Homogenization theory for designing graded viscoelastic sonic crystals

In this paper,we propose a homogenization theory for designing graded viscoelastic sonic crystals(VSCs) which consist of periodic arrays of elastic scatterers embedded in a viscoelastic host material.We extend an elastic homogenization theory to VSC by using the elastic-viscoelastic correspondence principle and propose an analytical effective loss factor of VSC.The results of VSC and the equivalent structure calculated by using the finite element method are in good agreement.According to the relation of the effective loss factor to the filling fraction,a graded VSC plate is easily and quickly designed.Then,the graded VSC may have potential applications in the vibration absorption and noise reduction fields.     

Viscoelastic Properties of Crude Oil Components at Oil‐Water Interfaces. 2: Comparison of 30 Oils

The dilational properties of diluted (0.7 vol/vol in toluene) and undiluted crude oil‐water interfaces have been studied using the oscillation drop method with the objective of understanding the properties contributing to the overall stability of crude oil emulsions. The importance of working with undiluted crude oils instead of model systems when dilational properties of real oil‐water systems are going to be reproduced in the laboratory setting has been discussed. For such studies, molecular exchange mechanisms and the aggregation state of asphaltenes are too dependent on concentration to justify the use of model compounds, i.e. fractionated asphaltenes diluted in a solvent. As expected in the low frequency range (0.01–1 Hz), molecular exchange from the bulk oil phase strongly affected the measured dilational parameters. For the diluted crude oils, the frequency dependence of the dilational modulus increased with its magnitude. The systems that exhibited particularly low magnitude of the dilational modulus were of the heaviest crude oils in the sample set, whereas the systems with greatest dilational modulus were among the lightest crude oils. The overall characteristic time of relaxation of the crude oil‐water interfaces was in the range below 10 seconds. The undiluted crude oil‐water interfaces had similar interfacial properties as the diluted samples except for slightly reduced magnitude of the dilational modulus. The crude oil‐water interfaces appeared to be soluble, but some observations pointed to intrinsic rheological properties of the interfaces. Intrinsic elasticity and viscosity of the films should be studied outside the frequency range used here at low (ω～0 Hz) and high (ω→500 Hz), respectively.     

Influence of Mg2+ Contents on the Thixotropy of HTlc/MT Suspensions

In this article, we found that the variation of Mg²⁺ content in the Fe-Al-Mg hydrotalcite-like compounds (HTlc) and the change of R values (R_defined = W_HTlc /W_MT) will influence the thixotropic and viscoelastic properties of HTlc/sodium montmorillonite (MT) suspensions. With increasing the content of Mg²⁺ in the HTlc samples, the storage modulus (G′) and the loss modulus (G″) of the HTlc/MT suspensions will increase gradually, which means the structure strength of the HTlc/MT suspension increases with increasing of Mg²⁺ content in the HTlc samples. In addition, the structure strength of the suspensions will increase with increasing of R values. Special emphasis has been laid on the phenomenon of thixotropy. As the gradual increase of the structure strength in the HTlc/MT suspension, the thixotropic types will change accordingly. The mechanism has been discussed emphatically.     

A Surface Rheological Study of Silwet L-77 Surfactant at the Air/Water Interface

The dilational viscoelastic properties of Silwet L-77, a trisiloxane surfactant of the general formula (Me₃SiO)₂SiMe(CH₂)₃(OCH₂CH₂)_7.5OH, at the air/water interface were investigated. Aqueous solutions of Silwet L-77 were spread onto a pendant drop and the dynamic surface tensions were measured by means of axisymmetric drop shape analysis. The surface dilatational elasticity, viscosity, and phase angle of the adsorption monolayer were also determined using the oscillating drop technique. The influences of frequency and concentration on the surface dilational properties were expounded. It is shown that the surface dilational modulus and elastic modulus increased with the increase of frequency, the viscous modulus was complex, while the phase angle decreased with the increase of frequency. Surface dilational modulus, elastic modulus, and viscous modulus passed through the maximum with the increase of bulk concentration and the phase angle increased with the increase of concentration.     

粘弹性饱和土体中半封闭圆形隧洞的动力响应分析

基于Biot波动方程,研究分析了粘弹性饱和土体中半封闭圆形隧洞的动力响应问题．假定衬砌材料为多孔介质,引入了更符合工程实际的半封闭边界条件．通过引入势函数,在Laplace变换域中得到隧洞边界上作用轴对称荷载和流体压力条件下应力、位移和超孔隙水压力的解答．利用Laplace数值逆变换得到时域中的解,分析了隧洞边界的半透水特性对隧洞动力响应问题的影响,结果表明：隧洞边界的半透水特性对应力、位移场的变化和超孔隙水压力的消散有很大的影响,透水和不透水下条件的解仅是本文的两个特例。     

基于等效黏性弹簧的黏弹性Timoshenko裂纹梁弯曲解析解

考虑裂纹的缝隙和黏性效应,将梁中横向裂纹等效为黏弹性扭转弹簧,利用广义Delta函数,给出了Laplace变换域内裂纹梁的等效抗弯刚度,得到了具有任意开闭裂纹数目且满足标准线性固体黏弹性本构的Timoshenko梁在时间域内的弯曲变形显式解析通解．在此基础上,通过两个数值算例,分析了时间、梁跨高比和裂纹深度等参数对黏弹性Timoshenko开裂纹梁弯曲变形的影响．结果表明：裂纹黏性对Timoshenko裂纹梁的弯曲具有显著的影响．相比于裂纹的弹性扭转弹簧模型,考虑裂纹黏性效应的黏弹性Timoshenko裂纹梁在裂纹处挠度尖点和转角跳跃现象十分明显．另外,由于横向剪切引起的附加变形,Timoshenko裂纹梁的稳态挠度与Euler-Bernoulli梁挠度的差值为常数,其大小与裂纹模型、梁跨高比或裂纹深度无关,这些结果对梁裂纹无损检测具有指导意义．     

粘弹性界面裂纹奇异场

对于许多粘弹性问题,通常可以利用对应性原理,即由弹性问题的结果得到对应的粘弹性问题在拉普拉斯变换域内的解,再通过反演变换求得最终时域中的解.但是,由于界面裂纹场存在着振荡奇异性,弹性问题解的形式就已经非常复杂,对应的粘弹性问题要通过反演变换直接求得准确的解析解几乎是不可能的.本文在利用对应性原理时做了更简单的准静态处理,即将弹性结果中的材料参数用粘弹性材料参数做对应替代,得到了粘弹性界面裂纹场近似的经典解,并与有限元分析结果作了比较.同时,利用Comninou接触模型,对粘弹性界面裂纹在远场拉剪混合加载情况下的裂尖应力场和接触区做了考察,并与经典解作了比较.     

黏弹性层合圆柱壳的临界轴压分析

基于经典屈曲理论,研究了轴向受压黏弹性复合材料层合圆柱壳的临界屈曲载荷. 利用Boltzmann线性积分型本构关系描述铺设单层的各向异性黏弹性行为. 结合解析与数值 方法,由Donnell型屈曲控制方程以及边界条件的Laplace变换导出相空间的特征方程,根 据Laplace逆变换的极值定理,获得层合圆柱壳的瞬时弹性临界载荷与持久临界载荷. 针对 多组铺设方式,通过数值算例重点分析了临界载荷随铺设角的变化特征,两种临界载荷的峰 值点差异程度与铺设方式、几何参数以及材料类型的关系,得到了一些对黏弹性层合圆柱壳 的优化设计有参考价值的结论.     

PEG‐Biscyanoacrylate Crosslinker for Octyl Cyanoacrylate Bioadhesive

PEG400 (polyethylene glycol, M_W 400) biscyanoacrylate is synthesized and copolymerized with 2‐octyl cyanoacrylate for potential use as bioadhesive. PEG400 biscyanoacrylate is synthesized from the esterification of anthracenyl cyanoacrylic acid where the anthracene unit serves as vinyl‐protecting group. Copolymerization increases the plasticity, mechanical strength, and resilience of the resulted polymer as determined by dynamic mechanical analysis. Peeling test confirms its superior bioadhesive properties. Surface morphology is characterized by SEM imaging. The formulations are cytocompatible and safe. This cyanoacrylate composition may provide improved bioadhesive cyanoacrylates.