Nonlinear response in glass fibre non-crimp fabric reinforced vinylester composites

This paper addresses the nonlinear stress-strain response in glass fibre non-crimp fabric reinforced vinylester composite laminates subjected to in-plane tensile loading. The nonlinearity is shown to be a combination of brittle and plastic failure. It is argued that the shift from plastic to brittle behaviour in the vinylester is caused by the state of stress triaxiality caused by the interaction between fibre and vinylester. A model combining damage and plasticity is calibrated and evaluated using data from extensive experimental testing. The onset of damage is predicted using the Puck failure criterion, and the evolution of damage is calibrated from the observed softening in plies loaded in transverse tension. Shear loading beyond linear elastic response is observed to result in irreversible strains. A yield criterion is implemented for shear deformation. A strain hardening law is fitted to the stress-strain response observed in shear loaded plies. Experimental results from a selection of laminates with different layups are used to verify the numerical models. A complete set of model parameters for predicting elastic behaviour, strength and post failure softening is presented for glass fibre non-crimped fabric reinforced vinylester. The predicted behaviour from using these model parameters are shown to be in good agreement with experimental results.     

高性能碳-玻璃纤维布加固混凝土梁的非线性响应

针对高性能碳-玻璃-环氧树脂(CF-GF-EP)混杂纤维布加固钢筋混凝土(RC)梁,研究了一种三维非线性混杂组合单元,并推导了统一单元模式,对RC梁的刚度退化、应力重分布等进行了分析。首先根据实体退化单元理论,研究了组合单元模拟RC梁,并利用选择积分技术推求了高性能CF-GF-EP纤维布单元对混杂组合单元刚度矩阵的贡献。运用Jiang屈服准则、Hinton压碎准则和Madrid强化准则等描述混凝土材料非线性,研制了相应的三维非线性分析程序。与试验资料对比分析可知,计算结果与试验数据吻合良好,表明本文构造的非线性单元模式的正确性和研制程序的可靠性,推导的混杂组合单元能准确用于混杂CF-GF-EP纤维布加固RC梁的非线性响应分析。加载初期高性能CF-GF-EP纤维布加固梁未有明显的刚度折减,其后达到屈服荷载和极限荷载时,加固梁刚度折减系数分别约为0.6和0.9。在开裂荷载前,高性能CF-GF-EP纤维布应力发展较为平缓且应力重分布系数变化较小,其后应力重分布系数逐渐增大直至结构失效。     

Nonlinear dynamical characteristics of situation awareness

An adequate situation awareness (SA) is recognised as one of the necessary elements for optimal decision making. At present, there are a large number of studies that recognise that SA is dynamic. However, these studies have rarely been able to advance further knowledge on SA dynamics. The studies have been unable to accommodate the inherently nonlinear and possibly chaotic behaviour of SA. Therefore, this study proposes a quantitative model to explore some of these nonlinear dynamical characteristics of SA. The study results suggest that the supply rate of useful information is an important parameter that influences the quality of SA. A higher rate of information supply does not necessarily improve SA. On the contrary, if the supply rate is increased beyond a certain threshold, SA starts bifurcating. A further increase in this rate results in SA becoming chaotic. Such bifurcation and chaotic behaviour result in SA degradation. Therefore, it is argued herein that the highest level of SA can be achieved by maintaining the supply rate of useful information just below the threshold where SA first starts bifurcating. Further, the implications of such a high level of SA on various spheres of optimal decision-making are discussed.     

Nonlinear dynamical model of compartment fire flashover

A new dynamical model predicting compartment fire flashover is presented. The development is motivated by apparent deficiencies of available models. Innovative interpretation of flashover in terms of thermal explosion theory is proposed. In contrast to previously developed flashover dynamical models, which can be interpreted in terms of classical thermal explosion, the present model is interpreted as a conjugate thermal explosion. Therefore, the general conjugate thermal explosion concept finds its natural application in compartment fire dynamics. Critical conditions for compartment fire flashover are derived for the present model, and compared to those available from earlier studies.     

Nonlinear finite element analysis of reinforced concrete beams strengthened by fiber-reinforced plastics

Numerical analyses are performed using the ABAQUS finite element program to predict the ultimate loading capacity of rectangular reinforced concrete beams strengthened by fiber-reinforced plastics applied at the bottom or on both sides of these beams. Nonlinear material behavior, as it relates to steel reinforcing bars, plain concrete, and fiber-reinforced plastics is simulated using appropriate constitutive models. The influences of fiber orientation, beam length and reinforcement ratios on the ultimate strength of the beams are investigated. It has been shown that the use of fiber-reinforced plastics can significantly increase the stiffnesses as well as the ultimate strengths of reinforced concrete beams. In addition, with the same fiber-reinforced plastics layer numbers, the ultimate strengths of beams strengthened by fiber-reinforced plastics at the bottom of the beams are much higher than those strengthened by fiber-reinforced plastics on both sides of the beams.     

空间RRRP机械臂的刚柔耦合非线性动力学特性研究

摘 要 机械臂的刚柔耦合非线性动力学特性分析是实现其运动精度控制的基础。针对某航天飞行器装载的RRRP型空间机械臂,采用模态组合函数描述各臂的弹性变形,将转动副角位移、移动幅线位移以及各臂的模态坐标作为广义自由度,利用Lagrange定理建立了空间RRRP机械臂的非线性动力学方程,采用4-5阶变步长 Longgekuta法,对非线性微分方程组进行了数值求解。研究了机械臂刚柔耦合的非线性特性及结构参数变化对末端振动的影响,为进一步实现其结构优化和精度控制奠定基础。     

Nonlinear dynamical analysis of eccentrically stiffened functionally graded cylindrical panels

Based on the classical shell theory with the geometrical nonlinearity in von Karman–Donnell sense and the smeared stiffeners technique, the governing equations of motion of eccentrically stiffened functionally graded cylindrical panels with geometrically imperfections are derived in this paper. The characteristics of free vibration and nonlinear responses are investigated. The nonlinear dynamic buckling of cylindrical panel acted on by axial loading is considered. The nonlinear dynamic critical buckling loads are found according to the criterion suggested by Budiansky–Roth. Some numerical results are given and compared with the ones of other authors.     

Experimental study of carbon fiber reinforced plastic with embedded optical fibers

Structural differences of interface between the embedded optical fiber and the resulting smart composite were studied. Quasi-isotropic composite laminated specimens were produced from T300 Carbon/epoxy prepregs. Optical fibers were embedded within the specimens at different locations. Micro photos of the specimens showed that ‘bridges’ formed around the embedded optical fibers. The configuration and the size of the ‘bridges’, namely, the interfacial structures, varied with the locations where the optical fiber was embedded. Tensile tests showed that the ultimate strength of the smart material was affected by different interfacial structures. As the location of the embedded fiber moved to the middle plane of the specimen, the size of the interface increased. This caused the drop of the tensile strength. Observation of the broken sections of different specimens showed that imbibitions between the coating of the optical fiber and the clad were not good enough, therefore, damages could originate at the interface between the coating and the clad foremost.     

Nonlinear numerical modelling of FRP reinforced glued laminated timber

Fibre-reinforced polymers (FRPs) are effective in the flexural stiffening and strengthening of structural members. Such systems can be optimised if accurate numerical models are developed. At present, limited information is available in the literature on numerical models that can predict with good accuracy the nonlinear behaviour of FRP reinforced low-grade glued laminated timber beams. This paper discusses the development of a finite element model, which incorporates nonlinear material modelling and nonlinear geometry to predict the load–deflection behaviour, stiffness, ultimate moment capacity and strain distribution of FRP plate reinforced glued laminated timber beams manufactured from mechanically stress graded spruce. Beams with and without sacrificial laminations are modelled and their performance is compared to unreinforced glued laminated timber beams. The model employed anisotropic plasticity theory for the timber in compression. The failure model used was the maximum stress criterion. Strong agreement was obtained between the predicted behaviour and the associated experimental findings. It was deduced from comparing the results from the numerical model with experimental findings that the FRP plate succeeds in increasing the performance of the adjacent timber significantly. The model is a useful tool for examination of the effect of reinforcement percentage and will be used for optimisation of the hybrid beam.     

碳纤维水泥基材料电阻的非线性研究

研究了碳纤维水泥基材料(CFRC) 非线性电阻的伏安特性,并着重讨论了不同的碳纤维掺量和温度对CFRC材料电压-电阻关系的影响.结果表明:在较小外加电压下(＜2V)CFRC材料的电压-电阻曲线会出现明显的平台区,随着电压的进一步增大,其电阻逐渐降低呈现非线性特性.相同纤维掺量的水泥基材料随温度的升高其电压-电阻曲线下降斜率基本保持不变,但初始电阻值下降;而在温度保持不变时,随碳纤维掺量的增加,电阻随电压下降的趋势逐渐减缓.     

CFRP加固钢筋混凝土T梁抗爆动力响应数值模拟

为了研究CFRP加固钢筋混凝土T梁的抗爆性能,采用ANSYS/LS-DYNA有限元软件,建立了钢筋混凝土T梁—CFRP材料—空气域—TNT炸药的全耦合模型,进行爆炸荷载下钢筋混凝土T梁的数值模拟,对钢筋混凝土T梁的动态力学性能进行了分析,分别研究了CFRP外贴形式、CFRP厚度对钢筋混凝土T梁的影响.结果表明:相比普通...     

Nonlinear response of a silicon waveguide enhanced by a metal grating

We theoretically demonstrate that nonlinear optical response in a thin silicon waveguide within a wide wavelength regime can be enhanced by a metal grating. Numerical simulation indicates that the enhancement factor of the four-wave mixing signal varies with the position. The largest enhancement factor of the four-wave mixing is more than 10⁴ at a certain position in the IR spectrum with proper geometric parameters. More importantly, the wavelength of four-wave mixing with the same enhancement factor can be controlled dynamically within a wide wavelength regime.     

Blast loading response of reinforced concrete panels reinforced with externally bonded GFRP laminates

The behavior of reinforced concrete panels, or slabs, retrofitted with glass fiber reinforced polymer (GFRP) composite, and subjected to blast load is investigated. Eight 1000 × 1000 × 70 mm panels were made of 40 MPa concrete and reinforced with top and bottom steel meshes. Five of the panels were used as control while the remaining four were retrofitted with adhesively bonded 500 mm wide GFRP laminate strips on both faces, one in each direction parallel to the panel edges. The panels were subjected to blast loads generated by the detonation of either 22.4 kg or 33.4 kg ANFO explosive charge located at a 3-m standoff. Blast wave characteristics, including incident and reflected pressures and impulses, as well as panel central deflection and strain in steel and on concrete/FRP surfaces were measured. The post-blast damage and mode of failure of each panel was observed, and those panels that were not completely damaged by the blast were subsequently statically tested to find their residual strength. It was determined that overall the GFRP retrofitted panels performed better than the companion control panels while one retrofitted panel experienced severe damage and could not be tested statically after the blast. The latter finding is consistent with previous reports which have shown that at relatively close range the blast pressure due to nominally similar charges and standoff distance can vary significantly, thus producing different levels of damage.     

Local bond stress-slip relationships of glass fibre reinforced plastic bars embedded in concrete

The bond relationships between glass fibre reinforced plastic bars with polyester matrix and concrete is investigated in order to obtain information concerning the possible use of these bars for the reinforcement of concrete constructions, where the use of ordinary steel bars could have certain disadvantages due to the possibility of corrosion. The results obtained are used to develop a model of the stress-slip relationship and to estimate the anchorage lengths needed to embed these bars in concrete casts.

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Resume On a effectué une recherche sur la relation d'adhérence entre des barres de résine polyester renforcée de fibres de verre (GRP) et le béton, dans le but d'obtenir des informations sur l'emploi éventuel de telles barres comme armatures pour les ouvrages en béton armé, dans lesquels l'emploi de barres ordinaires en acier présente plusieurs inconvénients, dont ceux liés au phénomène de la corrosion. Les résultats obtenus ont permis d'établir un modèle de la relation contrainte d'adhérence-glissement, et de formuler une estimation des longueurs nécéssaires pour l'ancrage de telles barres dans des coulée de béton.

     

Nonlinear free vibration of embedded double-walled carbon nanotubes with layerwise boundary conditions

This paper investigates the large-amplitude free vibration of a double-walled carbon nanotube (DWCNT) surrounded by an elastic medium in the presence of temperature change. Based on continuum mechanics, a nonlocal elastic beam model is employed in which nanotubes are coupled together via the van der Waals (vdW) interlayer interactions. The Pasternak foundation model and a nonlinear vdW model are utilized to describe the surrounding elastic medium effect and the vdW interlayer interactions, respectively. DWCNTs with different boundary conditions are analyzed utilizing the Timoshenko beam theory that considers the shear deformation and rotary inertia effects. The governing equations are derived from Hamilton’s principle; the Galerkin method is utilized to discretize the governing equations. The influences of the nonlocal parameter, spring constant, carbon nanotube aspect ratio, and temperature change on the nonlinear free vibration characteristics of a double-walled carbon nanotube with different boundary conditions are thoroughly investigated. It is deduced that the nonlocal parameter, spring constant, and the aspect ratio play significant roles for the value of the nonlinear frequency. Also, the temperature change and the type of boundary conditions have an effect on the nonlinear frequency.     

Nonlinear static response of laminated composite plates by discrete singular convolution method

In this paper, large deflection analysis of laminated composite plates is analysed. Nonlinear governing equation for bending based on first-order shear deformation theory (FSDT) in the von Karman sense is presented. These equations have been solved by the method of discrete singular convolution (DSC). Regularized Shannon’s delta (RSD) kernel and Lagrange delta sequence (LDS) kernel are selected as singular convolution to illustrate the present algorithm. The effects of plate aspect ratio, fiber orientation, boundary conditions, thickness-to-side ratio, and applied load on the nonlinear static response of the laminated plate are investigated.     

Compressive splitting response of glass-fiber reinforced unidirectional composites

An analytical and experimental study of the compressive behavior of unidirectional glass/epoxy composites loaded in the fiber direction has been carried out for a range of fiber volume fractions. It was observed experimentally that glass/epoxy composites failed predominantly by splitting at lower fiber volume fractions (V_f) and by a combination of splitting and kinking at higher V_f. In contrast, carbon/epoxy composites were found to fail by kinking only. A mechanical model developed by Lee and Waas is used to predict the compressive strength of the composites. The predicted compressive strengths were then compared with existing experimental data in the literature. The effectiveness of the model in including the effect of initial misalignment of fibers on the predicted compressive strengths has also been studied.     

Uniaxial tensile response of microfibre reinforced cement composites

Microfibre reinforced cement composites reinforced with high volume fractions of carbon, steel and polypropylene fibres were tested in uniaxial tension. Composites investigated included those with only one type of fibre (mono-fibre composites) and those with two or more types of fibres (hybrid-fibre composites) in the same mix. Considerable strengthening, toughening and stiffening of the host matrix due to microfibre reinforcement were observed. In the hybrid-fibre composites, different fibres appear to act as additive phases; i.e., they maintain their individual reinforcing capabilities. The composites were also impact tested in uniaxial tension using a newly designed instrumented impact machine. When compared with static test results, considerable sensitivity to stress rate was noted; composites were found to be stronger and tougher under impact and the improvements were more pronounced at higher fibre volume fractions. The potential of these composites for use in thin sheet products and other similar applications is recognized, and the need for continued research is stressed.