TRANSVERSAL INERTIAL EFFECT ON RELAXATION/RETARDATION TIME OF CEMENT MORTAR UNDER HARMONIC WAVE

Under dynamic loading, the constitutive relation of the cement mortar will be significantly affected by the transversal inertial effect of specimens with large diameters. In this paper, one-dimensional theoretical analysis is carried out to determine the transversal inertial effect on the relaxation/retardation time of the cement mortar under the harmonic wave. Relaxation time or retardation time is obtained by means of the wave velocity, attenuation coefficient and the frequency of the harmonic wave. Thus, the transversal inertial effect on the relaxation time from Maxwell model, as well as on retardation time from Voigt model is analyzed. The results show that the transversal inertial effect may lead to the increase of the relaxation time, but induce the decrease of the retardation time. Those should be taken into account when eliminating the transversal inertial effect in applications.     

Evolution of properties in hydration of cements—A numerical study

The present work develops a numerical method for analysis of the microstructure and property evolution in the hydration of the cement. A time-dependent micro-mechanical model is established to investigate the microstructure development and the effective property evolution of the cement paste, while the input parameters of the model are based on experimental data. It is assumed that the cement paste composite consists of the anhydrous cement particles, cement gel and pores. The cement particles have a periodically spatial array and are wrapped by the cement gel. The Young’s modulus and Poisson’s ratio of the cement paste are calculated by direct average method and two-scale expansion method. The comparisons between the numerical results and experimental data show that this model can simulate the evolution of the microstructure and properties during the hydration of the cements quite satisfactorily.     

ELECTRO-THERMAL EFFECTS AND DEFORMATION RESPONSE OF CARBON FIBER MAT CEMENT BEAMS

A carbon fiber mat is a sheet composed of intercrossing short carbon fibers, which has more stable and lower electrical resistivity compared with dispersed short carbon fiber mixed in cement. Thereby carbon fiber mat cement could exhibit obvious electro-thermal effect. When electrified, the temperature of composite structures made up of cement mortar and carbon fiber mat will rise rapidly. If the temperature field is not uniform, temperature difference will cause structures to deform, which can be used to adjust the deformation of structures. The temperature field and deformation response driven by the electro-thermal effects of a type of carbon fiber mat cement beams are studied. Firstly, the temperature and deformation responses are studied using theories of thermal conduction and elasticity. Secondly, experimental results are given to verify the theoretical solution. These two parts lay the foundation for temperature and deformation adjustment.     

Damage,plasticity and failure of ceramics and cementitious composites subjected to multi-axial state of stress

Experimental data on mechanical behavior of ceramics and cementitious composites subjected to triaxial state of stress and verification of the theoretical model capable to describe deformability and fracture of brittle rock-like materials are presented in the paper. To check the validity of the theoretical model the stress–strain curves and stresses at material fracture determined experimentally for brick and mortar were compared with the theoretical predictions. The limit surface at material fracture obtained experimentally from triaxial tests was used in numerical analysis of masonry specimens subjected to compressive loading. These numerical results obtained by employing the Finite Element Method software package Mafem3D were compared with experimental data available in the literature. Fairly good agreement of numerical predictions with experimental results for masonry specimens was observed.     

A GENERALIZED MODELING FOR CEMENT-BASED MATERIALS UNDER LARGE RANGE OF COMPRESSION STATE

The aim of this study is to describe the main behavior of cement-based materials under large compression state based on the recent experimental research. In this paper, the strainstress relations are firstly analyzed and confining pressure state is regarded as low/medium/high state. A generalized cup modeling is introduced by a coupled deviatoric shearing, pore collapse and damage mechanism within thermodynamic framework. A series of numerical simulations are performed for the considered cement paste and concrete. Comparisons between numerical predictions and experimental results show that the proposed model is able to describe the main features of mechanical behavior under large range of compression state.     

应用杆束系统研究水泥砂浆节理面的压剪动特性

基于分离式Hopkinson压杆系统提出一套Hopkinson束杆装置,研究水泥砂浆节理面在压剪复合加载下的动态界面滑移特性。应用三根小杆作为接收杆,分别采用单杆和三杆两种入射方式,对含有节理面的水泥砂浆试件进行冲击加载。在节理面发生滑动时,由三根接收杆测试得到水泥砂浆试件不同位置滑移状态,进而得出节理面的整体滑移速度。此研究初步揭示了节理面在发生滑移时的局部滑移状态以及整体滑移状态,为节理面的动摩擦特性研究提供了较可靠的实验技术。     

THEORETICAL ANALYSIS ON THE NUCLEATION OF MICRO-DAMAGE IN CEMENT MORTAR UNDER COMPRESSIVE LOADING AND SULFATE ATTACK

Effect of uniaxial compression on the nucleation of micro-damage in cement mortar under sulfate attack is investigated.Shape and size of micro-voids in cement mortar is detected using Micro Computed Tomography techniques.The formation of delayed ettringite crystal is analyzed using scanning electron microscope and energy disperse spectrum methods.Deformation of micro-voids and the distribution of stress at the surface of a micro-void are calculated.It is found that the nucleation of micro-cracks is caused by the tensile stress at the voids’ surface,and such damage nucleation will be speeded up by the remote uniaxial compressive load.     

空腔膨胀理论靶体阻力模型及其应用研究进展

从静/动态空腔膨胀模型的理论体系出发,介绍了空腔膨胀模型在不同方向上取得的成果,主要涉及理想侵彻条件的空腔膨胀压力计算模型及数值模拟方法和空腔膨胀模型在典型侵彻问题及复杂弹靶条件下的应用。在理想侵彻条件下的空腔膨胀压力计算模型中,主要讨论了靶体材料、屈服准则和状态方程对空腔边界应力的影响规律及空腔膨胀模型的适用性问题;根据数值模拟中初始条件的不同,介绍了空腔表面恒定速度/恒定压力两种数值模拟方法,证明了数值模拟方法的可靠性;整理了空腔膨胀模型的基本假设、适用范围、工程应用特点,列举了其在典型侵彻问题及多层复合靶板、约束靶体、弹体刻槽和异形截面形状弹体等复杂弹靶条件下的应用。针对空腔膨胀模型的研究现状,总结了目前空腔膨胀模型在冲击动力学领域的应用方向,归纳了空腔膨胀模型应用中尚存在的问题,展望了空腔膨胀模型下一步的重点发展方向。     

Fracture analysis of concrete using singular fractal functions with lattice beam network and confirmation with acoustic emission study

In the present study singular fractal functions (SFF) were used to generate stress-strain plots for quasi-brittle material like concrete and cement mortar and subsequently stress-strain plot of cement mortar obtained using SFF was used for modeling fracture process in concrete. The fracture surface of concrete is rough and irregular. The fracture surface of concrete is affected by the concrete’s microstructure that is influenced by water cement ratio, grade of cement and type of aggregate [1], [2], [3] and [4]. Also the macrostructural properties such as the size and shape of the specimen, the initial notch length and the rate of loading contribute to the shape of the fracture surface of concrete. It is known that concrete is a heterogeneous and quasi-brittle material containing micro-defects and its mechanical properties strongly relate to the presence of micro-pores and micro-cracks in concrete [1], [2], [3] and [4]. The damage in concrete is believed to be mainly due to initiation and development of micro-defects with irregularity and fractal characteristics. However, repeated observations at various magnifications also reveal a variety of additional structures that fall between the ‘micro’ and the ‘macro’ and have not yet been described satisfactorily in a systematic manner [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [15], [16] and [17]. The concept of singular fractal functions by Mosolov was used to generate stress-strain plot of cement concrete, cement mortar and subsequently the stress-strain plot of cement mortar was used in two-dimensional lattice model [28]. A two-dimensional lattice model was used to study concrete fracture by considering softening of matrix (cement mortar). The results obtained from simulations with lattice model show softening behavior of concrete and fairly agrees with the experimental results. The number of fractured elements are compared with the acoustic emission (AE) hits. The trend in the cumulative fractured beam elements in the lattice fracture simulation reasonably reflected the trend in the recorded AE measurements. In other words, the pattern in which AE hits were distributed around the notch has the same trend as that of the fractured elements around the notch which is in support of lattice model.     

水泥砂浆界面相弹性常数的反演计算

为了计算水泥砂浆界面过渡区的弹性常数,采用广义自洽方法(GSCM),根据水泥砂浆内部的微细观结构,建立了由水泥浆基体、岩石离散夹杂(骨料)、界面过渡区(ITZ)和有效弹性材料组成的四相复合材料模型.推导了界面相的体积模量和剪切模量方程.利用已知水泥砂浆材料的实验数据,计算了界面相的弹性常数.发现界面相剪切模量约为水泥浆基体剪切模量的50%.     

THE APPLICATION OF DISCRETE ELEMENT METHOD IN SOLVING THREE-DIMENTIONAL IMPACT DYNAMICS PROBLEMS

A three-dimensional discrete element model of the connective type is presented. Moreover, a three- dimensional numerical analysis code, which can carry out the transitional process from connective model (for continuum) to contact model (for non-continuum), is developed for simulating the mechanical process from continuum to non-continuum. The wave propagation process in a concrete block (as continuum) made of cement grout under impact loading is numerically simulated with this code. By comparing its numerical results with those by LS-DYNA, the calculation accuracy of the model and algorithm is proved. Furthermore, the failure process of the concrete block under quasi-static loading is demonstrated, showing the basic dynamic transitional process from continuum to non-continuum. The results of calculation can be displayed by animation. The damage modes are similar to the experimental results. The two numerical examples above prove that our model and its code are powerful and efficient in simulating the dynamic failure problems accompanying the transition from continuum to non-continuum. It also shows that the discrete element method (DEM) will have broad prospects for development and application.     

离散元法在求解三维冲击动力学问题中的应用

提出了三维连结型离散模型,建立了可实现连结型模型(用于连续介质)-接触型模型(用于非连续介质)转化的三维离散元计算程序,用来模拟连续介质转变为非连续介质的力学过程．利用该计算程序对冲击载荷下混凝土块体内(连续体情况下)的应力波传播过程进行了数值模拟．将计算结果的数值与LS-DYNA程序计算的结果进行比较,验证了该计算程序的计算精度．在此基础上,模拟了混凝土块体的动态破坏(连续介质向非连续介质转化)过程．其计算结果可用动画显示,得到的破坏形式与由实验得到的破坏形式相近．两个算例说明该离散元模型及其计算程序是模拟计算伴随有连续介质向非连续介质转变的动态破坏问题的有力工具．     

Electro-Thermal Effects and Deformation Response of Carbon Fiber

A carbon fiber mat is a sheet composed of intercrossing short carbon fibers, which has more stable and lower electrical resistivity compared with dispersed short carbon fiber mixed in cement. Thereby carbon fiber mat cement could exhibit obvious electro-thermal effect. When electrified, the temperature of composite structures made up of cement mortar and carbon fiber mat will rise rapidly. If the temperature field is not uniform, temperature difference will cause structures to deform, which can be used to adjust the deformation of structures. The temperature field and deformation response driven by the electro-thermal effects of a type of carbon fiber mat cement beams are studied. Firstly, the temperature and deformation responses are studied using theories of thermal conduction and elasticity. Secondly, experimental results are given to verify the theoretical solution. These two parts lay the foundation for temperature and deformation adjustment.     

不同失效模式下轴压管状结构的吸能特性比较

圆管、方管等管状结构受轴向的压缩作用,是工程上常见的一种结构受力形式.由于几何尺寸、边界条件和材料特性的不同,管状结构可以发生5种不同的失效模式:渐进屈曲、整体失稳、翻转、膨胀和劈裂.本文综述了近年来管状结构在轴压作用下不同失效模式的理论建模、实验测试和数值仿真研究结果,并对5种失效模式的力学响应、能量吸收特性进行了对比与分析.     

Nonlinear parameter identification of models for masonry

This paper investigates the parameter estimation problem for brick masonry models. An identification procedure is proposed in which the uncertainties of known parameters and/or errors of measurements are its elements of distinction. The minimization process of the discrepancies between experimental data and theoretical measurements takes place by means of a first order iterative method. The identification procedure is applied to two different problems: the calibration of an interface model for brick–mortar joint in its functional form through monotonic experimental tests; to evaluate the unknown parameters of a continuum model for brick masonry walls in its non-holonomic form by means of in-plane cyclic shear–compression test of masonry panels. The general framework of the non-linear estimate methodology, the parameter identification problems and the numerical results are presented.     

水泥水化的微结构与性能演化研究进展

水泥是一类重要的工程结构材料,其力学性能依赖于水泥的水化过程．综述了水泥水化的微结构和性能演化的研究进展;评述了研究水泥水化过程的实验方法和理论方法;详细介绍和评述了水泥水化的微结构仿真模型、力学分析模型以及建立微结构．性能关系的基本方法．分析和展望了未来水泥水化的主要问题和研究方向．     

水泥砂浆在主动围压下的动态力学性能

水泥砂浆等脆性材料的力学性能与所受围压的大小密切相关。为了研究水泥砂浆在围压下的动态力学性能,研制了适于分立式Hopkinson压杆加载的主动围压装置,最大预加载主动围压大于30 MPa。实验得到了水泥砂浆在不同围压、不同应变率下的轴向应力应变曲线,发现材料在围压作用下抗压强度和韧性大大提高并且整体进入了伪塑性,而材料的应变率效应也是显著的。     

石英玻璃圆环高速膨胀碎裂过程的离散元模拟

采用离散元算法模拟了石英玻璃圆环受到外加动态载荷时的力学行为. 首先基于flat-jointed粘结模型,通过标准的单轴拉压、三点弯曲等数值实验来标定了石英玻璃的微观参数. 在此模型基础上,数值模拟再现了石英玻璃圆环在不同应变率下的膨胀碎裂过程. 为定量分析数值模拟结果,需要准确确定圆环的碎裂发生时刻. 模拟发现:伴随着石英玻璃圆环的断裂,圆环外表面粒子径向膨胀速度的时程曲线会发生突然升高然后下降的跳动;详细分析表明,这种跳动源自周向的脆性断裂诱发的卸载波(周向拉伸应力急剧下降)以及伴随而来的泊松膨胀,这种径向速度跳动现象为实验中检测脆性断裂发生时刻提供了可能. 进一步的数值研究表明:(1)石英玻璃圆环的断裂应变随着应变率的提高而增大,与韧性金属材料的膨胀环实验结果一致;(2)石英玻璃圆环的碎片平均质量随着应变率的增大而减小;(3)数值计算获得的碎片平均尺寸与已有的理论和实验结果比较吻合. 利用液压膨胀环实验装置对石英玻璃圆环进行了验证性实验,回收得到的碎片形貌及碎片个数与数值模拟的结果基本一致.      

爆炸冲击震动对砖墙破坏作用的数值模拟

砖墙在爆炸冲击震动作用下的动力反应非常复杂,本构关系很难精确建立。本文阐述了砖墙几种常用的有限元模拟方法,分析各种方法的优缺点,确定采用一种砖块和砂浆分开的精细化建模的三维砖墙有限元模型;通过LS-DYNA软件,得到砖墙在水平爆炸冲击震动荷载下的破坏过程,计算结果与实验现象很好。研究表明:该种分析模型综合考虑了砖块和砂浆之间复杂的相互作用,并且对砂浆层进行了单独建模,保证了砖墙在数值模拟上的真实性和正确性,因此可以准确地模拟出实验中砖块之间砂浆层的损伤积累破坏。