Progressive damage and failure of mechanically fastened joints in CFRP laminates – Part II: Failure prediction of an industrial junction

In part II of this study, a methodology is presented to compute the failure of large-scale bolted joints in composite structures. This methodology is based both on a multilevel calculation strategy and on virtual testing. At the global level, coarse FE modelling of the structure is used to assess the load distribution between the fasteners. The most loaded fasteners are identified and the loads issued from the global calculation are used as boundary conditions for the local failure analysis, based on fast semi-empirical models. Nevertheless, in this work, instead of using experimental data, prevision of failure is achieved by fully numerical means. The parameters of the semi-empirical models are evaluated by virtual testing, using the refined FE model proposed in part I of this paper.     

Relative importance of uncertain structural parameters. Part II: applications

This second part describes the application of the methodology for assessing the relative importance of uncertain structural parameters. The emphasis is on the demonstration that the proposed method can indeed handle large-scale problems relevant to industrial users. Four examples are included, with growing complexity and degree of difficulty. While the first two examples are quite simple tutorial type problems, the remaining two examples deal with a large-scale application from aerospace engineering. The results demonstrate the remarkable efficiency of the method, even for problems with extremely high numbers of uncertain parameters.     

An embedded statistical method for coupling molecular dynamics and finite element analyses

The coupling of molecular dynamics (MD) simulations with finite element methods (FEM) yields computationally efficient models that link fundamental material processes at the atomistic level with continuum field responses at higher length scales. The theoretical challenge involves developing a seamless connection along an interface between two inherently different simulation frameworks. Various specialized methods have been developed to solve particular classes of problems. Many of these methods link the kinematics of individual MD atoms with finite element (FE) nodes at their common interface, necessarily requiring that the FE mesh be refined to atomic resolution. Some of these coupling approaches also require simulations to be carried out at 0 K and restrict modelling to two‐dimensional material domains due to difficulties in simulating full three‐dimensional material processes. In the present work, a new approach to MD–FEM coupling is developed based on a restatement of the standard boundary value problem used to define a coupled domain. The method replaces a direct linkage of individual MD atoms and FE nodes with a statistical averaging of atomistic displacements in local atomic volumes associated with each FE node in an interface region. The FEM and MD computational systems are effectively independent and communicate only through an iterative update of their boundary conditions. Thus, the method lends itself for use with any FEM or MD code. With the use of statistical averages of the atomistic quantities to couple the two computational schemes, the developed approach is referred to as an embedded statistical coupling method (ESCM). ESCM provides an enhanced coupling methodology that is inherently applicable to three‐dimensional domains, avoids discretization of the continuum model to atomic scale resolution, and permits finite temperature states to be applied. Published in 2009 by John Wiley & Sons, Ltd.     

结构静力分析的区间摄动有限元法

基于新的区间参数系统响应界值的评估方法,推导了基于Taylor展开的区间摄动有限元法和区间参数摄动有限元法的高阶求解方法。并提出了一种新的区间摄动有限元法,该方法将刚度矩阵的逆矩阵用一系列Neumann展开级数来表示,最终得到结构响应摄动量的上下界限,是对结构响应鲁棒性的一种直接评估,因此称之为区间鲁棒摄动有限元法。比较了三种区间摄动有限元法的计算精度和计算效率。算例结果表明:区间鲁棒摄动有限元法具有较好的精度,能够适用于大型航空航天结构的不确定分析和优化。     

Back analysis of model parameters in geotechnical engineering by means of soft computing

In this paper, a parameter identification (PI) method for determination of unknown model parameters in geotechnical engineering is proposed. It is based on measurement data provided by the construction site. Model parameters for finite element (FE) analyses are identified such that the results of these calculations agree with the available measurement data as well as possible. For determination of the unknown model parameters, use of an artificial neural network (ANN) is proposed. The network is trained to approximate the results of FE simulations. A genetic algorithm (GA) uses the trained ANN to provide an estimate of optimal model parameters which, finally, has to be assessed by an additional FE analysis. The presented mode of PI renders back analysis of model parameters feasible even for large‐scale models as used in geotechnical engineering. The advantages of theoretical developments concerning both the structure and the training of the ANN are illustrated by the identification of material properties from experimental data. Finally, the performance of the proposed PI method is demonstrated by two problems taken from geotechnical engineering. The impact of back analysis on the actual construction process is outlined. Copyright © 2003 John Wiley & Sons, Ltd.     

Numerical and exact models for free vibration analysis of cylindrical and spherical shell panels

The present paper shows a comparison between classical two-dimensional (2D) and three-dimensional (3D) finite elements (FEs), classical and refined 2D generalized differential quadrature (GDQ) methods and an exact three-dimensional solution. A free vibration analysis of one-layered and multilayered isotropic, composite and sandwich cylindrical and spherical shell panels is made. Low and high order frequencies are analyzed for thick and thin simply supported structures. Vibration modes are investigated to make a comparison between results obtained via the FE and GDQ methods (numerical solutions) and those obtained by means of the exact three-dimensional solution. The 3D exact solution is based on the differential equations of equilibrium written in general orthogonal curvilinear coordinates. This exact method is based on a layer-wise approach, the continuity of displacements and transverse shear/normal stresses is imposed at the interfaces between the layers of the structure. The geometry for shells is considered without any simplifications. The 3D and 2D finite element results are obtained by means of a well-known commercial FE code. Classical and refined 2D GDQ models are based on a generalized unified approach which considers both equivalent single layer and layer-wise theories. The differences between 2D and 3D FE solutions, classical and refined 2D GDQ models and 3D exact solutions depend on several parameters. These include the considered mode, the order of frequency, the thickness ratio of the structure, the geometry, the embedded material and the lamination sequence.     

适用于城市高层建筑群的震害预测模型研究

高层建筑是城市建筑的重要组成部分,现有建筑震害预测模型难以满足城市区域高层建筑群震害分析的要求。该文提出了一套非线性多自由度弯剪耦合模型（NMFS）以及其参数标定方法。该模型:1）能模拟高层建筑显著的弯剪耦合变形行为。与传统的剪切层模型对比,该模型能准确的模拟高层建筑的层间位移角包络,其结果与精细有限元模型的结果非常接近;2）具有非常高的计算效率。与精细有限元模型对比,该模型的计算加速比超过60000倍;3）参数标定简单。仅需要借助于少量的建筑属性信息（结构高度、建设年代、设计信息和结构类型）就能生成整个模型的弹塑性参数;4）能输出各层时程响应以及层间位移角情况,使未来基于工程需求参数（EDP）的区域高层建筑经济损失预测成为了可能。论文针对一栋典型高层结构详细展示了模型的建立与标定流程,并对标定参数的准确性进行了校验。最后对北京CBD地区高层建筑群进行了震害模拟,验证了该方法在城市区域中应用的可行性。该文的成果期望为未来区域高层建筑的地震损失预测提供参考。     

A comparative study of methodologies for vibro-acoustic FE model updating of cavities using simulated data

The structural dynamic modeling errors, which at times are difficult to eliminate in a structural FE model, can affect the accuracy and reliability of the vibro-acoustic FE models for NVH design of the cavities. A large number of methods have been proposed for structural finite element model updating. However, most of the studies conducted are mainly focused on structural dynamic applications and no work is reported on vibro-acoustic systems. The objective of this paper is to compare through a simulated study two recently proposed methodologies for vibro-acoustic FE model updating of cavities with weak acoustic coupling to address structural dynamic modeling errors. These methodologies utilize a direct and an iterative method of model updating developed for purely structural systems. A simulated example of a 2D rectangular cavity with a flexible surface is presented. Cases of incomplete and noisy data are considered. The comparison is done on the basis of accuracy of prediction of vibro-acoustic natural frequencies and the responses both inside and outside the frequency range of interest. It is concluded that both the methodologies give an accurate prediction of the vibro-acoustic natural frequencies and the response inside the updating frequency range. However, beyond this range, the predictions based on the direct updated vibro-acoustic models are not accurate. It is noted that the success of updating using IESM is dependent on the correct knowledge of the modeling inaccuracies, uncertainties or approximations and also on the choice of the suitable updating parameters, which could be very challenging for complex cavities. The vibro-acoustic FE model updating using the direct method could be handy in such situations where the iterative methods are difficult to be effectively applied.     

随机荷载作用下参数不确定结构的疲劳损伤估计

基于改进区间分析和频域疲劳计算方法,对参数不确定结构在平稳高斯荷载作用下的疲劳损伤进行研究,提出完全混合和简化计算两种方法。采用区间变量模型定义结构的不确定参数,功率谱密度描述外荷载的随机性;利用有理级数显式表示结构区间频响函数及在平稳高斯荷载作用下不确定结构的应力响应区间。通过数值方法验证疲劳损伤期望率关于不确定参数的单调性后,将应力响应中不确定参数的界限完全组合提出完全混合方法,准确估计参数不确定结构的疲劳损伤期望率区间;简化计算方法则将不确定参数的界限适当组合,由显式表达式近似计算结构的疲劳损伤期望率区间。算例表明,两种方法均具有较高计算精度,且大幅减少计算量。     

Identification of composite uncertain material parameters from experimental modal data

Stochastic analysis of structures using probability methods requires the statistical knowledge of uncertain material parameters. This is often quite easier to identify these statistics indirectly from structure response by solving an inverse stochastic problem. In this paper, a robust and efficient inverse stochastic method based on the non-sampling generalized polynomial chaos method is presented for identifying uncertain elastic parameters from experimental modal data. A data set on natural frequencies is collected from experimental modal analysis for sample orthotropic plates. The Pearson model is used to identify the distribution functions of the measured natural frequencies. This realization is then employed to construct the random orthogonal basis for each vibration mode. The uncertain parameters are represented by polynomial chaos expansions with unknown coefficients and the same random orthogonal basis as the vibration modes. The coefficients are identified via a stochastic inverse problem. The results show good agreement with experimental data.     

Bearing dynamic parameters identification for a sliding bearing-rotor system with uncertainty

Bearing dynamic parameters of a sliding bearing-rotor system are important factors to the vibration absorbing characteristics. It is a valuable method to identify bearing dynamic parameters based on the unbalance response. Nevertheless, in Engineer, the unbalance parameters are uncertain due to the structural complexity and manufacturing or measuring error. Thus, developing an efficient method to estimate the bearing dynamic parameters for the uncertain unbalance parameters seems more important and necessary. This paper presented an inverse method for identifying stably the bounds of bearing dynamic parameters with uncertain unbalance parameters based on dynamic load identification method and the interval analysis .In the method, bearing dynamic parameters identification problem is formulated as the oil film force reconstruction by considering the oil film supports as rotor load boundary conditions; the midpoint oil film force and the first derivative to each uncertain unbalance parameter need be calculated with the interval analysis; the bounds of bearing dynamic parameters can be finally identified based on the interval mathematics. Finally, the efficiency and robustness of the proposed method are verified by a numerical example.     

Research on deformation characteristics of JCOE forming in large diameter welding pipe

In the present work, the JCOE forming is investigated using the finite element (FE) method. A twodimensional FE model is established for the plane strain condition by FE code ABAQUS, and the FE model is validated by experiments. The aim of this research is to investigate forming quality states in the JCOE forming process; in particular, the effects of technological parameters on forming quality are evaluated. Taking the JCOE forming process of X80 steel φ1 219 mm×22 mm×12 000 mm welding pipe for instance, the deformation characteristics of JCOE forming are analyzed, in which the geometry of the formed pipe, residual stress distributions and effects of process parameters on JCOE forming quality can be obtained. Thus, the presented results of this research provide an effective approach to improve welding pipe forming quality.     

A multimedia, multiple pathway risk assessment of atrazine: the impact of age differentiated exposure including joint uncertainty and variability

This paper addresses the need to evaluate the differences in exposure to pesticides between children and adults. We present a framework for evaluating these differences in exposure to pesticides through multiple exposure pathways. The concentrations in all environmental media were determined as distributions in a companion paper. All parameter values utilized in the calculation of exposure and risk are considered to be distributions, resulting in distributions for output exposure and risk. A sensitivity analysis was completed to determine the relative importance of the parameters with respect to the outcome. A joint uncertainty and variability analysis was also completed to evaluate the relative contribution between uncertain and variable parameters. Exposure to atrazine by a mid-western farming family is presented as a case study. The predicted exposure for a child based on a 14-year exposure period was 1.6 times that calculated using lifetime averaged exposure parameters. This indicates the importance of considering children as a population sub-group when calculating the exposure and risk to pesticides.     

Damage assessment on plate-like structures using a global-local optimization approach

A new method for finite element model updating using simulated data is presented. A global-local optimization (GLO) approach was adopted to adjust the uncertain properties of the FE model by minimizing iteratively the differences between the measured dynamic modal parameters and the corresponding analytical predictions. In contrast with most of the existing updating techniques, which minimize modal force errors, objective functions based on Coordinate Modal Assurance Criterion (COMAC) and Frequency Response Assurance Criterion (FRAC) were employed. The GLO procedure was employed to minimize the norm of these vectors by updating the physical model variables (thickness, Young modulus, etc.). The proposed model updating procedure was applied to damage localization and quantification of structures, whose damage characteristics can be represented by a reduction of the element bending and axial stiffness. Results showed that a significant reduction in terms of computer run-time and improved damage assessment can be achieved. The procedure is illustrated on a plate-like structure by measuring dynamic properties before and after structural changes for four different damage cases.     

Reliability sensitivity analysis with random and interval variables

In reliability analysis and reliability‐based design, sensitivity analysis identifies the relationship between the change in reliability and the change in the characteristics of uncertain variables. Sensitivity analysis is also used to identify the most significant uncertain variables that have the highest contributions to reliability. Most of the current sensitivity analysis methods are applicable for only random variables. In many engineering applications, however, some of uncertain variables are intervals. In this work, a sensitivity analysis method is proposed for the mixture of random and interval variables. Six sensitivity indices are defined for the sensitivity of the average reliability and reliability bounds with respect to the averages and widths of intervals, as well as with respect to the distribution parameters of random variables. The equations of these sensitivity indices are derived based on the first‐order reliability method (FORM). The proposed reliability sensitivity analysis is a byproduct of FORM without any extra function calls after reliability is found. Once FORM is performed, the sensitivity information is obtained automatically. Two examples are used for demonstration. Copyright © 2009 John Wiley & Sons, Ltd.     

2.5维C/SiC复合材料弹性参数不确定性识别方法研究

提出了一种2.5维C/SiC编织复合材料弹性参数不确定性识别方法。采用刚度平均法获得复合材料等效弹性参数理论预测值。选取对结构动态特性影响较大的3个弹性参数E11,E22和G12作为待识别参数;在确定性识别结果基础上,采用拉丁超立方体采样构造随机试验样本,开展不确定性参数识别方法仿真研究。仿真结果表明,针对考虑弹性参数不确定性的2.5维C/SiC复合材料,采用所提出的方法能够准确识别材料弹性参数的均值与标准差,建立反映实际结构动态特性统计意义的精确动力学模型。     

A numerical integration approach for fractional-order viscoelastic analysis of hereditary-aging structures

In this paper, a novel numerical integration scheme is proposed for fractional-order viscoelastic analysis of hereditary-aging structures. More precisely, the idea of aging is first introduced through a new phenomenological viscoelastic model characterized by variable-order fractional operators. Then, the presented fractional-order viscoelastic model is included in a variational formulation, conceived for any viscous kernel and discretized in time by employing a discontinuous Galerkin method. The accuracy of the resulting finite element (FE) scheme is analyzed through a model problem, whose exact solution is known; and the most significant variables affecting the solution quality, such as the number of Gaussian quadrature points and time subintervals, are then investigated in terms of error and computational cost. Moreover, the proposed FE integration scheme is applied to study the short- and long-term behavior of concrete structures, which, due to the severe aging exhibited during their service life, represents one of the most challenging time-dependent behavior to be investigated. Eventually, also the Euler implicit method, commonly used in commercial software, is compared.     

偶应力反问题参数识别

利用有限元技术和最小二乘原理,建立了便于敏度分析的偶应力反问题数值求解模型,并采用高斯牛顿方法进行求解。模型考虑了材料的非均质、各向异性等因素,并在计及测量误差的情形下,对其进行了数值验证,得到了令人满意的结果。此外,还通过反问题的求解,对偶应力本构等效的网格材料的等效本构参数进行了估算,并给出相应算例。     

A topological optimization approach for structural design of a high‐speed low‐load mechanism using the equivalent static loads method

In high‐speed low‐load mechanisms, the principal loads are the inertial forces caused by the high accelerations and velocities. Hence, mechanical design should consider lightweight structures to minimize such loads. In this paper, a topological optimization method is presented on the basis of the equivalent static loads method. Finite element (FE) models of the mechanism in different positions are constructed, and the equivalent loads are obtained using flexible multibody dynamics simulation. Kinetic DOFs are used to simulate the motion joints, and a quasi‐static analysis is performed to obtain the structural responses. The element sensitivity is calculated according to the static‐load‐equivalent equilibrium, in such a way that the influence on the inertial force is considered. A dimensionless component sensitivity factor (strain energy caused by unit load divided by kinetic energy from unit velocity) is used, which quantifies the significance of each element. Finally, the topological optimization approach is presented on the basis of the evolutionary structural optimization method, where the objective is to find the maximum ratio of strain energy to kinetic energy. In order to show the efficiency of the presented method, we presented two numerical cases. The results of these analyses show that the presented method is more efficient and can be easily implemented in commercial FE analysis software. Copyright © 2011 John Wiley & Sons, Ltd.     

Prioritising the types of manufacturing flexibility in an uncertain environment

Researchers have stressed that manufacturing system flexibility research requires a quantitative model allowing a manufacturing system to prioritize its flexibility dimension and promote the performance of the manufacturing system. A quantification model presented in the present research is demonstrated to assess the degree of environmental uncertainty and illustrates a method for delivering the requirement of flexibility improvement for the manufacturing system so that the company is able to prioritize the types of manufacturing flexibility which a manufacturing system requires in an uncertain environment. Quantitative approaches including quality function deployment (QFD), analytical hierarchy process (AHP), and grey relational analysis (GRA) have been employed to find a means for improving the flexibility of a manufacturing system to cope with environmental uncertainty. QFD is the focal approach for the deployment of the integrated structure of the research. AHP is applied to explore the relative weighted importance of environmental uncertainty factors, while GRA is used to find out the relationships between manufacturing flexibility and environmental uncertainty. A combination of these approaches reveals a useful tool for managers to prioritize the types of flexibility which a manufacturing system requires for coping with an uncertain environment. In particular, the present research studied the manufacturing flexibility requirements of a food company in Taiwan.