Viscoelastic damping optimization of fuselage vibrations in aircraft

Kurzfassung

The focus of this thesis is the damping optimization of Constrained Layer Damping (CLD) treatments with viscoelastic materials (VEM) under mass constraints. The frequency dependent material properties of VEMs are characterised by a complex modulus. Under the assumption of Hooke’s law, the 1D-material model is transformed into a 3D-material model allowing for FE-calculations. Utilizing analytical models, the influence of the material and geometric properties on the modal loss factor of the first bending mode is analysed in a parameter study. It is demonstrated that the thickness of the core and constraining layers are possible design variables for damping optimization. Furthermore, an optimization process is established to determine optimal geometric parameters for damping maximization under predefined mass constraints. For this purpose, a Grid-Element-Generator is implemented in MATLAB and used for generating the associated FE-models. An interface to MSC Nastran solver is established for the analysis of the FE-model. Good agreements between analytical and numerical results confirm the applicability of the Nelder-Mead algorithm for damping optimization. Furthermore, the optimization algorithm is applied to a curved geometry of a highly simplified aircraft frame under free boundary conditions. The optimization is performed under predefined mass constraints for two different storage moduli representing two different temperature levels. A higher modal loss factor is achieved for a lower storage modulus. Considering different mass constraint, varying damping ratios are obtained for both storage moduli. As an alternative method for damping optimization, a cut-position optimization based on cutting of the core and constraining layers is performed. The results for the optimal location of the single cut contradict the rule of thumb established in literature. The optimal location of the single cut is dependent on the storage modulus of the VEM. Depending on the stiffness of a VEM, inserting a cut can either improve or reduce the damping effect.