混凝土化学-力学耦合作用的非局部损伤模型

提出了混凝土化学损伤和力学损伤的耦合模型.用损伤变量表示的本构关系模拟混凝土力学性能.分析了化学侵蚀下混凝土损伤发展过程.研究表明,应力软化造成混凝土局部损伤是结构失效的根源.局部化学损伤出现的时候,平衡微分方程不能满足.为了解决这个问题,采用了非局部损伤模型.试验和有限元计算结果表明,混凝土化学-力学耦合作用的非局部损伤模型能够较好地描述受化学侵蚀与荷载共同作用的损伤状态.     

Critical defect size distributions in concrete structures detected by the acoustic emission technique

Extensive research and studies on concrete fracture and failure have shown that concrete should be viewed as a quasi-brittle material having a size-dependent behavior. Numerous experimental techniques have been employed to evaluate fracture processes, and a number of modeling approaches have been developed to predict fracture behavior. A non-destructive method based on the Acoustic Emission (AE) technique has proved to be highly effective, especially to assess and measure the damage phenomena taking place inside a structure subjected to mechanical loading. In this paper, comparing AE frequency-magnitude statistics in solids subjected to damage processes with defect size distributions for disordered materials, critical parameters defining instability conditions for monitored structures are found. In addition, an experimental investigation conducted on concrete and RC structures by means of the AE technique is described. Experimental results confirm the described theories.     

The Use of Acoustic Emission Monitoring to Rank Paper Materials with Respect to Their Fracture Toughness

In this study, a simplified Acoustic Emission (AE) equipment, in essence an AE signal conditioner and a USB (Universal Serial Bus) data acquisition system, is used to study what happens in paper structures during mechanical loading. By the use of such equipment, some parameters that can be extracted are e.g. the stress and strain at onset of AE, the stress and strain at the onset of rapid AE defined as some numerical factor (larger then one) times the initial emission rate, the emission rate at the first stage of loading and the stress and strain at final failure i.e. when the specimen loses its load carrying ability.In this study however, the interest is focused on one particular parameter i.e. the elastic strain energy density W _c at onset of AE. This is a parameter with a clear physical meaning and in this study, the correlation between this parameter and a fracture toughness measure, is investigated.The conclusion is that when nine different paper materials (with a large span regarding properties) are considered, there is a correlation (however not linear) between these two parameters.     

基于应力波传播机理的混凝土无损检测研究综述

混凝土结构在基础设施中应用广泛,但环境与荷载导致材料和结构的老化损伤问题,势必影响材料性能及耐久性和结构完整性及安全性.构建有效的混凝土材料性能评估和结构损伤识别体系是当前研究热点.现有研究因对应力波传播机理不深入理解、损伤定位定量模型理想化、损伤形式简化等问题仍存在局限性.因此,开发普适全面、稳定可靠的无损检测系统,对在建或既有混凝土结构服役状况进行长期、实时跟踪,具有重要学术意义和工程应用价值.本文结合混凝土无损检测领域的国内外研究现状及进展,从智能压电材料振动机理研究、混凝土无损检测方法发展、基于应力波的无损检测研究进展这三个逻辑层面,即按照研究手段到研究方法再到具体研究方案的顺序,循序渐进地对混凝土材料性能评估和结构损伤识别的研究进行了成果总结和方法提炼;基于此进一步概述了该领域所存在的瓶颈问题、研究重难点,最后提出了针对这些局限性的混凝土无损检测研究展望.     

Identification of Cracking Mechanisms in Scaled FRP Reinforced Concrete Beams using Acoustic Emission

Acoustic emission was used to monitor the cracking mechanisms leading to the failure of scaled concrete beams having Glass Fiber Reinforced Polymer (GFRP) longitudinal reinforcement and no shear reinforcement. Dimensional scaling included that of the effective depth of the cross section, which is a key parameter associated with the scaling of shear strength; and maximum aggregate size, which affects the shear-resisting mechanism of aggregate interlock along shear (inclined) cracks. Five GFRP reinforced concrete (RC) beams with effective depth up to 290 mm and constant shear span-to-effective depth ratio of 3.1 were load tested under four-point bending. Two types of failures were observed: flexural, due to rupture of the GFRP reinforcement in the constant moment region; and shear, due to inclined cracking in either constant shear region through the entire section depth. Acoustic emission (AE) analyses were performed to classify crack types occurring at different points in the load history. The results of this study indicate that appropriate AE parameters can be used to discriminate between developing flexural and shear cracks irrespective of scale, and provide warning of impending failure irrespective of the failure mode (flexural and shear). In addition, AE source location enabled to accurately map crack growth and identify areas of significant damage activity. These outcomes attest to the potential of AE as a viable technique for structural health monitoring and prognosis systems and techniques.     

Microcracking and Fracture Process in Cement Mortar and Concrete: A Comparative Study Using Acoustic Emission Technique

This article reports the acoustic emission (AE) study of precursory micro-cracking activity and fracture behaviour of quasi-brittle materials such as concrete and cement mortar. In the present study, notched three-point bend specimens (TPB) were tested under crack mouth opening displacement (CMOD) control at a rate of 0.0004 mm/sec and the accompanying AE were recorded using a 8 channel AE monitoring system. The various AE statistical parameters including AE event rate $ \left( {\frac{dn }{dt }} \right) $ , AE energy release rate $ \left( {\frac{dE }{dt }} \right) $ , amplitude distribution for computing the AE based b-value, cumulative energy (ΣE) and ring down count (RDC) were used for the analysis. The results show that the micro-cracks initiated and grew at an early stage in mortar in the pre peak regime. While in the case of concrete, the micro-crack growth occurred during the peak load regime. However, both concrete and mortar showed three distinct stages of micro-cracking activity, namely initiation, stable growth and nucleation prior to the final failure. The AE statistical behavior of each individual stage is dependent on the number and size distribution of micro-cracks. The results obtained in the laboratory are useful to understand the various stages of micro-cracking activity during the fracture process in quasi-brittle materials such as concrete & mortar and extend them for field applications.     

An analytical homogenization method for heterogeneous porous materials

The objective of this work is to develop an analytical homogenization method to estimate the effective mechanical properties of fluid-filled porous media with periodic microstructure. The method is based on the equivalent inclusion concept of homogenization applied earlier for solid–solid mixture. It is assumed that porous media are described by the poroelastic constitutive law developed by Biot where porosity is a material parameter. By solving the governing equations of poroelasticity in Fourier transformed domain, the relation between periodic strain and eigenstrain in porous media is established. This relation is subsequently used in an average consistency condition involving both solid and fluid phase stresses and strains. The geometry of the porous microstructure is captured in the g-integral. This homogenization method can also be applied to estimate the equivalent properties of solid–fluid mixture where a pure solid and fluid can be modeled by assuming very low and high porosity, respectively. Several examples are considered to establish this new method by comparing with other existing analytical and numerical methods of homogenization. As an application, poroelastic properties of cortical bone fibril are estimated and compared with previously computed values.     

Crack propagation in concrete: Linear elastic fracture mechanics and boundary element method

Crack propagation in concrete is investigated by application of Linear Elastic Fracture Mechanics (LEFM) and the Boundary Element Method (BEM). Acoustic Emission (AE) measurement is made for detecting the load level at which unstable crack initiation occurs in a notched beam under bending. The corresponding critical stress intensity factor K_Ic does not vary appreciably with notch depth. This result is in contrast to that found by the conventional procedure.The critical stress intensity factor K_Ic is employed to analyze the process of crack propagation in an arbitraty direction. The BEM prediction is in good agreement with the experiments.The detection and assessment of crack initiation in concrete structures is of considerable technical interest.     

低周反复荷载下再生混凝土柱损伤演化模型研究

通过对现有的两种混凝土损伤评估模型的分析,本文以刚度退化和累积滞回耗能两个指标来描述在低周反复荷载下再生混凝土的损伤累积过程,提出了一种适用于再生混凝土柱的双参数地震损伤模型．通过24 组文献数据拟合出模型参数的合理值与耗能因子β 同再生混凝土多因素之间关系的表达式．同时对再生混凝土柱的损伤量化标准进行了细分．最后将本文提出的损伤模型同另外两种模型作了比较．对比表明,本文所提出的损伤模型计算出来的损伤值标准差较小,离散性较低,能够更好地反映再生混凝土柱的累积损伤过程．     

混凝土单轴受拉的非局部本构模型

混凝土受拉本构行为存在很强的局部软化现象,使得单轴受拉试验无法给出应力-应变关系,而只能给出应力-位移关系。本文根据内变量理论和等效应变假设建立了基于真实应变的混凝土单轴受力本构方程,并根据Weibull分布可以描述混凝土等脆性材料断裂过程的试验现象,建立了关于弹性应变的损伤演化规律。然后,通过假设平均应变与真实弹性应变的函数关系,在应力-平均应变的本构关系中采用平均弹性应变以描述其非局部行为,而在材料的损伤演化规律中采用真实弹性应变以描述其局部行为,由此建立了单轴受拉荷载条件下的非局部本构模型。最后,对一个单调受拉试验和一个反复受拉试验的仿真结果表明所提出的非局部本构模型可以准确地模拟试验结果。     

冲击载荷下混凝土材料的动态本构关系

利用改装的杆径为 74mm的直锥变截面式大尺寸Hopkinson压杆对混凝土材料进行冲击压缩实验 ,系统研究了混凝土的应变率硬化效应 ,采用一种新的方法损伤冻结法对混凝土材料在冲击载荷下的损伤软化效应进行了系统研究 ,给出了冲击载荷下混凝土的损伤演化方程 ;在对数据进行合理分析的基础上 ,结合粘弹性本构理论 ,得到混凝土材料的损伤型线性粘弹性本构关系。     

Acoustic emission for fatigue damage characterization in metal plates

Acoustic emission (AE) supplies information on the fracturing behavior of different materials. In this study, AE activity was recorded during fatigue experiments in metal coupons. The plates were characterized by a symmetric V-shape notch and were loaded in tension-tension fatigue until final failure with concurrent AE activity monitoring. The relatively broad bandwidth of the sensors enabled the recording of a wide range of frequencies up to 1 MHz. AE parameters like energy and duration exhibited a certain increase with the accumulation of damage although the hit rate was not significantly influenced. Furthermore the behavior of RA value (ratio of rise time to amplitude of the waveforms) which quantifies the shape of the first part of the AE signals and has been used for characterization of the cracking mode, showed a certain shift indicating the transition from tensile mode to shear which can be confirmed by the visual observation of the crack geometry after the experiment. The time history of RA is similar to the crack propagation rate (da/dN) curve but exhibits the rapid hyperbolic growth consistently about 1000 cycles earlier than final failure. Therefore, the use of acoustic emission parameters is discussed both in terms of characterization of the damage mechanisms, as well as a tool for the prediction of ultimate life of the material under fatigue.     

含孔复合材料层合板静拉伸损伤演化的实验和表征方法

为了深入探究复合材料层合板结构的损伤机理和损伤演化,应用声发射技术和图像相关技术同步实时监测含孔碳纤维复合材料层合板试样在静拉伸过程中的损伤演化。实验结果表明,试样表面应变场呈现局部化特征。对应变集中带在加载方向的应变值进行了统计分析,获得了应变场的特征统计量(标准差)随加载的演化模型。层合板损伤时产生声发射信号的峰值频率大小能够有效区分复合材料的损伤模式,由此,建立了基于损伤模式累积声发射数的损伤演化模型。通过对应变场演化模型和声发射损伤演化模型的分析,可以将复合材料的损伤演化分为损伤初始阶段、损伤平稳扩展期、损伤严重阶段三个部分。统计分析结果表明:在损伤严重阶段,基于声发射事件数的各种损伤的损伤变量和局部应变场标准差快速增长,因此局部应变场统计标准差可以作为后期局部损伤严重程度的识别指标。     

长期荷载作用下CFST界面粘结损伤试验研究

为考察在役钢管混凝土界面粘结性能与荷载作用时间的关系,以荷载作用时间段为基本参数,完成了7根圆钢管混凝土试件的推出试验.试验结果表明,钢管混凝土界面粘结力随荷载作用时间的延长而减小.根据试验结果,得出各个试件的荷载—滑移曲线.基于ANSI/ACI308-1992混凝土养护规程,得出每个试件在荷载作用下的混凝土徐变应变.从损伤力学的角度出发,定义界面损伤变量,推导了混凝土徐变应变与损伤的关系.通过回归得出含损伤变量的界面粘结—滑移本构关系.     

ABAQUS混凝土损伤塑性模型中损伤因子的率相关性及实现方法

ABAQUS程序中最常用的混凝土损伤塑性（concrete damage plasticity, CDP）模型无法实现损伤因子与应变率相关。为了准确描述混凝土材料在高应变率下的损伤特性,基于CDP模型定义了新的应变率场变量,编制了VUSDFLD用户子程序,开发了能够考虑损伤因子率相关性的改进的CDP（modified CDP,MCDP）模型。MCDP模型采用能量法求解混凝土拉压损伤因子,主求解程序能够随着应变率场变量的变化而自动更新不同应变率对应的损伤参数,计算得到的混凝土单轴静态加载结果与CDP模型吻合较好。MCDP模型对高应变率下动态压缩性能的模拟结果表明:混凝土材料在不同应变率下的拉压损伤对其动态力学性能有显著影响,编制的VUSDFLD子程序和MCDP模型能够有效地解决损伤应变率相关的模拟难题,可以准确地模拟爆炸荷载作用下钢筋混凝土梁的动态响应,为预测爆炸冲击等强动载作用下混凝土结构的响应和破坏提供了更可靠的技术途径。     

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.     

On the creep rupture of structures

A study is made of the failure times of structures made of materials which suffer creep deformation and which crack according to the Kachanov law in which strain and damage rates are given in terms of the existing stress and state of damage. For certain types of structures, it is shown that lower bound estimates of the failure time can be made by considering the variation of a certain global parameter.     

Fatigue crack growth model for assessing reliability of box-girders for cable-stayed bridge combining SHMS with strain data

Enhancement of the computer algorithm developed for the Runyang cable-stayed bridge (RYCB) in China is made by incorporating the fatigue crack growth analysis in contrast to the SN curve approach. Strain data obtained from the structural health monitoring system (SHMS) and finite element calculations are used. This provides the application of a deterministic method in addition to the probabilistic approach with the added feature of crack growth. The choice of selecting the two-parameter fatigue crack growth criterion was based on the definition of reliability index β such that the new results can be compared with those using the SN curves. A gradual drop of the reliability index β with time with an upper limit was obtained for the crack growth model in contrast to the linear time relation for the SN curve model that had no upper limit. This difference is significant and reveals the importance for selecting the fatigue failure criterion. Deterministic and probabilistic crack growth models are used to assess the differences. The results are based on the box-girder component strain history data of the Runyang Cable-stayed Bridge (RYCB) in China, stress history recorded by structural health monitoring system (SHMS) is analyzed using the monitored stress amplitude, mean stress and stress ratio. Finite element calculations are used to supplement data at locations not accessible for measurements. Additional improvement with reference to damage accumulation and the physical meaning of the reliability index will be studies in relation to the fatigue damage of box-girder of long cable-stayed bridges.     

基于声发射参量的不同深度灰岩塑性破坏识别研究

对某矿深部灰岩进行单轴压缩试验,测得试件破坏全过程的应力、应变和声发射参数。通过分析应力、应变和声发射参数之间的关系,可根据声发射振铃计数率的变化情况,将全过程分为三个阶段:第一阶段处于孔隙裂隙压密阶段,振铃计数率较多;第二阶段处于稳定变形阶段,声发射处于平静期;第三阶段岩石进入塑性变形阶段,振铃计数率明显增加。研究发现,深度越深,第一阶段终止时的应力水平越高;将声发射累计能率与累计振铃计数率的比值定义为能振比λ,λ的变化规律一定程度上反映出试件内部裂隙发展情况。研究λ的变化规律能够为识别岩体失稳提供依据,提高声发射技术监测预报的准确性。     

A multiaxial constitutive model for concrete in the fire situation: Theoretical formulation

This paper aims to develop a multiaxial concrete model for implementation in finite element software dedicated to the analysis of structures in fire. The need for proper concrete model remains a challenging task in structural fire engineering because of the complexity of the concrete mechanical behavior characterization and the severe requirements for the material models raised by the development of performance-based design. A fully three-dimensional model is developed based on the combination of elastoplasticity and damage theories. The state of damage in concrete, assumed isotropic, is modeled by means of a fourth order damage tensor to capture the unilateral effect. The concrete model comprises a limited number of parameters that can be identified by three simple tests at ambient temperature. At high temperatures, a generic transient creep model is included to take into account explicitly the effect of transient creep strain. The numerical implementation of the concrete model in a finite element software is presented and a series of numerical simulations are conducted for validation. The concrete behavior is accurately captured in a large range of temperature and stress states. A limitation appears when modeling the concrete post-peak behavior in highly confined stress states, due to the coupling assumption between damage and plasticity, but the considered levels of triaxial confinement are unusual stress states in structural concrete.