物理老化对玻璃态高聚物非线性蠕变行为的影响

在不同应力水平下对经历不同老化时间的有机玻璃(PMMA)进行常温蠕变测试,分析物理老化和应力对材料蠕变柔量函数的影响.分析表明,老化时间对PMMA蠕变行为的影响满足流变简单性规律,即不同老化时间的蠕变柔量曲线可以沿对数时间轴平移而叠加到参考曲线上.取最长的老化时间为参考状态,依时间-老化时间等效原理,得到了各应力水平下的蠕变柔量主曲线.老化移位因子与老化时间在双对数坐标图上呈现线性关系,其负斜率就是老化移位率.结果表明,老化移位率随应力的增高而减小.     

非线性粘弹体的时间-温度-应力等效原理及其应用

考虑粘弹性材料特征时间的应力相关性,认为应力水平与温度、压力、溶剂浓度、损伤及老化等材料特征时间的影响相似,有其等效性。依自由体积理论,推导了温度－应力移位因子的表达式,提出了时间－温度－应力等 效原理。应用此原理,可以将不同温度和应力水平下的儒变曲线移位成某一参考温度和参考应力水平下的主曲线,从而可以通过较高温度和应力水平下的短期蠕变行为来预测较低温度和应力水平下的较长期的蠕变行为。实例分析了高密度聚乙烯（HDPE）非线性蠕变行为的－应力等效性。     

应用自洽方法预测沥青混合料的蠕变行为

将沥青混合料看作是由沥青砂和粗骨料组成的两相复合材料,提出了应用自洽方法预测沥青混合料粘弹性性能的一般思路.采用四参量流变模型描述沥青砂的粘弹性能,通过 Laplace 变换将时间域的粘弹性问题转化为变换域的线弹性问题,然后经 Laplace 逆变换将变换域的解转化到时间域.利用不同温度、不同应力水平下沥青砂的单轴压缩蠕变实验,拟合四参量流变模型参数,预测了沥青混合料在不同温度和不同应力水平下的蠕变曲线,并与实验结果进行了比较,发现蠕变的早期阶段预测值与实验值基本符合.最后进一步分析了温度、应力水平对沥青混合料蠕变行为的影响.     

岩石三轴蠕变试验黏弹性解析及参数识别

基于岩石三轴蠕变试验,推导了相应的黏弹性解析解,并对三参量H-K流变模型给出 了黏弹性解析解的显式以及流变参数的识别方法. 在此基础上,针对花岗岩及片岩两种岩石 的室内蠕变分级加载试验曲线进行了流变参数的识别与反演. 分析表明,同一岩样在不同加 载级别下的弹性及黏弹性参数在量值上均有所不同,其中黏弹性模量和黏滞系数的量值随 载荷的增加出现损伤衰减,基于试验反演所获参数的解析蠕变曲线与试验蠕变曲线吻合较好.     

胶凝原油粘弹性的实验研究

用控制应力流变仪ＲＳ７５对新疆、胜利、长庆等胶凝原油进行了粘弹性实验,结果表明,胶凝原油具有典型的粘弹性固体特性,如具有蠕变／回复特性,以及复数模量│Ｇ＾＊│大,损耗角δ小的特性等,小振幅剪切振荡粘弹性参数,可反映原油不同状态下的流变结构特点。     

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．     

不同应力差下非线性蠕变研究

深部工程处于复杂的环境场(应力场、温度场和渗流场)之中,引起的地下工程围岩流变破坏事故屡见不鲜。本文通过三轴蠕变实验,根据岩体流变力学和蠕变原理对影响深部岩体蠕变的因素进行了研究。用实验数据进行数值计算,得出了深部软岩巷道围岩在高地应力作用下的非线性蠕变模型,建立了应力差-时间-蠕变的定量关系,利用MATLAB软件得到了不同情况下的蠕变-时间曲线;应力差-时间-蠕变曲面。从而验证了非线性流变理论,找到了引起蠕变的主要因素应力差。并利用应力差理论分析了深部巷道围岩的蠕变破坏机理。     

基于CDM的黏弹性薄板压屈失效分析

研究黏弹性薄板的压屈失效问题.基于连续介质损伤力学与Schapery对应原理,得到耦合各向异性蠕变损伤的黏弹性本构模型,构造板的分层模型以考虑损伤演化的空间特征,建立具初挠度板的非线性压屈控制方程与边界条件,并进行空间与时间域上的数值离散和迭代求解.计算结果与实验曲线的对比表明本文的分析方法是有效的,黏弹性材料的蠕变损伤将导致结构刚度不断削弱,在一定应力水平下,板发生变形快速增加的压屈失效.     

基于准点缺陷理论探索非晶合金蠕变机制

作为典型多体相互作用非平衡体系,如何明晰非晶合金多场耦合激励下变形机制,建立非晶合金变形行为、流动特性与微观结构特征本征关联始终是非晶合金力学性能的重要研究内容.本文以具有显著β弛豫行为的La_56.16Ce_14.04Ni_19.8Al₁₀非晶合金作为研究载体,通过开展宽温度应力窗口蠕变实验,着重考查了蠕变柔量、准稳态蠕变速率、特征弛豫时间、蠕变应力指数及蠕变激活能演变规律,系统研究了非晶合金蠕变行为与蠕变机制.基于准点缺陷理论分析了非晶合金蠕变行为由弹性向黏弹性及黏塑性逐步转变的过程,从微观结构演化角度构建了非晶合金蠕变行为完整物理图像.研究结果表明,非晶合金高温蠕变行为是一典型热力耦合、非线性过程,其潜在蠕变机制受控于温度、应力与加载时间.应力较低时,非晶合金蠕变机制对应于热激活单粒子流动.应力较高时,蠕变机制则对应于应力诱导局部剪切变形增强与温度诱导原子扩散等复杂耦合过程.非晶合金蠕变变形过程所涉及弹塑性转变源于非晶合金准点缺陷激活、微剪切畴经热力耦合激励形核长大、扩展与不可逆融合.     

人工冻结深部软岩单轴力学性能试验及蠕变模型1）

为了了解深部软岩在冻结条件下的单轴力学性能,以东北地区的原状泥砂岩为试验对象,利用自行研制的WDT-100型人工冻土试验仪器,对其进行不同温度下的人工冻土单轴抗压强度试验和单轴蠕变试验,得到泥砂岩单轴压缩应力-应变关系曲线,各温度下试样的单轴抗压强度以及蠕变曲线.单轴压缩试验结果表明：试样在给定温度和加载速率条件下,单轴压缩应力-应变关系曲线都有较为明显的屈服点,并且都在屈服点后,强度有所提高,出现硬化现象.单轴蠕变试验结果表明：单轴压缩蠕变曲线有非线性特征,单轴压缩蠕变的等时应力-应变曲线向应变轴靠拢;单轴压缩时蠕变模量随时间的增长而降低.最后采用遗传算法优化模型参数,得出泥砂岩蠕变经验方程.与试验结果对比,发现拟合情况较好.     

A new approach to model strain change of gelled waxy crude oil under constant stress

Deformation of gelled waxy crude oil with loaded stress is worthy of research for the flow assurance of pipelining system. A dispersion parameter was introduced to characterize the disruption degree of wax crystal structure in crude oil with shear action. Based on fractional calculus theory, a rheological model incorporating dispersion parameter was proposed to describe creep of gelled waxy crude. A discrete and numerical algorithm was proposed to solve the model. Combining with the experimental results of five kinds of waxy crude oil, the model parameters were regressed and found to change monotonously with test temperature. Multiple creep curves of gelled waxy crude oil at a certain temperature can be described with this model.     

Experimental study on yield behavior of Daqing crude oil

The yield behavior of Daqing crude oil was studied by means of multi-mode controlled-stress tests with a high-precision stress controlled rheometer. Two experiments of creep and constant stress loading rate were performed. The yield stress of gelled crude oil was dependent on the test conditions such as the stress loading time and the stress loading rate, but the yield strain did not change with test conditions. The yield strain did exist objectively and it can be used as the criterion for the yielding of the structure of gelled crude oil. The yield strain of gelled oil was studied through experiments of creep, constant stress loading rate, oscillatory shear stress increase, and constant shear rate. The yield strain of gelled crude oil decreased with the increasing gel strength. The experiment of constant speed increase of stress showed the strain softening phenomena for low gel strength oil.     

Time-temperature-stress equivalence and its application to nonlinear viscoelastic materials

Stress-dependence of the intrinsic time of viscoelastic materials is investigated. The influence of stress level on the intrinsic time is considered to be similar to that of temperature, pressure, solvent concentration, damage and physical aging. The time-temperature-stress equivalence principle is proposed, by employing which, the creep curves at different temperatures and stress levels can be shifted into a master curve at reference temperature and stress level. Thus the long-term creep behavior of viscoelastic materials at a lower temperature and stress can be predicted from the short-term one at a higher temperature and stress. As an example, the nonlinear creep behavior of high-density polyethylene (HDPE) at room temperature is studied using the time-temperature-stress equivalence principle presented. Project supported by the National Natural Science Foundation of China (No. 19632030, 50003005) and by the Education committee of Human Province (No. 99C122).     

Gelation of waxy crude oils by ultrasonic and dynamic mechanical analysis

A novel technique, based on low intensity ultrasonic wave propagation, has been applied to investigate the gelation of a waxy crude oil, caused by the crystallization of paraffin fractions as the temperature reduces below a threshold value, called WAT (wax appearance temperature). Because this phenomenon significantly affects the rheological behavior of crude oils, the knowledge of the conditions under which it occurs, during oil storage and/or transportation, is a topical issue in the oil industry. In this work, an ultrasonic equipment has been set up, able to propagate longitudinal waves in the MHz range and to display in real time the behavior of ultrasonic velocity and attenuation when the crude oil sample is subjected to heating and cooling cycles. When the ultrasonic probes alternatively rotate as parallel plates of a conventional rheometer, low intensity longitudinal waves (in the megahertz range), and shear oscillations (in the hertz range) are simultaneously applied on the sample, thus widely broadening the frequency range of investigation. On cooling, the crystallization of paraffin fractions and the consequent formation of a network structure in the oil matrix are responsible of the development and growth of the crude oil elastic response, which becomes dominant over the viscous response. This process can be reliably detected by dynamic mechanical analysis and by ultrasonic analysis through the increase of the storage modulus G′ and longitudinal velocity, respectively. The growth and further association between wax crystals causes a dissipation of acoustic energy, which is indicated by the increase of the wave attenuation. The combination of rheological and ultrasonic methods has provided a better insight both on the gel transition of crude oils and the viscoelastic behavior of gelled samples. The ultrasonic wave propagation has demonstrated to be a powerful tool for monitoring the sol–gel transition in waxy crude oils. Finally, the effect of ultrasonic waves with different intensity on the gel build-up has been also evaluated. A reduction of the gel strength with increasing wave intensity has been observed and the recovery of elastic response after removing ultrasonic irradiation has been monitored. This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006.     

Understanding thixotropic and antithixotropic behavior of viscoelastic micellar solutions and liquid crystalline dispersions. I. The model

A simple model consisting of the Upper Convected Maxwell constitutive equation and a kinetic equation for destruction and construction of structure, first proposed by Fredrickson in 1970, is used here to reproduce the complex rheological behavior of viscoelastic systems that also exhibit thixotropy and rheopexy under shear flow. The model requires five parameters that have physical significance and that can be estimated from rheological measurements. Several steady and unsteady flow situations were analyzed with the model. The model predicts creep behavior, stress relaxation and the presence of thixotropic loops when the sample is subjected to transient stress cycles. Such behavior has been observed with surfactant-based solutions and dispersions. The role of the characteristic time for structure built up, λ, in the extent and shape of the thixotropic loops is demonstrated.     

黏弹性薄板蠕变屈曲的载荷-时间特性研究

通过对黏弹性薄板压屈的稳定性分析,着重讨论了蠕变屈曲载荷－时间的特性,理论分析表明：黏弹性薄板蠕变屈曲与材料的力学性能密相关,屈曲载荷不像弹性薄板为一定值,而是与时间相关的;在一定载荷下,经过一段时间后出现延迟屈曲,相关的实验研究也证实这一重要结论;这种延迟失稳问题在工程中有其重要的意义。     

A MICROMECHANICAL FRAMEWORK WITH AGGREGATE-MASTIC INTERFACE EFFECT FOR PREDICTING UNIAXIAL COMPRESSION CREEP OF ASPHALT MIXTURE

According to the elastic-viscoelastic correspondence principle, an elastic microme- chanical framework taking the inclusion-matrix interface effect into account is extended for predicting viscoelastic properties of asphalt mixture, which is simply treated as elastic coarse aggregate inclusions periodically and isotropically embedded in a viscoelastic asphalt mastic matrix. The Burgers model is adopted for characterizing the matrix mechanical behavior, so that the homogenized relaxation modulus of asphalt mixture in compression creep is derived. After a series of uniaxial compression creep tests are performed on asphalt mastic in different temperature and stress conditions in order to determine the matrix constitutive parameters, the framework presented is validated by comparison with the experiment, and then some predictions of uniaxial compression creep behavior of asphalt mixture in different temperature and stress conditions are given.