Improving bending characteristics of FRP sandwich structures with reinforcement webs

Sandwich-structure materials consist of a high-strength skin material and a lightweight core material. The advantages of sandwich structures are known to include excellent mechanical properties and low weight. Sandwich structures are lightweight because of their lightweight core; meanwhile, the skin structure provides mechanical strength and bears bending stress. Carbon-fiber-reinforced plastic (CFRP) is a high-specific-strength and high-specific-rigidity material. In recent years, CFRP sandwich structures have been used in aerospace applications due to their lightweight properties. However, soft-core members such as plastic foam materials have low rigidity and therefore may not exhibit adequate function as a sandwich structure. Webs can make up for the lack of rigidity of soft core members. Consequently, sandwich structures with reinforcement webs offer higher strength than sandwich structures without reinforcement webs. This study focused on reinforcement webs suitable for use in CFRP sandwich structures by evaluating the bending characteristics of CFRP sandwich structures with reinforcement webs. Experimental results demonstrated that CFRP sandwich structures with reinforcement webs had improved bending strength. The effects of the spacing interval of reinforcement webs and the number of layers of carbon fiber fabric on the bending characteristics of CFRP sandwich structures were also examined. Finally, an optimal condition model was created for CFRP sandwich structures with reinforcement webs.     

Method to determine electrochemical potential gradients without reinforcement connection in concrete structures

A suitable condition assessment of a structure is usually based on comprehensive non-destructive measurements to locate the relevant critical areas. Electrochemical potential mapping is an approved method to detect areas with a high risk of chloride induced corrosion in concrete structures. A novel approach has been developed to locate critical areas without a reinforcement connection by using three external electrodes. As result a vector is calculated which points directly to the critical area near the sensor position, comparable with a compass. This method allows an immediate identification of critical areas on site. In this paper the methodology is explained and measuring results are compared to the conventional method.     

Topology optimization for reinforcement of no-tension structures

In the paper, the reinforcement of no-tension structures by the application of superposed high-strength sheets, or by the insertion of tensile bars, has been considered with the purpose to set up a design path aiming at the positioning of the new material according to some optimal criterion. In detail, no-tension models are adopted which are recognized as an effective tool for analyzing a wide class of structures (e.g., masonry and reinforced concrete members), and the equilibrium and the failure analysis of the reinforced body are developed with particular reference to its ultimate limit state of collapse. Finally, an approach through the “topologic optimization” is proposed for the identification of the optimal distribution of the reinforcement, and some of the obtained results are shown.     

On the Effect of Reinforcement Structure on the Load-Carrying Ability of Thermoelastic Cermet Rotary Disks

A direct problem of strength analysis for thermoelastic reinforced three-ply rotary disks has been formulated. On the basis of the direct problem and strength criterion for substructural elements of a disk, an inverse problem of determination of the load-carrying ability of a construction with a given shape of the disk profile and a given structure of the load-carrying ply reinforcement has been stated. Results of calculations of the load-carrying ability of disks with a fixed geometry made of different cermet compositions and having different reinforcement structures are presented. It has been shown that isotropic steel disks and quasi-isotropic reinforced disks, which are most commonly used, are of low efficiency as compared with spirally-reinforced constructions, for many of which a goal-directed optimization is worthwhile.     

Experimental investigation of the seismic performances of IMG reinforcement on curved masonry elements

In recent years, several new materials and technologies have been developed to limit the effects of earthquakes on the structures. In particular, for the structural reinforcement of masonry elements, the use of composite materials has shown to be effective.The experimental results of an innovative reinforcement technique based on inorganic matrixes, namely Inorganic Matrix composite Grid (IMG) are herein presented. The reinforcement has been applied to a full-scale masonry vault. Several shaking table tests (before and after the IMG reinforcement application) have been performed. The structural performance of the vault has been evaluated comparing the damages detected in the case of unreinforced and reinforced specimen.     

On the influence of the reinforcement structure of fibrous shells of revolution on the heat conduction in them

The initial boundary-value problem on the heat conduction in shells reinforced with fibers of constant cross section has been considered. It has been established that the specific, anisotropic, inhomogeneous properties of such a shell are determined by its heat conductivity dependent on the thermophysical properties of the phases of the composite material of the shell, the parameters of its reinforcement, and the geometry of this shell. The ways of reducing the three-dimensional problem on heat conduction to the two-dimensional one and the possibilities of reducing the dimension of this problem for thin shells of revolution reinforced symmetrically relative to their axis by two units have been determined. The stationary temperature fields of concrete thin shells of revolution with different Gaussian curvatures and different reinforcement structures have been compared. It is shown that the reinforcement structure and the geometry of a shell of revolution substantially influence the temperature distribution in this shell, which opens up a wide range of ways selecting designs of such shells with improved thermophysical parameters. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 2, pp. 145–155, March–April, 2006.     

Local fibre reinforcement of holes in composite multilayered plates

For various technical reasons, cutouts such as holes in thin-walled structures are inevitable and are of significant technical relevance. Unfortunately holes lead to an undesired stress concentration at the hole vicinity and a reduced strength of the structure. Therefore in practice a local reinforcement in the form of a ring is usually applied around the hole. The increasing requirements for modern structures in terms of low weight and high strength lead to the question of an optimal reinforcement design. The present paper addresses the new but well-approved techniques of the use of curved fibre format to determine the aforementioned optimal design of the reinforcement. The optimization of cutouts in laminated composite plates under bi-axial tensile loading conditions has been investigated using two approaches: the finite element and the Rayleigh-Ritz method. The used methodology implemented successfully theoretical results based on the complex potential theory. For the considered problems, the proposed methods were shown to successfully produce a constant objective function around the hole boundary under biaxial loading. The optimal reinforcement of holes in laminated composite plates illustrated that the optimum depends on the degree of orthotropy. Significant reduction of stress concentrations were demonstrated. The results obtained illustrate the necessity and usefulness of the applied optimization procedure.     

复杂分布动载荷识别技术研究

复杂结构的分布动载荷识别技术是动载荷识别技术的难点之一。本文推导了基于广义正交多项式特征技术动载荷识别模型，解决了在一定精度范围内通过有限测量点的振动信息识别具有无限未知量的分布动载荷关键技术，通过动态标定技术的研究和复杂结构有限元模型数值仿真计算，验证了方法的正确性、有效性和工程可用性，适用于具有确定性分布的稳态振动载荷识别．为分布动载荷识别技术的发展打下一定的基础。     

基于损伤容限优化的机身开口结构研究

将飞机结构设计的尽可能理想, 如重量轻、强度高、耐久性好等, 一直是飞机结构设计师的愿望。应用子模型分析技术, 从全局模型中提取感兴趣部位进行细节分析, 然后加入损伤容限约束条件, 对结构细节进行多目标优化设计。讨论了机身开口结构几何参数对结构寿命的影响, 结果显示加强结构附近剩余强度可能发生不减反增的现象, 裂纹扩展寿命对开口倒角半径并不很敏感, 增加带板厚度在一定范围内能够提高结构寿命。该文方法具有较好的精度和准确性, 同时为开口结构设计、优化和检修提供参考及指导。     

Integrated system identification and reliability evaluation of stochastic building structures

System identification and reliability evaluation play a significant role in structural health monitoring to ensure the serviceability and safety of existing structures. Although the development of system identification methods has attained much attention and some degree of maturity, reliability evaluation of existing structures still remains a challenging problem especially when uncertainties in measurement data and inherent randomness, which are inevitably involved in civil structures, are considered. In this regard, this paper presents a framework for integrated system identification and reliability evaluation of stochastic building structures. Two algorithms are proposed to respectively evaluate component reliability and system reliability of stochastic building structures by combining a statistical moment-based system identification method and a probability density evolution equation-based reliability evaluation method. System identification is embedded in the procedure of reliability evaluation of a stochastic building structure. The uncertainties in both the structure and the external excitation are considered. Numerical examples show that the structural component and system reliabilities of a three-story shear building structure with three damage scenarios can be effectively evaluated by the proposed methods.     

Integrated system identification and reliability evaluation of stochastic building structures

System identification and reliability evaluation play a significant role in structural health monitoring to ensure the serviceability and safety of existing structures. Although the development of system identification methods has attained much attention and some degree of maturity, reliability evaluation of existing structures still remains a challenging problem especially when uncertainties in measurement data and inherent randomness, which are inevitably involved in civil structures, are considered. In this regard, this paper presents a framework for integrated system identification and reliability evaluation of stochastic building structures. Two algorithms are proposed to respectively evaluate component reliability and system reliability of stochastic building structures by combining a statistical moment-based system identification method and a probability density evolution equation-based reliability evaluation method. System identification is embedded in the procedure of reliability evaluation of a stochastic building structure. The uncertainties in both the structure and the external excitation are considered. Numerical examples show that the structural component and system reliabilities of a three-story shear building structure with three damage scenarios can be effectively evaluated by the proposed methods.     

镁基复合材料界面调控研究进展EI北大核心

界面是影响镁基复合材料综合性能的关键因素,如何进行界面调控一直是镁基复合材料的研究热点。本文围绕镁基复合材料三种界面结构类型(共格界面、半共格界面和非共格界面),针对影响界面性能的两个关键问题(界面润湿性和界面反应),综述了界面优化方案的研究进展,提出了实现良好界面结合的界面结构设计与调控准则:良好润湿性与轻微界面反应。针对镁基复合材料的界面性能提升,可以考虑添加稀土元素,起到净化界面、改善润湿性的作用;根据工程需要选择基体和增强体,得到某方面性能优异的复合材料;开发新的增强体表面涂层,充分提高界面结合能力;通过第一性原理等计算模拟方法,深入探究界面结构与界面性能之间的关系。     

Bond behaviour of flat stainless steel rebars in concrete

In recent years there has been an increasing interest in applying stainless steel (SS) reinforcement in concrete structures to avoid corrosion derived durability problems. Concrete-steel bond behaviour and applicability of existing standards have not yet been extensively studied with respect to SS. Moreover, an interest in applying flat rebars as reinforcement elements has emerged. The idea is based on the optimization of the rebar surface in contact with the concrete as well as on the reinforcement optimization for shallow slabs. To advance in the evaluation of the bond between concrete and flat SS strips and to compare with the behaviour of standard carbon steel (CS) round reinforcement 72 bond tests have been conducted. The test program consists of pull-out tests to centrally reinforced concrete specimens. Traditional concrete and self compacting concrete are used for embedding. For a better analysis and understanding of the failure aspect of the specimens, fluorescent epoxy injection has been conducted. Results are compared to the CEB-FIP 1990 bond model. The authors conclude that, the use of SS instead of CS is not considerably influencing the bond capacity of the reinforcement when ribbed samples are used while parameters as reinforcement shape or rib pattern are more deterministic. An adaptation of the CEB-FIP 1990 bond model is proposed for flat reinforcements tested in this work.     

Post-strengthening of timber structures with CFRP's

Upgrading structures for higher working loads or restoring original design strength is an engineering task for structures of any material. A great number of existing buildings need specific interventions on site to restore the damages for renovation, including reinforcement or repair of timber structures due to previous overloading, insect and fungal attack. This article presents a study of reinforcement techniques for restoration and strengthening of existing timber floors under bending loads. Structural adhesives for application on the building site are used and the removal of the overhanging part of the structure as well as the inserted ceiling was not necessary. Commercial adhesives were tested. The tests showed the arrangement of the reinforcement and the stiffness of the materials transmitting the loads, i.e. wood, CFRP and the bonding agent, were of decisive influence for the overall strength of the specimen. The results obtained depend on the wood structure and the presence of defects e.g. knots and cracks. The investigations are currently going on and will be supplemented by using further practice related conditions.     

Patch repairs on reinforced concrete structures – Model investigations on the required size and practical consequences

For various reasons, world-wide numerous concrete structures have to be repaired due to corrosion problems of the steel reinforcement. In some cases, frequent use of road salt in past winters, not foreseen during the planning of older traffic structures, has led to damage; in other cases the causes lay in deficient planning and execution, as for example

• •irregular, insufficient concrete cover of the reinforcement;
• •unsuitable concrete mixture proportions for outdoor structures;
• •insufficient concrete curing, subsequently bad concrete quality in the concrete cover.

The model investigations described in this paper relate to construction practice, in which local damages due to reinforcement corrosion, e.g., spalls and cracks, are repaired solely in the area of visible surface damage. When damage is dealt with in this way, the carbonated or chloride contaminated concrete is often not removed completely, to avoid stability problems in the structure. In consequence, even after the repair measure there are areas of the reinforcement where there is no guarantee of sufficient protection against corrosion and a high corrosion risk remains. The reinforcement in the repaired area may even accelerate corrosion in unrepaired areas adjacent to it.     

Processing–microstructure-property predictions for short fiber reinforced composite structures based on a spray deposition process

Manufacturing of composite preforms by use of a programmed and controlled reinforcement spray deposition process presents itself as an attractive approach to produce short fiber reinforced composite structures. To predict properties of the final composite structure, simulations of the reinforcement deposition process are conducted to obtain the reinforcement orientation distribution. A micromechanics analysis incorporating the Mori–Tanaka method and texture tensors is used to predict the properties of the final consolidated composite parts. This processing–microstructure-property prediction scheme is applied to the analysis of composite structures in the carbon–carbon system. The effects of variations in reinforcement length in the spray deposited preform, and boundary effects as occurring in a near-net shape composite disk are discussed.     

地震动反演及结构参数识别的EKF算法

讨论了信息不完备的情况下剪切型结构的参数识别问题,利用不完备的测量信息,在系统载荷未知的条件下,采用最小二乘方法反演得到地震动响应时程,用扩展卡尔曼滤波(EKF)算法识别结构的动态物理参数,解决了扩展卡尔曼滤波算法需要输入的问题,取得了满意的参数识别结果。系统仿真算例表明,在引入20%的白噪声条件下,识别结果仍具有较高的计算精度,说明EKF算法用于剪切型结构参数识别时有较强的鲁棒性。     

考虑质量变化的空间拱结构小损伤识别方法研究

该研究针对空间拱结构损伤识别精度问题,提出了一种空间拱结构小损伤的动态识别方法。该方法在识别过程中考虑损伤区的质量变化的因素来提高了结构损伤识别的精度。在建立的空间拱结构损伤模型中,应用频率和振型的质量因子变化来表示损伤区质量的变化。应用Wittrick-Williams算法对损伤参数进行计算,根据计算结果来判定结构的损伤位置和程度。为了实现应用最少的测量模态和结构振动特性数据对结构损伤精确识别,建立了一个应用遗传算法的优化计算程序对损伤参数优化计算。在该优化程序中,应用初始的随机染色体群代表沿空间拱不同的损伤分布状况。通过对单一损伤、多个损伤区域和不同边界条件下空间拱进行损伤识别案例对所述方法进行验证,并分析解决了对称结构损伤定位唯一性的问题。结果表明,所提出的识别方法对空间拱结构的损伤具有较好的识别效果,所建立的损伤识别程序具有较高的识别精度。     

An inverse approach for pressure load identification

An inverse approach for the identification of pressure loading on a structure has been proposed and developed. In this approach, surface measurements of structural response (e.g. strain, displacement and velocity field measurements, such as can be measured with 3D digital image correlation) are utilized as input data and are combined with numerical simulations to identify the pressure load on a structure. The inverse approach has been verified by numerical benchmarks involving pressure identification under quasi-static as well as dynamic impulse loading conditions, and also been validated by an experiment involving a quasi-static pressure load. The results indicate that the proposed inverse method can identify not only the magnitude of the quasi-static pressure but also the impulsive pressure loading history. The developed inverse approach offers an opportunity to apply inverse analysis techniques to identify interactive pressure loads (such as those resulting from a blast wave) on structures in explosive events.