Strength of a structurally inhomogeneous material in axial compression

The process of failure of a two-phase structurally inhomogeneous material in axial compression is examined. Failure conditions for each phase of the material are formulated. It is shown that mechanical failure of the material is caused by the loss of strength by the continuous phase. The results are compared with experimental data.Nikolaevsk Shipbuilding Institute. Translated from Problemy Prochnosti, No. 9, pp. 56–59, September, 1989.     

FRP-confined concrete under axial cyclic compression

One important application of fiber reinforced polymer (FRP) composites in construction is as FRP jackets to confine concrete in the seismic retrofit of reinforced concrete (RC) structures, as FRP confinement can enhance both the compressive strength and ultimate strain of concrete. For the safe and economic design of FRP jackets, the stress–strain behavior of FRP-confined concrete under cyclic compression needs to be properly understood and modeled. This paper presents the results of an experimental study on the behavior of FRP-confined concrete under cyclic compression. Test results obtained from CFRP-wrapped concrete cylinders are presented and examined, which allows a number of significant conclusions to be drawn, including the existence of an envelope curve and the cumulative effect of loading cycles. The results are also compared with two existing stress–strain models for FRP-confined concrete, one for monotonic loading and another one for cyclic loading. The monotonic stress–strain model of Lam and Teng is shown to be able to provide accurate predictions of the envelope curve, but the only existing cyclic stress–strain model is shown to require improvement.     

Delamination buckling of cylindrical shells under axial compression

A model is developed for the study of delamination buckling of axially loaded cylindrical shells. Delamination is assumed to exist before load application, it spans the entire circumference, and it lies on the contact surface of neighboring laminae. The mathematical model employs Donnell-type kinematic relations and linearly elastic material behavior. Furthermore, each lamina is assumed isotropic, and the emphasis is on studying the effect of delamination size and position on the critical load. Two sets of boundary conditions are used with the model: simply supported and clamped. The study reveals several important conclusions. Among them, one may list the following: (a) the critical load is primarily controlled by the position of the delamination from the reference surface, provided that the delamination is not very close to the boundaries; and (b) for long delaminations (relative to the cylinder length), the critical load is not appreciably affected by the boundary conditions.     

Quasi-static and dynamic axial compression of glass/polyester composite hemi-spherical shells

An experimental investigation has been carried out to study the collapse mechanisms and the energy absorption capacities of the glass/polyester composite hemi-spherical shells under both quasi-static and drop hammer loading. The shells were made of randomly oriented glass fibre mats and polyester resin. Quasi-static tests were conducted at speed of 2 mm/min. and the impact velocities varied from 5 to 9 m/s. The radii of the shells varied from 53.5 to 106.1 mm and their thicknesses from 1.10 to 2.84 mm. Influence of these variables on the mechanism of deformation has been discussed. Experiments on 45 shells showed that their progressive crushing occurred due to the formation of successive zones of fracture. Based on these observations an analytical model has been developed for the prediction of load-deformation and energy-compression curves. The results thus obtained are found to match well with the experiments. It is seen that the ratio of the mean collapse loads recorded in impact and quasi-static tests for a given shell is greater than one but it decreases with the increase in thickness of the shell.     

Design of laminated composite cylindrical shells under axial compression

Approximate expressions describing shape efficiency are derived and two charts are shown that help with design. The first is a failure chart that shows the complete set of possible designs (and lay-ups) that allow the complexity of the problem to be presented in a simple way. The second is a chart showing optimal lay-ups for a cylindrical shell subject to axial compression. The load-bearing efficiency of cylindrical shells derives from both the properties of the material of which they are made and from the shape itself. Generally, thin-walled or slender shapes are more efficient (meaning lighter and more economical in the use of material) than thick-walled or solid sections. The limit in shape efficiency is either set by manufacturing constraints or, ultimately, by the properties of the material from which it is made. Laminated composite materials are more difficult to analyse than conventional isotropic materials, such as aluminium alloys, because of the additional need to consider the variation of ply orientation through the thickness of the laminate. These ultimate limits are examined and determined by a balance between competitive failure mechanisms.     

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.     

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.

     

Strength of a structurally inhomogeneous material under axial tension

The failure process in a two phase structurally inhomogeneous material under axial tension is investigated. The lengths of the most probable macrocracks caused by mechanical failure of the material are analyzed. The dependency of the ultimate strength of the material on the ultimate strength of the matrix is obtained. The calculated results are compared with the experimental.Translated from Problemy Prochnosti, No. 1, pp. 63–66, January, 1990.     

Cracks in glass under triaxial conditions

This experimental work documents the mechanical evolution of synthetic glass (SON68) under compressive triaxial stresses (hydrostatic and deviatoric conditions). The experimental setup enabled to monitor and vary independently confining pressure (range: [0, 50] MPa) and axial stress (up to 680 MPa) at room temperature. An optimized set of sensors allowed us to perform measurements during the experiments of: (i) axial and radial deformation, (ii) P- and S-elastic wave velocities, and (iii) acoustic emissions. In addition, in some samples, initial crack densities up to a value of 0.24 were introduced by thermal cracking. We compare the original synthetic glass data set to results obtained in the same experimental conditions on thermally cracked glass and on a basaltic rock with similar petrophysical properties (porosity, chemistry).Stress-strain data depict original linear elastic glass properties even up to an axial stress of 680 MPa (under 15 MPa confining pressure). A strong strength decrease (370 MPa at 15 MPa confining pressure) is observed for thermally cracked samples. Elastic wave velocity data highlight that cracks are mostly closed at a confining pressure of ∼30 MPa. The basaltic rock seems to correspond to an intermediate state between an original and a thermally treated glass. In all samples, damage was accompanied by dynamic crack propagation, producing large magnitude acoustic emissions. Thanks to a continuous recorder, we could locate a number of acoustic emissions in order to image the microcracking pattern evolution prior to failure.     

Constitutive behaviour of confined fibre reinforced concrete under axial compression

Steel fibre reinforced concrete is finding extensive use in field applications. The mechanism of delaying and arresting crack propagation by the fibres can be made use in passive confinement of concrete. Such concrete was termed as confined fiber reinforced concrete (CFRC). This paper presents an analytical model for predicting the stress–strain behaviour of CFRC based on the experimental results. A total of ninety prisms of size 150×150×300 mm were cast and tested under strain control rate of loading. The increase in strength and strain of CFRC were used in formulating the constitutive relation.     

复合材料圆柱壳轴压屈曲性能分析

对完整复合材料圆柱壳轴向压缩性能进行了试验研究,得到了圆柱壳结构的破坏载荷和各测量点的载荷-应变曲线,通过分析得出结构的破坏形式为屈曲破坏。利用ANSYS有限元软件建立了模型,对复合材料圆柱壳进行屈曲分析,将有限元计算的结构变形和屈曲载荷与试验结果进行对比,计算结果与试验结果一致,验证了模型的有效性。利用建立的有限元模型,分析了开口尺寸和铺层角度对含矩形开口的复合材料圆柱壳屈曲载荷的影响。在开口处加装复合材料口盖对结构进行补强,补强后的柱壳结构满足强度设计要求。     

Refractive index variation in compression molding of precision glass optical components

Compression molding of glass optical components is a high volume near net-shape precision fabrication method. In a compression molding process, a variation of the refractive index occurs along the radial direction of the glass component due to thermal treatment. The variation of refractive index is an important parameter that can affect the performance of optical lenses, especially lenses used for high precision optical systems. Refractive index variations in molded glass lenses under different cooling conditions were investigated using both an experimental approach and a numerical simulation. Specifically, refractive index variations inside molded glass lenses were evaluated by measuring optical wavefront variations with a Shack-Hartmann sensor system. The measured refractive index variations of the molded glass lenses were compared with the numerical simulation as a validation of the modeling approach.     

玻璃纤维增强聚合物复合材料约束壁式钢管混凝土短柱轴压性能试验

为研究玻璃纤维增强聚合物复合材料(GFRP)约束壁式钢管混凝土矩形短柱的轴压力学性能,对1组无GFRP约束试件和2组GFRP约束试件进行静力轴压试验;根据试验结果提出壁式柱的强弱约束模型,并基于双剪统一强度理论建立GFRP约束壁式钢管混凝土柱的轴压承载力计算公式;最后建立有限元模型,将计算结果与试验对比,并进行参数化分...