Nonlocal anisotropic damage model and related computational aspects for quasi-brittle materials

A three-dimensional damage model with induced damage anisotropy is proposed for quasi-brittle materials such as concrete. The thermodynamics framework is used, considering then a single second-order tensorial damage variable whatever the intensity and the sign of the loading. The quasi-unilateral conditions of micro-cracks closure are written on the hydrostatic stress only. Altogether with the consideration of damage laws ensuring a damage rate proportional to the positive part of the strain tensor this is sufficient to model a strongly different behavior due to damage in tension and in compression. A proof of the positivity of the intrinsic dissipation due to such an induced anisotropic damage is given.An efficient scheme for the implementation of the damage model in commercial Finite Element codes is then detailed and numerical examples of structural failures are given. Plain concrete, reinforced and pre-stressed concrete structures are computed up to high damage level inducing yielding of the reinforcement steels. Local and nonlocal computations are performed.A procedure for the control of rupture is proposed. It is a key point making the computations with anisotropic damage truly efficient.     

基于结构响应统计特征的神经网络损伤识别方法

提出一种采用结构动态响应的统计特征作为损伤指标的神经网络损伤识别方法,并对其进行了数值模拟和实验验证。首先,通过敏感性分析,分析了采用结构动力响应的统计特征作为损伤指标的可行性;然后数值模拟了一三跨连续梁采用结构位移方差作为损伤指标的神经网络损伤识别过程,其结果表明,经过训练的神经网络可以准确的识别出单损伤和多损伤工况中的损伤位置和损伤程度;最后进行一组两端固定的简支梁模型实验来验证所提出损伤识别方法的有效性。实验结果表明,对于单损伤工况,神经网络可以准确地识别出结构中损伤位置和损伤程度,对于双损伤工况,神经网络可以准确地识别出损伤位置,而损伤程度识别略有偏差。最后得出结论,采用结构动力响应的统计特征作为损伤指标的神经网络损伤识别方法是可靠有效的。     

Stochastic modeling of damage evolution in composites under environmental ageing

A mathematical model is proposed for predicting damage evolution and stiffness degradation in composite materials under environmental ageing. Environmental parameters such as moisture, temperature, and ultraviolet (UV) radiation cause hygrothermal loads on the structure, which leads to damage evolution. The present work establishes a constitutive model for treating the damage density as a random variable. A forward stochastic differential equation (FSDE) is proposed to predict the damage density evolution. Damage nucleation and annihilation rates are taken into consideration in terms of Brownian motions. A second-order damage tensor is developed using the solution of the FSDE. The damage tensor is incorporated into the constitutive model for predicting the elastic moduli. Finally, the proposed model is verified against experimental observations under certain hygrothermal conditions.     

基于加速度响应相关性的结构损伤识别方法

通过推导证明了加速度响应的相关特性包含结构的模态信息,与结构损伤的程度和位置相关,提出加速度响应相关特性作为结构损伤因子的损伤识别方法,以该指标作为BP神经网络神经元,判断结构的损伤程度和损伤位置。利用有限元分析证明了该方法的有效性,通过试验研究验证了该方法的适用性。     

Modeling of continuum damage for application in elastic-viscoplastic constitutive equations

A procedure is described for including isotropic and directional damage as load-history dependent softening variables in a set of elastic-viscoplastic constitutive equations. The evolution equation proposed for isotropic damage integrates to an exponential form for the case of constant stress. Directional damage is represented as a second-order symmetric tensor with a scalar effective value used in the constitutive equations. A method is proposed for treating directional damage in the case of non-proportional loading histories. Comparisons are given of uniaxial creep test results for an alloy at high temperatures with calculations based on the constitutive equations with the inclusion of isotropic damage.     

Unified damage description and risk assessment of buildings under extreme natural hazards

Internationally different classification schemes are available for the assessment of structural damage due to the different natural hazards. Due to their heterogeneity, these classification schemes are insufficiently suited for the consideration of damage in the sense of a multi hazard approach. A unified system is necessary for the evaluation of the building vulnerability and the damage prognosis due the different natural hazards. The unified system transfers repeatedly observed damage patterns into a scheme of damage grades. With this instrument, the structural damage can be uniformly evaluated and the relationship to the input parameters can be established. Following the concept of the European Macroseismic Scale 1998 (EMS‐98) for earthquake damage, it is distinguished between structural and non‐structural damage to which characteristic damage patterns can be assigned in the form of damage grades. A general classification scheme for building damage is presented, which enables a comparison of the damage due to the individual natural hazards. On the basis of real observed damage cases, the typical damage patterns for the natural hazards flood, tsunami and wind are highlighted and converted into harmonized classification schemes for damage grades. For the first time, a harmonized set of instruments is available for evaluation of damage cases on a building stock as a result of different natural hazards according to criteria standardized in engineering terms. The outlook refers to the damage prognosis due to the different natural hazards and their possible sequences.     

颗粒增强铝基复合材料细观损伤演化

研究了颗粒增强铝基复合材料韧性断裂的细观损伤演化特征,提出了细观损伤的最弱环模型,并建立了宏观损伤参数与细观损伤机理之间的联系。分析了颗粒尺寸、体积百分比和热处理状态对损伤演化的影响。研究结果表明:宏观损伤演化可以用幂指数形式来表示,细观最弱环模型能够揭示损伤机理,损伤演化模型计算的结果与实验结果吻合较好。     

An enhanced Lemaitre model formulation for materials processing damage computation

The Lemaitre damage model is now widely used to deal with coupled damage analyses for various mechanical applications. In this article, different extensions of the model are presented and discussed to deal with complex multiaxial configurations—such as multi-stages bulk forming processes. A specific treatment is done to account for compressive damage growth, and a stress triaxiality cut-off value is considered to avoid any damage evolution below a critical negative triaxiality. The damage potential is also modified to deal with highly ductile materials, and the plastic strain is split into a negative part and a positive part to differentiate damage growth for compressive states of stress and for tensile states of stress. Finally, an anisotropic damage approach based on the comparison between grain flow orientation and principal loading directions is defined. A combination of these extensions is achieved within a single Lemaitre formulation. Application on different examples show the robustness and accuracy of the model defined in this paper.     

主余震序列作用下结构增量损伤比研究

该文以非线性单自由度体系为研究对象,挑选了662条真实主余震序列作为地震动输入,采用Park和Ang提出的损伤指数作为结构损伤指标,对主余震序列作用下的结构主震损伤、余震增量损伤和主余震累积损伤在结构全周期范围内进行了分析。提出了增量损伤比参数(余震增量损伤/主震损伤),开展了增量损伤比与主余震强度比(余震强度/主震强度)的相关性分析,建立了以结构周期与主余震强度比为参数的增量损伤比预测模型,并对模型的预测精度及其受滞回模型的影响进行了讨论。结果表明:增量损伤比与主余震强度比之间存在较高的对数线性相关性,该文提出的增量损伤比预测模型精度较高,滞回模型对增量损伤比预测模型的精度影响较大。     

Numerical Simulation of Brittle Damage in Concrete Specimens

A framework for damage mechanics of concrete is applied to simulate the nonlinear elastic deformation behavior of concrete using finite element method (FEM). A rather simple isotropic damage model containing essentially no adjustable parameters is shown to produce results in remarkably good agreement with sample experimental data: the damage law requires only the fracture energy to be defined completely. The model is achieved by introducing a damage surface that is similar to the yield function in the conventional theory of plasticity. A special form of damage surfaces is constructed to illustrate the application of the model. A new damage criterion, defined as an equivalent strain norm, is proposed, in order to take into consideration the asymmetric behavior of concrete. For verifying the FEM program including the model, deformations predicted by this model are compared with both the experimental ones for the concrete structural model and the ones calculated without application of the continuum damage mechanics.__________Translated from Problemy Prochnosti, No. 3, pp. 57 – 74, May – June, 2005.     

Impact damage detection in composite laminates using nonlinear acoustics

The paper demonstrates the application of nonlinear acoustics for impact damage detection in composite laminates. A composite plate is monitored for damage resulting from a low-velocity impact. The plate is instrumented with bonded low-profile piezoceramic transducers. A high-frequency acoustic wave is introduced to one transducer and picked up by a different transducer. A low-frequency flexural modal excitation is introduced to the plate at the same time using an electromagnetic shaker. The damage induced by impact is exhibited in a power spectrum of the acoustic response by a pattern of sidebands around the main acoustic harmonic. The results show that the amplitude of sidebands is related to the severity of damage. The study investigates also the effect of boundary conditions on the results.     

A Probabilistic Method to Predict Fatigue Crack Initiation

A probabilistic method to predict macrocrack initiation due to fatigue damage is presented in this paper. Acoustic non-linearity is used to quantify pre-macrocrack initiation damage. This data is then used in a probabilistic analysis of fatigue damage. The probabilistic fatigue damage analysis consists of a suitably chosen damage evolution equation to model accumulated damage coupled with a procedure to calculate the probability of macrocrack initiation. The probability of macrocrack initiation is evaluated using the Monte Carlo Method with Importance Sampling. Numerical results for the probabilistic assessment of fatigue damage for a sample problem are presented and compared with experimental results.     

A model for anisotropic viscoelastic damage in composites

A model for continuous damage combined with viscoelasticity is proposed. The starting point is the formulation connecting the elastic properties to the tensor of damage variables. A hardening law associated with the damage process is identified from available experimental information and the rate-type constitutive equations are derived. This elastic damage formulation is used to formulate an internal variable approximation to viscoelastic damage in the form of a non-linear Kelvin chain. Elastic and viscoelastic equations are implemented into a finite element procedure. The code is verified by comparison with closed-form solutions in simplified configurations, and validated by fitting results of experimental creep tests.     

Effect of prior plastic deformation and creep strain on fatigue damage for a high-temperature alloy

A procedure is developed, an experimental study is provided, and a comparative evaluation is given for the effect of prior plastic deformation and creep strain on fatigue damage for high-temperature metallic materials on the example of alloy éI827. A different effect is demonstrated for prior plastic deformation and creep strain on fatigue damage of this alloy at 1073 K. In the case of prior creep a marked difference is established from the hypothesis of linear summation for damage in the direction of weakening during evaluation of the overall fatigue and static damage both in the form of relative endurance and relative strain.     

基于混沌激励与吸引子分析的结合面损伤识别方法

工程结构在使用寿命周期内,各种环境因素会导致结合面出现损伤,从而威胁结构的完整性和功能性,甚至诱发安全事故。研究了一种利用混沌激励与吸引子几何特性进行结合面损伤识别的方法,采用混沌振动信号激励待测结构,对采集到的加速度响应信号进行相空间重构,并构造了一种基于吸引子局部方差计算的特征参量用于损伤识别,同时研究了影响特征参量的主要参数。设计了悬臂梁结合面损伤识别实验,控制固定端螺栓预紧力的下降来模拟结合面损伤,利用上述方法对结合面的损伤状态进行了识别。结果表明：本文方法能够识别结合面的损伤状态,所构造的特征参量随损伤程度改变单调变化,响应测点配置、特征参量计算参数等对损伤识别的效果有影响。     

Damage mechanics of composite materials: I— Measurements of damage and strength

A new approach for modelling the strength of notched composites has been developed. The approach is based on the assumption that subcritical damage modifies the notch-tip stress field and that the state of subcritical damage just before failure, referred to as the terminal damage state (TDS), must have a significant influence on notched strength. The TDS was monitored for a wide range of cross-ply graphite reinforced epoxy specimens using real-time radiography. A finite element model incorporating the TDS was used to determine the modified notch-tip stress field. A simple tensile stress failure criterion has been found to predict failure very well provided that the effect of subcritical damage is considered in this way. The effect of both layup and notch size on strength can be entirely accounted for by the effect these parameters have on the terminal damage state. In the first paper of a four-part series, radiographs of c. 60 specimens have been used to characterize the notch-tip damage zone and to establish a qualitative relationship between terminal damage and notched strength.     

Nonlocal elastic damage near crack tip

A constitutive modeling for nonlocal elastic damage near crack tip is proposed. A calculation method for nonlocal elastic damage is introduced and the computational results for stress and damage are given by means of finite element method.     

Optical fibre damage detection for an aircraft composite leading edge

Recent developments towards the development of a fibre optic damage detection system for composite materials are described. Results of experiments designed to measure the influence of optical fibre orientation and depth on the sensitivity of the system are reported and optimal configurations have been determined for both. A surface treatment for controlling the damage sensitivity of the optical fibres to the point where they can detect barely visible damage is described. A study into the failure mechanisms of embedded optical fibres is also reported. The technique has been demonstrated to be capable of detecting both impact and quasi-statically induced damage and can be used to map the growth of a region of damage with increasing load.     

On-site indication of fatigue damage under complex loading

A fatigue damage indicator using a microcomputer is reported. The evaluation of damage was based on the rainflow algorithm, the S/N relation and the linear damage rule. Real-time, on-site fatigue damage evaluation was achieved by the investigation of a digital system-oriented rainflow algorithm, by the adoption of an input buffer and by the use of a table where the result of calculating fatigue damage per reversal of strain is stored. A hard-wired logic for the rainflow algorithm was established and was simulated by the microcomputer. Multi-channel monitoring and its applications to cross-analysis are briefly discussed.     

Analysis of fatigue damage evolution and associated anisotropic elastic property degradation in random short-fiber composite

A theoretical analysis of cyclic fatigue damage and associated anisotropic property degradation in a random short-fiber composite is presented. The fatigue damage takes various forms of microcracking, originated from microscopic stress concentrators in the highly heterogeneous material system. A probabilistic treatment of the microcracks is introduced to evaluate the statistical nature of the microscopic fatigue damage. Damage evolution and accumulation are analyzed through the development of probabilistic density functions of microcrack length and orientation during the cyclic loading history. Constitutive equations for the damaged fiber composite are then derived on the basis of a self-consistent mechanics scheme in conjunction with a three-dimensional elliptic crack theory and the microcrack density functions. Cyclic fatigue degradation and associated damage-induced anisotropy of composite material properties are determined and checked against experiments. The tensorial nature of material damage and composite stiffness changes during fatigue are evaluated explicitly. A power-law relationship between the rate of damage growth and the fatigue loading cycle is obtained. The rate of fatigue damage growth is found to decrease exponentially with load cycles—a phenomenon unique to the random short-fiber composite. This study provides a comprehensive analytical treatment of the homogeneous fatigue damage problem for random short-fiber composites. The fundamental mechanics and mechanisms of fatigue damage evolution and associated anisotropic property degradation of the composite are elucidated.