Fracture control for composite structures

A fracture control technique for composite structures is presented which takes advantage of the unique capability of composite materials to be tailored in stiffness and fracture toughness. Crack arrestment is achieved through the use of integral ‘buffer’ strips in the primary load-carrying laminate. Experimental uniaxial tension data obtained from damaged laminates containing such buffer strips indicate residual strength capacity in excess of the limit design stress for the selected laminate.     

A new numerical algorithm for sub‐optimal control of earthquake excited linear structures

Exact optimal classical closed–open‐loop control is not achievable for the buildings under seismic excitations since it requires the whole knowledge of earthquake in the control interval. In this study, a new numerical algorithm for the sub‐optimal solution of the optimal closed–open‐loop control is proposed based on the prediction of near‐future earthquake excitation using the Taylor series method and the Kalman filtering technique. It is shown numerically that how the solution is related to the predicted earthquake acceleration values. Simulation results show that the proposed numerical algorithm are better than the closed‐loop control and the instantaneous optimal control and proposed numerical solution will approach the exact optimal solution if the more distant future values of the earthquake excitation can be predicted more precisely. Effectiveness of the Kalman filtering technique is also confirmed by comparing the predicted and the observed time history of NS component of the 1940 El Centro earthquake. Copyright © 2001 John Wiley & Sons, Ltd.     

The optimum conditions for structures of work hardening elastoplastic materials

Problems of optimization of truss, beam, and membrane type structures of work hardening elastoplastic materials in static loading are studied. The optimum conditions are obtained and analyzed on the basis of variation principles. It is shown that these conditions determine the equal strength of structures.Translated from Problemy Prochnosti, No. 4, pp. 44–51, April, 1995.     

Calculated photonic structures for infrared emittance control

Using an available program package based on the transfer-matrix method, we calculated the photonic band structure for two different structures: a quasi-three-dimensional crystal of square air rods in a high-index matrix and an opal structure of high-index spheres in a matrix of low index, epsilon = 1.5. The high index used is representative of gallium arsenide in the thermal infrared range. The geometric parameters of the rod dimension, sphere radius, and lattice constants were chosen to give total reflectance for normal incidence, i.e., minimum thermal emittance, in either one of the two infrared atmospheric windows. For these four photonic crystals, the bulk reflectance spectra and the wavelength-averaged thermal emittance as a function of crystal thickness were calculated. The results reveal that potentially useful thermal signature suppression is obtained for crystals as thin as 20-50 microm, i.e., comparable with that of a paint layer.     

A new hybrid concept for sandwich structures

Sandwich structures are considered as optimal designs for carrying bending loads and can be either metal (aluminium faces and honeycomb or metal foam cores) or polymer structures (composite faces with polymer foam cores). In this paper, a new hybrid sandwich structure has been developed by combining most of the advantages of metallic and polymeric materials while avoiding some of their main disadvantages. For this new concept metal sheets are used at the outer surfaces to maximize rigidity while introducing in between lightweight cores adhesively bonded to keep the whole structure together. Furthermore, composite or wood layers may be used as intermediate layers to improve impact resistance. Potential methods for the manufacturing of this new structure are based on compression under vacuum. The results include the study of several panel configurations theoretically based on Finite element analysis and on the modified simplified equations and experimental results in the most representative cases of the study.     

Probabilistic bounded non-linear control algorithm for linear structures

Non-linear control algorithms with limited control force has been widely explored and has shown promising results, but the probabilistic analysis of such control algorithms has been made restrictively due to their non-linear nature and corresponding difficulty in obtaining analytical solution of the joint probability density function (PDF). In this paper, the method for the probabilistic analysis on the bounded non-linear control algorithm is proposed based on the equivalent non-linear system method and additional regression analysis. Numerical examples show that the proposed approximate joint PDF of the closed-loop system subjected to a Gaussian white noise and a Kanai–Tajimi filtered Gaussian white noise matches closely with the joint PDF obtained statistically. The effectiveness of the bounded non-linear control is also investigated utilizing the calculated approximate joint PDF. Time history analysis results indicate that the same control performance level as the linear controller is achieved when 50% of the maximum control force of the linear controller is used for the non-linear controller.     

建筑结构的鲁棒分散控制方法研究

传统集中式控制方法需同时采用系统所有测量信号,进而计算出所有作动器的控制力并发出信号进行振动控制,其控制系统复杂且可靠性较差。近年来出现的基于系统局部信息反馈的分散控制策略,设计简单及可靠性高,已经成为目前研究的热点之一。目前分散控制策略的作动器控制力仅采用建筑物的相邻两层测量信号进行控制,虽然能够控制住结构的响应,但信息过少,控制效果不好。为了达到更好的控制效果,在鲁棒控制策略的基础上,通过设置特殊矩阵的方式,提出了一种基于建筑物相邻四层信号的鲁棒分散控制方法。从动力仿真的结果来看,本文方法的控制效果比仅依靠相邻二层信号的控制效果好得多。     

结构离复位控制的可行性研究

发展了一种离复位控制策略,通过考虑与模糊控制理论相结合实施结构振动控制。这种控制方法不仅体现了模糊控制系统无需精确数学模型即可有效执行控制功能,具有反应迅速、超调量小和鲁棒性好等诸多优点,而且解决了其控制规则不易建立的难题。采用这种控制策略,研究了一个底层设有主动拉索控制系统的多层结构的地震反应控制问题。在算例中通过与线性二次型最优控制算法进行控制效果的对比,验证了它的可行性。     

A new axi-symmetric element for thin walled structures

A new axi-symmetric finite element for thin walled structures is presented in this work. It uses the solid-shell element’s concept with only a single element and multiple integration points along the thickness direction. The cross-section of the element is composed of four nodes with two degrees of freedom each. The proposed formulation overcomes many locking pathologies including transverse shear locking, Poisson’s locking and volumetric locking. For transverse shear locking, the formulation uses the selective reduced integration technique, for Poisson’s locking it uses the enhanced assumed strain (EAS) method with only one enhancing variable. The B-bar approach is used to eliminate the isochoric deformations in the hourglass field while the EAS method is used to alleviate the volumetric locking in the constant part of the deformation tensor. Several examples are shown to demonstrate the performance and accuracy of the proposed element with special focus on the numerical simulations for the beverage can industry.     

Precise integration strategy for aseismic LQG control of structures

Although LQG regulators have been used in many engineering fields, the relevant control technique still requires further development, particularly when the disturbances are random and non‐stationary. Therefore this paper presents a new method for non‐stationary seismic random excitations in which LQG control is applied to adjacent tall buildings which are connected together by active hydraulic actuators. The ground motion is induced by non‐stationary uniformly modulated white noise excitation of the bedrock which passes through a single layered soil which is regarded as a single‐peak filter. This represents an advance on other papers in the literature, which use stationary random white noise or filtered white noise as the ground excitation. The method uses the pseudo excitation method in conjunction with two different precise integration schemes to analyse structural non‐stationary seismic random responses via solution of the differential Riccati equations of this transient LQG control process. Unlike previous work in the literature, the performance index used has a finite time region, rather than an infinite one. Reasonable control effects are demonstrated via numerical simulation for two adjacent 20‐storey buildings which are connected by hydraulic activators. Copyright © 2006 John Wiley & Sons, Ltd.     

一种适用于大型工程结构的分散振动控制方法

基于瞬时最优控制算法,提出一种适用于大型结构振动控制的分散控制方法。根据大型结构的有限元模型,将其划分为若干子结构,相邻子结构间的作用力视为对子结构的"附加未知扰动"。在子结构加速度响应部分观测情况下,依次运用卡尔曼预测估计和最小二乘估计法,分别对子结构的状态和子结构间的作用力进行估计,并在此基础上基于瞬时最优控制算法确定各子结构的最优控制力,继而施加于结构对其进行振动控制。将提出的分散振动控制方法应用于美国土木工程师协会(ASCE)提出的20层控制基准问题(benchmark problem),并与传统的集中振动控制方法进行相关振动控制指标的比较。结果表明提出的分散振动控制方法能取得满意的振动控制效果。因此,提出的分散振动控制方法适合应用于对大型结构的振动控制。     

Stress-free displacement control of structures

Summary. The theorem on the unique decomposition of eigenstrain provides the tool to construct prescribed displacements inside and (or) at the surface of a linear elastic body in two novel ways (e.g., in connection with shape control). The first approach applies Maysels formula and renders the linear relationship between nodal displacements and impotent eigenstrain in discrete or discretized systems. The second, alternative approach can be applied to continuous systems as well, and allows controlling the shape of the body. Both approaches do not require the solution of the associated boundary value problem with prescribed eigenstrain, thus circumventing the possibly cumbersome (numerical) solution, which can present some difficulties with respect to its convergence.     

Production of new titanium alloy for orthopedic implants

The beta titanium alloys is one of the most promising groups of the titanium alloys. This fact is due to the good formability, mechanical properties and potential applications; moreover, these alloys present the highest level of mechanical, fatigue and corrosion resistance. The beta titanium alloys present the lowest elastic modulus, an interesting property for orthopedic implants. A β alloy recently developed for this application is Ti–35Nb–7Zr–5Ta. In this work, the alloy was produced by powder metallurgy, unique available alternative for obtaining parts with porous structure (until 50% of porosity), that is one important characteristic for the osteointegration. The Ti–35Nb–7Zr–5Ta samples were manufactured by blended elemental method from a sequence of uniaxial and cold isostatic pressing with subsequent densification by sintering among 900 at 1700 °C, in vacuum. The objective of this work is the analysis of alloy microstructural evolution from the elemental powders dissolution under the increase of the sintering temperature. The alloy was characterized by scanning electron microscopy, X-ray diffraction and Vickers microhardness measurements. Density was measured by Archimedes method. The results show that a β-homogeneous microstructure is obtained in the whole sample with the increase of sintering temperature. With the beginning of the β-stabilizers (Nb and Ta) dissolution, at low sintering temperatures, there is the formation of an intermediary Widmanstätten (α+β) phase.     

分布参数结构控制中溢出抑制的并行滤波法

提出了并行滤波控制方法抑制分布参数结构主动控制中的溢出。对无穷维分布参数结构,依据有限维降阶模型设计控制器会导致系统闭环性能变差,并有可能使系统失稳。文中针对分布参数结构特性设计了滤波增广结构,对增广系统设计带观测器的状态反馈构成控制器。在给出该方法原理的基础上,提出了并行滤波控制针对被控结构摄动的鲁棒稳定界。仿真算例表明,此方法在保证受控模态控制效果的同时有效地抑制了溢出。     

Piezoelectric actuators and sensors location for active control offlexible structures

A method for optimal positioning of piezoelectric actuators and sensors on a flexible structure is presented. First, a two-dimensional (2-D) model of a piezoelectric actuator bonded to a plate is obtained. Then, a Ritz formulation is used to find a state model of the system in view of its control. To define an optimal positioning strategy, an energy based approach is developed. This leads quite naturally to the study of controllability and observability properties of the overall dynamical model. A new criterion based on energy assessment is proposed to locate actuators and sensors