Buckling and postbuckling of anisotropic laminated cylindrical shells under combined axial compression and torsion

A postbuckling analysis is presented for an anisotropic laminated cylindrical shell of finite length subjected to combined loading of axial compression and torsion. The governing equations are based on classical shell theory with von Kármán–Donnell-type of kinematic nonlinearity and including the extension–twist, extension–flexural and flexural–twist couplings. The nonlinear prebuckling deformations and initial geometric imperfections of the shell are both taken into account. A singular perturbation technique is employed to determine interactive buckling loads and postbuckling equilibrium paths. The numerical illustrations concern the postbuckling response of perfect and imperfect, anisotropic laminated cylindrical shells for different values of load-proportional parameters. The results show that the postbuckling characteristics depend significantly upon the load-proportional parameter. The results reveal that in combined loading cases the postbuckling equilibrium path is unstable and the shell structure is imperfection-sensitive.     

Behaviour of ferrocement confined reinforced concrete (FCRC) under axial compression

An experimental investigation on the behaviour of ferrocement confined reinforced concrete (FCRC) under axial compression, by varying the specific surface factor (S_f) which controls the behaviour of ferrocement, is presented. A total of 270 prisms of size 150×150×300 mm were tested. The results indicated that additional confinement in the form of ferrocement shell improved the ultimate strength, strain at ultimate strength and the ductility of concrete. The improvement is in proportion to the S_f of the ferrocement shell for a given confinement index (C_i) of the lateral reinforcement.     

Buckling of defective single-walled and double-walled carbon nanotubes under axial compression by molecular dynamics simulation

Buckling of defective single-walled and double-walled carbon nanotubes (SWCNTs and DWCNTs, respectively) due to axial compressive loads has been studied by molecular dynamics simulations, and results compared with those of the perfect structures. It is found that single vacancy defect greatly weakens the carrying capacity of SWCNTs and DWCNTs, though it does slight harm to the effective elastic modulus of the tubes. The influence of defects on the buckling properties of nanotubes is related to the density of the defects, and the relative position of defects also plays an important role in buckling of DWCNTs. The van der Waals force among atoms in the inner and the outer tubes of short defective DWCNTs makes the critical buckling strain of DWCNTs greater than that of the inner tube.     

Plastic strain and stress deterioration of FRP-confined concrete prisms under axial cyclic compression

This paper presents the investigation and modeling of plastic strain and stress deterioration of FRP-confined concrete prisms under axial cyclic compression. Also the effect of the cross-section shape of specimens on these parameters is investigated. For this purpose, the experimental results of cyclic stress–strain behavior of circular, square and rectangular concrete prisms confined by FRP-composites are presented and examined. Besides, the effect of repeated complete cycles on the plastic strain and stress deterioration is studied. The results show that the relation between plastic strain and first unloading strain is linear and depends only on the two parameters of concrete strength and the shape of cross-section, but the stress deterioration depends only on the first unloading strain. The predictive formulations for plastic strain and stress deterioration are presented and compared with the experimental results.     

轴向压力作用下钢渣混凝土的压敏性研究

在不同加载速率、突然加载和卸载以及三向约束等加载条件下,钢渣混凝土具有良好的导电性和压敏性;随着钢渣掺量的增加,钢渣混凝土的压敏效应更加明显;施加压力较小时,电阻率随压力增大而迅速降低;压力增大到一定程度后,电阻率下降十分缓慢;压力达到极限荷载时,电阻率迅速升高;受到三向约束时,钢渣混凝土仍然具有压敏特性,相对于单轴受?其压力敏感性较低.     

Buckling analysis of moderately thick FG carbon nanotube reinforced composite conical shells under axial compression by DQM

A semi-analytical method is presented to study the buckling of moderately thick carbon nanotube reinforced composite conical shells under axial compression. Material properties are assumed to be graded across the shell thickness. In order to derive the equilibrium equations of the shell, first-order shear deformation theory is used. Then, the partial differential equations are transformed to algebraic type by using the differential quadrature method. To validate the results obtained in this study, comparisons are made with the outcomes of previous studies. Finally, the effects of the geometry of the shell, circumferential mode number, volume fraction of the carbon nanotube, and boundary conditions are studied.     

Buckling of composite circular plates under radial compression

Summary Axisymmetric nonlinear buckling equations are introduced for thin elastic circular plates composed of isotropic or orthotropic layers, uniformly compressed in the radial direction.The linear eigenvalue problem is solved in closed form for heterogeneous isotropic circular plates with clamped or simply supported circumference. Invariant buckling parameters are obtained which enable optimization. Several numerical examples are shown.

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Beulen von Kreisplatten aus Verbundwerkstoff unter radialem Druck

Zusammenfassung Für dünne, elastische, in radialer Richtung gedrückte Kreisplatten aus isotropen oder orthotropen Schichten werden drehsymmetrische nichtlineare Beulgleichungen hergeleitet.Das lineare Eigenwertproblem wird für am Rand eingespannte oder frei aufliegende heterogene isotrope Kreisplatten in geschlossener Form gelöst. Invariante Beulparameter, die eine Optimierung ermöglichen, werden erhalten und einige numerische Beispiele angegeben.

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Nomenclature a radius of plate - A _ij elastic area - A _ij ^* extensional rigidity - B _ij elastic statical moment - B _ij ^* , C _ij ^* extensional-flexural coupled rigidities - D _ij elastic moment of inertia - D _ij ^* modified flexural rigidity - flexural rigidity defined in equation (3.32.3) - E Young's modulus - h plate thickness - h ₀ distance to reference plane defined in equation (3.11) - J _i Bessel function of first kind and orderi - L _ij functional operator - m coefficient defined in equation (3.15) - M _r, M radial and circumferential couples, respectively - n constant defined in equation (3.21) - N _r, N radial and circumferential forces, respectively - N ₀ radial compression - p, q coefficients defined in equations (3.32) - Q transverse shear resultant - r radial coordinate - t invariant parameter defined in equation (3.44) - u, w radial and transverse displacements, respectively - x variable defined in equation (3.43) - Y ₁ Bessel function of second kind and first order - z vertical coordinate - change of slope in radial direction - _r0, ₀ radial and circumferential strain components, respectively atz=0 - circumferential coordinate - _r, curvatures in radial and circumferential directions, respectively - v Poisson's ratio - , slope angle before and after deformation, respectively - stress resultant function With 1 FigureThis study was supported by the Israel Academy of Sciences and Humanities and the Technion-Israel Institute of Technology. The numerical examples were carried out at the Technion's Computation Center by I. Smolash.Dedicated to the memory of Mr.Israel Stavsky, father of senior author.Visiting Professor, Washington University, St. Louis, Missouri., 1969/70.     

Modeling of temperature field of “Besser“ concrete blocks under standard fire conditions

The unsteady temperature field in a structural element of a load-bearing building wall constructed of vibropressed concrete blocks is investigated. Different variants of thermal loading under standard fire conditions are considered. Calculations made are based on the problem-oriented FEM-package NIKABT. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 2, pp. 330–339, March–April, 1997. .     

钢纤维混凝土动态压缩性能及全曲线模型研究

对钢纤维含量分别为0%,1%,2%,4%和6%的C30和C40混凝土进行了常三轴动态压缩试验,C30混凝土试件围压值为0,6,9,12,18和24 MPa,C40混凝土试件围压值为0,8,12,16,24和32 MPa;试验过程中采用位移控制模式下的正弦波分级加载.在此基础上,进行了应力应变全曲线表达式的选择与分析,并对各材料参数与诸因素同的相关性进行了分析.结果表明:(1)通过对Ezeldin等提出的钢纤维混凝土静态荷载作用下的应力-应变全曲线公式进行β参数修正,使该模型可以很好地描述钢纤维混凝土在动态常三轴压缩作用下的应力-应变全曲线关系;(2)混凝土基体强度越高,纤维含量对材料动态抗压强度的改善作用越显著,但受压状态下纤维含量对提高混凝土动态强度总的幅度较小;(3)混凝土动态峰值应力对围压大小较敏感,且随基体强度的提高,其敏感性有所下降.峰值应变与诸因素的相关关系基本上与峰值应力相同;(4)在动态荷载作用下钢纤维混凝土的割线弹性模量、钢纤维含量与割线模量间的相关性均随基体强度的提高而提高,(5)围压与纤维含量对混凝土材料在受压状态下的韧度指数ncmax影响不大.     

Buckling of composite orthotropic cylindrical shells under non-uniform axial loads

The static buckling of orthotropic composite cylindrical shells, under circumferentially non-uniform axial loads is investigated based on Flügge-type field equations. Use of a complex finite Fourier transform provides a simple method for handling any arbitrary non-uniform load but introduces modal coupling between the transformed equations. For simply supported boundaries (conditions SS3) the determination of the critical buckling load reduces to finding the eigenvalues of a finite matrix. Three different non-uniform loads are considered, having forms proportional to (1 + 2cosθ), cosθ and (θ^* − θ) where is the Heaviside function, θ is the circumferential coordinate and aθ^* is the width of an axially loaded strip of the shell of radius a. Computed results indicate the sensitivity of the critical buckling loads to the type of non-uniform load and the material lay-ups of the cylinders.     

Buckling of triangular Mindlin plates under isotropic inplane compression

Summary This paper considers the elastic buckling of triangular Mindlin plates under isotropic inplane compression. The total potential energy functional for the considered buckling problem is derived and the newly developedpb-2 Rayleigh-Ritz method is applied for the analysis. The method features thepb-2 Ritz function defined by the product of a two-dimensional polynomial function (p-2) and a basic function (b) formed from taking the product of the boundary equations with each equation raised to appropriate powers. New buckling results for isosceles and right-angled triangular Mindlin plates with various apex angles, thickness to width ratios and different combinations of edge conditions are presented. Such tabulated results are not only valuable to designers but also important to researchers as reference values for validating their numerical techniques and software packages.     

钢纤维增韧微膨胀钢管混凝土界面粘结性能研究

进行了30根钢纤维微膨胀钢管混凝土试件推出试验,系统研究了核心混凝土与管壁的界面粘结性能,阐述了推出试验过程中试件界面粘结破坏特征,并对界面粘结性能的影响因素进行了分析。结果表明,钢纤维微膨胀钢管混凝土界面粘结强度较普通钢管混凝土明显提高。径厚比和含钢率是钢纤维微膨胀钢管混凝土界面粘结强度的主要影响因素。钢纤维的掺加导致微膨胀钢管混凝土界面粘结性能降低,其体积掺量建议不宜超过0.75%。含钢率达到22%时,钢纤维掺量与界面长度对界面粘结性能影响不明显。     

Behaviour of recycled aggregate concrete filled stainless steel stub columns

Past research indicates that recycled aggregate concrete (RAC) could be successfully used in concrete-filled steel tubular (CFST) columns. Their yielded performance is almost as good as that of the traditional CFST columns. In addition, as a comparatively new construction material, stainless steel can be used to replace carbon steel for enhancing the durability and ductility of CFST columns. With an aim to combine the advantages of both RAC and stainless steel, RAC is proposed in this paper to be used as a filling material for stainless steel tubes. A test program is introduced in this paper to investigate the behaviour of RAC-filled stainless steel stub columns. For comparison purposes, reference specimens with carbon steel tubes are also tested. In the end, finite element analysis is conducted to simulate the current test results and those reported in the literature.     

Numerical simulation of the Split Hopkinson Pressure Bar test technique for concrete under compression

The Split Hopkinson Pressure Bar [SHPB] technique has been commonly used to investigate concrete compressive response under high strain rate. However, there appears to be a lack of agreement in the literature about a number of critical issues pertaining to this test method. In this paper, computational simulation models are employed to critique the technique and obtain a better understanding of it. Influential parameters are identified and attempts are made to shed light on some controversial issues surrounding the interpretation of high strain rate test data. The results show that significantly different strain rates can be obtained from the same SHPB test depending on the method used to estimate the strain rate value. Furthermore, comparing the results of simulations with pressure-independent and pressure-dependent constitutive material models show that strength increases associated with strain rate are strongly, but not totally, reliant upon the confinement introduced by lateral inertial effects and the frictional condition at the interface between the pressure bars and the specimen. Based on these observations, it is argued that the so-called ‘rate-enhanced’ models that explicitly account for strength increases as a function of strain rate should not be used in numerical simulations that already account for the effects of lateral confinement, since such models would tend to double-count the strain rate effect.     

Strength and complete stress-strain relationships for concrete tested in uniaxial compression under different test conditions

This report discusses the variations in strength and complete stress-strain behaviour which can result from varying some basic experimental conditions. The significance of varying the specimen slenderness ratio, ratio of specimen diameter to maximum size of aggregate, diameter of test platen used to transmit load from the testing machine to the test specimen, and the methods employed for strain measurement, have been evaluated.

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Résumé On examine les variations de résistance et des relations contrainte-déformation qu'on peut obtenir en faisant varier certaines des conditions expérimentales de base. On a étudié les incidences des variations du degré d'élancement de l'éprouvette, du rapport du diamètre de l'éprouvette à la dimension maximale du granulat, du diamètre du plateau d'essai utilisé pour transmettre la charge à l'éprouvette ainsi que de la méthode de mesure des déformations. Tous les essais ont été effectués sur des éprouvettes cylindriques carottées et sciées. Avec ce type d'éprouvette, on a obtenu une rupture uniforme par la suppression de la zone faible au voisinage de la surface de coulée. Les rapports d'élancement et les diamètres des carottes étaient respectivement compris entre 1–3 et 25–100 mm. On a maintenu une vitesse de déformation de 2,5×10⁻⁶/sec. Les principaux résultats de cette étude sont les suivants: a) la variation du comportement contraintedéformation en fonction du degré d'élancement montre différentes tendances selon que les déformations unitaires sont déterminées d'après la mesure des déformations globales ou d'après celle de la zone centrale des éprouvettes. Quoique ces différences diminuent avec l'augmentation du degré d'élancement, elles restent notables. b) Un degré d'élancement de 2,5 convient très bien à l'étude des propriétés du béton non armé. Pour des degrés d'élancement inférieurs, des ruptures se produisent aux extrémités fortement encastrées, tandis qu'à des degrés d'élancement plus élevés on constate un transfert indésirable d'énergie de déformation élastique depuis les régions de rupture aux extrémités vers la zone centrale de rupture. Le taux de variation de la résistance atteint aussi une valeur minimale. c) L'emploi de plateaux d'acier dont le diamètre excède celui de l'éprouvette détermine un accroissement de 3% de la déformation de rupture observée (à partir de la déformation globale); après application de la contrainte maximale le comportement est plus sérieusement affecté. d) Bien que la contrainte correspondant au début de la dilatation augmente avec le degré d'élancement, la contrainte latérale correspondant à ces contraintes de dilatation restait constante à 0,043% près. e) Avec une dimension maximale de granulat (D₀) de 9,52 mm, et pour des éprouvettes de diamètre (D) différent, on constate que la résistance atteint un maximum pour D/D₀=8. Le diamètre optimal d'une éprouvette, correspondant à sa résistance la plus élevée, pour la dimension maximale de granulat, peut être déterminé approximativement par: D/D₀=14,55−6,68 log₁₀D₀.

     

Failure of spheroplastics under axial tension and hydrostatic compression

The problem of determination of the external pressure and axial tensile force, which fail spheroplastics — a structurally inhomogeneous material consisting of hollow glass microspheres of different diameters, the latter being distributed in a random manner in a continuous epoxy matrix — is examined.Translated from Problemy Prochnosti, No. 9, pp. 59–61, September, 1990.