Association for Structural and Multidisciplinary Optimization in the UK (ASMO-UK) Effect of Aspect Ratio and Boundary Conditions on the Eigenfrequency Optimization of Composite Panels using Lamination Parameters Gokhan Serhat * and Ipek Basdogan † Department of Mechanical Engineering, College of Engineering, Koc University, Istanbul, 34450, Turkey Eigenfrequency optimization of laminated composite panels is a common engineering problem. This process mostly involves designing stiffness properties of the structure. Optimal results can differ significantly depending on the values of the model parameters and the metrics used for the optimization. Building the know-how on this matter is crucial for choosing the appropriate design methodologies as well as validation and justification of prospective results. In this paper, effects of aspect ratio and boundary conditions on eigenfrequency optimization of composite panels by altering stiffness properties are investigated. Lamination parameters are chosen as design variables which are used in the modeling of stiffness tensors. This technique enables representation of overall stiffness characteristics and provides a convex design space. Fundamental frequency and difference between fundamental and second natural frequencies are maximized as design objectives. Optimization studies incorporating different models and responses are performed. Optimal lamination parameters and response values are provided for each case and the effects of model parameters on the solutions are quantified. The results indicate that trends of the optima change for different aspect ratio ranges and boundary conditions. Moreover, convergence occurs beyond certain critical values of the model parameters which may cause an optimization study to be redundant. I. Introduction dvantageous properties of the composite laminates allowed their usage in the engineering structures to rise swiftly over the last decades 1 . Complex nature of such structures resulted in the development of more advanced modeling and optimization tools. Optimization of laminated composite structures for mechanical and vibro-acoustic requirements often involves alteration of stiffness properties. The results can change significantly depending on the values of the model parameters and the performance metrics used for the optimization. Building the know-how on this matter is important for choosing the appropriate design methodologies as well as validation and justification of prospective results. Hence, it is essential to investigate and quantify the effect of models and the response types on the optimization results. Vibrational characteristics of panel structures are strongly influenced by their stiffness properties. Hufenbach et al. showed that varying the fiber angles can alter dynamic properties of laminated composite panels and significantly change the sound fields resulting from panel vibrations 2 . Tinnsten and Esping used plate thickness and fiber orientation angle as design variables to carry out an acoustic optimization study to minimize sound intensity levels due to vibrating plates 3 . In vibro-acoustic design, one possible method is to reduce vibrations of the emitting structure by modifying its eigenfrequencies. Maximization of fundamental eigenfrequency can be used to avoid resonances occurring due to external excitation sources with the frequencies ranging from zero and up to the optimum fundamental eigenfrequency 4 . For example, Marburg and Hardtke improved the acoustic performance of a vehicle hat-shelf by increasing its fundamental frequency 5 . Narita and Robinson, obtained optimum laminate configurations yielding maximum fundamental frequencies for different boundary conditions and aspect ratios by following a layer-wise design approach 6 . Abdalla et al. also maximized the fundamental frequency of the laminated panels by using lamination parameters to model equivalent stiffness properties 7 . When the excitation frequencies lie in the range between two neighboring eigenfrequencies, maximization of the corresponding frequency gap is found to be more effective 4 . Honda et al. conducted a study to maximize the difference between fundamental and second natural frequencies by using lamination parameters as design variables 8 . Communicating Author: ibasdogan@ku.edu.tr * Ph.D. Candidate, Department of Mechanical Engineering, College of Engineering, Koc University. † Associate Professor, Department of Mechanical Engineering, College of Engineering, Koc University. A