橡胶隔振器静态力-位移关系计算方法的研究

测试了橡胶试片在单轴向拉伸、等双轴拉伸和平面剪切应力-应变状态下的应力-应变关系。利用测试得到橡胶试片在不同状态下应力-应变关系,和应用不同的本构模型,拟合得到了不同模型的材料常数。以两个橡胶悬置为研究对象,对橡胶隔振器进行三向静态力-位移特性分析,并与测试值进行对比。橡胶悬置1主要承受拉压变形或剪切变形,而悬置2则同时承受拉压和剪切变形。以悬置1和悬置2为研究对象,应用Mooney-Rivlin本构模型,计算分析了利用不同应力－应变状态得到的本构模型常数对橡胶隔振器力－位移关系计算结果的影响。以悬置2为研究对象,采用同一种应力－应变状态获取的本构模型常数,分析了本构模型对橡胶隔振器静态力-位移特性计算结果的影响。     

冲击荷载作用下岩石破碎分形特征

为探究冲击荷载作用下岩石破碎分形特征,选取花岗岩和砂岩开展分离式霍普金森压杆(SHPB)岩石动力学试验,得到了不同应变率下岩石的应力-应变曲线、破碎特性、强度参数和能量参数;利用标准筛对破碎后的岩块进行筛分,获取了岩石破碎块度分布曲线,并基于碎块粒径分布的质量分形模型计算出分形维数D;最后分析了分形维数与加载参数、破碎特性和耗能特性之间的关系。结果表明,岩石在冲击荷载作用下的破碎块度分布符合分形规律;动态抗压强度随应变率增大而增大,两者满足乘幂函数关系;加载过程中岩石应变率越大,岩石破碎程度越深,分形维数越大;分形维数与岩石破碎耗能密度之间满足乘幂函数关系。采用分形维数可实现对岩石在冲击荷载作用下的破碎特性、力学特性和破碎耗能特性的定量研究。     

钢筋混凝土微平面本构模型及其算法研究

阐述文献[1]的第二部分。第一部分在Ba ant Z.P.等人提出的混凝土微平面模型的基础上引入钢筋的影响提出了一个适用于配筋混凝土的微平面动态本构模型。混凝土模型采用能够反映各种复杂受力行为并被试验充分验证的微平面模型,钢筋采用Cowper-Symonds型率相关的双线性模型。该模型可用于钢筋混凝土的静力和动力显式分析。这一部分将给出模型的显式算法和试验验证。     

时频二维逼近及在故障分量提取中的应用

机械故障信号通常是具有非线性时频关系多分量信号,其频谱占有较宽的频带,且各分量的频谱常常相互交叠,给故障诊断带来了很大的障碍.在传统信号分解的基函数线性逼近方法和时频重排的基础上,提出了基于时频二维逼近的信号分量提取方法.该方法对所要提取的特征分量进行参数建模,并计算出分量模型的时频函数以及多分量信号的重排时频分布;然后采用模型的时频函数拟合逼近原始信号的时频分布,并采用非线性最小二乘法确定出模型的各个参数值;最后设置能量下降梯度阈值控制迭代次数,用拟合得出的参数模型重构出信号分量.仿真实例验证了,上述方法对比基于时频滤波的信号提取方法,只需要少数几次拟合就能提取出所需要的信号分量,分量的重构精度较高.这种方法在轴承故障冲击分量提取中的应用表明,其不仅可以较精确地对轴承故障进行定位,而且能为故障原因及故障程度提供准确的判断依据.     

考虑温度影响的饱和软黏土应变软化研究

循环荷载作用下饱和软黏土将产生应变软化现象,通过对宁波饱和软黏土开展温控动三轴试验,研究了温度、频率、初始偏应力、动应力和含水率对土体应变软化的影响,并在试验基础上建立了一个能够考虑温度影响的应变软化模型。结果表明:随着振动次数、初始偏应力、动应力和含水率的增加,土体软化加快,软化指数逐渐减小;随着频率和温度的提高,土体软化程度降低,软化指数增大;建立的软化模型可以较为合理地描述试验温度、频率、初始偏应力、动应力和含水率对土体应变软化特性的影响。     

考虑多分量气动导纳的抖振响应敏感性

对于类流线型箱梁桥梁断面,采用抖振频域分析方法和气动弹性节段模型测振风洞试验,初步验证了互谱法识别的多分量气动导纳函数结果的正确性和工程适用性。结合可靠度理论敏感性算法,系统评价了来流风速参数、模型参数、静风力系数、导纳函数和颤振导数等对于二维节段模型抖振响应的贡献率分布关系,更正了工程应用中采用单一分量导纳函数假定带来的对抖振气动力表达式认识上偏差。     

桥梁断面气动导纳互谱识别方法注记

现存的各种气动导纳函数识别方法,为了便于求解忽略了脉动风速互谱的作用,且假定脉动风水平和竖向分量对抖振力导纳函数分量作用等效,识别过程缺少对算法系统和试验采样误差影响的标定,导纳函数识别结果缺少验证等诸多不足。针对上述问题,采用改进的互谱导纳识别方法,考虑了多种影响因素的共同作用,详细标定了算法系统和试验测量误差的影响,基于测量稳定性和精度较高的节段模型高频天平测力方法识别了流线型箱梁桥梁节段Scanlan抖振气动力全部六个导纳函数分量。     

非轴对称移动载荷下有限长圆柱厚壳的动力响应

本文在介绍圆柱厚壳应力与位移理论分析发展的前提下,建立了有限长圆柱厚壳在非轴对称移动载荷作用下的三维力学模型,假设沿厚度方向径向剪切应变二次分布、径向正应变线性分布,且建立了为满足边界条件待定的位移表达式,采用Heaviside函数和Dirac函数表达移动和作用变化着的移动载荷的基础上,用最小势能原理建立了该圆柱厚壳的动力学微分方程组,应用Galerkin法和修正的Runge–Kutta–Fehlberg法,求得了非轴对称移动载荷作用下的圆柱厚壳的动力响应。通过具体算例,对有限长圆柱厚壳的位移响应和应力状态进行了分析,并将动态响应的理论解与ANSYS数值解进行了对比,从而相互印证了解的可靠性。     

形状记忆合金梁的建模及混沌阈值计算

从形状记忆合金（SMA）的等应变拉压实验数据出发,利用van-der-pol环模型模拟了形状记忆合金在加载和卸载过程中的应力应变迟滞环特性。并根据弹性理论和Galerkin方法建立了形状记忆合金简支梁在受轴向激励时的振动模型。随后得出了自由振动系统的分岔特性。在利用待定固有频率法研究了模型的非线性参数对系统固有频率的影响后,根据待定固有频率法的计算结果和时间尺度变化提出了系统Melnikov函数的改进表达式,提高了计算形状记忆合金梁模型在参数激励下发生混沌的阈值的精度。数值模拟的结果证明了该途径的有效性。     

基于应变和比能双控的钢结构损伤模型

损伤模型是对经历地震后结构的剩余承载力进行定量评价的重要工具。本文提出了一个基于等效塑性应变和比能双控的损伤模型,从材料的角度出发,考虑了三轴应力对地震作用下结构性能的影响,可用于空间结构强震作用下的非线性分析。应用该模型对一个九层的benchmark结构进行了地震倒塌模拟分析,结果显示基于应变与比能双控的损伤模型能够很好地评估强震下钢结构竖向构件及层的损伤发展过程。     

Microplane finite element analysis of tube‐squash test of concrete with shear angles up to 70°

Finite element analysis of the response of concrete structures to impact events such as missile penetration, explosive driving of anchors, blast, ground shock or seismic loading, requires knowledge of the stress–strain relations for concrete for finite strain at high pressures. A novel type of material test achieving very large shear angles of concrete at very large pressures, called the tube‐squash test, can be used to calibrate a concrete model taking into account plastic deformation at extreme pressures. A finite element analysis of such a test is performed by using a finite strain generalization of microplane models for concrete and steel. The results obtained are in good agreement with those previously obtained with a simplified method of analysis. Thus, they provide a validation of the microplane model, which is shown to be capable of reproducing the response of concrete not only for small strains at small pressures, which is predominantly brittle, but also for high pressures and large finite strains, which is predominantly frictional plastic. Copyright © 2001 John Wiley & Sons, Ltd.     

Shapes of loading surfaces of concrete models and their influence on the peak load and failure mode in structural analyses

This paper focusses on the influence of the deviatoric shape of the employed loading surface on the predicted failure of concrete. For this purpose, the Extended Leon Model (ELM) [1] and a multi-surface model are considered. The multi-surface model consists of three Rankine surfaces and the Drucker–Prager surface. Whereas the Drucker–Prager surface is characterized by a circular shape in the deviatoric plane, the ELM accounts for the dependence of the strength of concrete on the Lode angle. It is characterized by an elliptic loading surface in the deviatoric plane. The eccentricity e defines the out-off-roundness. It allows a smooth transition from a circular (e=1) to an almost triangular shape (e≈0.5) of the loading surface in the deviatoric plane.The significance of the deviatoric shape of the loading surface on the predicted peak load and failure mode of concrete structures is investigated by means of two example problems: a plane-strain compression test characterized by compressive failure and an anchor-bolt test characterized by shear failure.     

Failure of plain concrete beam at impact load: 3D finite element analysis

In the paper, the results of numerical failure analysis of plain concrete beams loaded by impact three-point bending load are presented and discussed. The theoretical framework for the numerical analysis is continuum mechanics and irreversible thermodynamics. The spatial discretization is performed by the finite element method using update Lagrange formulation. Green–Lagrange stain tensor is used as a strain measure. To account for cracking and damage of concrete, the beam is modeled by the rate sensitive microplane model with the use of the so-called co-rotational stress tensor. Damage and cracking phenomena are modeled within the concept of smeared cracks. To assure objectivity of the analysis with respect to the size of the finite elements, crack band method is used. The contact-impact analysis is based on the mechanical interaction between two bodies—concrete beam (master) and dropping hammer (slave) falling on the mid span of the beam. The contact constrains are satisfied by Lagrange multiplier method, which is adapted for the explicit time integration scheme. To investigate the influence of loading rate on the failure mode of the beam parametric study is carried out. The numerical results are evaluated, discussed and compared with test results known from the literature. It is shown that the beam resistance and failure mode strongly depend on loading rate. For lower loading rates beam fails in bending (mode-I fracture). However, with increasing loading rate there is a transition of the failure mechanism from bending to shear. The results are in good agreement with theoretical and experimental results known from the literature.     

NUMERICAL SMEARED FRACTURE ANALYSIS: NONLOCAL MICROCRACK INTERACTION APPROACH

A recently proposed new nonlocal concept based on microcrack interactions is discussed, its implementation in a smeared cracking finite element code for concrete is presented, numerical studies are reported, and comparisons with experimental results are made. The nonlocality is not merely a mathematical device to prevent excessive spurious localization into a zone of zero volume but is a necessary physical consequence of microcrack interactions. Since the constitutive law itself is strictly local, the new nonlocal concept can be combined with any type of constitutive law for strain-softening nonlocal damage, which is here chosen to be the microplane model. A simple method is formulated to approximately identify the material parameters in the model from the basic characteristics of concrete such as the tensile strength, fracture energy and maximum aggregate size. The results of finite element analysis are shown to be mesh insensitive, and good convergence is obtained. Cracking damage is found to localize into a volume whose size and shape depend on the macroscopic concrete properties as well as the current stress–strain state. Although the damage is considered to be tensile on the microlevel, due solely to mode I microcracks, the new nonlocal model can describe well not only mode I fracture tests but also complex shear-dominated and mixed-mode types of failure such a diagonal shear, and can do so for the same values of material parameters (which was not the case for previous nonlocal models). Most importantly, the new nonlocal model can correctly capture the size effect of quasibrittle fracture, in approximate agreement with Bažant's size effect law.     

A comparison of damage models formulated on different material scales

This contribution aims at characteristic damage mechanisms of materials at two levels of observation, (a) at the macro-scale in the sense of classical continuum damage mechanics, and (b) at the meso-scale utilizing the so-called microplane concept. The constitutive formulations are embedded in a thermodynamically consistent framework. The microplane formulation is based on a volumetric–deviatoric split (V–D), which is motivated from a macroscopic viewpoint. The main advantage of this formulation is that the material behavior at a material point is characterized by constitutive laws formulated on the individual microplanes, allowing to describe anisotropic material behavior in a very natural and simple way. One particular advantage of the present version of a microplane formulation is also its close relation to macroscopic models which enables the interpretation and identification of the microplane constitutive laws in terms of well-known macroscopic constitutive laws. The appropriate choice of the microplane loading functions is illustrated by a comparison of the microplane damage formulation with a macroscopic two-parameter damage model.     

静载和动载下不同龄期混凝土力学特性的试验研究

利用杆径为75mm的SHPB试验装置对5种不同龄期下的混凝土分别进行了冲击压缩试验,系统了解了冲击载荷对不同龄期支护混凝土力学特性的影响。为了进行对比,利用INSTRON系统也进行了相应龄期下的静载压缩试验。试验研究表明：静载下混凝土强度、割线弹性模量随龄期增长而增长,其中强度增长主要集中在龄期7d以前,割线弹性模量增长则集中在龄期14d以后,而峰值应变随龄期增长整体上呈减小的趋势;动载下混凝土强度、峰值应变以及单位体积吸收能随着龄期增长而增长,在各个龄期都表现出对应变率具有一定的敏感性,其中不同龄期混凝土的动态强度随应变率增加呈现指数函数增长趋势。不同龄期的混凝土在动载下以拉伸破坏为主,静载下基本呈现剪切破坏形式。     

Numerical predictions of ballistic limits for concrete slabs using a modified version of the HJC concrete model

Some modifications to the Holmquist–Johnson–Cook (HJC) model (1993) for concrete under impact loading conditions are proposed. First, the pressure-shear behaviour is enhanced by including the influence of the third deviatoric stress invariant to take into account the substantial shear strength difference between the tensile and compressive meridians. Second, the modelling of strain-rate sensitivity is slightly changed so that the strain-rate enhancement factor goes to unity for zero strain rate. Third, three damage variables describing the tensile cracking, shear cracking and pore compaction mechanisms are introduced. A critical review of the constitutive model with alternative proposals for parameter identification is given. The model parameters are obtained for two concrete qualities, and perforation of concrete slabs is considered numerically and compared with experimental results from the literature. Ballistic limit assessments with deviations under 8% when compared to the experimental results are obtained, indicating that the modified version of the HJC concrete model represents a good compromise between simplicity and accuracy for large-scale computations of concrete plates impacted by projectiles.     

Numerical modeling of mechanical regain due to self-healing in cement based composites

Recently there has been an increasing interest in self-healing materials which have the ability to retrieve their physico-mechanical properties once the material is damaged. This paper presents a numerical model for the self-healing capacity of cementitious composites capable of simulating the recovery of mechanical properties of the damaged (cracked) material. The recent SMM (Solidification-Microprestress-Microplane model M4) model for concrete, which makes use of a modified microplane model M4 and the solidification-microprestress theory, is able to reproduce the concrete time-dependent behavior, e.g. creep, shrinkage, thermal deformation, aging, and cracking from early age up to several years. The moisture and heat fields, as well as the hydration degree, are obtained from the solution of a hygro-thermo-chemical problem which is coupled with the SMM model. This numerical framework is extended to incorporate the self-healing effects and, in particular, the effect of delayed cement hydration, which is the main cause of the self-healing for young concrete. The new update model can also simulate the effects of cracking on the permeability and the opposite restoring effect of the self-healing on the mechanical constitutive law, i.e. the microplane model. A numerical example is presented to validate the proposed computational model employing experimental data from a recent test series undertaken at Politecnico di Milano. The experimental campaign has dealt with a normal strength concrete, in which (by means of three-point-bending tests performed up to controlled crack opening and up to failure, respectively before and after exposure to different conditioning environments) the recovery of stiffness and load bearing capacity has been evaluated.     

Numerical simulation of reinforced concrete beams with different shear reinforcements under dynamic impact loads

The behavior of concrete/reinforced concrete structures is strongly influenced by the loading rate. Reinforced concrete structural members subjected to impact loads behave quite differently as compared to the same subjected to quasi-static loading. This difference is attributed to the strain-rate influence on strength, stiffness, and ductility as well as to the activation of inertia forces. These influences are clearly demonstrated in experiments. Moreover, for concrete structures, which exhibit damage and fracture phenomena, the failure mode and cracking pattern depend significantly on loading rate. In general, there is a tendency that with the increase of loading rate the failure mode changes from mode-I to mixed mode. Furthermore, theoretical and experimental investigations indicate that after the crack reaches critical speed of propagation there is crack branching. The present paper focuses on 3D finite-element study of reinforced concrete beams with different amount of shear reinforcement under impact. The experiments reported in literature are numerically simulated using the rate sensitive microplane model as constitutive law for concrete, while the strain-rate influence is captured by the activation energy theory. Inertia forces are implicitly accounted for through dynamic finite element analysis. However, the impact was modeled not by explicit modeling of two bodies but by incrementing the load point displacement till the maximum value and at the rate reported from the test. The results of the numerical study show that the numerical analysis using the procedure followed in this work can very well simulate the impact behavior of reinforced concrete beams. The static and dynamic reactions, crack patterns and failure modes as predicted in analysis are in close agreement with their experimentally observed counterparts. It was concluded that under impact loads, of the order as simulated in this work (blunt impact with velocity of around 1 m/s), the shear reinforcement does not get activated and therefore the dynamic reactions, unlike static reactions, are almost independent of the amount of shear reinforcement in the beams. However, the presence of shear reinforcement significantly affects the crack pattern and the cracks are well distributed in the presence of shear reinforcement, thus avoiding the formation of shear plugs.