基于刚体─弹簧模型的混凝土微裂纹行为模拟

本文在细观上将混凝土看成是由粗骨料颗粒与硬化水泥浆体组成的两相非均质复合材料，以随机颗粒模型代表混凝土的细观结构，研究了一种对随机颗粒模型结构进行全自动网格剖分的方法，采用适宜处理微裂纹行为的刚体一弹簧模型（RigidBodySpringModel），探讨了预先存在的微裂纹对混凝土性能的影响，建立起混凝土的细观结构与宏观性能之间的关系，同时也对混凝土在拉伸荷载下的裂纹传播行为进行了研究。     

考虑孔隙及微裂纹影响的混凝土宏观力学特性研究

混凝土是一种典型的多孔介质材料,孔隙分布错综复杂,孔径尺寸跨越微观尺度和宏观尺度,对混凝土弹性模量及强度等力学参数产生巨大影响.认为混凝土是由骨料、孔隙及砂浆基质组成的三相复合材料,采用Monte Carlo 法将孔隙、微裂纹及微缺陷与骨料颗粒随机投放在砂浆基质中.根据三相球模型及中空圆柱形杆件模型得到含孔材料的有效力学性质,并推导得到含孔材料的等效本构模型.建立含孔隙混凝土试件的细观单元等效化力学模型,对二级配含孔隙混凝土试件在单轴拉伸及压缩条件下的反应进行了非线性分析.结果表明:孔隙、微裂纹的存在对混凝土宏观弹性模量、强度及残余强度等力学性质都有很大影响,在对混凝土宏观力学特性分析及研究混凝土损伤断裂时不应忽略其影响.     

华南风化花岗岩劈拉断裂行为的试验与细观模拟研究

为研究不同风化程度花岗岩在劈拉受载下的破损行为,该文对广东从化地区新鲜、微风化、中风化三种风化程度的花岗岩进行微观特征测量及巴西圆盘试验研究。分别基于考虑空间相关特征的随机模型和数字图像技术两种方法实现对岩样细观结构的表征,结合细观参数反演技术,建立反映岩石细观非均质组构特性的颗粒离散元模型,对花岗岩劈拉试验进行数值仿真试验研究。结果表明,随着风化级别的提高,花岗岩中粘结力较强的长石等矿物向粘土矿物转化,结晶强度降低,微孔隙等软弱结构增多,岩石宏观力学性能不断劣化,劈拉破坏由单一裂纹主导转为多条分叉状裂纹,岩样脆性也逐渐减弱。基于两种方法建立的细观随机力学模型仿真结果均表明岩样的劈拉强度随风化级别的提高逐渐降低,与试验结果变化趋势相符,分析得到不同风化花岗岩的劈拉破裂响应特征与试验结果一致,且从细观层面对花岗岩样受劈拉作用的渐进破裂响应提供了深入认识。     

孔隙率变化规律及其对混凝土变形过程的影响

混凝土内部随机分布的微裂纹和孔洞等细观缺陷影响其破坏机理及宏观力学性能,且孔隙率随着外荷载的变化而不断变化。该文中将含缺陷(微裂纹、各种空隙等)混凝土复合材料简化为空心球力学模型,基于弹性力学理论推导并获得了混凝土当前孔隙率与材料初始孔隙率及体应变之间的定量关系;推导并得到了含孔隙混凝土的有效弹性模量、有效泊松比及峰值应变等与孔隙率的定量关系,进而得到了单轴加载条件下含孔隙混凝土细观单元的等效多折线本构关系模型。最后,采用细观单元等效化力学模型,研究了单轴加载(拉伸和压缩)情况下不同孔隙率混凝土材料的破坏过程及宏观力学性能,探讨了孔隙率变化规律及其对混凝土变形过程的影响。     

冲击荷载作用下混凝土动态本构模型的研究

基于混凝土冲击荷载作用下的实验研究,以修正Ottosen四参数破坏准则为屈服法则,引入损伤,构造了一个动态本构模型用于描述混凝土材料的冲击特性。宏观上,假设混凝土材料是一个均匀连续体;而从细观角度来看,混凝土材料内部存在大量随机分布的微裂纹损伤。假设微裂纹均匀分布,且符合理想微裂纹体系统条件,定义含裂纹材料中单位体积内微裂纹所占的比例来表征微裂纹损伤所引起的混凝土材料宏观力学性能的劣化,并给出了损伤的演化方程。通过模型计算模拟结果与实验结果比较发现,模拟曲线与实验曲线拟合良好,因而可以用该模型模拟混凝土材料在冲击荷载下的动态特性。     

基于颗粒流离散元模型的弹丸侵彻细观混凝土数值模拟方法研究

利用颗粒离散单元法,研究弹丸侵彻细观混凝土模型中弹丸受到介质的阻应力与侵彻速度的关系。采用蒙特卡罗法随机生成并投放混凝土骨料且骨料的粒径分布满足级配曲线。通过对混凝土颗粒离散元细观力学模型进行单轴压缩实验、巴西劈裂实验和双轴压缩实验的参数反演,确定细观模型参数,能使细观混凝土模型具有和一般混凝土等效的力学性能。分析了骨料、过渡层和砂浆三相材料各细观参数对混凝土单轴压缩应力应变关系影响以及锥形弹和平头弹弹丸直径对侵彻阻应力的影响。将颗粒离散元细观力学模型方法计算的弹丸阻应力与空腔膨胀理论计算模型相比较,表明计算离散元方法具有良好的精度和实用性。     

橡胶颗粒抑制蒸养混凝土热损伤机制

通过向蒸养混凝土中掺入橡胶颗粒制备蒸养橡胶混凝土来抑制蒸养过程中混凝土产生的热损伤。通过试验测试了蒸养橡胶混凝土的抗压强度；建立了考虑界面过渡区的橡胶混凝土随机骨料模型，基于ABAQUS，模拟研究了橡胶颗粒对降温阶段混凝土温度损伤应力的影响，从细观角度研究橡胶颗粒抑制蒸养混凝土中微裂纹发展规律，并将温度损伤应力作为初始缺陷，模拟了橡胶混凝土的抗压性能，验证了模拟结果的可靠性；通过压汞(Mercury intrusion porosimetry,MIP)测试研究了橡胶颗粒对蒸养混凝土孔结构的影响；通过超景深显微镜研究了橡胶与水泥石之间的结合情况。研究结果表明：橡胶颗粒掺入可以抑制蒸养混凝土的热损伤，减少强度损失。橡胶颗粒可以有效降低蒸养混凝土试件的总孔隙率，蒸养橡胶混凝土试件有害孔径较未掺加橡胶颗粒的普通蒸养混凝土下降了3.1%，同时改善了橡胶和水泥基体的粘结状况。     

混凝土试样在静态载荷作用下断裂过程的数值模拟研究

提出了一个模拟混凝土断裂过程的细观力学模型,并应用该模型从混凝土的细观非均匀性结构出发,对混凝土试样在单轴和双轴静态载荷作用下的断裂过程进行了数值模拟,给出双轴载荷作用下混凝土的强度包络面。数值模型结果较好地模拟了混凝土试样从裂纹萌生、扩展到宏观裂纹形成的整个断裂过程,与实验结果表现出较好的一致性。     

基于细观单元等效化方法的混凝土动态破坏行为分析

混凝土材料具有明显的应变率效应,对其力学性质增强机理的认识还不统一。在细观随机骨料模型基础上,采用特征单元尺度划分试件网格,推导了考虑材料拉/压强度应变率效应的细观单元等效本构关系,建立了非均质混凝土材料的细观单元等效化数值模型。基于二维模型对Dilger等混凝土动态压缩试验进行了数值模拟,获得的数值结果与试验数据及随机骨料模型结果吻合良好,证明了细观单元等效化方法的准确性;进而对三维混凝土试件动态单轴拉伸和压缩破坏模式及宏观力学性质的加载速率效应进行了研究。数值结果表明:随着加载速率的增加,混凝土裂纹(损伤)数量增大,混凝土破坏将耗散更多的能量,是混凝土动态强度提高的主要原因。     

加载速率对混凝土拉伸破坏行为影响观数值分析

探讨了加载速率及细观结构非均质性对混凝土破坏模式及宏观力学性能的影响。考虑到混凝土细观结构非均质性的影响,将混凝土看作由骨料和砂浆基质组成的两相复合材料。考虑材料的应变率效应,采用塑性损伤模型来描述砂浆基质的动态力学行为;由于骨料具有较高强度,假定不会产生断裂,设定为弹性体。对单边缺口的混凝土试件及L形试件在不同加载速率下的动态拉伸破坏模式进行了细观数值研究。数值结果表明:1) 混凝土动态破坏模式及裂纹扩展方向具有明显的加载速率相关性;2) 随着加载速率的提高,混凝土破坏模式从I-型模式到混合型模式转变;3) 混凝土细观结构越复杂,组分间相互作用越复杂,裂纹扩展路径越复杂,裂纹分支现象越为明显;4) 随着加载速率的提高,混凝土破坏时产生更多的裂纹扩展路径(分支裂纹),且损伤区域宽度增大,导致混凝土在高应变率作用下消耗更多的能量,可认为是混凝土材料动态强度提高的主要原因。     

Elastic microcracked bodies with random properties

The stochastic properties of a ‘dense’ distribution of microcracks in an elastic body are analyzed by using a multifield continuum model describing the influence of the microcracks on the gross mechanical behavior of the body. Numerical examples are presented: the strain localization phenomena that have been already found in deterministic bodies are confirmed, and similar patterns are shown to exist also for the stochastic moments of the displacements. In particular, the patterns in the portraits of skewness and kurtosis become stronger when the correlation of the distance between neighboring microcracks increases. Such a distance is considered as a uni-variate non-Gaussian random field. Strain localization is an indicator toward the irreversible growth and coalescence of microcracks.     

Dynamic tensile test of fly ash concrete under alternating tensile–compressive loading

In order to obtain the post-peak stress–strain curve of plain concrete, the strain-controlled uniaxial tensile tests are carried out on cylindrical specimens. The tensile cyclic tests are performed between a lower compressive load and an upper load to the envelope curve. Test results have shown that residual strain accumulates slowly when the concrete is subjected to alternating tensile–compressive loading, during which the microcracks increase and aggregate. Meanwhile, the rates of stiffness degradation with cycles are similar in all cases. In the study, an analytical model of the post-peak cyclic behavior of concrete is proposed. The strain is decomposed into classical linear elastic strain and non-classical strain described in the Presach–Mayergoyz model, which describes the hysteresis behavior of concrete well from a microscopic point of view. A good agreement is obtained between the test data and calculated results.     

利用声发射信号与速率过程理论对混凝土损伤进行定量评估

混凝土性能退化过程伴随着混凝土材料中的孔隙、微裂缝和裂缝的发展,而声发射是材料不均匀变形或裂缝开裂及扩展过程的伴生现象。在损伤力学和声发射速率过程理论的基础上建立了在单轴受压状态下混凝土材料的声发射特征参数与损伤演化间关系的方程,从而实现运用测量得到的声发射特征参数最终量化评估混凝土的损伤大小。对某实际桥梁混凝土芯样进行了单轴压力下的声发射试验,得出了混凝土的实际损伤量;通过对声发射Kaiser效应和Felicity效应的分析,评价了混凝土芯样的受力历史;试验结果证明了声发射技术在混凝土损伤评估应用中的有效性和准确性。     

Damage mechanics of concrete

A process model, also known as continuum damage model, for a compressed concrete containing randomly distributed penny-shaped microcracks is formulated. The damage is computed from Fracture Mechanics principles applied to interfacial microcracks. The formulation is restricted to the monotonically increasing quasi-static compressive loads.     

Study on fracture behavior of particulate reinforced magnesium matrix composite using in situ SEM

The fracture behavior of SiCp/AZ91 magnesium matrix composite fabricated by stir casting is investigated using the in situ SEM technique. Experimental results show that (1) the dominant microcrack nucleation mode is interface decohesion in particle-dense regions because of the weak interface formed during the solidification process of the composite and large stress concentrations caused by particle segregation, (2) microcracks coalesce by the failure of matrix ligaments between microcracks while additional microcracks are initiated in the particle-dense region ahead of the coalesced microcracks, and (3) cracks propagate by coalescence of microcracks or along the particle/matrix interface. And so we come to the conclusion that the fracture mechanism of SiCp/AZ91 composite is interface-controlled. The in situ SEM observations are verified by complementary SEM studies of the fractured specimens of conventional tensile tests. And so, the in situ SEM observations can be qualitative representation on the fracture behavior of bulk SiCp/AZ91 composite.     

Insight into the inherent randomness of concrete properties using the stochastic micromechanics

The concrete properties inherently fluctuate even using the same manufacturing process. A Legendre polynomial based micromechanical framework is proposed to investigate the inherent randomness of the concrete’s elastic behavior. The three-phase composite model is employed to represent the concrete material at microscale level. The volume fractions of different constituents are reached using a simplified analytical approach. Predictions with and without particle interactions for the concrete properties are obtained through the two-level homogenization process. By describing the microstructures with high-dimensional random vector, the stochastic micromechanical framework is obtained based on the presented three-phase model to incorporate the concrete’s inherent randomness. The unbiased probability density functions (PDFs) of the properties are attained using efficient simulation framework consisting of the Legendre polynomial approximations, Monte Carlo simulations and the linear transformations. Numerical examples indicate that the proposed framework is able to describe the probabilistic characters of concrete properties from microscale level and obtain the material’s unbiased PDFs with high efficiency. Finally, the statistical effects of ITZ are discussed on the concrete’ s macroscopic probabilistic behaviors.     

Nondestructive Techniques for Studying Fracture Processes in Concrete

Some laboratory nondestructive evaluation techniques have been invaluable in studying fracture processes in concrete. Several nondestructive evaluation methods including acoustic emission (AE), computer vision, digital speckle pattern interferometry (DSPI), and X-ray microtomography (XMT) were used to examine the fracture behavior of concrete in tension and compression. Acoustic emission testing was used in an attempt to characterize the fracture properties of individual microcracks. As the specimens were loaded, AE waveform data was recorded, and analyzed for source location and source characterization. While DSPI analysis is limited to the specimen surface, the resolution is detailed such that microcracks on the order of 0.25 μm can be detected. Computer vision is a very useful method to measure crack openings for multiple crack development. It also can be used in conjunction with a hydraulic testing machine, which often generates vibration problem for some sensitive techniques. Crack patterns in cement-based materials under various material compositions and testing conditions are examined. This revised version was published online in July 2006 with corrections to the Cover Date.