Analysis modal of sandwich structures honeycomb core with finite element method (Master thesis)

Ψυχογυιός, Χαράλαμπος

In the present work, the free vibration of honeycomb cored sandwich plates is studied. The main purpose is to obtain the results of natural frequencies and eigenmodes that appear during the dynamical behavior of plates. The analytical methods for their determination for this kind of structures are usually impossible. Thus, numerical methods are applied, the finite element method being the most significant. The modelling and simulation of the structure are carried out in ANSYS commercial package. In the first place, a healthy sandwich panel is analyzed in order to take the proper decisions for its design. In the second place, the behavior of the same panel containing a debonding between the skin and the core is examined in order to investigate the damage influence on the panel’s modal characteristics and to find a signal that informs about the presence of the damage. In the first chapter, a general presentation of cellular and honeycomb materials takes place along with a summary of their manufacturing process giving the reasons for their wide application in lightweight structures. In the second chapter, the mechanical behavior of honeycomb materials is presented along with their most common homogenization technique based on Ashby – Gibson theory. The resulted effective properties can simplify the analysis of honeycomb structures. In the third chapter, the architecture, the properties and the most common types of failure of sandwiches are presented, making a brief reference on their mechanics. In the fourth chapter, an extensive presentation of finite element numerical method takes place, for both static and dynamic analysis of structural components. After a synopsis of vibrations phenomenon, the whole procedure of the method is developed through elasticity energy theorems and the solution of its equation is explained, both for static and dynamic problems. In the fifth chapter, the two main categories of equivalent macro – mechanical behavior of honeycomb composite plates are presented. The equivalent plate model, formed by a plate of isotropic and homogenous material, belongs in the first category of equivalent single layer models, while, the equivalent sandwich model, formed by every layer considering its effective properties, belongs in the second category of discrete layer models. In the sixth chapter, the steps of equivalent models’ creation and simulation in ANSYS are presented, along with the formation of the exact geometrical model of honeycomb sandwich plate, utilizing the program’s capabilities. The types of finite elements used for the discretization of the layers are also presented. Those elements are considered the most suitable for the kinematics of layers. In the seventh chapter, the results of the modal analysis of the previous models are presented for three different case studies, in order to verify the validity of the models for the prediction of the real modal behavior. In the eighth chapter, the most common damage that appears in the free vibration state is mentioned, as well as the attempts for its finite element modelling and its detection through natural frequencies shift before it poses a threat for the safety of the structure. In the ninth chapter, the natural frequency results of a sandwich containing a debonding are presented. It seems that the presence of damage leads to the natural frequencies’ reduction, which can be treated as a ‘signal’ of its existence. The influence of skin to core debonding is investigated in conjunction with its size, its shape and the plate’s boundary conditions so to gather adequate information for a non-destructive method scheme. In the case of healthy honeycomb sandwich plate, we can come to the conclusion that the model which exhibits the best accuracy in combination with the number of finite elements is the equivalent sandwich model. Even more, this model can grasp the special features of each layer and embed new ones, just like the adhesive layer. This model is suitable for the conduction of plate design optimization, as well as, for the investigation of damage influence. In regard with the modal behavior of sandwich panels containing a skin to core debonding, it is clearly observed that the natural frequencies are decreased with the increase of damage area and the restraint of the boundary conditions. This attitude is eminent especially in higher frequencies. Moreover, as an indication of a damage existence can be the sudden appearance of phenomena, just like the “cross – over” where the inversion of eigenmodes series shows up. Lastly, the plate with circular debonding zone is more sensitive in comparison with the plates with the square or through-the-width zone.
Institution and School/Department of submitter: Πανεπιστήμιο Ιωαννίνων. Πολυτεχνική Σχολή. Τμήμα Μηχανικών Επιστήμης Υλικών
Subject classification: Sandwich construction
Keywords: Δόμες sandwich,Κυψελοειδή υλικά,Μέθοδος πεπερασμένων στοιχείων,Βλάβη αποκόλλησης,Sandwich structures,Honeycomb core,Finite element analysis,Debonding
Appears in Collections:Διατριβές Μεταπτυχιακής Έρευνας (Masters)

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