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.     

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 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.     

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.

     

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.     

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.     

The stability of composite material stiffened conical shells under axial compression

The stability of composite material stiffened conical shells under uniform axial compression and with classical clamped boundary conditions is investigated. The effects of stiffeners are uniformly distributed over the whole surface of the shell, and the shell is treated as an equivalent orthotropic shell. A method of solution is developed by using energy principles and Rayleigh-Rize approximations. It is shown that the approach proposed in this paper is practical, effective and satisfactory.     

Mechanical instabilities of individual multiwalled carbon nanotubes under cyclic axial compression

Individual multiwalled carbon nanotubes with a range of aspect ratios are subjected to cyclic axial compression to large strains using atomic force microscopy. Distinct elastic buckling and post-buckling phenomena are observed reproducibly and are ascribed to Euler, asymmetric shell buckling (i.e., kinking), and symmetric shell buckling. These show agreement with continuum theories that range from approximate to remarkable. Shell buckling yields reproducible incremental negative stiffness in the initial post-buckled regime.     

Strength and fracture of lithium hydride crystals under uniaxial compression

We determine the conditions of fracture of lithium hydride crystals under uniaxial compression and establish the character of deformation and fracture of tested specimens and the ultimate compressive loads that can be withstood by the specimens. We also discovered and statistically analyzed a specific size effect on the fracture processes in crystals that is connected with the fact that the dimension of the crystal in the direction of applied compressive stresses is the principal factor that determines its compression strength. Russian Federal Nuclear Center, All-Russia Scientific Research Institute for Experimental Physics, Sarov, Russia. Translated from Problemy Prochnosti, No. 5, pp. 64–69, September–October, 1999.     

Failure prediction on advanced grid stiffened composite cylinder under axial compression

Axial compression test is performed on an advanced grid stiffened (AGS) composite cylinder and the progress of failure is recorded by strain sensors and LVDT sensors. Failure prediction on the AGS composite cylinder is accomplished by a cyclical symmetrical finite element (FE) model which can improve the efficiency of numerical simulation. Numerical results and experimental data are compared to verify the proposed model. Various failure prediction criterion based on degradation principles for the AGS composite cylinder are employed to investigate the accuracy of the proposed model.     

Efficient modelling of delamination buckling in composite cylindrical shells under axial compression

Composite cylindrical shells and panels are widely used in aerospace structures. These are often subjected to defects and damage from both in-service and manufacturing events. Delamination is the most important of these defects. This paper deals with the computational modelling of delamination in isotropic and laminated composite cylindrical shells. The use of three-dimensional finite elements for predicting the delamination buckling of these structures is computationally expensive. Here combined double-layer and single-layer of shell elements are employed to study the effect of delamination on the global load-carrying capacity of such systems under axial compressive load. It is shown that through-the-thickness delamination can be modelled and analysed effectively without requiring a great deal of computing time and memory. A parametric study is carried out to study the influence of the delamination size, orientation and through-the-width position of a series of laminated cylinders. The effect of material properties is also investigated. Some of the results are compared with the corresponding analytical results. It is shown that ignoring the contact between the delaminated layers can result in wrong estimations of the critical buckling loads in cylindrical shells under compressive load.