of cellular morphology on creep behavior

The ability to control material properties in space and time for functionally graded viscoelastic materials makes them an asset where they can be adapted to different design requirements. The continuous microstructure makes them advantageous over conventional composite materials. Functionally graded porous structures have the added advantage over conventional functionally graded materials of offering a significant weight reduction compared to a minor drop in strength. Functionally graded porous structures of acrylonitrile butadiene styrene (ABS) had been fabricated with a solid‐state constrained foaming process. Correlating the microstructure to material properties requires a deterministic analysis of the cellular structure. This is accomplished by analyzing the scanning electron microscopy images with a locally adaptive image threshold technique based on variational energy minimization. This characterization technique of the cellular morphology is analyst independent and works very well for porous structures. Inferences are drawn from the effect of processing on microstructure and then correlated to creep strain and creep compliance. Creep is strongly correlated to porosity and pore sizes but more associated to the size than to porosity. The results show the potential of controlling the cellular morphology and hence tailoring creep strain/compliance of ABS to some desired values. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 795–803     

一种新的岩石非线性黏弹塑性蠕变模型研究

为了克服传统元件组合模型不能描述岩石蠕变过程中非线性特征的缺陷，首先根据加速蠕变阶段的应变和应变率随蠕变时间急剧增大的特点，建立黏塑性应变与蠕变时间的指数函数关系并提出非线性黏塑性体．将该非线性黏塑性体与广义Burgers蠕变模型串联，建立可以描述岩石全蠕变过程的非线性黏弹塑性蠕变模型，根据叠加原理得到一维应力状态下的轴向蠕变方程．然后基于塑性力学理论指出岩石三维蠕变本构方程建立过程中的不足之处，并给出非线性黏弹塑性蠕变模型合理的三维蠕变方程．最后采用不同应力水平下砂岩轴向蠕变试验对模型合理性进行验证，结果表明：拟合曲线与试验曲线吻合度较高，所建蠕变模型能够很好地描述砂岩在不同应力水平下的蠕变变形规律，尤其对加速蠕变阶段的非线性特征描述效果很好，验证了模型的合理性．     

热粘弹波在变温非均匀合金熔体中的传播

机械波在金属凝固过程中传播的定量计算一直是一个难题,主要原因就是在这个过程中的熔体结构非常复杂.本研究考虑到熔体的变温、非均匀和粘弹性的特点,采用Kelvin粘弹性介质模型,建立了具有粘热损失特性的热粘弹性波动方程,通过隐式有限差分方法对波动方程进行求解,并以ZL203A合金熔体为研究对象,探究了热粘弹波在变温非均匀介质中的传播规律.结果表明:热粘弹波从合金熔体的低温区向高温区传播时,非均匀的温度场对波的传播有较大影响;相反,当波从合金熔体的高温区向低温区传播时,非均匀的温度场对波的传播几乎没有影响.热粘弹波在合金熔体中的衰减系数随频率的增大呈线性增大,而随温度的升高先增大后减小,在熔体的枝晶搭接温度附近达到最大值.     

White and some colored marbles as alternative radiation shielding materials for applications

ABSTRACT

In this paper, the radiation shielding parameters such as linear attenuation coefficients (LAC, µ), mass attenuation coefficients (MAC, µ/ρ), effective atomic numbers (Z_eff), effective electron densities (N_eff), half value of layers (HVL), mean free paths (MFP) and buildup factors (exposure (EBF) and energy absorption (EABF)) were investigated for cream (M1), pink (M2), white (M3), maroon (M4) and green (M5) marbles. Attenuation coefficients were measured in the energy region 31.18–661.66 keV photon energies. The values of Z_eff and N_eff were then calculated using these coefficients with logarithmic interpolation method, and HVLs and MFPs were calculated using the values of LAC of marble samples at the same photon energies. The experimental results were compared with the theoretical values obtained from WinXCom program, and good agreements were observed between the experimental and theoretical results. HVLs and MFPs of all marble samples were compared with those of some concretes, glasses and commercial radiation shielding glasses (SCHOTT Co.). The studied marbles were better radiation shielding materials than standard shielding concretes due to lower HVL and MFP values lower than the ordinary concrete. Finally, EBFs and EABFs of the marbles were calculated in the energy region 0.015–1?MeV up to penetration depths of 40 mfps by Geometric Progression method (G-P), and the results were discussed in terms of photon energies and chemical compositions of the marbles.     

Reflection of plane waves in generalized thermoelasticity of type III with nonlocal effect

The generalized thermoelasticity theory based upon the Green and Naghdi model III of thermoelasticity as well as the Eringen's nonlocal elasticity model is used to study the propagation of harmonic plane waves in a nonlocal thermoelastic medium. We found two sets of coupled longitudinal waves, which are dispersive in nature and experience attenuation. In addition to the coupled waves, there also exists one independent vertically shear-type wave, which is dispersive but experiences no attenuation. All these waves are found to be influenced by the elastic nonlocality parameter. Furthermore, the shear-type wave is found to face a critical frequency, while the coupled longitudinal waves may face critical frequencies conditionally. The problem of reflection of the thermoelastic waves at the stress-free insulated and isothermal boundary of a homogeneous, isotropic nonlocal thermoelastic half-space has also been investigated. The formulae for various reflection coefficients and their respective energy ratios are determined in various cases. For a particular material, the effects of the angular frequency and the elastic nonlocal parameter have been shown on phase speeds and the attenuation coefficients of the propagating waves. The effect of the elastic nonlocality on the reflection coefficients and the energy ratios has been observed and depicted graphically. Finally, analysis of the various results has been interpreted.     

Effect of transition elements on dispersoid formation and elevated-temperature mechanical properties in 6082 aluminum alloy

ABSTRACT

The effect of transition elements, specifically Mn, Cr, V, and Mo, on dispersoid formation and mechanical properties in 6082 aluminum alloy was studied. The elevated-temperature mechanical properties were evaluated based on the compressive yield strength and creep resistance. The results indicated that the addition of Mn to the 6082 alloy resulted in the formation of a large number of the thermally stable α-Al(MnFe)Si dispersoids, thereby significantly improving the elevated-temperature mechanical properties of the alloy. Subsequent additions of Cr, V, and Mo increased the amount of Mn-bearing intermetallic phases, which decreased the supersaturation levels of Mn and Si in the α-Al, and consequently decreased the volume fraction of the dispersoids. The alloys containing Cr, V, and Mo exhibited similar yield strengths at 300°C and higher yield strengths at room temperature compared to the alloy containing only Mn. The size effect of the smaller dispersoids containing Cr, V, and Mo together with the solid-solution hardening of these elements could balance out the strength decrease resulting from the decreased volume fraction of the dispersoids. The additions of Cr, V, and Mo significantly increased the creep resistance of the Mn-containing 6082 alloy. Vanadium induced the highest creep resistance followed by Cr and Mo. Solute atoms of these elements with low diffusivity in the aluminum matrix contributed significantly to increasing the creep resistance at 300°C.     

A statistical approach to characterize the viscoelastic creep compliances of a vinyl ester polymer

The objective of this study was to develop a model to predict the viscoelastic material functions of a vinyl ester (VE) polymer with variations in its experimentally obtained material properties under combined isothermal and mechanical loading. Short-term tensile creep experiments were conducted at three temperatures below the glass transition temperature of the VE polymer, with 10 replicates for each test configuration. The measured creep strain versus time responses were used to determine the creep compliances using the generalized viscoelastic constitutive equation with a Prony series representation. The variation in the creep compliances of a VE polymer was described by formulating the probability density functions (PDFs) and the corresponding cumulative distribution functions (CDFs) of the creep compliances using a two-parameter Weibull distribution. Both Weibull scale and shape parameters of the creep compliance distributions were shown to be time and temperature dependent. Two-dimensional quadratic Lagrange interpolation functions were used to characterize the Weibull parameters to obtain the PDFs and, subsequently, the CDFs of the creep compliances for the complete design temperature range during steady state creep. At each test temperature, creep compliance curves were obtained for constant CDF values and compared with the experimental data. The predicted creep compliances of the selected VE polymer in the design space are in good agreement with the experimental data for all three test temperatures.