Damage model and implementation in nonlinear dynamic problems

Microcracking, damage and subsequent softening in materials introduce higher levels of nonlinearity than those for materials characterized by nonlinear elastic or classical plasticity models. Hènce, implementation of such advanced models that allow for the foregoing effects require special considerations in terms of the analysis of the characteristics of the model, convergence during plastic deformations, and time integration schemes that consider the nonlinearity.This paper describes a damage model, a special scheme involving drift correction and the generalized time finite element (GTFEM) scheme for time integration for dynamic analysis. The main objective is to examine the model and develop schemes that can lead to consistent and reliable predictions from computational procedures. Toward this aim, (1) the damage model is analyzed with respect to its convergence behavior with mesh refinement, (2) a special drift correct scheme is implemented for the plasticity based model, (3) the generalized time finite element method (GTFEM) is implemented in the nonlinear dynamic finite element procedure for time integration and compared with the Newmark method, and (4) the damage model, the drift correction scheme and the GTFEM are verified by solution of representative static and dynamic problems involving a material (concrete) that experiences damage and softening, including verification with respect to behavior of concrete in the laboratory.     

Experimental study on scaling of RC beams under close-in blast loading

Damage effects analysis and assessment of buildings under blast loading is an important problem concerned by the area of explosion accident analysis, blast-resistant design, anti-terrorist and military weapon design.The damage character of RC beam under close-in blast loading is investigated through experiments. The damage modes and damage levels of RC beams are studied under different blast loads. The results show that the spallation area increases with the decrease of the scaled distance. The concrete beams are prone to be damaged in flexure mode with concrete crushed on the front face, concrete spallation on the back surface and concrete flake off on the side surface. The scaling of the dynamic response of reinforced concrete beams subjected to close-in blast loadings is also studied. The test results showed a similar macrostructure damage and fracture in all experiment conditions. But the local damage degree of RC beams with smaller size has been reduced a little as compared with that of beams with larger size. Based on the results, empirical equations of the center deflection to height ratio are proposed to correct scaling model considering size effects.     

Mesoscale modelling and analysis of damage and fragmentation of concrete slab under contact detonation

It is interesting and important for researchers to understand the damage process in order to reliably predict fragment distribution of concrete material under blast loading. In the present study, a mesoscale concrete model is developed to simulate the dynamic failure process of a concrete slab under contact detonation. In the mesoscale model, the concrete material is assumed to consist of two phases, that is, the high strength coarse aggregates and the low strength mortar matrix, randomly distributed in the structure components. Each coarse aggregate is assumed to be circular with a random radius in a given distribution range following the Fuller's curve. The mesoscale model together with a dynamic plastic damage material model is incorporated into the hydrocode AUTODYN. The dynamic damage process of the concrete slab under contact detonation is numerically simulated. Based on the numerical results, the fragment size distribution is estimated by an image analysis program. Two different random aggregate distributions are assumed in the present simulations. Numerical results from the two different cases are compared, and the results from the mesoscale model are compared with that from the homogeneous concrete material model. The fragment size distributions obtained from numerical simulations are also compared with those from the empirical statistic formulae.     

聚丙烯纤维对混凝土损伤渗透特性的影响

为研究损伤开裂后聚丙烯纤维/混凝土(PPF/PC)的渗透特性,通过圆盘劈裂试验预制了不同宽度的裂缝(100~400 μm),比较了PPF掺量对裂缝曲折度的影响。利用自行设计的渗透试验装置对混凝土损伤后的渗透率进行了研究,分析了不同PPF掺量、不同水压力下混凝土的损伤渗透率的变化规律。通过研究发现,加入PPF后,混凝土脆性较低,内部裂缝更易控制,裂缝曲折度相对于PC更高,且与PPF掺量成正比;相同水压力,相同有效裂缝宽度条件下,随着PPF掺量的增加,混凝土损伤渗透率降低,PPF的存在能够有效提高混凝土损伤后的抗渗性能;相同水压力下,相同PPF掺量的混凝土试件损伤渗透率整体上与有效裂缝宽度成正比;不同水压力下,相同PPF掺量的PPF/PC损伤渗透率在同一有效裂缝宽度情况下,随水压力增加而减小;修正后的泊肃叶渗流模型可以更好地评价PPF/PC损伤渗透特性。      

钢筋混凝土剪力墙应变率效应试验与基于动力塑性损伤模型的模拟

应变率对混凝土材料的力学性能有着重要影响。该文在对2 片剪力墙构件进行快速加载试验的基础上,在混凝土塑性损伤模型中引入应变率效应,建立了考虑应变率效应的塑性损伤模型。应用该模型对该文所描述的快速加载下的剪力墙构件以及在拟静力作用下的另外3 个构件进行了有限元模拟,结果表明考虑应变率效应的混凝土塑性损伤模型较好地描述了钢筋混凝土剪力墙在快速加载下的非线性行为。最后,运用所建立的考虑应变率效应的混凝土塑性损伤模型对不同轴压比和加载速率下剪力墙构件的非线性性能进行了模拟与分析。     

基于材料损伤的钢筋混凝土构件损伤模型

基于地震作用下钢筋混凝土结构的损伤机理, 以钢材和混凝土材料损伤模型为基础, 推导出损伤状态下钢筋混凝土构件刚度和材料自由能退化表达式, 提出一种以刚度退化和自由能退化线性组合的钢筋混凝土构件损伤模型. 对钢筋混凝土柱的拟静力试验进行模拟分析, 结果表明该模型可以较好的描述钢筋混凝土柱的损伤演化过程, 且能够定量确定其在单向荷载作用下与之正交方向的损伤程度;对钢筋混凝土桥墩的振动台试验数据和钢筋混凝土刚构桥的数值算例进行了模拟分析, 结果表明基于材料损伤的钢筋混凝土构件损伤模型可以描述钢筋混凝土桥墩抗震能力的退化过程以及预测结构的失效模式, 可用于地震作用下桥梁结构的损伤分析.     

高性能混凝土研究进展Ⅱ:耐久性能及寿命预测模型

综述了高性能混凝土耐久性能方面的研究进展,包括高性能混凝土的抗氯离子渗透性能、抗冻融性能、抗碳化性能、抗盐侵蚀性能以及多种因素耦合作用下的耐久性能等,介绍了与高性能混凝土耐久性相关的损伤模型和寿命预测模型,并分析了高性能混凝土耐久性能研究现存的一些问题。分析发现:高性能混凝土的耐久性能受材料种类、掺量、实验条件等因素的影响,适量的矿物掺合料和外加剂能够减少混凝土内部有害孔的数量,增加结构的密实度,提高混凝土的耐久性能。多种因素耦合作用下的混凝土耐久性及损伤模型、寿命预测模型等更接近结构所处环境的实际情况,这将会成为高性能混凝土耐久性方面研究的一个热点。     

钢筋混凝土柱基于易损性曲线的地震损伤评估

为评估地震作用下结构构件的损伤程度,建立基于竖向剩余承载力的损伤指标,提出钢筋混凝土柱基于易损性曲线的地震损伤评估方法。该方法对钢筋混凝土柱采用精细化分析模型,避免单自由度体系假设,通过增量动力分析获得对数正态分布形式的易损性曲线,进行地震损伤程度评估。研究表明:所提出的地震损伤评估方法,计算简单,能有效评估钢筋混凝土柱在不同地震动强度指标下的损伤程度,为震后钢筋混凝土结构的安全评估和修复加固提供了理论依据。     

基于有效耗能的改进Park-Ang双参数损伤模型及其计算研究

Park-Ang双参数地震损伤模型考虑了地震作用的首次超越破坏与累积损伤破坏两方面因素,较好的定义了结构的破坏,被后续研究广泛应用,但无法区分不同幅值作用下结构破坏的差异。分析了不同位移幅值下钢筋混凝土柱的破坏特点,研究了结构损伤与结构耗能之间的关系。试验表明弹性阶段(位移幅值小于一倍屈服位移)的试件几乎不发生破坏,造成钢筋混凝土柱破坏的能量集中在相持阶段和破坏阶段,定义非弹性阶段引起结构破坏的滞回耗能为导致结构破坏的有效耗能。基于有效耗能假设引入有效耗能因子e,提出了改进的Park-Ang双参数地震损伤模型。对建议模型进行了21组钢筋混凝土柱的试验数据验证,计算结果表明:有效耗能因子可以反映相同耗能下不同位移幅值引起的结构破坏差异,有效耗能因子e物理意义明确,改进后的双参数地震损伤模型计算精度高,离散性小,能够区分不同位移幅值下钢筋混凝土柱的破坏差异,较好地评估了RC结构的损伤性能。     

一种新的混凝土各向异性弹塑性损伤本构模型及其数值实施

该文的目的是建立一种新的、相对简单的混凝土各向异性塑性损伤本构模型,以方便的模拟混凝土结构的破坏行为。为了更好地描述混凝土在拉、压荷载作用下的不同损伤机制,建立了拉、压不同的两种损伤演化方程,用于确定各向异性的拉、压损伤变量。另外,根据应变等效假设,假定有效构型和损伤构型的应变相等,该方法不仅大大简化了模型的推导过程,而且可方便的通过解耦算法进行有效应力和损伤及名义应力的计算,也即塑性部分计算可通过现有的隐式算法实现,损伤部分及名义应力的计算则可通过较为简便的显式算法实现,从而可大大提高计算效率。模型结果与试验结果的对比分析表明：该模型能较好地描述混凝土在三维应力状态下的非线性行为;对双边开口四点弯曲梁试件的模拟也表明：该模型能反应混凝土损伤各向异性的特点,计算结果相比ABAQUS软件自带的混凝土损伤塑性本构模型(CDP模型)更符合实际情况,计算效率也更高。     

考虑过渡区界面影响的混凝土宏观力学性质研究

混凝土材料的宏观力学特性及破坏机理由其细观组分来决定,界面过渡区是影响混凝土断裂破坏路径及宏观力学特性的重要因素。认为界面过渡区是区别于远处砂浆基质的一层含较高孔隙率的近场砂浆材料,采用“两步等效法”得到了混凝土细观单元的等效本构关系模型。最后基于细观单元等效化方法分析了在单轴拉伸、单轴压缩及弯拉载荷条件下混凝土试件的破坏过程及宏观力学性质,探讨了界面过渡区对混凝土力学特性的影响,并与随机骨料模型分析结果进行了对比。结果表明:界面相的存在对混凝土的弹性模量、强度及残余强度等力学性质有很大影响,在对混凝土宏观力学特性及细观断裂破坏过程进行研究时不可忽略其影响。     

冲击荷载作用下混凝土结构破坏过程的近场动力学模拟

混凝土在冲击、侵彻等动载荷作用下产生损伤和破坏的过程,其实质是力学模型从连续体到非连续体的转变过程。传统的连续介质理论基于连续性假设并运用偏微分方程求解问题,难以直接用于计算和模拟材料及结构发生破坏的整个过程。近场动力学(Peridynamics,PD)是一种新兴的基于非局部模型描述材料特性的数值计算方法。该方法假定位于连续体内的粒子通过有限的距离与其它粒子相互作用,通过积分计算在一定近场范围(horizon)内具有一定影响域的材料点之间的相互作用力,而不论位移场的连续与否,避免了传统的局部微分方程求解在面临不连续问题时的奇异性和现有多尺度算法的复杂性。该文概述了PD方法的理论基础,描述了其建模思路及计算体系,给出了用近场动力学方法模拟结构受冲击荷载的计算格式。算例结果表明:PD方法可以很好地刻画和模拟材料及结构的损伤累积与渐进破坏过程。最后讨论了PD方法在理论、计算和应用等方面有待进一步研究的问题。     

非均质混凝土材料破坏的三维细观数值模拟

在随机骨料模型的基础上,采用特征单元尺度对网格进行剖分,建立了非均质混凝土材料损伤破坏及宏观力学特性研究的三维细观单元等效化模型。对单轴拉伸、单轴压缩条件下湿筛混凝土试件的破坏过程及宏观力学性能进行了模拟分析;研究了混凝土梁的三分点弯拉力学特性,并与平面模型结果作了对比。研究表明:1) 与平面计算模型相比,三维模型更真实地模拟混凝土材料在外荷载作用下的损伤破坏过程,更准确的描述非均质混凝土材料的宏观力学性能,且与实验结果吻合;2) 骨料空间分布形式基本不影响混凝土材料的宏观弹性模量及强度,但影响其破坏过程和破损路径;3) 与随机骨料模型等细观力学方法相比,该方法具有高效性。     

地铁隧道钻爆法施工影响下混凝土结构损伤预测方法研究

结合青岛地铁隧道下穿建筑物的工程实践,通过数值模拟的方法,分析了爆破振动影响下混凝土结构的应力分布,基于Ottosen和过-王2种混凝土破坏准则,推导了适用于混凝土材料的屈服接近度模型,进而得出了混凝土结构的损伤分布范围和演化过程。以地表沉降实测数据为基础,利用随机介质理论方法反分析地表移动参数,并利用所得的参数对下穿建筑物时进行了沉降预测,由此实现了对不同沉降量和爆破振速共同影响下建筑结构开裂损伤的量化评估与控制研究。结果表明：在建筑结构的开裂损伤演化中,地表沉降的影响程度远大于爆破振动的影响程度。     

混凝土随机损伤本构模型研究新进展

综述了混凝土微观损伤机理与随机损伤本构模型研究现状,介绍了本课题组在混凝土随机损伤本构模型方面取得的研究进展:进行了考虑mode-II微裂缝的微观损伤机理分析,提出并验证了混凝土随机损伤本构模型——束-链模型,发展了测定多轴受压混凝土损伤变量的X-射线CT方法。最后,得到了相关研究结论。     

Damage prediction of concrete gravity dams subjected to underwater explosion shock loading

The damage prediction of concrete gravity dams under blast loads has gained importance in recent years due to the great number of accidental events and terrorist bombing attacks that affected engineering safety. It has long been known that an underwater explosion can cause significantly more damage to the targets in water than the same amount of explosive in air. While the physical processes during an underwater explosion and the subsequent response of structures are extremely complex, which involve lots of complex issues such as the explosion, shock wave propagation, shock wave-structure interaction and structural response. Hence a sophisticated numerical model for the loading and material responses would be required to enable more realistic reproduction of the underlying physical processes. In this paper, a fully coupled numerical approach with combined Lagrangian and Eulerian methods, incorporating the explosion processes, is performed. The RHT (Riedel–Hiermaier–Thoma) model including the strain rate effect is employed to model the concrete material behavior subjected to blast loading. Detailed numerical simulation and analysis of a typical concrete gravity dam subjected to underwater explosion are presented in this study. In terms of different TNT charge weights, the structural response and damage characteristics of the dam at different standoff distances are investigated. Based on the numerical results, critical curves related to different damage levels are derived.     

混凝土结构冻融损伤理论及冻融可靠度分析

该文基于疲劳累积损伤理论,对混凝土的冻融损伤特性进行了可靠性分析。把混凝土结构冻融破坏近似看作由不同正负峰值温度差顺序作用产生的疲劳累积损伤破坏,给出了常幅温差及变幅温差作用下基于疲劳累积损伤的冻融可靠度计算分析模型,并对某一混凝土构件的冻融损伤可靠度进行了计算分析。     

Constitutive modeling of creep-fatigue interaction for normal strength concrete under compression

Conventional approaches to model fatigue failure are based on a characterization of the lifetime as a function of the loading amplitude. The Wöhler diagram in combination with a linear damage accumulation assumption predicts the lifetime for different loading regimes. Using this phenomenological approach, the evolution of damage and inelastic strains and a redistribution of stresses cannot be modeled. The gradual degration of the material is assumed to not alter the stress state. Using the Palmgren–Miner rule for damage accumulation, order effects resulting from the non-linear response are generally neglected.In this work, a constitutive model for concrete using continuum damage mechanics is developed. The model includes rate-dependent effects and realistically reproduces gradual performance degradation of normal strength concrete under compressive static, creep and cyclic loading in a unified framework. The damage evolution is driven by inelastic deformations and captures strain rate effects observed experimentally. Implementation details are discussed. Finally, the model is validated by comparing simulation and experimental data for creep, fatigue and triaxial compression.     

Modeling of plain concrete structures based on an anisotropic damage formulation

In this contribution, a strain-based constitutive law for concrete within the framework of continuum damage mechanics is proposed. The model allows for the multi-axial simulation of predominantly tensile loaded plain concrete structures. A second-order integrity tensor is chosen as the internal damage variable to consider the phenomenon of load-induced anisotropy. The model is implemented in the finite element method. Hence, a fracture-energy based regularization approach is included to overcome the mesh-dependence of the computational results. The proposed model is applied to the simulation of well-known experiments with plain concrete specimens, which are often used to demonstrate the applicability of damage constitutive laws. In this context, attention is focused especially on problems regarding the accurate implementation of the complex loading and boundary conditions. Oversimplifications are discussed and proposals for a more realistic involvement of the test setups in finite element models are submitted.     

3D constitutive model of anisotropic damage for unidirectional ply based on physical failure mechanisms

A 3D anisotropic continuum damage model is developed for the computational analysis of the elastic–brittle behaviour of fibre-reinforced composite. The damage model is based on a set of phenomenological failure criteria for fibre-reinforced composite, which can distinguish the matrix and fibre failure under tensile and compressive loading. The homogenized continuum theory is adopted for the anisotropic elastic damage constitutive model. The damage modes occurring in the longitudinal and transverse directions of a ply are represented by a damage vector. The elastic damage model is implemented in a computational finite element framework, which is capable of predicting initial failure, subsequent progressive damage up to final collapse. Crack band model and viscous regularization are applied to depress the convergence difficulties associated with strain softening behaviours. To verify the accuracy of the damage model, numerical analyses of open-hole laminates with different lay-up configurations under tension and compression were performed. The numerical predictions were compared with the experimental results, and satisfactory agreement was obtained.