用振动梁方法测量粘弹性材料的复模量和粘弹性参数

本文利用共振梁方法,通过对一点响应、相位滞后和共振频率的测量,可以获得粘弹性材料的复模量以及标准线性固体模型的三个参数,即松弛模量、松弛系数和蠕变系数.实验结果表明,实验原理正确,测试系统可靠而且简单实用.     

基于复数神经网络的粘弹性材料动态模量拟合方法

固体火箭发动机药柱粘弹性材料除具有弹塑性特性,还具有粘滞性,这一特性使得材料变形具有明显的时间效应,本构关系复杂,进行动态力学分析时,动态模量难以有效拟合．本文提出了一种基于(Levenberg-Marquardt, L-M)算法的复数神经网络拟合粘弹性材料动态模量的方法．通过广义Maxwell模型推导得到材料的动态模量表达式,以此构造未定网络参数为复数的神经网络,从而提供了一种输入、输出样本均为复数的神经网络解决方法．将实数L-M训练算法进行改进,衍生到复数领域,提出复数L-M训练算法．通过粘弹性材料实验,将实验数据时温等效转换,获得复数神经网络的训练及测试样本．通过对神经网络进行训练,实现粘弹性材料动态模量的高精度拟合．数值算例表明,与传统神经网络拟合方法相比,所提方法在训练速度和泛化能力方面都有其优越性．     

均匀化方法在粘弹性多层复合材料中的应用

主要研究了由各向同性线弹性加强体和各向同性线粘弹性基体组成的多层复合材料的问题,在已有的线弹性多层材料的均匀化方法的基础上,应用弹性一粘弹性对应原理,在Carson域中求解粘弹性多层材料的问题。通过Burgers模型表示线粘弹性基体材料,反演得到了多层材料的有效松弛模量和有效泊松比在时间域中的表达式,并且与实验结果和其他结果进行了比较。     

恒应变率下航空有机玻璃耗能模量温度谱的实验研究

通过动态实验和准静态实验测得了恒应变率下3~#航空有机玻璃(PMMA)在一定温度范围内的割线模量与温度的关系。通过付氏变换,转换成耗能模量与温度的关系,再与内耗变频温度谱进行了直接的比较,得到了两种实验条件下的实际对应关系,为非线性粘弹性高聚物的力学松弛谱中应变率敏感峰和耗能模量峰之间的关联提供了实验论证。文章还提出了温度负敏感性现象。     

介电流变弹性体的电致变形与电流变效应研究

介电弹性体(Dielectric Elastomer,DE)具有显著的电致变形效应,刚度无法调控是制约其应用的关键瓶颈问题.本文将具有电致变模量特性的电流变弹性体(Electrorheological Elas-tomer,ERE)与商用介电弹性体VHB4910层叠复合,并涂覆柔性电板,制备介电流变弹性体(Dielec...     

裂纹扩展稳定性的实验研究

对五种金属材料进行了裂纹扩展稳定性的实验研究,试件分别采用三点弯曲试件(3PB 试件)和紧凑拉伸试件(CT 试件).试验结果分别由载荷 P——位移Δ曲线斜率的变化,(dp)/(dΔ)——Δ曲线,和撕裂模量曲线验证“撕裂失稳准则”.实验表明该准则是非保守的,同时还表明材料撕裂模量曲线 T_(MAT)-Δ(?)有相当离散度.     

天然橡胶基磁流变弹性体的研制与表征

磁流变弹性体又称磁敏高弹体,是一种由高分子聚合物和磁性颗粒构成的新型智能材料,它的力学、电学、磁学等诸性能可以由外加磁场来控制,因此磁流变弹性体在舰船、振动控制等领域具有广泛的应用前景.但目前国际上研制的磁流变弹性体存在机械性能不够好和磁致效应不够强的问题,这制约了基于磁流变弹性体器件的设计和应用.为了制备出实用型磁流变弹性体,本文对其制备条件进行了研究,包括基体类型、预结构化时磁场强度和温度、增塑剂和磁性颗粒含量对磁流变弹性体磁致效应的影响.结果表明,以天然橡胶为基体的磁流变弹性体,在高于600mT外加磁感应强度下,当磁性颗粒含量为80%(质量比)时,剪切模量的相对增量达133%;而当磁性颗粒含量为90%时,剪切模量的绝对增量达4.5MPa.本文还对磁流变弹性体应用环境进行了实验研究,结果表明磁流变弹性体在小应变下显示出更强的磁致效应,而激励频率不改变材料的磁致模量.     

近似不可压缩粘弹性结构动力响应的有限元分析

采用近似不可压缩有限元方法,分析了粘弹性结构的动力响应.针对粘弹性材料近似不可压缩性质和准静态分析的局限性,基于Hamilton变分原理,推导出了一种计算近似不可压缩粘弹性结构动力响应的增量有限元方法,考虑不同工况和松弛模量,计算了固体火箭发动机药柱结构的动力响应.理论分析和算例表明该方法简单实用、通用性强,有重要的工程价值.     

一种适用于低体模量材料的被动围压SHPB实验设计

基于动态三轴被动加载实验技术,建立了一种可测量吉帕量级及以下低体模量材料压强-体应变关系的被动围压SHPB实验设计方法。在该实验设计中确定了样品、封装垫块、围压套管的尺寸以及尺寸间的匹配,并对实验压强进行了限制。通过比较传统SHPB实验和被动围压SHPB实验测量LC4铝合金等效应力的方式,验证了被动围压SHPB实验压强测量的有效性;通过实验和数值模拟分析,验证了体应变测量的有效性。将设计的被动围压SHPB实验方法应用于铈,得到了铈在伽马→阿尔法相变区间完整显示的压强-体应变演化信息,且相变起始和终止压强、相变体积变化量均与静高压实验结果基本一致。这说明设计的被动围压SHPB实验方法适用于测量低体模量材料的压强-体应变关系。     

不同拉压模量连续梁的解析解

拉压不同模量的材料在工程中应用很广,特别是近几年发展起来的复合材料都具有明显的拉压不同模量性质.本文对复杂应力状态下不同模量连续梁提出了中性轴判断定理,并用分段积分方法推导出不同模量结构的中性层计算表达式及应力的解析解.通过对实例的计算及分析,得出不同模量与经典力相同模量两种方法在结构应力计算上的差异,最后提出对该类结...     

复合材料动态粘弹性能的细观研究

利用细观力学的Eshelby等效夹杂方法研究了颗粒增强复合材料的动态粘弹性力学性能,分析了材料复模量随夹杂体积分数、载荷频率之间的变化规律,给出了许多有意义的结论,为复合材料结构的优化设计及应用提供了理论基础。     

On the Measurement of two Independent Viscoelastic Functions with Instrumented Indentation Tests

In the present paper, a methodology for complete characterization of linear isotropic viscoelastic material with spherical instrumented indentation test is proposed. The developed method allows for measuring two independent viscoelastic functions, shear relaxation modulus and time-dependent Poisson’s ratio, from the indentation test data obtained at non-decreasing loading, but otherwise arbitrary. Finite element modelling (FEM) is relied upon for validating the proposed methodology and for quantifying the influence of experimental variables on the measurements accuracy. Spherical indentation experiments are performed on several viscoelastic materials: polyoxymethylene, bitumen and bitumen-filler mastics. The viscoelastic material functions obtained with the indentation tests are compared with the corresponding results from the standard mechanical tests. Numerical and experimental results presented indicate that the methodology proposed allows mitigating the machine compliance and loading rate effects on the accuracy of the viscoelastic indentation tests.     

Measurements of Two Independent Viscoelastic Functions by Nanoindentation

Current nanoindentation measurement techniques normally assume that one material function (such as the Poisson's function) is a constant, and measures just one material function, such as the creep compliance in shear. For materials with significant viscoelastic effects and unknown viscoelastic functions, assuming a constant for one material function is not satisfactory. Accurate measurements require simultaneously determining two independent material functions. This paper provides a method to use nanoindentation to measure both bulk and shear relaxation functions. Two different nanoindenter tips, namely Berkovich and spherical indenters, are used for nanoindentation on polymers. Any two independent viscoelastic functions, such as bulk relaxation modulus and shear relaxation modulus, have different representations in the load–displacement curves obtained with these two indenters so that the two independent viscoelastic functions can be separated and determined. Two polymers, poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) were used in nanoindentation. Nanoindentation measurements were conducted on PVAc above glass transition temperature (Tg) and on PMMA below Tg. Both shear and bulk relaxation functions determined from nanoindentation were found in a reasonably good agreement with data obtained from conventional tests, providing validation of the method presented. The new method can be applied in measurements of two independent viscoelastic functions at sub-micron scale of very small amounts of materials such as polymeric films on a substrate, heterogeneous materials such as bones, tissues, and nanocomposites.     

仪器柔度标定误差对两种压入测试方法的精度影响

针对目前最具代表性的两种压入测试方法:Oliver-Pharr方法和Ma方法,通过有限元数值模拟分析了仪器柔度标定误差对测试精度的影响.结果表明仪器柔度的标定精度直接影响压入测试结果的准确度,尤其是当材料较软且压入深度较大时更为显著;对同一材料,压入载荷越大,由仪器柔度标定误差引入的压入测试结果误差越大;在同一压入深度下,针对不同材料,由仪器柔度标定误差引入的压入测试结果误差差别不大;就测试方法而言,Ma方法具有比Oliver-Pharr方法更高的精度和更低的仪器柔度敏感性.     

Influence of excitation amplitude on the characteristics of nonlinear butyl rubber isolators

The purpose of this study is to explore the advantages and characteristics of nonlinear butyl rubber (type IIR) isolators in vibratory shear by comparison with linear isolators. It is known that the mechanical properties of viscoelastic materials exhibit significant frequency and temperature dependence, and in some cases, nonlinear dynamic behavior as well. Nonlinear characteristics in shear deformation are reflected in mechanical properties such as stiffness and damping. Furthermore, even when the excitation amplitude is small the response amplitude may often be large enough that nonlinearities cannot be ignored. The treatment involves developing phenomenological models of the effective storage modulus and effective loss factor of a rubber isolator material as a function of excitation amplitude. The transmissibility of a nonlinear viscoelastic isolator is compared with that of a linear isolator using an equivalent linear damping coefficient. Forced resonance vibration and impedance tests are used to characterize nonlinear parameters and to measure the normalized transmissibility. It is found that as the excitation amplitude of the nonlinear viscoelastic isolator increases, the response amplitude decreases and the transmissibility is improved over that of the linear isolator for excitation frequency that exceeds a particular value governed by the temperature and excitation amplitude. The method of multiple scales and numerical simulations are used to predict the response characteristics of the isolator based on the phenomenological modeling under different values of system parameters.     

Measurement of the viscoelastic properties of bituminous materials using an oscillating needle technique

An embedded oscillating needle is used to measure the dynamic viscoelastic properties of a stiff bituminous material. A Micro-Fourier Rheometer was used to cause the embedded needle to undergo pseudorandom small amplitude oscillations in the axial direction with measurement of the instantaneous resistance force. The phase and magnitude of the force signal are used to calculate the storage and loss moduli. A theoretical framework for this technique is developed from the Mindlin solution coupled with slender body theory, and the correspondence principle of linear viscoelasticity. Experiments are performed on neat bitumen binders as well as mixtures of glass spheres in bitumen; the results show that the presence of the glass spheres dramatically increases the viscoelastic response functions. The results agree reasonably well with those obtained using the parallel plate squeezing mode. Received: 31 March 1999 Accepted: 14 July 1999     

微振激励下黏弹性阻尼器微观链结构力学模型

减小微振动对高精密仪器至关重要,利用黏弹性阻尼器进行微振动抑制是一个新兴而又具有挑战性的课题.本文采用分子链网络模型方法分析了黏弹性材料的微观分子链结构,综合考虑材料分子链结构中的网络链和自由链对黏弹性材料力学性能的影响,提出一种基于材料微观分子链结构的微振激励下黏弹性阻尼器力学模型.模型分别采用标准线性固体模型和Maxwell模型来描述网络链和自由链中单个链的力学性能,并分别采用8链网络模型和3链网络模型考虑两种类型分子链的综合效应,引入温频等效原理描述温度对微振激励下黏弹性阻尼器力学性能的影响.该模型能够描述温度和频率对黏弹性阻尼器动态力学性能的影响,并能够反映黏弹性材料的微观结构与材料力学性能的关系.为验证所提模型的有效性及考察黏弹性阻尼器在微振激励下的耗能能力和动态力学性能,在微振条件下对黏弹性阻尼器进行了动态力学性能试验.研究结果表明黏弹性阻尼器具有较好的微振耗能能力,其动态力学性能受温度和频率影响较大,所提的力学模型能够精确地描述微振激励下黏弹性阻尼器动态力学性能随温度和频率的变化关系.      

Isothermal viscoelastic properties of PMMA and LDPE over 11 decades of frequency and time: a test of time–temperature superposition

Many instruments used to measure viscoelastic properties are only capable of subjecting a sample to a limited range of loading frequencies. For thermorheologically simple materials, it is assumed that a change in temperature is equivalent to a shift of the viscoelastic behavior on the log frequency or time axis. For many materials, time–temperature superposition appears to work well for modulus or compliance curves over three decades of time or frequency, but some deviations are known if the window is expanded to five or six decades. To apply a more stringent test of the validity of time–temperature superposition, broadband viscoelastic spectroscopy is used to isothermally study polymethylmethacrylate and low-density polyethylene at several temperatures in the glassy region. Shear modulus and damping (tan δ) are measured isothermally over a wide range (up to 11 decades) of time and frequency. Results indicate that, while modulus curves can be approximately superimposed, the damping (tan δ) curves change in height and shape with temperature.     

Separating viscoelasticity and poroelasticity of gels with diferent length and time scales

Viscoelasticity and poroelasticity commonly coexist as time-dependent behaviors in polymer gels.Engineering applications often require knowledge of both behaviors separated;however,few methods exist to decouple viscoelastic and poroelastic properties of gels.We propose a method capable of separating viscoelasticity and poroelasticity of gels in various mechanical tests.The viscoelastic characteristic time and the poroelastic difusivity of a gel define an intrinsic material length scale of the gel.The experimental setup gives a sample length scale,over which the solvent migrates in the gel.By setting the sample length to be much larger or smaller than the material length,the viscoelasticity and poroelasticity of the gel will dominate at diferent time scales in a test.Therefore,the viscoelastic and poroelastic properties of the gel can be probed separately at diferent time scales of the test.We further validate the method by finite-element models and stress-relaxation experiments.