岩石非定常Burgers蠕变模型及其参数识别

针对岩石蠕变的非线性特征,提出了一种非线性黏壶元件,并分别替换Burgers模型中的两个线性黏壶元件,从而建立了一种非定常参数Burgers模型。推导了非定常Burgers模型的一维蠕变方程,分析模型参数λ₁与λ₂取值范围对蠕变方程的影响,从理论上确定了非定常模型能够描述蠕变的3个阶段。在此基础上,将非定常Burgers模型的蠕变方程推广至三维应力状态下,并基于Levenberg-Marquardt算法对向家坝岩石的三轴压缩蠕变试验曲线进行拟合及参数识别。通过对比定常与非定常Burgers模型的拟合曲线与相关系数,可以发现非定常Burgers模型拟合效果更好,且能够准确描述包括加速蠕变在内的岩石三阶段蠕变特性,验证了所构建非线性模型的适用性与合理性。     

中等时间尺度下弹道明胶的线性黏弹性模型

在不同时间尺度下,明胶表现出不同的黏弹性行为。应用蠕变和应力松弛模式研究质量分数为10%的弹道明胶在中等时间尺度下的线性黏弹性行为。低剪切速率下的连续加载-卸载实验显示,在常规感知的时间尺度下(～1min)弹道明胶接近完全弹性体,其线性区应变范围为0～0.25。对蠕变实验对应的中等时间尺度(1h),选择样品老化24h作为实验起点,这时明胶结构缓慢老化带来的影响可以忽略,从而能够构建恒定参数的本构方程来描述弹道明胶的线性黏弹性行为。对1组蠕变实验结果进行分析,可确定Burgers模型的4个粘弹性参数,然后使用该组参数的模型预测另外2组蠕变实验以及应力松弛实验。结果表明Burgers模型能较好地表征弹道明胶在中等时间尺度下的线性黏弹性行为。     

聚丙烯打包带蠕变特性研究

采用四元件Burger模型对常温下聚丙烯(PP)打包带的拉伸蠕变特性进行了研究。结果表明:Burger模型可以用于模拟PP打包带的短期蠕变行为;瞬时弹性变形系数与应力水平无明显关系;Kelvin模型中延时弹性变形系数和粘性系数以及Maxwell模型中的粘性系数均随应力水平的增加而下降;松弛时间和延迟时间均随应力的增加而下降;拉伸蠕变速率随应力上升而急剧上升,该结论与滞后时间的分析一致。     

玻纤增强聚丙烯复合材料的应变率敏感特性

采用层合热压实验法将玻璃纤维基毡与聚丙烯薄膜复合制成不同玻纤含量的玻纤增强聚丙烯(GF/PP)复合板材,在不同的加载速率下进行拉伸测试,研究GF/PP复合材料的应变率敏感特性,分析Burgers模型对该材料本构关系拟合预测的可行性。结果表明:GF/PP复合材料在低应变率范围内对应变率是敏感的,随应变率的增加,其断裂应力和抗拉强度增大;随玻璃纤维含量的增加,其所对应的应变率效应反而有所下降。同时,Burgers模型能够有效地拟合预测出该材料的拉伸应力-应变曲线,与实验曲线相比,进一步验证了GF/PP复合材料的应变率敏感特性及其变化趋势。     

中等时间尺度下弹道明胶的线性黏弹性模型EI北大核心CSCD

在不同时间尺度下,明胶表现出不同的黏弹性行为。应用蠕变和应力松弛模式研究质量分数为10%的弹道明胶在中等时间尺度下的线性黏弹性行为。低剪切速率下的连续加载-卸载实验显示,在常规感知的时间尺度下(～1min)弹道明胶接近完全弹性体,其线性区应变范围为0～0.25。对蠕变实验对应的中等时间尺度(>1h),选择样品老化24h作为实验起点,这时明胶结构缓慢老化带来的影响可以忽略,从而能够构建恒定参数的本构方程来描述弹道明胶的线性黏弹性行为。对1组蠕变实验结果进行分析,可确定Burgers模型的4个粘弹性参数,然后使用该组参数的模型预测另外2组蠕变实验以及应力松弛实验。结果表明Burgers模型能较好地表征弹道明胶在中等时间尺度下的线性黏弹性行为。     

重组竹顺纹受压蠕变性能及预测模型

为研究重组竹顺纹受压蠕变性能,对12个重组竹受压试件进行了蠕变试验,通过分析蠕变增长率和Burgers模型中各参数,得到重组竹受压试件受压蠕变变化规律。结果表明,重组竹在持续受压荷载作用下,应力水平较低时,蠕变变形发展稳定,不会发生承载力破坏;应力水平较大时(达到60%极限载荷水平),将会直接导致重组竹受压屈曲破坏,蠕变变形呈非稳定性增长;重组竹受压蠕变应变增长率随时间增加而逐渐增大,其增长速率则呈减速状态,受压蠕变的应变增长率随应力比增加而增大。基于试验结果,分析了重组竹受压蠕变试验的弹性变形、黏弹性变形和黏性变形的变化规律,对不同荷载水平的Burgers模型参数进行确定,并进一步推导了Burgers模型参数随应力水平的变化规律。基于此,建议考虑荷载水平的重组竹受压蠕变预测模型,模型预测结果与试验曲线一致。      

基于Burgers模型对缓冲材料EPE压缩蠕变行为的分析

目的 为解决缓冲材料发泡聚乙烯因蠕变引起保护失效的问题,基于Burgers模型,分析载荷、应力保持时间对EPE压缩蠕变行为的影响。方法 根据不同应力下的不同保持时长压缩蠕变试验结果,拟合EPE不同应力下的压缩蠕变试验数据,分析不同应力、应力保持时间下的EPE蠕变过程中蠕变总量、弹性应变、黏弹性应变和黏性应变的变化规律。结果 Burgers四元件力学模型对EPE不同应力下的压缩蠕变实验数据拟合度为0.992 4～0.998 9。在同一应力保持时间下,随着应力从3.3 kPa增加到5.3 kPa,弹性模量E₁、弹性模量E₂、黏度系数η₂、黏度系数η1都逐渐减小,蠕变总量、黏性应变呈非线性显著增加,弹性应变、粘弹性应变呈线性微量增加;在同一应力下,随着应力保持时间从20 d增加到120 d,弹性应变一直保持不变,占蠕变总量的比例降低,粘弹性应变先微量增加后保持不变,占蠕变总量的比例降低,黏性应变增加,占蠕变总量的比例增加。结论 Burgers模型可较准确地模拟EPE不稳定蠕变阶段和稳定蠕变阶段的压缩蠕变行为;应力的增加导致EP...     

黏土蠕变非线性特性及其分数阶导数蠕变模型

针对黏土蠕变的非线性性质,以成都黏土为研究对象展开蠕变试验,发现黏土变形包括瞬时弹性变形、衰减蠕变变形、稳态蠕变变形和加速蠕变变形;黏土长期弹性模量随时间和应力的增加非线性软化;黏滞系数随应力的增加非线性软化,随时间的增加非线性硬化。基于流变学理论、分数阶微积分理论和Harris衰减函数,分别构建了分数阶导数元件、非线性弹性元件和非线性黏滞元件,从而建立了形式简单、参数较少和概念清晰的非线性分数阶导数蠕变模型。将非线性分数阶导数蠕变模型和Burgers蠕变模型进行对比拟合分析,发现非线性分数阶导数蠕变模型各阶段的拟合结果更好,对黏土非线性蠕变的描述更合理,可准确地反映黏土蠕变全过程,表明了所建立非线性分数阶导数蠕变模型的科学合理性。     

船用高分子材料压缩蠕变特性研究

研究了四种船用高分子材料的蠕变特性,分析了其压缩蠕变曲线特点,以及试验中弹性变形、粘弹性变形及粘性变形在总变形中所占比例,最后建立材料压缩蠕变的Burgers模型,得到了试验数据的拟合方程和拟合曲线。由实测曲线和拟合曲线可知,利用Burgers模型可以准确分析试验期间材料的蠕变性能,并能较好地预测材料后期的蠕变行为。     

沥青混合料非线性黏弹塑性蠕变模型

本研究采用非线性黏弹塑性蠕变模型,更好地来模拟沥青混合料蠕变全过程。该模型由Burgers模型和非线性黏塑性元件串联而成。通过对沥青混合料的单轴压缩蠕变实验,得到不同应力作用下应变随时间的变化规律以及不同温度条件下沥青混合料蠕变柔量随时间的变化规律。利用origin软件对实验数据进行拟合,得到非线性黏弹塑性蠕变模型的六个力学参数。模型模拟结果与实验结果一致,表明本文所建立的非线性黏弹塑性蠕变模型的正确与合理性。此模型可较好地描述沥青混合料加速蠕变阶段,弥补了Burgers模型的不足。     

New achievements in visco-elastoplastic constitutive model and temperature sensitivity of glasphalt

This paper focuses on the experimental investigation on temperature sensitivity and visco-elastoplastic behaviour of glasphalt. Classic Burgers creep model could only describe the viscoelastic behaviour of materials before the third creep-phase, so a viscoplastic string is added in series with classic Burgers model in order to predict the visco-elastoplastic behaviour of glasphalt. In this research, the effects of loading stress and temperature on creep behaviour of glasphalt under dynamic loading are investigated. In addition, some methods were used to solve model parameters and then predictions from proposed model were compared with experimental results. It was shown that creep testing curves coincided well with theoretic curves, validating that modified Burgers model can completely characterise creep behaviour of glasphalt. Besides, temperature sensitivity of glasphalt was evaluated by using indirect tensile stiffness modulus test, and stiffness modulus behaviour model of glasphalt was presented based on the experimental results and numerical analysis.     

Creep rupture and viscoelastoplasticity of polypropylene

Observations are reported on isotactic polypropylene in tensile tests with various strain rates, relaxation tests at various strains, and creep tests with various stresses at room temperature. Constitutive equations are derived for the viscoelastic and viscoplastic responses of semicrystalline polymers at three-dimensional deformations with small strains. The stress-strain relations involve eight material constants that are found by fitting the experimental data. The model is applied to the numerical analysis of creep failure of polypropylene under various deformation modes (uniaxial tension, equi-biaxial tension, shear, multiple-step creep tests).     

Analytical approach of mechanical behavior of carpet yarn by mechanical models

The viscoelastic models for describing mechanical behavior of polypropylene carpet yarn are presented in this paper. Since carpet yarn is subjected to tensile and frictional forces during tufting, the mechanical properties are important material features especially in vibration analysis. In this paper, the standard linear model and the standard nonlinear model are developed to simulate the polypropylene yarn tensile, stress relaxation and creep behaviors. The obtained formulas are fitted with experimental data and the ideal model parameters are determined by applying least squares. The results showed that the standard linear model fits well the tensile, relaxation and creep experimental curves, and will be employed to analyze the vibration characteristics of yarn during the tufting.     

聚丙烯木塑复合材料蠕变行为的模拟与预测

通过三点弯曲蠕变试验,研究了聚丙烯木塑复合材料在23℃左右室内环境五种应力水平下的短期蠕变行为,并采用四元件模型对其蠕变行为进行模拟。依据时间-温度-应力等效原理,以5MPa为参考应力水平,将其它四种应力水平下的蠕变曲线移位成5MPa应力水平下的主曲线,从而可以预测聚丙烯木塑复合材料在5MPa应力水平下大约3年内的蠕变行为。     

Fiber orientation dependence of continuous carbon/epoxy composites nonlinear viscoelastic behavior

The effect of fiber orientation on the nonlinear viscoelastic response of a unidirectional fiber polymeric system is examined. Based on Schapery’s nonlinear viscoelastic equation and a model for the prediction of the nonlinear parameters, the nonlinear behavior of carbon/epoxy composite off-axis specimens is studied in order to achieve a complete characterization of an orthotropic system. The stress dependence of the parameters of the nonlinearity, introduced in the model is achieved and applied through a generic function developed and already confirmed [Compos. Sci. Technol. 58 (6) (1998) 883; Composites Part A 30 (7) (1999) 839; Progress in Durability Analysis in Composites Systems Conference Proceedings, DURACOSYS 97, Blacksburg, Virginia, USA, 14–17 September, 1997, Balkema Publishers, pp. 271–278; Compos. Sci. Technol. 59 (9) (1999) 1311] in the isotropic case. Verification of the model in the case of orthotropy is performed through a series of creep tests in composite specimens at various fiber orientations. The estimated values indicate an increase in the nonlinearity as the applied stress increases, while satisfactory predictions of the nonlinear parameters can be achieved, using the proposed model.     

Thermo-viscoelastic responses of multilayered polymer composites: Experimental and numerical studies

This study presents experimental works and finite element (FE) analyses of nonlinear thermo-viscoelastic behaviors of multilayered (pultruded) composites under tension. Creep tests are conducted on E-glass/polyester composites having 0°, 45° and 90° off-axis fiber orientations at various temperatures and stresses. Isochronous creep curves show that the nonlinear stress–strain responses increase with time and temperature for composites tested at higher temperatures (75–125 °F) while there is no particular trend seen at lower temperatures (0–50 °F). A convolution integral model is used for the time–stress–temperature dependent responses. The nonlinear viscoelastic model is implemented in FE framework for analyzing responses of viscoelastic pultruded structures. Sensitivity analysis is conducted to examine error in measuring strains during experiments by simulating the creep tests using FE.     

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。     

正交铺设陶瓷基复合材料单轴拉伸行为

采用细观力学方法对正交铺设陶瓷基复合材料单轴拉伸应力-应变行为进行了研究。采用剪滞模型分析了复合材料出现损伤时的细观应力场。采用断裂力学方法、 临界基体应变能准则、 应变能释放率准则及Curtin统计模型4种单一失效模型确定了90°铺层横向裂纹间距、 0°铺层基体裂纹间距、 纤维/基体界面脱粘长度和纤维失效体积分数。将剪滞模型与4种单一损伤模型结合, 对各损伤阶段应力-应变曲线进行了模拟, 建立了复合材料强韧性预测模型。与室温下正交铺设陶瓷基复合材料单轴拉伸应力-应变曲线进行了对比, 各个损伤阶段的应力-应变、 失效强度及应变与试验数据吻合较好。分析了90°铺层横向断裂能、 0°铺层纤维/基体界面剪应力、 界面脱粘能、 纤维Weibull模量对复合材料损伤及拉伸应力-应变曲线的影响。