高温后聚丙烯纤维混凝土力学性能试验研究

采用液压伺服试验系统,对120块不同聚丙烯纤维掺量的混凝土立方体试件进行高温后力学性能试验研究.分析了不同纤维掺量,不同目标温度对混凝土峰值应变、弹性模量、抗压强度等力学性能的影响,此外还探讨了高温后聚丙烯纤维混凝土的表观形态.试验结果表明,高温作用后混凝土试件物理结构发生了很大的变化.随着温度的升高聚丙烯纤维混凝土与普通混凝土一样,弹性模量与抗压强度都呈下降趋势,并且在300oC以后显著降低,而峰值应变上升.在相同温度下,随着纤维掺量的增加混凝土试件的抗压强度与弹性模量下降,峰值应变升高.通过对试验数据的统计分析,建立了不同掺量下聚丙烯纤维混凝土的相对抗压强度随温度变化的关系式,为聚丙烯纤维混凝土在各种实际工程中的应用提供了一定的参考价值.     

沥青混凝土级配曲线的数值设计方法与对有效弹性模量影响的研究

沥青混凝土有效性能的预测和评估是其在道路工程与大坝防渗等应用中的关键问题。本文利用细观随机建模技术和有限元方法研究了级配曲线对沥青混凝土有效弹性模量的影响。首先,基于改进的蒙特卡洛法分别实现了服从工程筛分级配曲线或给定函数级配曲线的沥青混凝土细观随机模型的高效生成;然后,利用建立的模型分析了骨料的含量、弹性模量和试样尺寸对沥青混凝土的有效弹性模量的影响,并与实验结果进行对比,验证了所建模型的可靠性;从而,预测了在AC-13级配范围内级配曲线的凹凸程度,即粗细骨料的组成比例对沥青混凝土有效弹性模量的影响。结果表明,级配曲线的凹凸程度对其有效弹性模量影响存在较大差异,级配曲线的下凹(粗骨料占比增大)会导致有效弹性模量迅速增大,级配曲线的上凸(细骨料占比增大)也会引起有效弹性模量略有上升;另外提出的随机建模方法可实现对级配曲线,即对粗细骨料组成比例的定制设计.     

压力作用下锂渣混凝土氯离子渗透性研究

研究混凝土纤维掺量、掺合料种类和所受压应力大小等三种因素对混凝土抗氯离子渗透性能的作用,对于氯离子侵蚀环境中的混凝土结构耐久性设计有非常重要的意义。本文通过设计正交试验,将56d龄期混凝土试块置于能够给混凝土施加持续压力作用的特制混凝土氯离子渗透性能测试装置下进行氯离子电通量试验。试验结果表明:聚丙烯纤维的掺入可以提高混凝土抗氯离子渗透性能,且最佳纤维掺量为0.1%;掺合料对混凝土抗氯离子渗透性作用效应由强到弱依次为:锂渣锂渣粉煤灰复掺粉煤灰普通混凝土硅灰,锂渣的掺入能够明显改善混凝土抗氯离子渗透性能;施加压应力初期混凝土抗氯离子渗透性能得以增强,但压应力过大反而降低了混凝土抗氯离子渗透性能,有益于混凝土抗氯离子渗透性的压应力大小不能超过混凝土极限强度的50%。通过对正交试验结果进行极差分析发现:掺合料种类为影响混凝土渗透性的最大因素,纤维掺量与压应力大小对混凝土渗透性的作用效应相当。     

考虑老化的混凝土粘弹性分数导数模型

混凝土是一种具有分形结构的材料。采用分数微积分模型来研究具有分形结构材料的老化规律目前尚未见到。本文的目的是采用含分数阶导数的类标准线性体来模拟考虑老化的混凝土的蠕变和松弛规律。给出了分数导数与Abel核之间的关系。讨论了类标准线性体的蠕变柔量和松弛模量及其在考虑老化的混凝土中的应用。与传统的混凝土流变模型相比较表明，类标准线性体可以更好地同时拟合混凝土在不同龄期的蠕变和松弛曲线。而且其形式简单、统一，在计算过程中需要调整的参数很少。可以预见，类标准线性体在混凝土的结构设计和计算中将有着广泛的应用前景。     

毛竹纤维细胞壁的黏弹性力学性能研究

以毛竹纤维细胞壁(BFCW)为研究对象,运用纳米压痕技术对BFCW的蠕变性能和松弛性能进行了研究。通过设计不同的加载方式,得到了纳米压痕的载荷与压入深度的关系曲线;通过拟合不同应变率的实验结果数据,计算得出了BFCW的蠕变应力指数。研究了BFCW在不同压入深度、加载速率、保载时间下的松弛行为,分析了不同的载荷、加载速率、保载时间对BFCW蠕变行为的影响。结果表明:BFCW纵向和横向具有不同的力学性质,在纵向表现出更明显的松弛特性和更强的抗蠕变变形能力;BFCW的蠕变行为随着压入载荷的增大愈加明显,表现为蠕变位移和蠕变速率增大;BFCW纵向的压入深度和蠕变位移量均比横向的小,在最大压入载荷为15mN时,其差值分别达到了24.96%和32.25%;BFCW的松弛模量、载荷松弛量与压入深度呈正比;BFCW纵向的松弛能力比横向的强,在加载速率为50nm/s时纵向载荷松弛量较横向高34.58%。     

聚丙烯纤维加固土融沉特性的离散元数值模拟研究

针对掺入纤维对土体冻融特性的影响进行离散元数值模拟,首先自研了过程感知融化固结系统,基于融沉试验,提出了微观参数折减法模拟融化过程,分析了纤维掺量、纤维长度、土样高度的影响规律。结果表明：离散元方法可对融沉过程进行较为有效的模拟,并可得到最优纤维掺量,对于模拟工况,最优纤维掺量为0.1%;从融化压缩模量看,纤维长度与掺量为敏感影响因素,土样高度为次要因素;掺入纤维对土体强度与融沉特性的提升效果具有一致性。     

纤维混杂效应对混凝土弯曲韧性的改善

本文采用在C30混凝土中以不同的方式掺入玄武岩纤维与聚丙烯纤维,通过对8种单掺纤维混凝土试件和16种混杂纤维混凝土试件的弯曲性能进行试验研究,分析玄武岩纤维、聚丙烯纤维及其混杂纤维对混凝土弯曲力学特征的影响。试验结果表明,玄武岩纤维、聚丙烯纤维及玄武岩-聚丙烯混杂纤维的掺入对基体混凝土弯曲破坏时承受的最大荷载、破坏点挠度值和韧性指数均有很大的改善,其中,混杂纤维优于单掺纤维。混杂纤维对基体混凝土韧性指数提高最大达到18.1%。本文提出"纤维混杂效应函数"概念,并利用Matlab数据拟合出玄武岩-聚丙烯混杂纤维改善基体混凝土弯曲韧性的混杂效应函数。通过对混杂效应函数求极值,得到玄武岩-聚丙烯混杂纤维对基体混凝土弯曲韧性改善的最佳的体积掺加率,该值与试验数据基本一致。     

基于Riesz势空间分数阶算子的非局部粘弹性力学元件

将幂函数引入Eringen非局部线粘弹性本构,导出Riesz势形式的应力-应变关系。利用该关系,构造非局部弹簧和非局部阻尼器两类元件;利用元件的串联和并联,建立非局部Kelvin和非局部Maxwell粘弹性模型,推导模型的松弛模量和蠕变柔量。进一步,给出非局部粘弹性模型在生物组织超声波耗散建模中的应用。     

复合纤维增强混凝土阻尼测试装置开发与试验研究

纤维与聚合物的掺入可以明显改善混凝土材料的阻尼性能。本文首先给出了正弦交变激励下粘弹性材料三点弯曲梁阻尼特性关系,其次首次自主开发了大尺寸材料的阻尼性能测试装置,然后利用开发的装置在频率(0.5~2.0Hz)条件下测定了6种不同配比复合纤维增强阻尼混凝土的损耗因子与储存模量,最后对纤维的阻尼增强机理进行了初步探讨。试验结果表明:复合纤维增强阻尼混凝土与素混凝土相比,提高了混凝土的损耗因子80%~200%。主要原因是聚合物分子在外力作用下的内耗增加了普通混凝土的阻尼能力,而纤维的阻尼增强机理在于纤维的掺入增加了纤维与水泥基材的界面摩擦力。     

非定常微分型黏弹性本构模型

在应力作用下, 材料的力学参数随着微观结构的变化而变化, 需要考虑参数的时间效应. 利用黏滞系数随时间变化的黏性元件, 构造出非定常Maxwell模型、非定常Kelvin模型和非定常Zener模型. 求解非定常模型的微分型本构方程得到它们的松弛模量、蠕变柔量和卸载方程. 结果表明, 可以把常见的经验松弛函数和经验蠕变函数视为非定常微分型本构模型.      

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.     

Waterhammer modelling of viscoelastic pipes with a time-dependent Poisson's ratio

Poisson's ratio in viscoelastic materials is a function of time. However, recently developed waterhammer models of viscoelastic pipes consider it constant. This simplifying assumption avoids cumbersome calculations of double convolution integrals which appear if Poisson's ratio is time-dependent. The present research develops a mathematical model taking the time dependency of Poisson's ratio into account for linear viscoelastic pipes. Poisson's ratio is written in terms of relaxation function and bulk modulus which is assumed to be constant. The relaxation function is obtained from creep function given as the viscoelastic property data of pipe material. The results obtained from the present waterhammer model are compared with the experimental data for two different flow rates. The comparison reveals that with the application of the time-dependent Poisson's ratio and unsteady friction, the viscoelastic data of mechanical tests can directly be used for waterhammer analysis with less need for the calibration of the flow configuration. It was also shown that the creep curve calibrated based on the present model is closer to the actual creep curve than that calibrated based on previous models.     

Relations Between Relaxation Modulus and Creep Compliance in Anisotropic Linear Viscoelasticity

The results obtained previously for scalar and class P completely monotone relaxation moduli are extended to arbitrary anisotropy. It is shown for general anisotropic viscoelastic media that, if the relaxation modulus is a locally integrable completely monotone function, then the creep compliance is a Bernstein function and conversely. The elastic and equilibrium limits of the two material functions are related to each other. The relaxation modulus or its derivative can be singular at 0. A rigorous general formulation of the relaxation spectrum in an anisotropic viscoelastic medium is given. The effect of Newtonian viscosity on creep compliance is examined. Put some makeup on him and lay him to rest. Anonymous     

低密度开孔泡沫蠕变松弛特性分析

利用三维Voronoi模型和有限元方法分析了胞壁材料具有粘弹特性的低密度开孔泡沫的蠕变和应力松弛行为.采用了三参量标准线性固体模型来描述胞壁材料的粘弹特性.所得结果表明.低密度开孔泡沫具有与其胞壁材料相同的松弛时间,当相对密度较低时(低于1%)开孔泡沫的松弛模量与胞壁材料的松弛模量和泡沫相对密度平方成正比.此外,计算结果还表明,低密度开孔泡沫在较小的初始应力条件下具有与其胞壁材料相同的延迟时间.其蠕变柔度与胞壁材料的蠕变柔度和泡沫相对密度平方倒数基本成正比.但随着初始应力值的增大,泡沫的延迟时间将会显著增加.     

高温环境下纤维复合材料蠕变损伤的细观机理研究

首先利用复合材料纤维断裂单胞模型，编制蠕变损伤子程序，对单胞模型进行蠕变损伤分析。分析了纤维／基体弹性模量比对蠕变变形、蠕变损伤以及应力场的影响。从计算结果发现，蠕变损伤首先在纤维断裂尖端起始，然后沿着一定的角度向基体外围延伸，直至完全损伤，而且纤维／基体模量比对高温环境下的复合材料蠕变损伤产生很大的影响；纤维与基体的模量相差越大，复合材料越容易变形，抵抗蠕变变形的能力就越小，蠕变损伤越严重。经过对不同韧性的基体材料进行研究，发现基体韧性低的复合材料蠕变损伤明显高于高韧性基体复合材料，表明低韧性基体复合材料抵抗蠕变破坏的能力较低。     

A class of linear viscoelastic models based on Bessel functions

In this paper we investigate a general class of linear viscoelastic models whose creep and relaxation memory functions are expressed in Laplace domain by suitable ratios of modified Bessel functions of contiguous order. In time domain these functions are shown to be expressed by Dirichlet series (that is infinite Prony series). It follows that the corresponding creep compliance and relaxation modulus turn out to be characterized by infinite discrete spectra of retardation and relaxation time respectively. As a matter of fact, we get a class of viscoelastic models depending on a real parameter \(\nu > -1\). Such models exhibit rheological properties akin to those of a fractional Maxwell model (of order 1/2) for short times and of a standard Maxwell model for long times.     

Modeling and measuring visco-elastic properties: From collagen molecules to collagen fibrils

Collagen is the main structural protein in vertebrate biology, determining the mechanical behavior of connective tissues such as tendon, bone and skin. Although extensive efforts in the study of the origin of collagen exceptional mechanical properties, a deep knowledge of the relationship between molecular structure and mechanical properties remains elusive, hindered by the complex hierarchical structure of collagen-based tissues. Understanding the viscoelastic behavior of collagenous tissues requires knowledge of the properties at each structural level. Whole tissues have been studied extensively, but less is known about the mechanical behavior at the submicron, fibrillar and molecular level. Hence, we investigate the viscoelastic properties at the molecular level by using an atomistic modeling approach, performing in silico creep tests of a collagen-like peptide. The results are compared with creep and relaxation tests at the level of isolated collagen fibrils performed previously using a micro-electro-mechanical systems platform. Individual collagen molecules present a non-linear viscoelastic behavior, with a Young's modulus increasing from 6 to 16 GPa (for strains up to 20%), a viscosity of 3.84±0.38 Pa s, and a relaxation time in the range of 0.24–0.64 ns. At the fibrils level, stress–strain–time data indicate that isolated fibrils exhibit viscoelastic behavior that could be fitted using the Maxwell–Weichert model. The fibrils showed an elastic modulus of 123±46 MPa. The time-dependent behavior was well fit using the two-time-constant Maxwell–Weichert model with a fast time response of 7±2 s and a slow time response of 102±5 s.