Çok katmanlı yüzeylere sahip kalınlıkça sivrilen sandviç plakların anlık basınç yüklemesi altındaki lineer olmayan dinamik davranışı

A sandwich structure is a class of composite material which is manufactured by attaching two thin laminated stiff faces to a core which is lighter and less stiff. Sandwich structures have a wide application area in aerospace, defence and marine industry. Sandwich and laminated composite components such as panels can have variable thickness in some cases while the others have constant thickness according to design, geometric necessities and minimum weight requirements. A tapered effect in thickness can be sometimes sufficient to obtain the variable thickness. The crucial point to design more durable sandwich structures is predicting dynamic responses of them when they are subjected to various types of transient dynamics loadings. This study focuses on the sandwich structure as a plate and the transient dynamic load as explosive air blast waves. Air blast loads occur on supersonic and hypersonic flights due to turbulences, sonic boom and shock waves. Sonic boom is a phenomenon and aerospace structures are subjected to sonic booms that cause shock waves while aircrafts, rockets and missiles reach and pass the speed of sound on flights. On the other hand, an explosion is another phenomenon which is the result of a rapid release of energy because of an explosive event. Chemical explosions like bombs, nuclear explosions, fuel explosions, etc. can be a blast source which causes overpressure. In this study, nonlinear dynamic behaviour of composite sandwich plates with variable thickness subjected to time dependent pressure pulse has been investigated by an analytical model, experimental model and finite element method (FEM). Tapered plate subjected to air blast loading is considered as a design model for the variable thickness and pressure pulse. The tapered sandwich plate has a honeycomb core and laminated composite face sheets. In the first part of study, the tapering thickness is varying for both sandwich core and face sheets together in case of simply supported boundary conditions for all edges. Moreover, a tapered sandwich plate which has a thickness varying for only sandwich core, is analysed for clamped boundary conditions for all edges to include the experimental model into the second part of study. Clamped sandwich plate with both tapered sandwich core and tapered face sheets is also considered. The theoretical method is based on a sandwich plate theory including the large deformation effects, such as geometric nonlinearities, in-plane stiffness and inertias, and shear deformation. The geometric nonlinearity effects are taken into account by using the von Kármán large deflection theory of thin plates. The classical sandwich plate theory for plates with constant thickness which have one-layered face sheets found in the literature is developed to analyse the tapered sandwich plates with multi-layered face sheets. The equations of motion for the plate are derived by the use of the virtual work principle. Approximate solution functions are assumed for the space domain and substituted into the equations of motion. The Galerkin method is used to obtain the nonlinear differential equations in the time domain. The finite difference method is applied to solve the system of coupled nonlinear equations. Finally, the equations of motion are reduced into a form that can be easily solved by one of the methods for solution of linear equation systems such as LU decomposition. The displacement-time and strain-time histories are obtained on certain points through the tapered direction. Then, the results obtained by using the present method are compared with the ones obtained by using a commercial finite element package ANSYS and the experimental method. The effects of taper ratio, the stacking sequence and the fiber orientation angle on the dynamic responses of the tapered sandwich plates are also investigated for the simply supported boundary condition. The simply supported and clamped tapered plates are discretized using 28x28 eight nodded layered shell elements (Shell281) which have the geometric nonlinearity capability for the FEM. Shell 281 has six degrees of freedom which are at each node. Three degrees of freedom is in translation while three is in rotation. These types of shell elements are not only typically described as geometrically planar elements, but also they are spatially 3D elements. Shell elements are very used to model thin and thick shell and plate structures which are subjected to bending. The total number of elements, 784 is satisfactory and is chosen as based on the mesh sensitivity analysis. The individual laminas (laminated faces and core) are assumed to be perfectly bonded in the finite element model. Finite element analysis is examined for only tapered sandwich plate that has both a tapering core and tapered faces because of the restrictions of defining the tapering function in FEM. The experimental studies contain manufacturing a tapered sandwich plate, determining mechanical properties of the constituents and finally simulating an environment of the blast effect with a two stage procedure. At first, blast load tests for transient pressure measurement on a plexiglass model plate are done. Then, air blast load tests on the real tapered sandwich clamped plate are carried out for measuring strain and acceleration at certain points. Acceleration data work useful on obtaining displacement-time histories experimentally. The experimental model presented here can reduce the period of time and can be defined as low cost in comparison with conventional tests by using the mentioned two stage procedure. Polyester film membranes for bursting which are used in this study are also very practical and cheap for the blast testing. There is a good correlation between not only the results of theory and FEM for simply supported tapered sandwich plate but also the results of the experimental model and theory with FEM for the clamped tapered plate. The closed form solution presented here costs much lower computer time compared to the FEM. The experimental studies are a troublesome process anyway which have a long period of time. However, the testing process is strongly needful for the validation of the theory. The code is once validated with the test, then the theory can be carried out the desired design of plates. Because, it is very easy to change design parameters for the theory. The method present here can be used in preliminary design of sandwich and laminated plates with variable thickness. The method is very capable of predicting the nonlinear dynamic response of the tapered sandwich plate with multi-layered faces. The thickness variation is taken as a linear function of coordinate axis in this study. However, the thickness variation could be taken as the second or third degree functions of coordinate axis. The blast pressure is uniformly distributed on the tapered plate. Tapered sandwich plates could be more functional under the effect of non-uniform pressure distribution. The thicker part of the tapered plate could be subjected to higher pressure load while the thinner part is subjected to lower pressure load.