大理岩三点弯曲梁弹塑性损伤破坏研究

在应变空间内,推导出弹塑性损伤增量本构方程,通过自编程序实现该本构方程,同时将此程序实现了并行算法;建立了三点弯曲梁二维及三维细观尺度数值试样,分别进行相应地弹塑性损伤破坏数值模拟,作了相应的对比分析,在数值模拟中,提出破坏单元网格消去法,模拟裂纹扩展。研究发现:数值试验同物理试验较吻合,且与理论分析是一致的;由于三维试样细观单元的非线性比二维在空间上更为离散,由三维细观单元弹塑性损伤非线性反映宏观试样的非线性,更能深入地研究细观破坏机理;提出的破坏单元网格消去法为有限元清晰模拟裂纹开辟了新的途径;并行算法程序的实现为应用于大型三维水利水电工程奠定了基础。     

基于网格自适应的钢筋混凝土随机有限元分析

为了在钢筋混凝土结构有限元分析模型中对裂缝进行描述,建立了基于网格自适应的有限元裂缝分析模型,可以比较直观地模拟裂缝的发生和开展过程;同时引入随机模拟方法,采用蒙特卡罗(Monte Carlo)法处理与裂缝有关的参数。用该方法能够模拟钢筋混凝土结构裂缝出现位置和间距的随机性,而且能改进计算数据的后处理。对跨中受力的钢筋混凝土简支梁进行计算分析,所得裂缝分布和位移均与试验结果吻合良好。结果表明,用该方法对钢筋混凝土结构进行分析是合理的,也是可行的。     

Influence of Semi‐Rigid Connections and Local Joint Damage on Progressive Collapse of Steel Frameworks

Abstract: Semi‐rigidly connected steel frames are more vulnerable than rigidly connected frames to resist progressive collapse due to abnormal loading events. This article extends the threat‐independent method for progressive‐failure analysis of rigid frames to analysis accounting for semi‐rigid connections. The influence of joint damage caused by disengagement of member(s) is also considered in the analysis, and the degree of damage is modeled by a health index. A compound element model is employed to include the contributions of nonlinear behavior of beam‐to‐column connections, connection and member‐end damage, member inelasticity, member shear deformation, and geometrical nonlinearity to structural response. Four beam collapse modes are illustrated for the progressive collapse analysis associated with debris loading generated when disengaged structural components fall onto lower parts of the structure. The impact effect is taken into account for the quasi‐static nonlinear analysis by utilizing an impact amplification factor according to GSA and DoD guidelines. Any progressive collapse occurring thereafter involves a series of collapse events associated with topological changes of the frame. The analysis procedure is illustrated for the progressive collapse behavior of two planar steel frames. The results demonstrate that the proposed method is potentially an effective tool for the progressive collapse analysis of semi‐rigid steel frames under abnormal loading events.     

Modeling of Tension Stiffening Behavior in FRP‐Strengthened RC Members Based on Rigid Body Spring Networks

Abstract: Allowing for the tension stiffening effects resulting from the bond between steel reinforcement and surrounding concrete leads to effective deformation analysis of reinforced concrete (RC) members when using a nonlinear finite element analysis modeled on the smeared crack concept. Nowadays, externally bonded fiber reinforced polymer (FRP) composites are widely used for strengthening existing RC structures. However, it remains unclear to what extent the tension stiffening of postcracking concrete is quantitatively influenced by the addition of FRP composites, as a result of the bond between the FRP and the concrete substrate. This article presents a discrete model, which is based on rigid body spring networks (RBSN), for investigating the tension stiffening behavior of concrete in FRP‐strengthened RC tensile members. A two‐parameter fracture energy‐based model was deployed to represent the bond‐slip behavior of the FRP‐to‐concrete interface. The reliability of the RBSN model was verified through comparisons with previous test results. Further parametric analysis indicates that the tension stiffening of concrete is hardly influenced by the addition of FRP composites before the yield of steel reinforcement has occurred although concrete crack patterns and crack widths may be influenced by the bond‐slip behavior of the FRP‐to‐concrete interface.     

Effects of local rock heterogeneities on the hydromechanics of fractured rocks using a digital-image-based technique

A digital-image-based (DIB) finite element approach is developed based on the numerical code rock failure process analysis (RFPA) to characterize micro-scale rock heterogeneity, and to understand the impact of micro-scale rock heterogeneity on the macro-scale hydromechanical response of rocks. The DIB technique incorporates small-scale spatial variability of initial deformation modulus, strength and permeability directly into a coupled hydromechanical model. Variability in Young's modulus, strength, and permeability is applied by a property map defined from the pixel-scale of a digital image. In the RFPA, mechanical deformation is followed, including the accumulation of damage applied in individual elements, which modifies modulus, strength, and permeability with the intensity of damage. The RFPA simulates progressive failure in fractured rocks, representing both the growth of existing fractures and the formation of new fractures, without having to identify crack tips and their interaction explicitly. In this DIB simulation approach, image voxels are used to give equivalent mechanical and flow properties. These property maps are ported to the model capable of solving directly for the evolving deformation, and fluid flow fields. The model is validated through comparisons of the simulated results with phenomenological observations documented in previous studies. The validated model is then applied to investigate the hydromechanical response of fractured rock characterized by digital image. The model is able to reproduce the spatial evolution of damage in the sample, the coalescence of existing cracks, and the formation of new cracks.     

Simulation of cohesive crack growth by a variable-node XFEM

A new computational approach that combines the extended finite element method associated with variable-node elements and cohesive zone model is developed. By using a new enriched technique based on sign function, the proposed model using 4-node quadrilateral elements can eliminate the blending element problem. It also allows modeling the equal stresses at both sides of the crack in the crack-tip as assumed in the cohesive model, and is able to simulate the arbitrary crack-tip location. The multiscale mesh technique associated with variable-node elements and the arc-length method further improve the efficiency of the developed approach. The performance and accuracy of the present approach are illustrated through numerical experiments considering both mode-I and mixed-mode fracture in concrete.     

应变硬化水泥基复合材料加固钢筋混凝土构件弯曲破坏分析方法研究

依据应变硬化水泥基复合材料单向拉伸试验结果,提出评价该类材料单向拉伸特性的考虑断裂能的三折线模型。提出采用虚拟材料的零跨拉伸模型反映应变硬化水泥基复合材料加固层紧邻既有钢筋混凝土构件裂缝处的细小裂缝扩展过程,得到应变硬化水泥基复合材料加固既有钢筋混凝土构件时加固层数值单元的单向拉伸平均应力-应变曲线,该曲线考虑紧邻混凝土梁某裂缝的加固层细小裂缝(置于同一单元内)对单元单向拉伸应力-应变曲线的影响,继而以该平均应力 应变曲线作为加固层单元的应变硬化水泥基复合材料单向拉伸本构关系。该方法可避免不同单元尺寸对数值分析结果的影响,因此,对该类材料加固既有钢筋混凝土构件可以采用较大尺寸的单元划分进行数值分析。     

Path-dependent nonlinear analysis of a concrete reactor containment vessel subjected to internal pressure using a volume control technique

The so-called volume control method has overcome the drawbacks of the load controlled method and the displacement controlled method by adding a pressure node which has an increment of pressure as an additional degree of freedom to a finite element. In this paper, a nonlinear volume control finite element method has been developed for the nonlinear analysis of reinforced concrete containment reactor vessels (RCCV) and prestressed concrete containment reactor vessels (PCCV), which can predict the ultimate internal pressure capacity of a containment reactor vessel subjected to accidental internal pressure. A multi-layered shell element with a pressure node is adopted for analysis by utilizing a path-dependent crack model, and an orthogonal two-way fixed crack model along with an averaged in-plane 2D RC model and a prestressing tendon model for the layered shell elements. A reinforced concrete panel subjected to biaxial tension, and RCCV and PCCV subjected to internal pressure are also analyzed. Both applicability and validity of the proposed method are shown by comparing the results of volume control method with both experimental results and existing analysis results.     

混凝土局部损伤裂缝带模型研究

为消除混凝土开裂使有限元分析结果对单元网格尺寸产生的依赖性,在基于局部本构模型的材料非线性有限元分析中,建立了混凝土局部损伤裂缝带模型。结果表明：该模型所采用的三参数幂函数型拉伸损伤演化方程,可满足不同单元网格尺寸下应力-应变关系调整的需要;在局部损伤本构模型中引入断裂能和与单元网格尺寸相关的裂缝带宽度,可在保证断裂能客观性的前提下依据初始应力-开裂应变关系推导不同单元网格尺寸下的应力-开裂应变关系;提出基于异步粒子群智能算法的优化反演方法,可实现在有限元分析中依据应力-开裂应变关系自适应,从而确定不同单元网格尺寸下的模型参数。在ABAQUS软件平台上采用所建立的模型对算例进行了数值模拟,验证了模型的有效性与程序编制的正确性。     

Local damage and crack band model for concrete

为消除混凝土开裂使有限元分析结果对单元网格尺寸产生的依赖性，在基于局部本构模型的材料非线性有限元分析中，建立了混凝土局部损伤裂缝带模型。结果表明：该模型所采用的三参数幂函数型拉伸损伤演化方程，可满足不同单元网格尺寸下应力-应变关系调整的需要；在局部损伤本构模型中引入断裂能和与单元网格尺寸相关的裂缝带宽度，可在保证断裂能客观性的前提下依据初始应力-开裂应变关系推导不同单元网格尺寸下的应力-开裂应变关系；提出基于异步粒子群智能算法的优化反演方法，可实现在有限元分析中依据应力-开裂应变关系自适应，从而确定不同单元网格尺寸下的模型参数。在ABAQUS软件平台上采用所建立的模型对算例进行了数值模拟，验证了模型的有效性与程序编制的正确性。     

A two-dimensional linear variation displacement discontinuity method for three-layered elastic media

A new displacement discontinuity method is developed for the analysis of multi-layered elastic media. This approach is based on a novel superposition scheme and the analytical solution to the problem of a displacement discontinuity element within bonded half-planes. A three-layered elastic region is obtained by superposing two sets of bonded half-planes and subtracting one infinite plane. The advantages of this approach are: (1) it is not necessary to introduce elements at the interface, (2) the method is applicable for three dimensional modelling and (3) it can be extended to an N (N>3) layer system easily. The accuracy of the model is illustrated by comparing the numerical results with the analytical solutions for a hole in an infinite strip in tension and with the numerical solution for a pressurized crack within a three-layered system. In order to show the efficacy of the developed model, the simulation results of a typical South African mining problem and general tunnel excavation problems are also presented.     

Effect of interfacial imperfection on bending behavior of composites and sandwich laminates by an efficient C0 FE model

The bending behavior of composites and sandwich plates having imperfections at the layer interfaces is investigated by a refined higher order shear deformation plate theory (RHSDT) and a Least Square Error (LSE) method. In this theory, the in-plane displacement field is obtained by superposing a globally varying cubic displacement field on a zig-zag linearly varying displacement field. This plate theory represents parabolic through thickness variation of transverse shear stresses which satisfy the inter-laminar continuity condition at the layer interfaces and zero transverse shear stress condition at the top and bottom of the plate. In this plate model, the interfacial imperfection is represented by a liner spring-layer model. Finite element method is adopted and an efficient C⁰ continuous 2D finite element (FE) model is developed based on the above mentioned plate theory for the static analysis of composites and sandwich laminates having imperfections at the layer interfaces. In this model, the first derivatives of transverse displacement have been treated as independent variables to circumvent the problem of C¹ continuity associated with the above plate theory (RHSDT). The LSE method is applied to the 3D equilibrium equations of the plate problem at the post-processing stage, after in-plane stresses are calculated by using the above FE model based on RHSDT. The proposed model is implemented to analyze the laminated composites and sandwich plates having interfacial imperfection. Many new results are also presented which should be useful for the future research.     

A hybrid approach to detect crack parameters using measured natural frequencies in thin walled angle section steel beams

Present study outlines a hybrid approach using Finite Element Method (FEM)–Response Surface Method (RSM)–Genetic Algorithm (GA) to predict the crack parameters, namely crack position and crack depth ratio with the help of only the measured natural frequencies of cracked thin walled beams. Numerical experimental trials of cracked beams have been conducted based on design of experiment (DOE) approach using an improved Finite element model. The improvement has been achieved by consideration of warping stiffness in cracked angle section beam. Thereafter, regression analysis has been conducted to construct Response Surface Function (RSF). Optimum crack parameters were then calculated using GA by minimizing an objective function which has been formed as the root mean square (RMS) of the residuals between RSFs and measured frequencies. Results of the study indicate that, the proposed approach performs with excellent accuracy and it does not require the response of an uncracked beam as benchmark information.     

Composite wedge failure using photogrammetric measurements and DFN-DEM modelling

Analysis and prediction of structural instabilities in open pit mines are an important design and operational consideration for ensuring safety and productivity of the operation. Unstable wedges and blocks occurring at the surface of the pit walls may be identified through three-dimensional (3D) image analysis combined with the discrete fracture network (DFN) approach. Kinematic analysis based on polyhedral modelling can be used for first pass analysis but cannot capture composite failure mechanisms involving both structurally controlled and rock mass progressive failures. A methodology is proposed in this paper to overcome such limitations by coupling DFN models with geomechanical simulations based on the discrete element method (DEM). Further, high resolution photogrammetric data are used to identify valid model scenarios. An identified wedge failure that occurred in an Australian coal mine is used to validate the methodology. In this particular case, the failure surface was induced as a result of the rock mass progressive failure that developed from the toe of the structure inside the intact rock matrix. Analysis has been undertaken to determine in what scenarios the measured and predicted failure surfaces can be used to calibrate strength parameters in the model.     

点腐蚀裂纹的应力强度因子分析

由点腐蚀诱发的裂纹分为穿透型裂纹和表面型裂纹两种形式,通过半解析半数值方法,分别给出两种裂纹形式的应力强度因子表达式。对于穿透型裂纹,其应力强度因子表达式基于有限宽板上的圆孔周界散射出的裂纹的应力强度因子推导得到,并采用有限元方法进行验证,验证结果表明表达式与有限元方法吻合良好;对于表面型裂纹,首先采用有限元方法进行系列计算,根据计算结果进行回归分析并得到应力强度因子的回归公式,然后对回归公式进行显著性检验,检验结果表明回归公式是显著成立的。     

Use of Ritz method for damage detection of reinforced and post-tensioned concrete beams

Damage-induced changes in modal characteristics can be detected using experimental modal analysis. In this article, based on changes in natural frequency, mode shapes, and damping ratios, a methodology for detecting damage location and severity is presented. The damage was induced by application of point load at half span location on the reinforced and post-tensioned concrete beams. The load was gradually increased to obtain different crack patterns to be used in simulation of damage scenarios. Experimental modal analysis was performed on the undamaged and damaged beams. The natural frequency and mode shapes were used to determine the location of damage. The approach is developed at an element level with a conventional finite element (FE) model by Ritz method, which is called Ritz damage detection method (RDDM). The mathematical model for both damped and undamped damaged structures have been established through the eigenvalue equations. The singular value decomposition (SVD) technique is used for determination of damage or sound index. These indexes are sensitive to the change of dynamic characteristics due to damages. This approach is applied to five simply supported post-tensioned concrete beams. The numerical results show that the exact location and severity of damage for different simulated damage scenarios could be efficiency found by the present methodology.     

Analysis of fatigue crack growth behavior in asphalt concrete material in wearing course

Fatigue crack is a main form of structural damage in flexible pavements. Under the action of repeated vehicular loading, deterioration of the asphalt concrete materials in pavements, caused by the accumulation and growth of micro and macro cracks, gradually takes place. The indirect tensile fatigue testing (ITFT) was carried out on three types of asphalt concrete materials comprising a high stiffness Gilsonite modified wearing course (GM-ACWC), stone mastic asphalt (SMA) and conventional asphalt concrete wearing course (ACWC) to establish fatigue damage model and failure criteria. Fatigue damage model, based on continuum damage approach, to describe the formation of micro-cracks and crack propagation was developed for the wearing course materials. With the fatigue damage model, finite element analysis was carried out to study the crack resisting performance of the three wearing course materials in a flexible pavement structure.     

基于连续介质离散元的双重介质渗流应力耦合模型

 为模拟边坡在库水涨落和降雨作用下的渐进破坏过程,发展基于连续介质离散元的双重介质渗流应力耦合模型。它分为固体计算模型、孔隙渗流应力耦合模型和裂隙渗流应力耦合模型3个部分。固体计算模型能够反映地质体的破坏规律,可以模拟从连续到非连续的破坏过程;孔隙渗流应力耦合模型可以方便地计算出自由水位线(浸润线)的位置;裂隙渗流应力耦合模型可以避免由于不连通裂隙(孤立裂隙)存在所导致的收敛性问题;假设只考虑库水涨落和降雨的最终状态,裂隙渗流产生的水头分布和由库水涨落所引起的水头改变作为孔隙渗流的变边界条件,从而实现孔隙渗流场和裂隙渗流场的耦合。典型算例计算结果表明,基于连续介质离散元的双重介质渗流应力耦合模型对于库区古滑坡的研究是很有效的。     

基于水平集坐标的二维压剪节理动态扩展过程无网格法模拟研究

 发展能有效模拟岩体节理拉压破坏动态扩展过程的无网格模拟方法,构造岩体节理张开和闭合接触摩擦行为的无网格法近似函数,采用水平集坐标描述和捕捉节理在扩展过程中的几何信息。当节理张开时,引入断裂线,并采用衍射准则引入位移跳跃项。当节理闭合时,提出类似于Goodman接触摩擦单元的无网格近似函数构造方法,考虑节理面上、下两侧的相对滑动和法向接触。与Goodman单元或以往的无网格模拟不同,本文的接触方法基于材料点而非结点,并与水平集坐标更新算法结合,在处理节理模型生成和动态扩展后的模型更新上更灵活、简便,且具有通用性。通过测试拉剪、压剪破坏单节理和雁行节理的静态和动态破坏扩展过程的算例,验证了本文方法的正确性和有效性。     

基于压电主动传感技术的高温后PVA-ECC梁冲击损伤监测研究

研究了低速冲击荷载作用下具有高温损伤的聚乙烯醇纤维增强水泥基复合材料（PVA-ECC）梁的损伤特征,对比分析不同温度损伤与室温下的PVA-ECC梁,在同一跌落高度和落锤质量上进行了一系列的落锤低速冲击试验,模拟冲击能量对梁的影响结果。采用压电陶瓷智能骨料传感器的主动监测方法和扫频波信号,监测落锤低速冲击作用下PVA-ECC梁的裂纹产生、发展、断裂全过程与波衰减的规律。基于小波包能量法分析重复冲击试验下的PVA-ECC梁裂缝发展演化。建立了PVA-ECC梁三维有限元模型,通过有限元分析得出300 ℃高温加热后的PVA-ECC梁低速冲击裂纹开展全过程,并与实测结果进行对比。结果表明:高温损伤造成PVA-ECC梁的抗冲击性能减弱;室温下的PVA-ECC梁有一定抗冲击能力,但当温度达到PVA纤维的熔点（230 ℃）时,PVA-ECC梁中的PVA纤维消失,产生孔隙,形成素水泥砂浆梁,不具有抗冲击能力;高温损伤造成PVA-ECC丧失了高强度、韧性、耐疲劳的能力特性。