Browsing by Author "Kara, Ali"

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Experimental and numerical approaches to evaluate the crushing behavior of combined geometry core sandwich structures against blast
(Izmir Institute of Technology, 2015-07) Kara, Ali; Taşdemirci, Alper; Güden, Mustafa
In this study, novel sandwich structures containing combined geometry structures as core materials were designed and developed for blast protection applications. The proposed combined geometries consist of a hemispherical geometry attached seamlessly to a cylindrical segment. Deep drawing method was used to obtain four different types of combined geometries having two different radii from blanks with two different initial thicknesses. The mechanical properties of the blank material were obtained by conducting tensile experiments at quasi-static and high strain rate regimes. Thereafter, crushing and energy absorption behavior of core units were determined by tests at quasi-static and low velocity regimes, experimentally. Before crushing simulations, manufacturing method was simulated to have realistic residual stress/strain and thickness variations of numerical specimens. Having accurate deformation history, crushing experiments were simulated and a good agreement was reached proving the realistic modeling of the manufacturing effects. The effect of heat treatment on the crushing behavior of combined geometry shells was also investigated both experimentally and numerically and there was a good agreement noted. After, cross-shaped sandwich structures of one type of combined geometry were prepared. Static, low velocity and high velocity crushing behavior of sandwiches were investigated. Study on sandwich structures also included confined experiments in order to account for the interaction between the core units and between the core units and surrounding environment; such a case might be a bigger sandwich in which adjacent cores could exert forces to each other. Numerical study was validated by comparing experimental and numerical results of three different loading regimes for sandwiches. Having well-verified numerical models, numerical study was extended to investigate strain rate and inertial effects on sandwich structures by simulations at high crushing velocities. With complete knowledge on crushing and energy absorption of single geometries and sandwiches, behavior of sandwiches under blast was investigated by using ConWep function. Various types were proposed for arrangements of sandwiches to have higher energy absorption and lower transmitted forces to the protected structures.
In-plane vibrations of curved beams having variable curvature and cross-section
(Izmir Institute of Technology, 2009) Kara, Ali; Yardımoğlu, Bülent
In this study, vibration characteristics of curved beams having variable curvatures and cross-sections are investigated. For convenience and progressive requirements, vibration characteristics of curved beams having; constant curvature and cross-section, variable curvature and constant cross-section, constant curvature and variable cross-section are also examined. The governing differential equations have derivatives with variable coefficients except for constant curvature and cross-sectioned case. Due to the fact that the solutions of differential equations with variable coefficients are analytically impossible except for special combinations of coefficients, in the investigation of eigenvalues of differential equations with variable coefficients usage of a numerical solution technique becomes necessary. At this point, the Finite Difference Method (FDM) is used to have the eigenvalues by converting continuous eigenvalue problem into discrete eigenvalue problem. Numerical solutions of the equations of motion with variable coefficients based on Finite Difference Method are carried out by using a symbolic program developed in Mathematica. The accuracy and numerical precisions of the developed program are evaluated by comparing the results with the analytical results given in literature. Good agreement is obtained in the comparisons of the present results with analytical results given in tabular form. Then, the effects of selected taper and curvature functions of beams on natural frequencies are found. The results are presented in tabular and graphical forms.