基于数字图像的混凝土破坏过程的数值模拟

把混凝土看作是由水泥砂浆为基相,粗细骨料为分散相的复合材料。在细观尺度上,应用数字图像处理技术表征混凝土材料中由骨料的形状、大小和分布对混凝土材料造成的非均匀性,在组成相材料内部细微观尺度上,采用统计方法来描述材料的非均匀性,建立了细-微观尺度耦合分析的混凝土损伤数值模型,模拟了混凝土单轴载荷作用下的破坏过程。数值模拟结果能反映出骨料分布和组成相材料的非均匀性对混凝土力学行为的影响,较好的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破坏过程。     

基于统计损伤理论的硫酸盐侵蚀混凝土本构模型研究

基于统计损伤理论及宏观试验现象,该文建立了考虑硫酸盐侵蚀影响的混凝土单轴、双轴压缩统计损伤本构模型。混凝土变形破坏被理解为细观断裂、屈服两种损伤模式的连续累积演化过程。硫酸盐侵蚀效应改变了混凝土微结构的组成成分和力学特征,进而改变了微裂纹萌生、扩展的形态以及损伤的累积演化过程,可通过改变断裂和屈服两种细观损伤机制演化过程的概率分布形态来模拟。分析结果表明:在硫酸盐侵蚀环境下,侵蚀程度的加深显著改变了混凝土细观损伤累积演化过程,最终导致混凝土宏观力学性能呈现先“强化”后“弱化”的现象。在此过程中,细观损伤演化过程呈现出明显地规律性,可由统计损伤模型中5个特征参数的变化规律表征。该文模型为复杂环境下侵蚀混凝土细观损伤过程预测和分析提供了新的方法和工具。     

基于两相随机介质的混凝土破坏全过程模拟

通过两相随机介质对混凝土细观结构进行建模,给出了混凝土破坏全过程模拟。首先,为了反映混凝土水泥砂浆和骨料的随机分布,引入新近发展的两相介质随机场对混凝土进行建模。采用随机点集生成随机有限元与内聚单元,以考虑裂纹分布的随机性,形成混凝土材料破坏分析的数值模型。数值模拟给出了混凝土单轴受拉裂纹开展全过程。数值模拟结果表明:当骨料断裂能明显大于水泥砂浆断裂能时,采用两相随机介质,能够较好地描述在受拉状态下,混凝土裂纹主要产生于水泥砂浆及骨料以及水泥砂浆的界面处,较少骨料破坏,这一典型特征。最后,通过模拟所得的均匀化应力-应变关系与混凝土单轴受拉试验结果对比,进一步验证了该文所给出的两相随机介质模拟的正确性。     

三维机织碳/碳复合材料双轴压缩载荷下的力学行为

基于三维机织碳/碳复合材料的细观结构特征, 设计平板十字形试样, 在材料双轴力学性能试验机上开展了复合材料单轴、 双轴加载压缩试验, 对比分析了三维机织碳/碳复合材料在双轴压缩载荷下的力学行为。研究表明: 三维机织碳/碳复合材料的压缩行为表现为非线性、 脆性断裂; 双轴载荷作用下非线性特征更为显著, 压缩模量随应力的增加而增大, 强度与模量相较于单轴有较大幅度增加, 双轴压缩载荷作用下材料的强化效应显著; 试样破坏位置并未出现在试样中心区, 而是发生在试样的加载端部或十字形试样的加载分枝根部, 主要表现为基体开裂、 纤维断裂和层间脱粘, 碳布及其层间界面剪切强度的强弱直接影响材料的压缩强度。     

基于断裂力学的沥青混凝土抗裂性能与研究方法进展

针对断裂力学在沥青混凝土的应用背景以及目前研究的不足，分别从沥青混凝土断裂力学基本理论和沥青断裂性能的研究进展以及沥青混凝土断裂测试方法三个方面总结了相关成果和进展。阐述了线弹性断裂力学、弹塑性断裂力学及疲劳断裂力学在沥青混凝土中的应用条件和评价参数；从材料组成、再生和温拌工艺、路面层间黏结以及细观模拟等方面总结了沥青混凝土抗裂性能的研究现状；列举了目前评价沥青混凝土断裂性能的试验方法；指出了目前研究的不足和以后研究的建议。研究结果表明：沥青混凝土的低温和中温断裂一般分别用线弹性断裂力学和弹塑性断裂力学理论来评价和分析，有必要将线弹性断裂力学和弹塑性断裂力学结合，从而提出一个统一的宽温度域的评价指标；目前针对单调荷载作用下沥青混凝土断裂性能的研究较多，针对循环荷载作用下沥青混凝土断裂行为的研究还较少，有必要通过疲劳试验明确沥青混凝土的裂纹扩展规律；沥青混凝土断裂的细观模拟可以从细观角度揭示裂纹发生和扩展的机理，但破坏模型的选取和细观参数的确定是影响模拟结果准确性的重要因素，需要进一步重视。     

基于对混凝土破损过程的模拟研究

从细观力学层次出发,把混凝土看作是由骨料、砂浆基质及两者之间的界面过渡区组成的三相复合材料,通过引入细观损伤变量及损伤变量的演化规律模拟了混凝土的破坏过程。首先生成骨料位置随机分布且颗粒级配符合Fuller曲线的数值试件,然后对每一相材料分别赋以服从Weibull分布的随机力学参数(弹模、强度和泊松比等)并规定相应的破坏准则,将通过上述途径获得的混凝土数值试件导入有限元软件Abaqus进行数值模拟。与实验结果的对比显示,提出的随机损伤数值模型能够较好地模拟混凝土板式试件在单拉、单压下的应力-应变关系曲线以及混凝土中裂纹的萌生、开裂过程。     

钢筋混凝土梁抗冲击性能研究的细观数值模拟方法

为更真实地描述钢筋混凝土梁在冲击荷载作用下复杂的破坏过程,结合混凝土细观结构非均质性,假定混凝土是由骨料颗粒、砂浆基质以及界面过渡区(ITZ)组成的三相复合材料,考虑混凝土细观组分的率效应,将钢筋网嵌入至素混凝土梁,建立了钢筋混凝土梁抗冲击力学行为研究的三维细观尺度数值模型。基于该数值分析模型与方法,研究了落锤速度对钢筋混凝土梁抗冲击性能的影响规律。将细观数值模拟结果与宏观模型结果以及已有试验结果进行了对比分析,包括破坏模式、冲击力、跨中位移以及支座反力等,发现模拟结果与试验结果吻合良好,验证了细观数值分析模型在钢筋混凝土梁抗冲击性能模拟中运用的可行性。     

基于三维细观数值方法的钢筋混凝土悬臂梁抗剪行为研究

从细观角度出发,结合混凝土内部组成的微/细观结构特征,考虑钢筋与混凝土之间的非线性粘结-滑移行为,建立了钢筋混凝土悬臂梁力学行为研究的三维细观尺度数值分析模型与方法。基于建立的数值方法,对已有关于钢筋混凝土悬臂梁在循环往复荷载作用下的抗剪破坏试验进行模拟,模拟结果与试验结果的良好吻合表明了细观数值方法的可行性。进而在此基础上,拓展模拟了不同箍筋率下钢筋混凝土悬臂梁的剪切破坏过程,揭示了箍筋对悬臂梁滞回特性及抗剪承载力的影响规律。研究表明:1）细观数值分析模型能较好的模拟构件的宏观力学性能,且能细致的描述裂缝在混凝土中的发展过程;2）随着配箍率的增加,梁抗剪承载力及延性增加,但随着配箍率增加,钢筋混凝土梁抗剪承载力增加趋势减缓。建立的细观尺度数值方法可为有/无腹筋混凝土梁抗剪/抗弯破坏尺寸效应提供理论支持。     

混凝土拉伸损伤演变的细观力学研究

对二维平面应力问题,根据能量耗散等价假设,即当产生微裂纹所需能量等于形成损伤所需的能量时,细观损伤状态等价于宏观损伤状态,建立了单轴拉伸与双轴拉伸情况下混凝土宏观损伤变量与细观损伤参数(微裂纹密度)的关系。微裂纹扩展准则采用复合型准则。计算结果表明,混凝土双轴受拉发生破坏时的损伤值随横向拉应力的增大而增大。本文求得的有效弹性模量与细观力学的SCM结果吻合较好,与忽略相互作用的Taylor方法结果也基本吻合。     

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。     

3D meso-mechanical analysis of concrete specimens under biaxial loading

In recent years, the mechanics of materials group at ETSECCPB‐UPC has developed an approach for meso‐mechanical analysis of concrete using zero‐thickness interface elements in 2D and more recently in 3D. In this methodology, the meso‐structure is generated with in‐house developed computer programs based on Voronoï/Delaunay theory. In the analysis, continuum elements are assumed linear elastic. Non‐linearity and fracture phenomena are made possible by the systematic use of zero‐thickness interface elements inserted on a priori determined potential fracture planes. In this paper, the results obtained for a 3D specimen under biaxial loading are presented. The results turn out to be very satisfactory and, in particular, it is observed that even the specimens which contain a reduced number of aggregates (14 in the present calculations) lead to a realistic failure envelope under biaxial loading, and they also capture the tendencies of cracking and fracture orientations observed in experiments for different rates of biaxial loading. The special limit case of biaxial loading under restrained out‐of‐plane deformations is also analysed, leading to practically elastic behaviour as shown by available experimental evidence.     

Biaxial tension fatigue response of concrete

Concrete structures such as rigid airport pavements are subjected to repeated high-amplitude loads resulting from passing aircraft. The resulting stress-state in concrete is a biaxial combination of compression and tension. It is of interest to understand the response of plain concrete to such loading conditions, which will enable development of realistic material models for implementation in mechanistic pavement design procedures.The objective of this work is to characterize the quasi-static and low-cycle fatigue response of concrete subjected to biaxial stresses in the biaxial tension region, where the principal tensile stress is larger than or equal in magnitude when compared with the principal compressive stress. An experimental investigation of material behavior in the biaxial tension region is conducted. The experimental setup consists of the following test configurations: (a) notched concrete beams tested in three-point bend configuration, and (b) hollow concrete cylinders subjected to torsion.Failure of concrete in the biaxial tension region is shown to be a local phenomenon under quasi-static and fatigue loading, wherein the specimen fails owing to a single crack. The crack propagation is studied using the equivalent elastic crack concept. It is observed that the crack growth rate in constant amplitude fatigue loading exhibits a two-phase process: a deceleration phase followed by an acceleration stage. The crack growth in the acceleration stage is shown to follow Paris law. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the considered biaxial fatigue response.     

Analytical model for prediction of load capacity of RC columns confined with CFRP under uniaxial and biaxial eccentric loading

This paper presents an analytical model for prediction of the load carrying capacity of reinforced concrete (RC) columns confined with carbon fiber reinforced polymers (CFRP) under uniaxial and biaxial eccentric loading. The model is based on realistic material laws and accounts for the non-linear stress–strain behavior of both unconfined and CFRP-confined concrete. Under uniaxial eccentric loading, the column cross-section is discretized into finite layers along the section depth. For a symmetric square cross-section subjected to biaxial eccentric loading with equal eccentricity about each principal axis, the column cross-section is discretized into finite layers along the diagonal of the column cross-section. For a given strain distribution in a direction perpendicular to the neutral axis, the sectional forces are integrated numerically and the load capacity of the column is predicted using an iterative process. For rectangular and non-symmetric square cross-sections subjected to biaxial eccentric loading, the load capacities under uniaxial eccentric loading along each principal axis are first derived independently. The column load capacity under concentric axial loading is calculated. The determined distribution of the column load capacities under uniaxial eccentric loading and concentric loading are then utilized to compute the load capacity under biaxial eccentric loading using the reciprocal load equation. An experimental study was carried out to examine the effectiveness of the CFRP-confinement to improve the load capacity and ductility of RC columns under biaxial eccentric loading. The accuracy of the proposed analytical model was demonstrated by comparing the model predictions to results of the current experimental study in addition to experimental data published in the literature.     

Fatigue behavior of concrete subjected to biaxial loading in the compression region

Concrete hollow cylinders subjected to combined compression and torsion were used to simulate concrete airport pavements subjected to biaxial fatigue loading in the compression region. It was found that the increase in the compliance in the post-peak period is due to the damage evolution of the specimen. The static failure mechanisms was explained by fracture mechanics. Similar failure was observed in fatigue loading. It was found that with the crack growth as a parameter, the static response acts as an envelope for the fatigue failure response. The rate of the crack growth under fatigue loading follows a two-stage process: a deceleration stage followed by an acceleration stage up to failure. In the deceleration stage, the growth is governed by the R-curve of the specimen. In the acceleration stage, it is governed by the Paris Law. The previously proposed model in the biaxial tension region was extended to the biaxial compression region. In the biaxial compression region, static and fatigue behaviors under pure compressive loading were modelled in terms of inelastic displacement, rather than crack length.     

Analysis of the effect of biaxial loading on the fracture toughness of reactor pressure-vessel steels

The stress-strain state at the crack tip and its relation to the crack opening displacement and the J-integral under biaxial loading have been studied by solving elastoplastic problems in a geometrically nonlinear formulation by the finite-element method. Numerical investigations have been performed for various cracks and two modes of biaxial loading (tension and bending) under conditions of both small-scale and large-scale yielding. For prediction of the influence of biaxial loading on fracture toughness (at brittle fracture) a procedure has been developed that is based on established laws of stress-strain state formation at the crack tip under biaxial loading and a criterion of brittle fracture proposed earlier. The effect of biaxial loading on fracture toughness is predicted as applied to reactor pressure-vessel steels. Calculated results are compared with avilable experimental data. Alternative approaches to prediction of the effect of biaxial loading on fracture toughness are discussed. TsNII KM “Prometei,” St. Petersburg, Russia. Translated from Problemy Prochnosti, No. 5, pp. 5–26, September–October, 1999.     

Fatigue crack propagation model and size effect in concrete using dimensional analysis

It is well known that fatigue in concrete causes excessive deformations and cracking leading to structural failures. Due to quasi-brittle nature of concrete and formation of a fracture process zone, the rate of fatigue crack growth depends on a number of parameters, such as, the tensile strength, fracture toughness, loading ratio and most importantly the structural size. In this work, an analytical model is proposed for estimating the fatigue crack growth in concrete by using the concepts of dimensional analysis and including the above parameters. Knowing the governed and the governing parameters of the physical problem and by using the concepts of self-similarity, a relationship is obtained between different parameters involved. It is shown that the proposed fatigue law is able to capture the size effect in plain concrete and agrees well with different experimental results. Through a sensitivity analysis, it is shown that the structural size plays a dominant role followed by loading ratio and the initial crack length in fatigue crack propagation.     

Application of the biaxial Iosipescu method to mixed-mode fracture of unidirectional composites

In this paper the biaxial Iosipescu test method has been used, employing specimens with a central precrack placed along the notch-root axis, to study the intralaminar failure properties of a unidirectional carbon/epoxy composite under mixed-mode (dominated by shear) loadings. A linear finite element analysis has been performed to determine the energy release rates and stress intensity factors for the central crack under various biaxial loading conditions. In addition, a series of simple and biaxial fracture experiments have been performed on the composite material. Numerical results indicate that the method is capable of generating a wide range of mixed-mode loading conditions at the crack tip for various loading angles and crack lengths. Using the numerical results, in conjunction with experimental data, the biaxial intralaminar failure process in the cracked Iosipescu specimens has been explained.     

Strain rate effects on dynamic fracture and strength

An experimental procedure and accompanying theoretical analysis is presented to produce a well-characterized technique for quantifying dynamic fracture properties of quasi-brittle materials. An analytical and experimental investigation of mode I fracture of concrete was conducted under the dynamic loading of a split Hopkinson pressure bar. Fracture specimens in the form of notched-cavity splitting tension cylinders were subjected to stress wave loading that produced strain rates nearing 10/s. Fracture parameters were extracted by the application of the two-parameter fracture model, a nonlinear fracture model for quasi-brittle materials. Finite element analysis verified the experimental configuration and addressed inertial contributions within the dynamic environment. Ultra-high-speed digital photography was synchronized with the fracture process to provide additional validation and insight to the experimental technique. Results show that the effective fracture toughness and specimen strength both increase significantly with loading rate. The numeric and photographic results validate the experimental technique as a new tool in determining rate dependent material properties.     

The characteristics of an inclined crack in anisotropic paperboard under biaxial loading

In this paper, a fracture mechanics method was applied for the evaluation of crack behaviour in anisotropic paperboard subjected to biaxial uniform loading. The experiment was performed to determine the crack propagation angle and the fracture strength of paperboard under biaxial loading with the cruciform specimen optimized by FEM simulation. The effects of biaxial loads on the critical stress ratio and crack propagation angle for various inclination angles were investigated. The experimental results were compared with theoretical results, which were calculated by using the Normal Stress Ratio Criteria. The experimental results for crack propagation angle and critical stress show good agreement with theoretical results.     

双轴受压状态下的高延性纤维增强水泥基复合材料本构模型

基于Darwin和Pecknold考虑混凝土双轴力学行为的方法,建立一个同时考虑双轴受压状态下非线性力学行为和抗压强度变化的高延性纤维增强水泥基复合材料(ECC)二维正交各向异性本构模型。在因双轴加载而产生的正交各向异性的2个方向上引入等效单轴应变,建立非线性应力-等效单轴应变关系以考虑ECC的双轴非线性行为,并采用一条双轴强度包络线确定2个方向上的抗压强度。推导模型的显式数值算法,编写包含该算法的用户自定义材料子程序UMAT,并嵌于有限元计算程序ABAQUS v6.14中。通过对两组不同配合比的ECC试件在不同应力比下的双轴受压加载试验进行数值分析验证本模型的有效性。数值计算得到的主压应力方向上的应力-应变曲线及预测的抗压强度与试验结果吻合较好,表明该文提出的本构模型能够有效地预测ECC在双轴受压状态下的非线性力学行为和破坏强度。