Cohesive fracture simulation and failure modes of FRP–concrete bonded interfaces

A work-of-fracture method using three-point bend beam (3PBB) specimen, commonly employed to determine the fracture energy of concrete, is adapted to evaluate the mode-I cohesive fracture of fiber reinforced plastic (FRP) composite–concrete adhesively bonded interfaces. In this study, a bilinear damage cohesive zone model (CZM) is used to simulate cohesive fracture of FRP–concrete bonded interfaces. The interface cohesive process damage model is proposed to simulate the adhesive–concrete interface debonding; while a tensile plastic damage model is used to account for the cohesive cracking of concrete near the bond line. The influences of the important interface parameters, such as the interface cohesive strength, concrete tensile strength, critical interface energy, and concrete fracture energy, on the interface failure modes and load-carrying capacity are discussed in detail through a numerical finite element parametric study. The results of numerical simulations indicate that there is a transition of the failure modes controlling the interface fracture process. Three failure modes in the mode-I fracture of FRP–concrete interface bond are identified: (1) complete adhesive–concrete interface debonding (a weak bond), (2) complete concrete cohesive cracking near the bond line (a strong bond), and (3) a combined failure of interface debonding and concrete cohesive cracking. With the change of interface parameters, the transition of failure modes from interface debonding to concrete cohesive cracking is captured, and such a transition cannot be revealed by using a conventional fracture mechanics-based approach, in which only an energy criterion for fracture is employed. The proposed cohesive damage models for the interface and concrete combined with the numerical finite element simulation can be used to analyze the interface fracture process, predict the load-carrying capacity and ductility, and optimize the interface design, and they can further shed new light on the interface failure modes and transition mechanism which emulate the practical application.     

混凝土K&C模型材料参数分析与模拟

混凝土K&C模型材料参数一般取国外文献中的原始数值,没有根据混凝土强度等级和单元尺寸的不同而作相应的调整．根据相关的试验研究成果,提出了一种确定K&C模型强度参数值的方法,并阐述了K&C模型损伤参数值的调整方法,使得数值计算结果更加合理．运用有限元显式动力分析软件ANSYS/LSDYNA,采用流固耦合方法模拟爆炸荷载作用下钢筋混凝土板的动态响应,混凝土K&C模型取本文确定的参数值,计算结果与试验结果吻合较好,从而验证了K&C模型材料参数取值的正确性．     

混凝土爆破损伤的SPH-FEM耦合法数值模拟

为了提高计算效率以及更好展现爆炸荷载下混凝土破坏过程,采用SPH-FEM耦合法对混凝土爆破成坑进行模拟。首先结合前人给出的C30混凝土Holmquist-Johnson-Cook（HJC）部分本构参数,通过理论推导等方法确定出剩余的参数;然后代入模型中计算,将数值解与实测数据进行对比;最后以峰值压力和峰值加速度作为考察对象,对HJC模型中21个参数敏感性进行分析。结果表明:SPH-FEM耦合法能直观地模拟爆炸荷载作用下爆坑的发展全过程,且能够较好地处理SPH边界问题;基于所给出的C30混凝土HJC本构参数,采用SPH-FEM耦合法对混凝土爆破破坏进行模拟,计算结果与实测数据吻合度高,表明HJC本构参数的确定具有合理性。此外,还发现HJC本构参数对爆破问题结果的敏感度各不相同,指出对峰值压力和峰值加速度均有较大影响的参数在确定的时候需引起足够的重视。     

抗战斗部侵彻爆炸作用的混凝土遮弹层设计

准确评估战斗部侵彻爆炸作用下混凝土遮弹层的损伤破坏可为防护工程设计提供重要参考。首先基于Karagozian&Case(K&C)模型框架建立了新型混凝土动态损伤本构模型,其中强度面综合考虑了静水压力、Lode角、应变率和损伤;独立描述了拉伸和压缩损伤,并考虑了拉压之间的连续过渡以及剪切变形和体积压缩对损伤的贡献。随后,开展了半无限厚混凝土靶体的105 mm口径弹体侵彻爆炸联合作用试验。进一步通过对上述试验和已有有限厚混凝土靶板的预制孔埋置装药爆炸试验进行数值仿真分析,验证了所建立的本构模型、参数取值和有限元分析方法在描述混凝土动态阻力、损伤演化和开裂行为方面的准确性。最后,确定了SDB、WDB-43/B和BLU-109/B三种典型战斗部以声速侵彻爆炸普通混凝土的临界贯穿和临界震塌厚度。结果表明：SDB、WDB-43/B和BLU-109/B战斗部侵彻爆炸作用下混凝土的临界贯穿厚度分别为1.4、3.4和3.8 m,临界震塌厚度分别为3.6、6.3和8.3 m;由于携带炸药量的差异,不同战斗部侵彻爆炸下的临界贯穿和临界震塌厚度与侵彻深度的比值非定值,相应的比值范围分别为1.4...     

近爆下泡沫混凝土复合结构在地下洞室的抗爆特性数值研究

为研究近爆作用下带泡沫混凝土复合结构的地下洞室的抗爆性能,基于LS-DYNA非线性有限元软件,建立了TNT炸药-围岩-锚杆-衬砌结构-空气的爆炸效应全耦合模型。应用ALE多物质流固耦合算法对比分析了普通衬砌结构洞室和带泡沫混凝土夹层的衬砌结构洞室的重要控制测点的应力、空气层压力和位移。测点峰值应力与TM5-855-1的经验公式吻合良好,证明了数值模拟的可靠性。数值研究结果表明,相比普通混凝土夹层,泡沫混凝土夹层能够有效地削弱爆炸冲击波的传播,大幅降低洞室内部空气的压力峰值,降低超压对人体器官的损伤,提高洞室内部人员的生存几率;采用泡沫混凝土夹层的衬砌需加强拱顶区域的刚度以降低顶底板相对位移差。研究结果为地下洞室抗爆设计提供了参考依据。     

结构爆炸毁伤的浸没多介质有限体积物质点法

结构在爆炸载荷作用下的毁伤现象涉及强非线性激波、固体结构极端变形和破坏破碎、强流固耦合, 给数值计算方法带来了极大的困难与挑战. 针对结构爆炸毁伤问题, 建立了浸没多介质有限体积物质点法(iMMFV-MPM), 采用基于黎曼求解器的多介质有限体积法(MMFVM)模拟爆炸产物和空气的多介质流体, 采用物质点法(MPM)模拟固体结构, 并将提出的基于拉格朗日乘子的连续力浸没边界法(lg-CFIBM)扩展到多介质流体中以处理流固耦合边界条件. 该算法在每个时间步严格满足流固耦合界面处的速度边界条件及动量守恒方程, 不需要重构流固耦合界面, 能够有效地模拟近场爆炸下爆炸产物与结构的相互作用、激波与结构的相互作用和演化以及结构的动态断裂和拓扑变化. 利用iMMFV-MPM对近场爆炸下方形钢筋混凝土靶板的失效模式、外爆载荷下建筑物的毁伤现象以及多腔室内爆炸试验进行了模拟, 模拟结果与相关实验数据吻合良好, 验证了所建立的流固耦合算法的有效性及精度.      

混凝土重力坝爆炸荷载数值分析及抗爆性能研究

利用LS-DYNA非线性有限元程序,基于Eulerian和Lagrangian耦合的方法,研究了RHT本构模型模拟的混凝土板在爆炸荷载下的动力反应,并且将数值结果与现场实验结果进行比较,由此说明了RHT本构模型模拟爆炸荷载下混凝土动力反应的有效性。研究了2 t TNT炸药在距离坝体上游面10 m不同起爆深度的情况下,有泡沫混凝土保护层和无泡沫混凝土保护层的坝体动力响应及其损伤状况。计算结果表明,无泡沫混凝土保护层时,坝体上游面主要损伤区域位置总是随起爆深度的增加向坝体底部移动;当上游表面有泡沫混凝土保护层时,坝体上游表面的损伤明显变小,下游面的损伤较无保护层情况也明显减小。表明泡沫混凝土能够有效减小混凝土大坝在爆炸荷载下的损伤,在提高混凝土大坝的抗爆性能方面起到很好的保护作用。     

An application of a damage constitutive model to concrete at high temperature and prediction of spalling

A characteristic feature of concrete under uniaxial compression is the development of cracks parallel to the loading direction. A damage constitutive model proposed by Ortiz [Ortiz, M., 1985. A constitutive theory for the inelastic behaviour of concrete. Mech. Mater. 4, 67–93] can predict the transverse tensile stress responsible for these cracks by considering the interaction between the aggregate and the mortar and the development of damage in the latter. When concrete is exposed to high temperature, as is the case during fire, the failure mode is thermal spalling. In order to improve the prediction of the stresses involved in this failure Ortiz’s model is extended to account for the effects of high temperature. Published experimental results for uniaxial and biaxial compression at high temperatures are used to calibrate the temperature dependence of some of the material properties. The transient creep strain is accounted for by modifying the constrained thermal strain. The stress analysis is coupled with hygro-thermal analysis of heat, mass transfer and pore pressure build-up. The effect of pore pressure on the damage evolution is modeled by applying a body force in the stress analysis module proportional to the pressure gradient. A numerical example of concrete under fire is solved and the computed results are discussed. Spalling is predicted when the damage variable reaches its maximum value of unity. The predicted depth and time of spalling for a range of variation of permeability and initial liquid water content are presented. They are in good agreement with published experimental results.     

土中爆炸作用下箱涵动力响应的SPH-FE耦合分析

采用SPH 与FE耦合的数值分析技术,SPH 法用于模拟爆破近域土体,FE法用于模拟远场土体 和钢筋混凝土箱涵,研究了土中爆炸作用下钢筋混凝土箱涵的动力响应。研究结果表明:箱涵损伤区域及损 伤程度与土中爆炸位置密切相关;不同爆炸位置下箱涵受力和变形规律存在较大差异,尤其是边墙中间位置, 受土体变形流动影响,爆炸位置的不同会引起明显的二次加载和拉压变化。SPH 与FE耦合分析技术可以满 足土中爆炸对结构物影响问题的计算效率和工程分析要求。     

Micromechanics-based elastic damage modeling of particulate composites with weakened interfaces

A micromechanical framework is proposed to predict the effective elastic behavior and weakened interface evolution of particulate composites. The Eshelby’s tensor for an ellipsoidal inclusion with slightly weakened interface [Qu, J., 1993a. Eshelby tensor for an elastic inclusion with slightly weakened interfaces. Journal of Applied Mechanics 60 (4), 1048–1050; Qu, J., 1993b. The effect of slightly weakened interfaces on the overall elastic properties of composite materials. Mechanics of Materials 14, 269–281] is adopted to model spherical particles having imperfect interfaces in the composites and is incorporated into the micromechanical framework. Based on the Eshelby’s micromechanics, the effective elastic moduli of three-phase particulate composites are derived. A damage model is subsequently considered in accordance with the Weibull’s probabilistic function to characterize the varying probability of evolution of weakened interface between the inclusion and the matrix. The proposed micromechanical elastic damage model is applied to the uniaxial, biaxial and triaxial tensile loadings to predict the various stress–strain responses. Comparisons between the present predictions with other numerical and analytical predictions and available experimental data are conducted to assess the potential of the present framework.     

A penny-shaped cohesive crack model for material damage

A novel micromechanics based damage model is proposed to address failure mechanism of defected solids with randomly distributed penny-shaped cohesive micro-cracks (Barenblatt–Dugdale type). Energy release contribution to the material damage process is estimated in a representative volume element (RVE) under macro hydrostatic stress state. Macro-constitutive relations of RVE are derived via self-consistent homogenization scheme, and they are characterized by effective nonlinear elastic properties and a class of pressure sensitive plasticity which depends on crack opening volume fraction and Poisson’s ratio. Several distinguished features of the present model are compared with Gurson model and Gurson–Tvergaard–Needleman (GTN) model, showing that the proposed model can better capture material degradation and catastrophic failure due to cohesive micro-crack growth and coalescence.     

Experimental investigations and modeling of volume change induced by void growth in polyamide 11

Polymers are known to be sensitive to hydrostatic pressure. The influence of stress triaxiality ratio on cavitation and damage has been highlighted in numerous studies. This paper proposes experimental investigations allowing the control of both the stress triaxiality ratio and the void distribution via microscopic observations of microtome-cut surfaces from interrupted tests. With the help of a finite element code, the Gurson–Tvergaard–Needleman model was calibrated by using these multi-scale experimental data. Then comparison between both numerical and analytical models and experimental data was performed. Bridgman formulae were reported to be valid up to the peak load. Moreover, a better understanding of the time evolution of significant parameters such as the porosity (volume change) and the stress triaxiality ratio (hydrostatic pressure), was highlighted.     

基于微-细观机理的混凝土疲劳损伤本构模型

该文致力于混凝土疲劳损伤发展机理的微细观解释. 以速率过程理论为基础,通过考虑裂纹断裂过程区中的水分子动力作用,在细观尺度上建立了具有物理机理的疲劳损伤能量耗散表达式. 结合细观随机断裂模型,以宏观损伤力学为框架,建立了疲劳损伤演化方程. 通过数值模拟,计算了单轴受拉时的疲劳损伤演化以及不同加载幅度下的疲劳寿命. 与相关试验结果的对比显示出该文模型能够很好地表现混凝土材料的疲劳损伤演化过程.      

Control of a submerged jet in a thin rectangular cavity

An innovative method is presented for control of an oscillatory turbulent jet in a thin rectangular cavity with a thickness to width ratio of 0.16. Jet flow control is achieved by mass injection of a secondary jet into the region above the submerged primary jet nozzle exit and perpendicular to the primary nozzle axis. An experimental model, a 2-D and a 3-D computational fluid dynamics (CFD) model are used to investigate the flow characteristics under various secondary injection mass flow rates and injection positions. Two-dimensional laser Doppler anemometry (LDA) measurements are compared with results from the CFD models, which incorporate a standard k–ε turbulence model or a 2-D and 3-D realisable k–ε model. Experimental results show deflection angles up to 23.3° for 24.6% of relative secondary mass flow are possible. The key to high jet control sensitivity is found to be lateral jet momentum with the optimum injection position at 12% of cavity width (31.6% of the primary nozzle length) above the primary nozzle exit. CFD results also show that a standard k–ε turbulence closure with nonequilibrium wall functions provides the best predictions of the flow.     

Stochastic damage model for concrete based on energy equivalent strain

Starting with the framework of conventional elastoplastic damage mechanics, a class of stochastic damage constitutive model is derived based on the concept of energy equivalent strain. The stochastic damage model derived from the parallel element model is adopted to develop the uniaxial damage evolution function. Based on the expressions of damage energy release rates (DERRs) conjugated to the damage variables thermodynamically, the concept and its tensor formulations of energy equivalent strain is proposed to bridge the gap between the uniaxial and the multiaxial constitutive models. Furthermore, a simplified coupling model is proposed to consider the evolution of plastic strain. And the analytical expressions of the constitutive model in 2-D are established from the abstract tensor expression. Several numerical simulations are presented against the biaxial loading test results of concrete, demonstrating that the proposed models can reflect the salient features for concrete under uniaxial and biaxial loading conditions.     

高聚物牺牲包层对钢筋混凝土板的爆炸毁伤缓解效应

为研究高聚物牺牲包层对钢筋混凝土结构的爆炸毁伤缓解效应,开展了带高聚物牺牲包层钢筋混凝土板的接触爆炸试验,同时设置了普通钢筋混凝土板作为对照组,对比分析了高聚物牺牲包层对钢筋混凝土板毁伤特征的影响。此外,运用AUTODYN软件建立了现场爆炸试验的SPH-FEM耦合模型,通过与试验结果的对比,验证了所建耦合模型的可靠性。在此基础上,通过参数敏感性分析,探究了炸药量和高聚物牺牲包层密度、厚度对带高聚物牺牲包层钢筋混凝土板毁伤特性以及吸能特性的影响。结果表明：接触爆炸下,高聚物牺牲包层能够有效地分散爆炸荷载,缓解爆炸荷载对钢筋混凝土板的冲击作用,具有良好的防护性能;药量在一定范围内增大时,高聚物牺牲包层依然能维持较高的吸能水平,增大包层密度和厚度有利于增强高聚物牺牲包层的吸能特性,包层厚度的变化会造成被保护钢筋混凝土板毁伤模式的改变。     

混凝土中爆炸数值仿真算法研究

用AUTODYN动力学软件对装药在混凝土介质靶中爆炸作数值仿真,研究了不同的网格描述方法,以及网格划分方法对计算结果的影响。通过实际算例比较了Lagrange算法和SPH算法以及不同的网格划分细度对靶径向、轴向不同距离的最大冲击压力、靶背过载以及损伤区、炸坑大小的模拟计算结果。分析得到了混凝土介质靶中爆炸问题数值仿真的合适算法,即拉氏算法对炸点近处的较高的最大冲击压力有较好的敏感能力,SPH算法对仿真计算爆坑形态以及抛掷体速度等具有明显的优势。对于算例问题,拉氏算法10 mm网格和SPH算法3 mm粒子即可满足一定的计算精度。     

钢筋混凝土排架结构的抗爆破坏等级

为了研究钢筋混凝土排架结构在大当量爆炸冲击波下的破坏规律,依据最大TNT当量为3 t的爆炸试验,对排架主体结构的抗爆破坏等级进行数值模拟研究。通过量纲分析得到1/2缩比模型的荷载参数和结构尺寸。基于Abaqus有限元软件,利用CONWEP方法实现爆炸加载,分别计算装药0.5 t爆距33 m和装药3 t爆距33 m两种工况下排架结构的破坏形态,并与试验结果进行对比。进一步通过控制药量和距离,计算不同超压和冲量下缩比模型的破坏形态。研究结果表明,排架的关键破坏特征为中间承重柱的倾覆转动;数值计算与试验破坏形态吻合较好,特征位移和特征转角的最大相对误差分别为5.6%和4.6%。以承重柱的倾覆角作为划分依据,将计算结果分为3种破坏等级,拟合得到的超压-冲量曲线和药量-距离曲线可用于厂房安全距离和仓库容量设计以及意外爆炸下的破坏程度预估。     

THE FATIGUE CONSTITUTIVE MODEL OF CONCRETE BASED ON MICRO-MESO MECHANICS

该文致力于混凝土疲劳损伤发展机理的微细观解释. 以速率过程理论为基础,通过考虑裂纹断裂过程区中的水分子动力作用,在细观尺度上建立了具有物理机理的疲劳损伤能量耗散表达式. 结合细观随机断裂模型,以宏观损伤力学为框架,建立了疲劳损伤演化方程. 通过数值模拟,计算了单轴受拉时的疲劳损伤演化以及不同加载幅度下的疲劳寿命. 与相关试验结果的对比显示出该文模型能够很好地表现混凝土材料的疲劳损伤演化过程.     

NDE of metal damage: ultrasonics with a damage mechanics model

This research describes a nondestructive method for the quantitative estimation of property variations due to damage in metal materials. The method employs a damage mechanics model, which accounts for stiffness degradation and damage evolution of a metal medium with a measurement of ultrasonic velocity. In order to describe the progressive deterioration of materials prior to the initiation of macrocracks, we have developed a new damage mechanics model. Thereafter, a finite element model valid for numerically describing such damage process has been developed by ABAQUS/Standard code, and correlations between damage state, elastic stiffness and plastic strain could be found by the results of the finite element simulation. The property variations due to damage evolution are calculated based on the Mori–Tanaka theory, and then the ultrasonic velocity can be predicted by Christoffel’s equation. When the measured velocity is coupled with the theoretically predicted velocity, the unknown damage variable is solved, from which other residual properties are determined by the predictions of damage model. The proposed technique is performed on type 304 stainless steel bars. The numerical results obtained by the simulation were compared with experimental ones in order to verify the validity of the proposed finite element model and good agreement was found. It is shown that the damaged properties of metals can be estimated accurately by the proposed method.