Numerical–experimental crack growth analysis in AA2024-T3 FSWed butt joints

This paper deals with a numerical and experimental investigation on the influence of residual stresses on fatigue crack growth in AA2024-T3 friction stir welded butt joints. The computational approach is based on the sequential usage of the Finite Element Method (FEM) and the Dual Boundary Element Method (DBEM). Linear elastic FE simulations are performed to evaluate the process induced residual stresses, by means of the contour method. The computed stress field is transferred to a DBEM environment and superimposed to the stress field produced by a remote fatigue traction load applied on a friction stir welded cracked specimen; the crack propagation is then simulated according to a two-parameter growth model. Numerical results have been compared with experimental data showing good agreement and evidencing the predictive capability of the proposed method. The obtained results highlight the influence of the residual stress distribution on crack growth.     

A partition of unity enriched dual boundary element method for accurate computations in fracture mechanics

We introduce a novel enriched Boundary Element Method (BEM) and Dual Boundary Element Method (DBEM) approach for accurate evaluation of Stress Intensity Factors (SIFs) in crack problems. The formulation makes use of the Partition of Unity Method (PUM) such that functions obtained from a priori knowledge of the solution space can be incorporated in the element formulation. An enrichment strategy is described, in which boundary integral equations formed at additional collocation points are used to provide auxiliary equations in order to accommodate the extra introduced unknowns. In addition, an efficient numerical quadrature method is outlined for the evaluation of strongly singular and hypersingular enriched boundary integrals. Finally, results are shown for mixed mode crack problems; these illustrate that the introduction of PUM enrichment provides for an improvement in accuracy of approximately one order of magnitude in comparison to the conventional unenriched DBEM.     

Analyses of large quasistatic deformations of inelastic bodies by a new hybrid-stress finite element algorithm: Applications

In an earlier paper [1] the authors described a new hybrid-stress finite element algorithm suitable for the analysis of large quasistatic deformations of inelastic bodies. However, that paper focused on the subtleties of the algorithm, and only finite homogeneous deformation problems were presented as examples. The present paper is concerned with the algorithm's implementation and application to problems of technological interest.     

Recent advances in computerized aerospace structural analysis and design

This paper presents accomplishments at the Boeing Commercial Airplane Company in the areas of (1) large problem solutions, (2) practical structural design automation and (3) a hypothesis for stable crack growth.A turbine blade (3200 nodes 350 elements, 9500 freedoms, 15 million words of storage) and a sports stadium (3400 nodes, 9600 elements, 20,000 freedoms, 70 million words of storage) are presented in terms of problem definition, strategy of approach, data management and computer resources.The Boeing automated strength design capability in the ATLAS System is discussed in terms of user specified constraints on the automated strength resizing and local optimization, strength criteria, user control and convergence criteria. Cost and weight data are presented from large aircraft design studies (20,000 design variables).A new concept of crack stability based on non-linear stress/strain analysis is presented. Stable crack growth is modeled by a suitable failure criterion in the stress analysis procedure. A crack is extended by a simultaneous unloading of the newly created crack surface and the loading of the uncracked region. Test comparisons and numerical experiments support a new hypothesis based on material strength characteristics, plastic zone size/history and the residual plastic strains.     

Recursive evaluation of time convolution integrals in the spectral boundary integral method for mode III dynamic fracture problems

The shear crack, propagating spontaneously on a frictional interface, is a useful idealization of a natural earthquake. However, the corresponding boundary value problems are quite challenging in terms of required memory and processor power. While the huge computation amount is reduced by the spectral boundary integral method, the computation effort is still huge. In this paper, a recursive method for the evaluation of convolution integrals was tested in the spectral formulation of the boundary integral method applied to 2D anti-plane crack propagation problems. It is shown that analysis of a 2D anti-plane crack propagation problem involving N_t time steps, based on the recursive evaluation of convolution integrals, requires O(αN_t) computational resources for each Fourier mode (as opposed to O(N_t²) for a classical algorithm), where α is a constant depending on the implementation of the method with typical values much less than N_t. Therefore, this recursive scheme renders feasible investigation of long deformational processes involving large surfaces and long periods of time, while preserving accuracy. The computation methodology implemented here can be extended easily to 3D cases where it can be employed for the simulation of complex spontaneously fault rupture problems which carry a high computational cost.     

The pervasiveness of evolution in GRUMPS software

This paper describes the evolution of the design and implementation of a distributed run‐time system that itself is designed to support the evolution of the topology and implementation of an executing, distributed system. The three different versions of the run‐time architecture that have been designed and implemented are presented, together with how each architecture addresses the problems of topological and functional evolution. In addition, the reasons for the rapid evolution of the design and implementation of the architecture are also described. From the lessons learned in both evolving the design of the architecture and in trying to provide a run‐time system that can support run‐time evolution, this paper discusses two generally applicable observations: evolution happens all the time, and it is not possible to anticipate how systems will evolve as designs; and large, run‐time systems do not follow a predictable path. In addition to this, rapid prototyping has proved to be extremely useful in the production of the three architectures; this kind of prototyping has been made much easier by designing the core set of Java abstractions in terms of interfaces; and building an architecture that allows as many decisions as possible to be made at run‐time which has produced a support system that is more responsive to the user as well as the distributed environment in which it is executing. Copyright © 2003 John Wiley & Sons, Ltd.     

Large scale evolutionary optimization using cooperative coevolution

Evolutionary algorithms (EAs) have been applied with success to many numerical and combinatorial optimization problems in recent years. However, they often lose their effectiveness and advantages when applied to large and complex problems, e.g., those with high dimensions. Although cooperative coevolution has been proposed as a promising framework for tackling high-dimensional optimization problems, only limited studies were reported by decomposing a high-dimensional problem into single variables (dimensions). Such methods of decomposition often failed to solve nonseparable problems, for which tight interactions exist among different decision variables. In this paper, we propose a new cooperative coevolution framework that is capable of optimizing large scale nonseparable problems. A random grouping scheme and adaptive weighting are introduced in problem decomposition and coevolution. Instead of conventional evolutionary algorithms, a novel differential evolution algorithm is adopted. Theoretical analysis is presented in this paper to show why and how the new framework can be effective for optimizing large nonseparable problems. Extensive computational studies are also carried out to evaluate the performance of newly proposed algorithm on a large number of benchmark functions with up to 1000 dimensions. The results show clearly that our framework and algorithm are effective as well as efficient for large scale evolutionary optimisation problems. We are unaware of any other evolutionary algorithms that can optimize 1000-dimension nonseparable problems as effectively and efficiently as we have done.     

A two-parameter model for crack growth simulation by combined FEM–DBEM approach

This paper describes the application of a two-parameters crack growth model, based on the usage of two threshold material parameters (ΔK_th and K_max,th) and on the allowance for residual stresses, introduced at the crack tip by a fatigue load spectrum or by material plastic deformations. The coupled usage of finite element method (FEM) and dual boundary element method (DBEM) is proposed in order to take advantage of the main capabilities of the two methods.The procedure is validated by comparison with available experimental results, in order to assess its capability to predict the retardation phenomena, introduced by a variable load spectrum or by a plastic deformation introduced with a tool on the panel (indentation).In particular two different tests are made: the first test involve a CT specimen undergoing a load spectrum and the second one involve a dented panel undergoing a constant amplitude fatigue load. In both cases a satisfactory numerical–experimental correlation will be proved.The main advantages of the aforementioned procedure are: the simplicity of the crack growth law calibration (few constant amplitude tests are sufficient without the need for any non-physical calibration parameters), and the possibility to simulate residual stress effects on crack propagation with a simplified approach, based on linear elastic fracture mechanics.     

基于文档实例的中文信息检索

传统的信息检索系统基于关键词建立索引并进行信息检索.这些系统存在查询返回文档集大、准确率低和普通用户不便于构造查询等不足.为此,该文提出基于文档实例的信息检索,即以已有文档作为样本,在文档库中检索与样本文档相似的所有文档.文中给出了基于文档实例的中文信息检索的解决方法和实现技术.初步实验结果表明该方法是行之有效的.     

Analysis of Stokes flow in microchannels with superhydrophobic surfaces containing a periodic array of micro-grooves

Superhydrophobic surfaces have been demonstrated to be capable of reducing fluid resistance in micro- and nanofluidic applications. The objective of this paper is to present analytical solutions for the Stokes flow through microchannels employing superhydrophobic surfaces with alternating micro-grooves and ribs. Results are presented for both cases where the micro-grooves are aligned parallel and perpendicular to the flow direction. The effects of patterning the grooves on one or both channel walls are also analyzed. The reduction in fluid resistance has been quantified in terms of a dimensionless effective slip length, which is found to increase monotonically with the shear-free fraction and the periodic extent of each groove–rib combination normalized by the channel half-height. Asymptotic relationships have been derived for the normalized effective slip length corresponding to large and small limiting values of the shear-free fraction and the normalized groove–rib period. A detailed comparison has been made between transverse and longitudinal grooves, patterned on one or both channel walls, to assess their effectiveness in terms of enhancing the effective slip length. These comparisons have been carried out for small and large limiting values, as well as finite values of the shear-free fraction and normalized groove–rib period. Results for the normalized effective slip length corresponding to transverse and longitudinal grooves are further applied to model the Stokes flow through microchannels employing superhydrophobic surfaces containing a periodic array of micro-grooves inclined at an angle to the direction of the applied pressure gradient. Results are presented for the normalized effective slip lengths parallel to the direction of the applied pressure gradient and the normalized cross flow rate perpendicular to the direction of the applied pressure gradient.     

Non-linear MSD crack growth by DBEM for a riveted aeronautic reinforcement

A special specimen is created cutting a rectangular notched area from the surrounding of the upper left corner of a wide body aircraft door. Then a constant amplitude fatigue traction load is applied by a special servo-hydraulic machine, in order to induce a Multi Site Damage (MSD) scenario.The Dual Boundary Element method (DBEM), as implemented in a commercial code, is adopted for a three-dimensional MSD crack growth simulation of such multi-layer and multi-material component. To this aim, the cracked part of a pre-existing global two-dimensional model is extracted and “extruded” in order to generate a three-dimensional submodel, whose boundary conditions are imposed displacements, calculated by the two-dimensional model, along a virtual line corresponding to the submodel boundary. Non-linear contact conditions are applied between the mating plate surfaces in the area surrounding the cracks, in order to precisely model the plate interactions in the area of interest.The three-dimensional approach is aimed to improve, with respect to the two-dimensional approach, the correlation between numerical and experimental results (e.g. by an accurate assessment of the secondary bending effects). The obtained improvements on crack growth rates, in the initial part of the crack propagation, justify the increased computational effort that a three-dimensional non-linear approach involves.The proposed numerical procedure, based on DBEM, is successfully validated for the virtual testing of a complex aeronautic reinforcement.     

Parallel eigenvalue reordering in real Schur forms

A parallel algorithm for reordering the eigenvalues in the real Schur form of a matrix is presented and discussed. Our novel approach adopts computational windows and delays multiple outside‐window updates until each window has been completely reordered locally. By using multiple concurrent windows the parallel algorithm has a high level of concurrency, and most work is level 3 BLAS operations. The presented algorithm is also extended to the generalized real Schur form. Experimental results for ScaLAPACK‐style Fortran 77 implementations on a Linux cluster confirm the efficiency and scalability of our algorithms in terms of more than 16 times of parallel speedup using 64 processors for large‐scale problems. Even on a single processor our implementation is demonstrated to perform significantly better compared with the state‐of‐the‐art serial implementation. Copyright © 2009 John Wiley & Sons, Ltd.     

A generic genetic algorithm for product family design

Product family design (PFD) has been well recognized as an effective means to satisfy diverse market niches while maintaining the economies of scale and scope. PFD essentially entails a configuration problem by “combination," where combinatorial explosion always occurs and is known to be mathematically intractable or NP-hard. Although genetic algorithms (GAs) have been proven to excel in solving combinatorial optimization problems, it is difficult to adopt the traditional GA to deal with the complex data and interrelationships inherent in the PFD problem. This paper proposes a generic genetic algorithm (GGA) for PFD. A generic encoding scheme is developed to adapt to diverse PFD scenarios. A hybrid constraint-handling strategy is proposed to handle complex and distinguishing constraints at different stages along the evolutionary process. The design and implementation procedures of the GGA are discussed in detail. An application of the proposed GGA to motor family design is reported. The GGA efficiency is also tested through efficiency analysis in terms of the probability of generating feasible solutions, as well as through analysis of the GGA complexity.     

Distributed control design for spatially interconnected systems

This paper deals with analysis, synthesis, and implementation of distributed controllers, designed for spatially interconnected systems. We develop a state space framework for posing problems of this type, and focus on systems whose model is spatially discrete. In this paper, analysis and synthesis results are developed for this class of systems using the l/sub 2/-induced norm as the performance criterion. The results are stated in terms of linear matrix inequalities and are thus readily amenable to computation. A special implementation of the resulting controllers is presented, which is particularly attractive for distributed operation of the controller. Several examples are provided to further illustrate the application of the results.     

Improving enterprise system support—a case-based approach

Many organisations have implemented or are implementing large enterprise systems, like ERP and PDM, for integrating their business functions and streamlining the flow of information. Implementing such systems is very complex, however. In many companies the results envisioned have not or have only partly been achieved. In the literature many explanations can be found for insufficient or failing implementation efforts. A large part of the problems encountered appears to be human and organisational in nature, while it has been stressed that implementing an enterprise system requires and involves organisational change.Success and failure factors and do's and don'ts, as can be found in the literature, provide necessary, but not sufficient, preconditions for starting an implementation project. The complexity of such a project makes full anticipation and control of potential problems impossible. An organisation needs to be prepared to encounter disturbances and take corresponding actions. Knowledge on the dynamics of enterprise system implementation processes is, however, scarce and scattered. Moreover, to what extent the context in which an enterprise system is implemented influences implementation is not yet fully known.In this article, an approach is presented to gather knowledge on implementation process dynamics. The approach builds on theoretical and practical contributions in search for a structured human and organisational approach. The knowledge gathered has been analysed by means of a comprehensive reference framework. After this step a tool has been developed to support a consultant in assessing the maturity of a company to start an enterprise system implementation project. The knowledge, used directly in the tool, makes it possible to suggest improvement actions sensitive to the situation and context. Through statistical analysis, interesting context differences have been identified, which may also help in further refining the advice. Initial validation results are promising.     

Feedforward Neural Network Implementation in FPGA Using Layer Multiplexing for Effective Resource Utilization

This paper presents a hardware implementation of multilayer feedforward neural networks (NN) using reconfigurable field-programmable gate arrays (FPGAs). Despite improvements in FPGA densities, the numerous multipliers in an NN limit the size of the network that can be implemented using a single FPGA, thus making NN applications not viable commercially. The proposed implementation is aimed at reducing resource requirement, without much compromise on the speed, so that a larger NN can be realized on a single chip at a lower cost. The sequential processing of the layers in an NN has been exploited in this paper to implement large NNs using a method of layer multiplexing. Instead of realizing a complete network, only the single largest layer is implemented. The same layer behaves as different layers with the help of a control block. The control block ensures proper functioning by assigning the appropriate inputs, weights, biases, and excitation function of the layer that is currently being computed. Multilayer networks have been implemented using Xilinx FPGA "XCV400hq240." The concept used is shown to be very effective in reducing resource requirements at the cost of a moderate overhead on speed. This implementation is proposed to make NN applications viable in terms of cost and speed for online applications. An NN-based flux estimator is implemented in FPGA and the results obtained are presented     

MHD flow in a rectangular duct with a perturbed boundary

The unsteady magnetohydrodynamic (MHD) flow of a viscous, incompressible and electrically conducting fluid in a rectangular duct with a perturbed boundary, is investigated. A small boundary perturbation $\epsilon$ is applied on the upper wall of the duct which is encountered in the visualization of the blood flow in constricted arteries. The MHD equations which are coupled in the velocity and the induced magnetic field are solved with no-slip velocity conditions and by taking the side walls as insulated and the Hartmann walls as perfectly conducting. Both the domain boundary element method (DBEM) and the dual reciprocity boundary element method (DRBEM) are used in spatial discretization with a backward finite difference scheme for the time integration. These MHD equations are decoupled first into two transient convection–diffusion equations, and then into two modified Helmholtz equations by using suitable transformations. Then, the DBEM or DRBEM is used to transform these equations into equivalent integral equations by employing the fundamental solution of either steady-state convection–diffusion or modified Helmholtz equations. The DBEM and DRBEM results are presented and compared by equi-velocity and current lines at steady-state for several values of Hartmann number and the boundary perturbation parameter.     

Genetic Algorithm for Boolean minimization in an FPGA cluster

Evolutionary algorithms are an alternative option to the Boolean synthesis due to that they allow one to create hardware structures that would not be able to be obtained with other techniques. This paper shows a parallel genetic programming (PGP) Boolean synthesis implementation based on a cluster of FPGAs that takes full advantage of parallel programming and hardware/software co-design techniques. The performance of our cluster of FPGAs implementation has been compared with an HPC implementation. The experimental results have shown an excellent behavior in terms of speed up (up to ×500) and in terms of solving the scalability problems of this algorithms present in previous works.     

Thin crack observation in a reinforced concrete bridge pier test using image processing and analysis

In reinforced concrete (RC) structural experiments, the development of concrete surface cracks is an important factor of concern to experts. One conventional crack observation method is to suspend a test at a few selected testing steps and send inspectors to mark pen strokes on visible cracks, but this method is dangerous and labor intensive. Many image analysis methods have been proposed to detect and measure the dark shadow lines of cracks, reducing the need for manual pen marking. However, these methods are not applicable for thin cracks, which do not present clear dark lines in images.This paper presents an image analysis method to capture thin cracks and minimize the requirement for pen marking in reinforced concrete structural tests. The paper presents the mathematical models, procedures, and limitations of our image analysis method, as well as the analysis flowchart, the adopted image processing and analysis methods, and the software implementation. Finally, the results of applying the proposed method in full-scale reinforced concrete bridge experiments are presented to demonstrate its performance. Results demonstrate that this method can capture concrete surface cracks even before dark crack lines visible to the naked eye appear.