波形钢腹板箱梁的约束扭转剪应力分析

基于周边不变形理论,结合闭口薄壁杆件约束扭转的计算分析,研究了波形钢腹板箱梁在约束扭转时混凝土悬臂板上扭转剪应力的分布,并进行了计算.通过对悬臂板在约束扭转中剪力流计算公式的推导,进一步阐述了其自由扭转剪应力及翘曲扭转剪应力的分布,指出了相关文献在这部分计算中存在的问题.通过一个简支波形钢腹板组合箱梁算例,将该文方法计算结果与ANSYS有限元计算结果进行比较.结果表明:在波形钢腹板箱梁截面中,主要由波形钢腹板承受扭转剪应力,其次是混凝土底板,底板剪应力最大值发生在底板中心处,其数值近似等于腹板剪应力的一半,而混凝土顶板和悬臂部分的扭转剪应力很小;该文计算的扭转剪应力结果在总体上符合有限元得到的扭转剪应力分布规律,在悬臂自由端为0,随着离开悬臂自由端距离的增大,扭转剪应力逐渐增大并达到峰值.     

Behavior of concrete cylinders confined by a carbon composite 3. Deformability and the ultimate axial strain

Results of an experimental investigation into the mechanical properties of concrete cylinders confined by a carbon-epoxy composite wrapping are presented. It is shown that, for all the con fined con crete spec i mens tested, the loading paths in the normalized stress space follow a single master curve, what ever the concrete strength and confinement intensity. At stresses in the confined concrete exceeding the strength of plain concrete, the tangent modulus was found to depend on the slope angle of the master curve and the asymptotic value of the differential Poisson ratio (the first derivative of the lateral strain with respect to the axial one). Formulas for predicting the ultimate axial strain and the tangent modulus are derived and compared with the corresponding fib (fédération internationale du béton) recommendations. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 4, pp. 433–448, July–Au gust, 2006.     

Nonlinear finite element analysis and modeling of a precast hybrid beam–column connection subjected to cyclic loads

In this work, a detailed three-dimensional (3D) nonlinear finite element model is developed to study the response and predict the behavior of precast hybrid beam–column connection subjected to cyclic loads that was tested at the National Institute of Standards and Technology (NIST) laboratory. The precast joint is modeled using 3D solid elements and surface-to-surface contact elements between the beam/column faces and interface grout in the vicinity of the connection. The model takes into account the pre-tension effect in the post-tensioning strand and the nonlinear material behavior of concrete. The model response is compared with experimental test results and yielded good agreement at all stages of loading. Fracture of the mild-steel bars resulted in the failure of the connection. In order to predict this failure mode, stress and strain fields in the mild-steel bars at the beam–column interface were generated from the analyzed model. Such fields of stresses and strains are hard to measure in experimental testing. In addition, the magnitude of the force developed in the post-tensioning steel tendon was also monitored and it was observed that it did not yield during the entire loading history. Successful finite element modeling will provide a practical and economical tool to investigate the behavior of such connections.     

Calculation of the long-term strength of thin-walled polymeric tubes under biaxial loading

The problem on calculating the failure time of thin-walled polymer tubes in creep under internal pressure, internal pressure with tension, pure torsion, and torsion with tension is solved. The solution is constructed based on the concept of equivalent stresses. A mixed long-term failure criterion taking into account the sings of principal stresses is used as the equivalent stress. The calculation results are in a satisfactory agreement with experimental data.Translated from Mekhanika Kompozitnykh Materialov, Vol. 40, No. 6, pp. 811–826, November–December, 2004.     

平面应力状态断裂强度的椭圆准则分析

简要分析了近年来提出的一个断裂准则——椭圆准则的基本特征,导出了它在主应力坐标系下的基本方程.根据所导出的基本方程,获得了平面应力条件下椭圆准则理论断裂强度曲线的完整描述关系,并分析讨论了破坏发生的方位及断裂形式与材料本征力学性质参数之间的联系.与既有理论结果及实验现象的对比解释了椭圆准则在材料相关参数确定方面的局限性.当应力状态相关材料特征参数在拉伸区和压缩区均作为常数时,获得了铸铁和混凝土平面应力状态下的断裂强度曲线.与相关实验数据的对比表明,它们在拉伸区能较好地吻合,但压缩区的差异十分显著,进一步证实了材料特征参数随应力状态变化规律对椭圆准则发展的必要性.     

Uniform analysis of a stabilized hybrid finite element method for Reissner-Mindlin plates

This paper presents a low order stabilized hybrid quadrilateral finite element method for ReissnerMindlin plates based on Hellinger-Reissner variational principle,which includes variables of displacements,shear stresses and bending moments.The approach uses continuous piecewise isoparametric bilinear interpolations for the approximations of the transverse displacement and rotation.The stabilization achieved by adding a stabilization term of least-squares to the original hybrid scheme,allows independent approximations of the stresses and moments.The stress approximation adopts a piecewise independent 4-parameter mode satisfying an accuracy-enhanced condition.The approximation of moments employs a piecewise-independent 5-parameter mode.This method can be viewed as a stabilized version of the hybrid finite element scheme proposed in [Carstensen C,Xie X,Yu G,et al.A priori and a posteriori analysis for a locking-free low order quadrilateral hybrid finite element for Reissner-Mindlin plates.Comput Methods Appl Mech Engrg,2011,200:1161-1175],where the approximations of stresses and moments are required to satisfy an equilibrium criterion.A priori error analysis shows that the method is uniform with respect to the plate thickness t.Numerical experiments confirm the theoretical results.     

Different problems associated with the stress trajectories element method (Poster Presentation)

This study deals with the determination of stresses in statically determined solids of arbitrary rheology. The stress trajectory concept is used as the basic element in numerical formulations. Different types of data can be employed, which necessitates consideration of different mathematical problems associated with the stress identification. Three classes of data are discussed and an example from geodynamic is presented. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

An experimental-analytical hybrid model for the elasto-plastic stresses at a crack

The primary obstacle preventing the analytical determination of physically sensible stresses at a crack tip is the presence of a mathematical singularity there. This singularity is best known in its elastic form; however it persists even in elasto-plastic crack-tip stresses. To overcome the difficulty we adopt the following strategy: we attempt to capture initial elastic stresses experimentally, than track subsequent elasto-plastic stress distributions analytically.We infer a finite stress at a crack tip from the experimental behaviour of cracked specimens at fracture when the specimens are made of a truly brittle material. Given a size-independent result, we argue that the crack-tip stress at fracture must equal the ultimate stress for such a material; thus dividing by the applied stress at the same point gives a measure of the stress concentration factor, K_T. The approach is checked for size independence and against hole configurations with known theoretical, yet physically reasonable, K_T. Then the effective experimental KT are taken as inputs for the second phase of the study in which we model the crack as being a smooth notch having the same stress concentration factor as found experimentally. In this way our configuration initially shares the same stresses at the crack tip as we inferred physically. Next we track effects of incremental plastic flow on a set of finite element grids. Satisfactory resolution in return for modest computational effort is obtained by employing a substructuring method. The accuracy in both the elastic and the elasto-plastic regime is checked against trial problems with exact solutions. Thereafter, physically interpretable stress distributions ahead of the crack are determined for a range of materials and for varying load levels.     

Effective elastic moduli of two-dimensional solids with multiple cracks

An accurate method which directly accounts for the interactions between different microcracks is used for analyzing the elastic problem of multiple cracks solids. The effective elastic moduli for randomly oriented cracks and parallel cracks are evaluated for the representative volume element (RVE) with microcracks in infinite media. The numerical results are compared with those from various micromechanics models and experimental data. These results show that the present method is simple and provides a direct and efficient approach to dealing with elastic solids containing multiple cracks. Project supported by the National Natural Science Foundation of China (Grant No. 19704100), and the National Natural Science Foundation of Chinese Academy of Sciences (Project KJ951-1-201).     

Numerical aspects of microplane constitutive models for concrete

This paper presents a comprehensive study on the numerical aspects of a class of microplane constitutive models for concrete, with emphasis on the most popular one, i.e., the model M4 developed by Bažant and co-workers. The effect of the computational procedures for the microplane shear stress components, the strain increment magnitude, the integration scheme and the loading direction on the model responses are investigated in detail. Several problems in the responses of the model, from the computational point of view, are detected and discussed. Some procedures to enhance the numerical performance of the model are then proposed. These include a removal of the tensile volumetric stress boundary in the original formulation, the introduction of a novel semi-explicit numerical algorithm for the microplane volumetric and deviatoric stresses, and a method to minimise the sensitivity of the model responses to the loading direction through a simple meshing-based integration scheme. Finally, some practical considerations for the choice of numerical integration scheme in the finite element calculations of large-scale concrete structures with the microplane model M4 are presented.     

界面弧形裂纹对混凝土开裂强度的影响研究

混凝土由于水分蒸发、干缩、泌水以及骨料与砂浆变形不一致等原因会导致骨料与砂浆的界面层中产生弧形裂纹,从而对混凝土开裂强度产生很大影响.从细观角度将混凝土视作由粗骨料和水泥砂浆组成的两相复合材料,并将界面层视为粗骨料与水泥砂浆的接触层进行分析.首先基于相互作用直推估计(interaction direct derivative, IDD)法,考虑混凝土中骨料颗粒的相互作用,将施加在混凝土表征体积元的远场外荷载等效为无限大基体中含单一骨料的等效外荷载.然后,将等效外荷载转化为最大和最小主应力,基于断裂力学理论得到界面层中弧形裂纹的应力强度因子,并根据复合型裂纹幂准则判断弧形裂纹是否发生开裂,进而来研究混凝土开裂强度的变化规律.通过与数值模拟结果的比较,验证了界面弧形裂纹应力强度因子解析解的有效性,参数分析结果表明,当裂纹与最大主应力垂直或与最小主应力呈45°夹角时,骨料周围弧形裂纹最易发生开裂破坏.随着裂纹长度增加,混凝土受拉和受压开裂强度先减小后增大,且均存在最不利的裂纹长度.混凝土开裂强度随着骨料体积分数的增加而增大,随着骨料粒径的增大而减小.在裂纹长度较小时,增大骨料的弹性模量有利于提高混凝土开裂强度.骨料周围承受同号应力可以提高混凝土的开裂强度,反之,异号应力会降低开裂强度.     

Model for cover cracking due to corrosion expansion and uniform stresses at infinity

The mechanical model of the cover layer cracking in reinforced concrete structures due to corrosion expansion of reinforcement and uniform stress at infinity is established in this paper. The principle of superposition and the series expansion technique of the theory of complex potential established by Muskhelishvili are applied. The complex stress potentials are assumed to be in the form of Taylor and Laurent series expansions, and the unknown coefficients are determined by the boundary conditions and the stress state at infinity. Finally the analytical solution for hoop stresses in concrete is derived. Referring to the previous studies in the literature, the equation for time of concrete cracking due to corrosion expansion of reinforcement and uniform stresses at infinity is established. It is found that the change of stress state at infinity may accelerate or decelerate the initiation of crack. In addition, compared with the case without corrosion, the existence of corrosion products can alter the location of cracking. Further analyses indicate that the effect of the ratio between reinforced bar and concrete on the cracking is insignificant, and that the possibility of cover layer cracking increases with increasing penetration of corrosion.     

Non-linear seismic response of concrete gravity dams to near-fault ground motions including dam-water-sediment-foundation interaction

This study focuses on non-linear seismic response of a concrete gravity dam subjected to near-fault and far-fault ground motions including dam-water-sediment-foundation rock interaction. The elasto-plastic behavior of the dam concrete is idealized using Drucker–Prager yield criterion based on associated flow rule assumption. Water in the reservoir is represented by 9-noded isoparametric quadrilateral fluid finite elements while the dam, the foundation rock and the sediment layer are modeled by using 8-noded isoparametric quadrilateral solid finite elements. The program NONSAP modified for elasto-plastic analysis of fluid-structure systems using the Lagrangian fluid finite element is employed in the response calculations. The fluid element includes the effects of surface waves and sloshing behavior of fluids. Non-linear seismic analyses of the selected concrete dam subjected to both near-fault and far-fault ground motions are performed. The results obtained from linear and non-linear analyses are compared with each other.     

有限变形下的混凝土动态本构关系研究

讨论有限变形和小变形假设下本构关系的区别,并将其运用于混凝土的弹-粘塑性本构关系研究,提出了一个应变率相关的动态力学模型．模型基于Ottosen的4参数屈服准则,分别考虑混凝土在硬化阶段和软化阶段加载面的不同变化规律,建立冲击荷载下的混凝土本构关系．该模型可以应用于冲击载荷下混凝土材料响应的模拟．引进Green-Naghdi客观率建立有限变形的混凝土模型．根据大量实验结果对应变率和材料强度的关系提出合理假设,使模型可以反映混凝土大变形的动态力学行为,为相关工程问题的研究提供有益的思路和有效的工具．     

Yield surfaces for non-homogeneous anisotropic shells of revolution

The state of stress in a rotationally symmetric shell is characterized by the direct stresses and moments in the circumferential and longitudinal directions. It is assumed that the material of the shell is rigid perfectly plastic and that the yield stress of the material varies over the thickness of the shell. The material has different yield stresses in tension and compression and the yield stresses in the principal directions have different values. The yield condition for the shell is obtained in terms of the stress resultants assuming that the material of the shell obeys the maximum shear stress criterion.     

Optimum ramp-stress accelerated life test for m identical repairable systems

This paper proposes optimum ramp accelerated life test (ALT) of m identical repairable systems using non-homogeneous power law process (PLP) under failure truncated case. An ALT with linearly increasing stress is a ramp test. In particular, a ramp test with two different linearly increasing stresses is a simple ramp test. The optimum ramp test with different stress rates is formulated by determining the proportions of test systems allocated to each stress rate using D-optimality criterion. D-optimality criterion minimizes the reciprocal of the determinant of the Fisher information matrix of the model parameters. The method developed is illustrated using two stress rates and three stress rates. It has been found that it takes much longer to obtain same estimated expected no. of failures at baseline condition than at stress levels.     

混凝土材料的粘塑性损伤本构模型

基于不可逆热力学,引入运动硬化、等向硬化和损伤内变量,构造了相应的自由能函数和流动势函数,推导出了混凝土材料的粘塑性损伤本构模型．数值模拟的结果表明,该模型能够避开屈服面和破坏准则的基本假设来描述混凝土材料的以下特性:压缩载荷作用下的体积膨胀现象;应变率敏感性;峰值后由损伤和破坏引起的应力软化和刚度退化现象A·D2由于此模型避开了根据各种变形阶段选择与其相应的本构模型的繁琐计算,因此更便于纳入复杂工况下应力分析有限元程序中．     

基于NARX神经网络方法的汽轮机转子关键部位应力预测

汽轮机启动过程中,对转子进行应力预测具有重要意义.为满足在线预测国产某350 MW超临界汽轮机转子关键部位应力的需要,提出了一种基于具有外部输入的非线性自回归(NARX)神经网络的应力预测方法.根据转子实际尺寸建立二维轴对称有限元模型,确定了相应的边界条件,并对有限元计算结果进行验证,得到了转子在冷启动工况下的温度场和应力场,确定了转子的危险点.再根据冷启动规则,构建了288个典型启动工况,并对每个启动工况分别进行有限元计算,获取了每个工况下机组运行参数以及转子危险点处应力的时间历程数据,进而建立了神经网络训练样本库,并使用NARX神经网络对危险点处应力进行预测.此基于NARX神经网络的预测方法可以准确预测出汽轮机转子危险点处的应力变化趋势,其预测值与有限元仿真结果吻合良好,可以满足转子应力在线预测的需求.     

弹性平面孔洞形状优化复变函数方法

本文提出了弹性平面孔洞形状优化的复变函数方法,充分利用了复变函数方法分析孔洞应力的有效性,进行应力分析.对孔洞形状的优化,是将保角变换函数中的一些系数做为设计变量,采用敏度分析和梯度法降低绝对值最大的周向应力,同时使绝对值次大的周向应力不超过绝对值最大的周向应力(这两个周向应力实际是周向应力的两个极值点处的周向应力值)逐次迭代修正保角变换函数中的系数值,直至绝对值最大的周向应力降低至绝对值次大的周向应力相等为止.这个方法保证了应力解在边界上的连续性、可微性和高精度性,比差分法和有限元法有着明显的优越性.     

The closed form solutions for axisymmetric modeling of thermal stress due to repetitive pulse laser heating

Repetitive pulse laser heating is increasingly being used in the laser processing industry such as laser welding, laser cutting and laser drilling due to its energy accumulation effect. Modeling of thermo-mechanical coupling effect of repetitive pulse laser heating can enhance the understanding of its thermo physical process. In this paper, an axisymmetric modeling of thermal stress for repetitive pulse laser heating solid materials is introduced, and its closed form solutions are obtained based on a semi-analytical method. The accuracy and efficiency of the closed form solutions are verified by comparison with the existing numerical modeling software based on the finite element method. Distributions of thermal stress for different radial locations, axial locations and duty cycles are calculated, and effects of these parameters on temporal profile of the thermal stress distributions are analyzed. In the region of laser irradiation, three components of thermal stress are all shown as compressive stress. And outside the region of laser irradiation, the hoop component is shown as tensile stress. On the material surface, the change of thermal stresses is very sharp as a result of direct absorption of incident laser energy. With the increasing of the depth, the rises and decreases of thermal stresses become smooth gradually. The duty cycle has a significant effect on the profiles of thermal stresses. In the same conditions, the maximum value of thermal stresses gets higher as the duty cycle decreases. Results of this study can provide some theoretical basis for parameter inversion and optimization of repetitive pulse laser heating, as well as some corresponding experimental research.