92负刚度蜂窝结构压缩性能分析

为准确开展以黏弹性材料为载体的负刚度蜂窝结构数值模拟研究,提出一种基于黏弹性广义Maxwell模型的负刚度蜂窝结构压缩性能数值模拟法。进行了尼龙12的动态机械分析(DMA)测试,基于广义Maxwell模型拟合实测动态模量数据,得到反映尼龙12动态黏弹性的无量纲模量g_i和松弛时间τ_i。建立了负刚度蜂窝结构的有限元模型,基于实测的动态黏弹性参数,进行压缩性能的数值模拟研究,并同压缩试验结果进行比较,验证了数值模拟的准确性,利用数值模拟研究几何参数对结构压缩性能的影响规律。结果表明,基于黏弹性广义Maxwell模型的负刚度蜂窝结构数值模拟分析法可较准确模拟结构的压缩性能,为预测负刚度蜂窝结构的力学性能提供帮助。     

采用间接拉伸蠕变试验评价沥青混合料低温抗裂性能

为深入评价沥青混合料的低温抗裂性能,对3种(粗、中和细)级配AC-20沥青混合料进行了3个温度(0,-10和-20℃)条件下的间接拉伸低温蠕变试验。根据幂函数模型计算得到了沥青混合料的低温蠕变柔量,并基于时-温等效原理,以-10℃为参考温度建立了幂函数模型蠕变柔量主曲线,分析了沥青混合料的低温蠕变性能和低温抗裂性能。结果表明,温度在0℃时,中间级配混合料的低温抗裂性能最好,而随着温度降低到-20℃时,粗级配混合料的低温抗裂性能变最好;采用幂函数模型能够较好地描述沥青混合料扩宽时间域的低温蠕变柔量主曲线,其参数幂指数m能较好地表示沥青混合料的低温松弛能力,在宽时间域内,细级配混合料的低温松弛性能增大,相应的其低温抗裂性也最好。     

沥青混合料低温临界开裂温度的确定

为了同时考虑沥青混合料在降温过程中温度应力的累积和松弛作用,确定临界开裂温度,对试件进行了线收缩系数试验,并利用间接拉伸试验确定其抗拉强度和蠕变柔量,由蠕变柔量和松弛模量的关系得到松弛模量的Prony系列表达式;由Boltzmann叠加原理,得到温度应力公式,计算出不同降温速率下产生的温度应力,根据低温抗拉强度曲线,确定出沥青混合料的临界开裂温度,并对结果予以验证。结果表明:该方法考虑了应力累积和松弛二者的综合作用,能够较好地反映沥青混合料的低温开裂特性,其计算结果与约束试件温度应力试验结果相近;该方法不仅适用于恒定降温速率,还适用于现场连续变速降温工况;随温度的降低或降温速率的增加沥青混合料内部温度应力累积速度加快,临界开裂温度随降温速率增加而升高。      

生物可降解材料PLGA的应力松弛性能

针对基于生物可降解材料聚乳酸-羟基乙酸共聚物(PLGA)的热压成型工艺仿真研究中,粘弹性材料模型参数缺乏的问题,通过单轴拉伸应力松弛试验,研究了生物可降解材料PLGA在5种温度状态下的应力松弛性能。基于时间-温度等效原理,平移得到60℃的松弛模量主曲线,并采用广义Maxwell模型对此主曲线进行拟合。结果表明:随着温度的升高,PLGA材料的松弛时间明显缩短,且拟合曲线与原数据误差较小,说明广义Maxwell模型能较理想地模拟PLGA材料的应力松弛行为。这些数据的获得为后续仿真研究提供了参考。     

沥青混合料热粘弹性本构关系试验测定及其力学应用

基于热粘弹性力学理论,就不同的温度条件下沥青混合料的应力松弛特征开展了试验研究,应用热流变简单材料的时温等效原理对试验结果进行了分析和参数拟合,根据试验结果建立了描述沥青混合料粘弹特性的广义Maxwell模型;通过理论推导提出了沥青混合料非定常和非均匀变温条件下增量型热粘弹性本构关系,在此基础上,给出应用本构关系进行沥青路面热粘弹性力学分析的数值实现方法;通过对TSRST试验的模拟,对得到的沥青混合料热粘弹性本构关系及其数值实现方法的合理性进行了验证,并给出一个工程计算实例。     

基于蠕变试验的沥青粘弹性损伤特性

通过BBR-弯曲梁流变仪在不同温度下进行沥青小梁蠕变试验,得到了不同温度下的蠕变柔量曲线。通过移位,按WLF公式推导得到了不同温度下的移位因子和各个温度下的蠕变柔量主曲线簇。用Weibull函数来描述沥青内部缺陷的分布,将Burgers粘弹性模型与连续损伤因子模型二者耦合建立了沥青的粘弹性损伤模型。理论模型和试验结果比较吻合,表明在沥青粘弹性性能评价过程中考虑损伤效应的影响是必要的。     

基于细观力学的纤维沥青混凝土有效松弛模量

为了研究纤维沥青混凝土的本构模型,将其视为以沥青混合料为粘弹性基体,纤维为弹性夹杂的两相复合材料。对基于复合材料细观力学理论建立的有效模量表达式进行了修正,提出了纤维沥青混凝土的割线有效松弛模量。以聚酯纤维沥青混凝土为例进行了有效松弛模量的解析分析和模拟蠕变实验的有限元分析,分析结果与试验数据的比较表明,该文提出的割线有效松弛模量模型对于纤维沥青混凝土粘弹性力学行为具有很好的预测能力。应用该模型对路面弯沉变形进行了有限元分析,结果表明:纤维的加入有效的改善了沥青混凝土路面的粘弹性性能。     

纤维增强复合材料黏弹性行为的预测模型

根据标准线性固体模型构造了一种预测纤维增强复合材料黏弹性行为的模型, 推导出该模型的本构方程与松弛模量和蠕变柔量表达式, 该模型经有限元仿真验证具有较高的精度。利用该模型研究了纤维几何特性对蠕变柔量和松弛模量的影响。结果表明, 复合材料蠕变柔量与纤维比长度呈线性关系, 而当纤维比半径增大到临界值后, 其变化对材料的松弛模量和蠕变柔量影响减小, 该临界值随纤维弹性模量的增大而减小; 当纤维模量与基体模量相差较大时, 复合材料的增强系数和减柔系数几乎不受时间变化的影响。      

基于黏弹性理论的多聚磷酸改性沥青低温性能

基于黏弹性理论,对不同质量分数(0.5wt%,1.0wt%,1.5wt%,2.0wt%)的多聚磷酸改性沥青及基质沥青进行低温弯曲流变试验(BBR),结合Burgers模型对低温弯曲梁流变试验得到的蠕变数据进行非线性拟合,利用Burgers模型的四个参数(E_1、η_1、E_2、η_2分别是Maxwell和Kelvin模型中弹性模量和黏性系数)确定出低温性能指标,对多聚磷酸改性沥青低温性能进行分析。结果表明:多聚磷酸的添加使沥青中的黏性和弹性都得到了一定的改善,沥青中松弛时间减少,储存能减少,耗散能增加,应力松弛能力得到提高。在相同温度下,多聚磷酸改性沥青延缓了沥青进入蠕变稳定期的时间,但随着温度的降低,达到蠕变稳定期的时间缩短。多聚磷酸改性剂的添加可以提高沥青低温性能,且随质量分数的增加,低温改善效果更好;但随着温度的降低,多聚磷酸对沥青的低温改善程度减小,且不同质量分数对其影响的差异也减小,在-24℃以上适合使用多聚磷酸改性,而在-24℃以下掺入多聚磷酸改性沥青意义不大。     

沥青混合料动态模量预估模型研究进展

由于沥青路面结构受到车辆荷载、环境等因素的不断变化作用,它的实际工作状态与静态体系在材料性质等方面存在较大差距。因此,针对动态荷载作用下沥青路面结构的动态参数和动力特性的研究十分必要。沥青混合料动态模量是沥青路面设计的重要参数之一,通过室内试验测得沥青混合料不同温度、频率下的动态模量,然后绘制主曲线,可准确预测不同温度、频率及极端条件下沥青混合料的动态模量。然而,目前沥青混合料模量测试方法复杂、试验成本较高,因此寻求简便的方法获得或预估沥青混合料的模量成为近年来研究的热点。为此,已有众多学者针对沥青混合料动态模量预估模型进行了研究。典型的预估模型包括Witczak 1-37A模型、NCHRP 1-40D模型和Hirsch模型等,尽管这些模型是在大量试验数据的基础上拟合得到,但由于世界不同地区的材料、试验方法、环境条件等存在差异,致使三种经验预估模型在不同地区的适用性也不尽相同。与此同时,随着计算机技术的发展,研究人员逐渐从微观角度建立模型来预测混合料的动态模量,而在沥青混合料数值模拟方面,离散元法(DEM)具有其他数值模拟方法无可比拟的优势。在典型预估模型的适用性方面,国外针对沥青混合料动态模量的预估做了大量研究,对几种典型的动态模量预估模型的适用性进行了系统分析,并提出了具体的修正模型。但目前我国在预估模型方面的研究只集中于个别省份和地区,而全国范围内沥青混合料动态模量的综合测试较少,缺乏有效的预估模型基础数据,因此针对我国不同地区沥青混合料动态模量的预估模型有待进一步深入研究。在沥青混合料细观模型方面,基于CT图像处理技术的离散元仿真试验,可建立以真实试件为依据的虚拟几何模型,其中集料形状、分布以及空隙特征都可与实际情况一致,从而进行良好的虚拟仿真试验。本文归纳了沥青混合料动态模量预估模型的研究进展,分别对沥青混合料动态模量预估模型以及各预估模型在不同地区的适应性进行了分析,并介绍了基于DEM的动态模量预测方法。最后,分析了沥青混合料动态模量预估模型研究面临的问题并展望了其应用前景,以期为沥青混合料动态模量预估模型在我国沥青路面设计中的研究和应用提供参考。     

Universal linear viscoelastic approximation property of fractional viscoelastic models with application to asphalt concrete

The paper presents a comprehensive linear viscoelastic characterization of asphalt concrete using fractional viscoelastic models. For this purpose, it is shown that fractional viscoelastic models are universal approximators of relaxation and retardation spectra. This essentially means that any spectrum can be mathematically represented by fractional viscoelastic models. Characterization of asphalt concrete is performed by constructing the dynamic modulus master curve and determining the parameters of the generalized fractional Maxwell model (GFMM). This procedure is similar to the widely used one of determining the master curve of asphalt concrete using a statistical function such as the sigmoidal model. However, from the GFMM, the relaxation modulus, creep compliance, continuous relaxation spectrum, and Prony series parameters can be determined analytically. A further advantage of the GFMM is that unlike the sigmoidal model, which only gives a representation of either the dynamic modulus or the storage modulus, the GFMM gives a representation of both the storage modulus and loss modulus (and therefore also the dynamic modulus and phase angle). The procedure was successfully applied to ten different mixes used in the State of Virginia.     

Continuous relaxation and retardation spectrum method for viscoelastic characterization of asphalt concrete

This paper presents a simple and practical approach to obtain the continuous relaxation and retardation spectra of asphalt concrete directly from the complex (dynamic) modulus test data. The spectra thus obtained are continuous functions of relaxation and retardation time. The major advantage of this method is that the continuous form is directly obtained from the master curves which are readily available from the standard characterization tests of linearly viscoelastic behavior of asphalt concrete. The continuous spectrum method offers efficient alternative to the numerical computation of discrete spectra and can be easily used for modeling viscoelastic behavior. In this research, asphalt concrete specimens have been tested for linearly viscoelastic characterization. The linearly viscoelastic test data have been used to develop storage modulus and storage compliance master curves. The continuous spectra are obtained from the fitted sigmoid function of the master curves via the inverse integral transform. The continuous spectra are shown to be the limiting case of the discrete distributions. The continuous spectra and the time-domain viscoelastic functions (relaxation modulus and creep compliance) computed from the spectra matched very well with the approximate solutions. It is observed that the shape of the spectra is dependent on the master curve parameters. The continuous spectra thus obtained can easily be implemented in material mix design process. Prony-series coefficients can be easily obtained from the continuous spectra and used in numerical analysis such as finite element analysis.     

Characterization of linear viscoelastic properties of asphalt concrete subjected to confining pressure

This paper presents a triaxial storage modulus master curve model and a continuous relaxation spectrum model for characterizing the linear viscoelastic (LVE) properties of asphalt concrete subjected to confining pressure. The triaxial master curve model relates the reduced storage modulus to the reduced frequency by performing both horizontal and vertical shifting on storage modulus test results. The horizontal shifting is used to characterize the time- and temperature-dependent behavior of asphalt concrete, while the vertical shifting is to characterize the pressure-dependent behavior. The vertical shift factor has the form of a sigmoidal function and varies with the reduced frequency and confining pressure. A relaxation spectrum model as well as a long-time equilibrium modulus model has been derived from the triaxial master curve model via integral transforms of basic LVE equations and complex algebra. The model predictions agree well with the laboratory test results of three asphalt mixtures indicating the proposed models can accurately characterize the LVE behavior of asphalt concrete under confinement.     

湿度对玻璃微珠增强硬质聚氨酯复合泡沫塑料黏弹力学性能影响

实验研究了温度与湿度因素对玻璃微珠增强硬质聚氨酯复合泡沫塑料(玻璃微珠/RPUF)黏弹力学性能的影响。结果表明, 湿度对黏弹力学性能有显著的影响, 随着湿度的增加, 玻璃微珠/RPUF的储能模量减小, 刚度下降, 力学损耗因子值增加, 蠕变柔量增大, 柔韧性增强。湿度对玻璃微珠/RPUF弯曲蠕变性能的影响具有类似于时间-温度等效原理的等效关系, 并给出了60 ℃/50%RH参考温湿度下玻璃微珠/RPUF的蠕变主曲线和平移因子。     

Viscoelastic Characterisation of the Temporomandibular Joint Disc in Bovines

Abstract: The aim of this paper was to develop a biomechanical model to reproduce the viscoelastic behaviour of the material of the bovine temporomandibular joint (TMJ) disc, as a substitute material to be applied in subsequent experimental studies of possible human TMJ disorders. As the mechanical properties of the disc change due to different factors, this study is focussed on evaluating its viscoelastic response under compression in stress–relaxation, creep and cycling loading tests, using an equipment that allows us to simulate the real function conditions of the material in a liquid medium at 37 °C. In order to fit the data obtained from the relaxation and creep tests, generalised Maxwell and Kelvin models of eight terms are proposed, leading to Prony’s series of practical interest for subsequent numerical calculations.     

Micromechanical finite element framework for predicting viscoelastic properties of asphalt mixtures

A micromechanical finite element (FE) framework was developed to predict the viscoelastic properties (complex modulus and creep stiffness) of the asphalt mixtures. The two-dimensional (2D) microstructure of an asphalt mixture was obtained from the scanned image. In the mixture microstructure, irregular aggregates and sand mastic were divided into different subdomains. The FE mesh was generated within each aggregate and mastic subdomain. The aggregate and mastic elements share nodes on the aggregate boundaries for deformation connectivity. Then the viscoelastic mastic with specified properties was incorporated with elastic aggregates to predict the viscoelastic properties of asphalt mixtures. The viscoelastic sand mastic and elastic aggregate properties were inputted into micromechanical FE models. The FE simulation was conducted on a computational sample to predict complex (dynamic) modulus and creep stiffness. The complex modulus predictions have good correlations with laboratory uniaxial compression test under a range of loading frequencies. The creep stiffness prediction over a period of reduced time yields favorable comparison with specimen test data. These comparison results indicate that this micromechanical model is capable of predicting the viscoelastic mixture behavior based on ingredient properties.     

高聚物材料动态模量测量与分析

高聚物材料具有良好的阻尼结构特性,因此在减振降噪中发挥很大作用。高聚物材料的模量是复数,并且随着系统的频率和环境温度呈非线性变化。将某高聚物材料在动态热机械分析仪上进行动态力学行为实验,并对实验结果采用分数导数维模型进行拟合。结果表明,分数导数维Maxwell模型可以同时精确的拟合高聚物材料的存储模量和损耗模量随频率变化的曲线,而且其形式简单,统一,计算所需参数较少,研究结果为实际应用提供了参考依据。