Yiska Goldfeld

Buckling of Laminated Conical Shells Given the Variations of the Stiffness Coefficients

The buckling behavior of filament-wound laminated conical shell is thoroughly investigated by consideration of the variation of the stiffness coefficients. To date, all analyses of laminated conical shells have been undertaken with constant stiffness coefficients in the laminate constitutive relations, usually under the assumption of nominal material properties taken from the midlength of the cone. The main object of the study is to investigate the influence of the variation of the stiffness coefficients on the buckling behavior of laminated conical shells. An analytical and computational model was developed to calculate the variation of the stiffness coefficients under the assumption that, in the case of filament-wound truncated conical shells, the fiber orientation changes using a geodesic path. The model was added to the computer code STAGS-A to calculate the buckling behavior of the laminated conical shell.

Imperfection Sensitivity of Conical Shells

The sensitivity of isotropic conical shells to imperfection is considered, via the initial postbuckling analysis, on the basis of three different shell theories: Donnell’ s, Sanders’ s, and Timoshenko’ s. The conical shell was chosen as a representative case exhibiting the entire range of imperfection sensitivity. The procedure involves nonlinear partialdifferential equations,whichareconverted into a sequenceofthreelinearsets.Thelatteraresolvedwiththe variablesexpandedinFourierseriesinthecircumferentialdirectionandine nitedifferencesintheaxialdirections. A general code is developed and used in studying the effect of higher exactness of the shell theory on the sensitivity behaviorandinparametricanalysesofthesensitivity ofconicalshells,especiallywithrespecttotheconesemivertex angle.

Buckling of Laminated Cylindrical Shells in Terms of Different Theories and Formulations

Bifurcation buckling analysis of laminated cylindrical shells is presented on the basis of three different shell theories: Donnells (Donnell, L. H., Stability of Thin-Walled Tubes under Torsion, NACA TR-479, 1933), Sanderss (Sanders, J. L., Jr., Nonlinear Theories of Thin Shells, Quarterly Journal on Applied Mathematics, Vol. 21, No. 1, 1963, pp. 21-36), and Timoshenkos (Timoshenko, S., Theory of Elastic Stability, McGraw-Hill, New York, 1961). Formulations in terms of the displacement components and of the Airy stress function and normal displacement are examined. The partial differential equations are derived via the variational principle and solved with variables expanded in Fourier series in the circumferential direction and presented as finite differences in the axial direction. The buckling behavior of an angle-ply laminated cylindrical shell under different modes of loading was investigated parametrically, showing-in contrast to its isotropic counterparts-a discrepancy between the two formulations.

Sensory carbon fiber based textile-reinforced concrete for smart structures

This article investigates the feasibility of intelligent textile-reinforced concrete structural elements with sensing capabilities. The concept is based on dual use of glass and carbon fiber textiles as reinforcement and, at the same time, as a sensory agent. Experimental investigation demonstrates the feasibility of the concept in two applications: detecting strains in a mechanically loaded textile-reinforced concrete beam and monitoring the interaction of the structural element with a wet environment. By detecting the changes to the integrative electrical resistance of the carbon tow, the ability of the textile to sense strain and exposure to water is demonstrated. For strain sensing, the hybrid reinforcing textile provides electro-mechanical sensing with a gauge factor of the order of 1 and a detectable correlation with the load, strain, and displacement responses. For the detection of wetting, the implementation of the carbon tow in a Wheatstone bridge detects fractional resistance changes in the order of 10−5, a figure that is effectively detected by monitoring the voltage across the bridge. The response to wetting, which is conditioned by the cracking of the beam and the exposure to ionic conductive solutions, provides a mean to monitor the functionality and the structural health of the textile-reinforced concrete beam.

Damage Identification in Reinforced Concrete Beams Using Spatially Distributed Strain Measurements

AbstractBrillouin optical time domain reflectometry or analysis (BOTDR/A) is one of the strain measurement technologies that is suitable for smart monitoring of civil engineering infrastructures. Although the technology has the advantage of supplying spatially distributed data, it is currently limited to a spatial resolution of approximately 1xa0m. This infers that the technology may lack the ability to identify the exact type and source of damage; that is, different geometrical configurations of cracking within a concrete beam may lead to similar BOTDR/A readings, and hence, the exact nature of cracking might not be resolved. This study suggests different crack indicators and analytically and experimentally examines their correlations with BOTDR/A readings of damaged RC beams. The analytical part entails a finite-element based statistical analysis of hundreds of cracking cases in fractured RC beams and their effects on the simulated BOTDR/A readings. It is found from the analysis that the increase of curva...

Shell Theory Accuracy with Regard to Initial Postbuckling Behavior of Cylindrical Shell

are the normal strains in the prebuckling state. The buckling anal- ysis for the nondebonded imperfect beams is then similar to that presented earlier for the free-vibration problem. In the case of a generally debonded beam, an exact buckling solution cannot be obtained and approximate treatments should be employed. How- ever, if the beam is with through-length delamination at the kth interface, for example, an exact solution can be obtained, just as it can for the nondebonded imperfect beam, but with K (k)

Elastic Buckling and Imperfection Sensitivity of Generally Stiffened Conical Shells

The sensitivity of stiffened conical shells to imperfection is considered, via the initial postbuckling analysis. Unlike stiffened cylindrical shells, in the case of stiffened conical shells the stiffeners inclination and the distance between the stiffeners vary with the shell coordinates, which complicates the problem considerably. The main objective of the study is to investigate the influence of the stiffeners on the buckling load and on the imperfection sensitivity. It is felt that by finding the various parameters that influence the shells imperfection sensitivity, it is possible to improve the behavior of the whole structure. A special computer code had been developed to calculate the classical buckling load and the imperfection sensitivity via Koiters theory of generally stiffened conical shells with consideration to the variation of the material properties in the shells coordinates. In the present work the shell is assumed to be closely stiffened and a smeared approach is adopted. Therefore, only global buckling behavior is considered.

Measures for identifying cracks within reinforced concrete beams using BOTDR

BOTDR is one of the strain measurement technologies that is suitable for smart monitoring of civil engineering infrastructures. While the technology has the advantage of supplying spatially distributed data, it is currently limited to a spatial resolution of about 1m. This infers that the technology may lack the ability to identify the exact type and source of damage; that is, different geometrical configurations of cracking within a concrete beam may lead to similar BOTDR readings, and hence the exact nature of cracking might not be resolved by the BOTDR. This study suggests different crack indicators, and examines, both analytically and experimentally, their correlation with BOTDR readings of damaged reinforced concrete beams. The analytical part entails statistical analysis of hundreds of cracking cases in fractured reinforced concrete beams and their effect on the simulated BOTDR readings. The analysis is conducted within COMSOL-Multiphysics, and is aimed to understand the correlation between different crack indicators and the beam curvature as would be obtained by the BOTDR. The experimental part consists of a controlled load test of a reinforced beam instrumented by BOTDR fibers, and is aimed to support the analytical findings.

Discontinuities in the Sensitivity Curves of Laminated Cylindrical Shells

The discontinuity in the sensitivity of laminated cylindrical shells is investigated via the initial post-buckling analysis. A general procedure for sensitivity, based on Koiters parameters and using the Donnell and Sanders shell theories, is developed and used for parametric study of the discontinuity phenomenon. It was found that the discontinuity occurs at points of change of the circumferential wave number.

Optimum Design of Filament-Wound Laminated Conical Shells for Buckling Using a Response Surface Methodology

Optimum laminate configuration for minimum weight of filament-wound laminated conical shells subject to buckling load constraint is investigated. In the case of a laminated conical shell the thickness and the ply orientation (the design variables) are functions of the shell coordinates, influencing both the buckling load and its weight. These effects complicate the optimization problem considerably. The first level of complexity is attributed to the correlation between the volume and the buckling load and their dependence on the fiber configuration. The second level of complexity is associated with the high computational cost involved in the calculation of the buckling load. Thus, the main objective of this study is to solve the optimization problem as well as to reduce the computational cost associated with it. Based on the characteristic buckling behavior of laminated conical shells an adaptive response surface technique is developed.