Dynamic response of a reinforced concrete slab subjected to air blast load

Reinforced concrete is the principal material for military engineering and nuclear power plant containment. However, impacts and explosions could completely destroy such structures, causing tremendous casualties and property loss. Hence, this study conducts an analysis on the propagation law of a blast pressure wave and the dynamic response of reinforced concrete structures under explosive pressure wave effects. This study uses proper state material parameters and equations and then applies the nonlinear finite element analysis software LS-DYNA to conduct a numerical simulation of a free-field explosion model. After comparison with the computed results from empirical equations and validating the reliability of the numerical analysis model, the destruction and influencing factors on reinforced concrete slabs, under the effects of a blast pressure wave, are investigated. The results can serve as a reference for future analysis and design.     

Linear and Nonlinear Analyses of the Onset of Buoyancy-Induced Instability in an Unbounded Porous Medium Saturated by Miscible Fluids

This study analyzes the stability of an initially sharp interface between two miscible fluids in a porous medium. Linear stability equations are first derived using the similarity variable of the basic state, and then transformed into a system of ordinary differential equations using a spectral expansion with and without quasi-steady-state approximation (QSSA). These transformed equations are solved using the eigenanalysis and initial value problem approach. The initial growth rate analysis shows that initially the system is unconditionally stable. The stability characteristics obtained under the present QSSA are quantitatively same as those obtained without the QSSA. To support these theoretical results, numerical simulations are conducted using the Fourier-spectral method. The results of theoretical linear stability analyses and the numerical simulations validate to each other.     

Debonding of FRP-plated reinforced concrete beam,a bond-slip analysis. I. Theoretical formulation

External bonding of FRP plates or sheets has emerged as a popular method for strengthening reinforced concrete. Debonding along the FRP–concrete interface can lead to premature failure of the structure. In this study, a bond-slip model is established to study the interface debonding induced by a flexural crack in a FRP-plated concrete beam. The reinforced concrete beam and FRP plate are modeled as two linearly elastic Euler–Bernoulli beams bonded together through a thin layer of FRP–concrete interface. The interface layer is essentially modeled as a large fracture processing zone of which the stress–deformation relationship is described by a nonlinear bond-slip model. Three different bond-slip models (bi-linear, triangular and linear-damaging) are used. By dividing the debonding process into several stages, governing equations of interfacial shear and normal stresses are obtained. Closed-form solutions are then obtained for the interfacial shear and normal stresses and the deflection of the beam in each stage of debonding. In such a way, the proposed model unifies the whole debonding process, including elastic deformation, debonding initiation and growth, into one model. With such a superior feature, the proposed model provides an efficient and effective analytical tool to study FRP–concrete interface debonding.     

Aeroelastic stability analysis of a flexible over-expanded rocket nozzle using numerical coupling by the method of transpiration

The aim of this paper is to present and compare two different approaches for aeroelastic stability analysis of a flexible over-expanded rocket nozzle. The first approach is based on the aeroelastic stability models developed in a previous work, while the second uses the numerical fluid–structure coupling via the transpiration method. The aeroelastic frequencies of the nozzle obtained by various stability models are compared with those extracted from the numerical coupling by the method of transpiration. Both set of results show an overall good agreement.     

Decomposition methods for time‐domain Maxwell's equations

Decomposition methods based on split operators are proposed for numerical integration of the time‐domain Maxwell's equations for the first time. The methods are obtained by splitting the Hamiltonian function of Maxwell's equations into two analytically computable exponential sub‐propagators in the time direction based on different order decomposition methods, and then the equations are evaluated in the spatial direction by the staggered fourth‐order finite‐difference approximations. The stability and numerical dispersion analysis for different order decomposition methods are also presented. The theoretical predictions are confirmed by our numerical results. Copyright © 2007 John Wiley & Sons, Ltd.     

Elastic analyses of heterogeneous hollow cylinders

Two different kinds of heterogeneous elastic hollow cylinders are studied in the present paper. One is a multi-layered cylinder with different values in different layers for both elastic modulus and Poisson’s ratio. Another is an elastic hollow cylinder with continuously graded material properties. By introducing two recursive algorithms, the extrusion stresses between two neighbor layers in the multi-layered cylinder submitted to uniform pressures on the inner and outer surfaces can be simply determined. Then the exact solutions of the multi-layered structure can be found based on Lamé’s solution. For the hollow cylinder with continuously graded properties, the displacement method is used. Both Whittaker equation and hyper-geometric equation are derived and successfully solved, and then the exact solutions are found. The results obtained in the present paper are compared with the numerical solutions and good agreements are found. At the end of the present paper, some inherent properties of these two different kinds of heterogeneous elastic hollow cylinders are presented and discussed. The results obtained in the present paper are useful in the design and analysis for composites reinforced by unidirectional fiber layers.     

Development and assessment of a coupled strategy for conjugate heat transfer with Large Eddy Simulation: Application to a cooled turbine blade

Although Large Eddy Simulation (LES) is identified today as the most promising method for turbulent flow problems, few applications of LES coupled to heat transfer solvers in solids have been published. This paper describes a coupling strategy of a LES solver and a heat transfer code within solids on parallel architectures. The numerical methods used in both solvers are briefly recalled before discussing the coupling strategy in terms of physical quantities to exchange (fluxes and temperatures), stability and parallel efficiency. The stability study is performed using an amplification matrix analysis on a one-dimensional case and allows the determination and optimization of coupling parameters. The coupled tool is then applied to a cooled turbine blade model where results demonstrate both the efficiency of the parallel implementation and the quality of the results. Coupled and non-coupled simulations are compared to experimental results and discussed in terms of cooling efficiency and flow structures.     

Numerical modeling of damage detection in concrete beams using piezoelectric patches

Research and development of active monitoring systems for reinforced concrete structures should lead to improved structural safety and reliability. Numerical models of active monitoring and damage detection systems can help in the development and implementation of these systems. Modeling of damage detection process in a concrete beam with piezoelectric sensors/actuators based on wave propagation is investigated in this paper. Numerical modeling process is divided into two parts: (1) piezoelectric smart aggregates (SA), and (2) wave propagation models. Displacement obtained in the SA model is used as an input parameter for the modeling of wave propagation. Wavelet analysis is used as a signal processing tool and the damage index is calculated based on the wave energy. In this paper root-mean-square deviation (RMSD) damage index is used. Damage indices obtained by this numerical analysis are compared with experimental results. Very good fit between the finite element (FE) results and experimental results confirm a good FE approach of this problem.     

采用黏弹性人工边界时显式算法稳定性条件

黏弹性人工边界是处理无限域波动问题常用的数值模拟方法。采用显式时域逐步积分算法进行计算时,受黏弹性人工边界的阻尼、刚度等影响,人工边界区的稳定性比内部计算域的更严格,尚无明确、实用的稳定性判别准则,这限制了黏弹性人工边界在显式动力分析中的应用。针对二维黏弹性人工边界,利用基于局部子系统的稳定性分析方法和基于传递矩阵谱半径的稳定性判别准则,给出了可代表整体模型局部特征的不同边界子系统的稳定性条件解析解。通过对比分析不同计算区域的稳定性条件及其影响因素,证明了整体模型的稳定性由角点子系统控制。在此基础上,获得了含黏弹性人工边界的整体模型在显示动力计算中的统一稳定性判别准则和简化实用计算方法。在实际应用中,令积分时间步长满足稳定性条件,即可顺利完成整体模型的动力计算。以上研究可为将黏弹性人工边界应用于显式动力计算时积分时间步长的合理选取提供参考。     

Cluster synchronization analysis of complex dynamical networks by input-to-state stability

Cluster synchronization is an interesting issue in complex dynamical networks with community structure. In this paper, we study cluster synchronization of complex networks with non-identical systems by input-to-state stability. Some sufficient conditions that ensure cluster synchronization of complex networks are provided. We show that the cluster synchronization is difficult to achieve if there are some links among different clusters. The analysis is then extended to the case where the outer coupling strengths are adaptive. Finally, numerical simulations are given to validate our theoretical analysis.     

三维非稳定渗流自由面边界积分项的精确数值计算

利用固定网格法分析三维非稳定渗流问题时,将要面对两项积分难题:以自由面及单元表面为边界的空间积分及以自由面为边界的曲面积分。针对常用的任意8结点6平面三维普通单元,提出采用坐标变换及等参变换技术求取空间积分项的精确数值解;至于曲面积分项,建议改用单元非饱和区部分表面作为积分边界,经过坐标变换及等参变换处理积分边界后,利用高斯数值积分可求出曲面积分项的精确数值解。通过一个普通单元及一项均质半无限边界堤坝的实例分析,表明此方法的精确性和稳定性良好。     

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通过对拱顶储罐罐壁承受轴向载荷、初始几何缺陷及轴压失稳状况研究,指 出在固定顶罐设计、建造和运行各阶段都应进行罐壁轴压稳定性校核. 根据圆柱薄壳稳定性 理论和轴压失稳临界应力数值分析计算结果,提出固定顶罐罐壁轴压稳定性校核方法和数学 模型,并运用回归分析方法建立罐壁轴压失稳临界应力计算公式. 对几种常用规格的拱顶罐 有初始挠度缺陷罐壁轴压稳定性分析表明：随储罐容积和罐壁初始挠度增大,罐壁轴压稳定 性呈减弱趋势.     

Stability analysis of a thin-walled cylinder in turning operation using the semi-discretization method

In this work, the stability of a flexible thin cylindrical workpiece in turning is analyzed. A process model is derived based on a finite element representation of the workpiece flexibility and a nonlinear cutting force law. Repeated cutting of the same surface due to overlapping cuts is modeled with the help of a time delay. The stability of the so obtained system of periodic delay differential equations is then determined using an approximation as a time-discrete system and Floquet theory. The time-discrete system is obtained using the semi-discretization method. The method is implemented to analyze the stability of two different workpiece models of different thicknesses for different tool positions with respect to the jaw end. It is shown that the stability chart depends on the tool position as well as on the thickness.     

The viscous curtain: General formulation and finite-element solution for the stability of flowing viscous sheets

The stability of thin viscous sheets has been studied so far in the special case where the base flow possesses a direction of invariance: the linear stability is then governed by an ordinary differential equation. We propose a mathematical formulation and a numerical method of solution that are applicable to the linear stability analysis of viscous sheets possessing no particular symmetry. The linear stability problem is formulated as a non-Hermitian eigenvalue problem in a 2D domain and is solved numerically using the finite-element method. Specifically, we consider the case of a viscous sheet in an open flow, which falls in a bath of fluid; the sheet is mildly stretched by gravity and the flow can become unstable by ‘curtain’ modes. The growth rates of these modes are calculated as a function of the fluid parameters and of the geometry, and a phase diagram is obtained. A transition is reported between a buckling mode (static bifurcation) and an oscillatory mode (Hopf bifurcation). The effect of surface tension is discussed.     

加固需求度分析在某水电站右岸边坡的应用

边坡工程需要合理确定加固区与非加固区,在施工过程中,要动态调整设计方案。加固需求度方法将影响边坡稳定的主要因素与现场实际的位移监测结果结合起来,依靠专家经验知识,能够得到比较好的结果。在边坡加固需求度的基础上,引入影响权值,对某水电站右岸边坡稳定性进行分析,计算出一段位移监测期内边坡各部分的加固需求程度,得到了比较好的应用。     

多自由度参激系统稳定性分析的数值解法

现有参激系统的动力稳定性问题研究主要集中在主不稳定区域上。为获得组合不稳定区域,基于Floquet方法,采用Bolotin方法在不同周期数下设解形式,结合特征值分析法得到确定多自由度参激系统动力不稳定区域的数值解法。对一个两自由度受周期轴向力的旋转轴系算例的稳定性分析,发现通过增加设解近似项数可获得高阶不稳定区域,且各阶不稳定区域边界随近似次数的增加逐渐趋于稳定,此外,增大阻尼可使各不稳定区域边界变得更加平滑。本文方法可用于一般多自由度周期参激阻尼系统,是一种简明易操作的直接数值解法。     

Limit Analysis of Plates and Shells: Research over Two Decades

This paper summarizes results obtained by the author and his associates and students over the last two decades on limit analysis and optimal plastic design of plates and cylindrical shells. Circular metal plates are first considered, including minimum weight plastic design. Rectangular plates are then treated, both in metal and reinforced concrete. Limit analysis of cylindrical metal shells with reinforcing rings is the next subject, followed by optimal design of such shells. It is concluded that computer-aided analytical methods remain of value to complement purely numerical approaches.     

基于损伤分布模型的震损RC框架力学性能分析与实证研究

震损结构的剩余荷载位移响应分析十分重要,是准确评估震后钢筋混凝土(RC)框架结构剩余性能的基础,然而震损结构损伤的不均匀分布为其荷载位移响应的定量分析带来了困难。本文基于纤维梁柱模型提出了一种能够考虑地震损伤不均匀分布的震损RC框架数值模型。基于震损RC框架的损伤现象对其损伤分布进行了量化,考虑了材料损伤沿柱高度与截面深度的分布;基于量化结果分别确定了RC框架中各纤维梁柱单元中不同位置处纤维的损伤本构关系,并对其剩余荷载位移响应进行了定量分析;以一个一榀三层四跨的RC框架试验为算例,对模型进行了验证。结果表明,本文模型能够较为准确地模拟震损RC框架的剩余荷载位移响应,其中最大剩余承载力、刚度和屈服强度等各项指标计算误差均值均在10%以下,因此本文模型能够为震后RC结构的评估工作提供一种新的思路。     

CFRP加筋板剪切稳定性试验和数值分析模型研究

为研究数值模型对碳纤维增强复合材料(CFRP)加筋板面内剪切稳定性试验计算结果的影响,采用画框式夹具对CFRP加筋板进行了屈曲试验,获得了初始屈曲载荷和载荷-应变曲线;同时基于ABAQUS有限元软件建立了四种数值模型,分别进行线性屈曲和非线性屈曲分析,通过将计算结果与试验结果的比较,确定了有效的数值模型．在此基础上,通过对屈曲前、后有效区边界上的内力分布的比较分析,明确了夹具的传力效果、试件真实的受力状态,以及偏差产生原因．     

Meso cell model of fiber reinforced composite: Interface stress statistics and debonding paths

The primary goal of this work is to develop an efficient analytical tool for the computer simulation of progressive damage in the fiber reinforced composite (FRC) materials and thus to provide the micro mechanics-based theoretical framework for a deeper insight into fatigue phenomena in them. An accurate solution has been obtained for the micro stress field in a meso cell model of fibrous composite. The developed method combines the superposition principle, Kolosov–Muskhelishvili’s technique of complex potentials and Fourier series expansion. By using the properly chosen periodic potentials, the primary boundary-value problem stated on the multiple-connected domain has been reduced to an ordinary, well-posed set of linear algebraic equations. The meso cell can include up to several hundred inclusions which is sufficient to account for the micro structure statistics of composite. The presented numerical examples demonstrate an accuracy and high numerical efficiency of the method which makes it to be a promising tool for studying progressive damage in FRCs. By averaging over a number of random structure realizations, the statistically meaningful results have been obtained for both the local stress and effective elastic moduli of disordered fibrous composite. A special attention has been paid to the interface stress statistics and the fiber debonding paths development, which appear to correlate well with the experimental observations.