Application of structural modification method to nonlinear vibration analysis of bladed disks

Şayin, Burcu
High cycle fatigue failure of turbine blades is one of the most important problems in the design of gas turbine engines; hence, bladed-disk assemblies have been studied extensively for more than half a century. Damping design becomes an important issue in order to attenuate the blade vibration. For bladed-disk systems, friction damping concept is a common strategy to decrease vibration levels. There are different strategies in order to add friction damping to the system: blade-to-blade dampers such as shrouds and underplatform dampers such as wedge dampers. In the design of friction dampers, geometry of the friction contact becomes an important issue for maximizing energy dissipation. In order to determine the optimum damper geometry, i.e. shroud angle for shroud contacts and wedge surface angles for wedge dampers, nonlinear forced response analysis should be repeated by changing the finite element models in order to incorporate these geometry changes. Repeating the finite element analysis for each geometry change requires significant amount of time in the case of large finite element models used in real life examples. Moreover, in order to accurately model frictional contacts, high number of friction elements are required and utilizing modal superposition approach the number of nonlinear equations can be decreased significantly compared to receptance methods. Therefore, in this thesis, in order to decrease the computational time required for the finite element analysis, a new structural modification method with additional degrees of freedom is developed. The developed method is capable of determining the modal data of the modified structure by using modal data of the original structure and system matrices of the modifying part. The developed structural modification method is compared with the available modification methods in terms of computational time required and it is observed that the developed modification method is computationally more efficient than the existing methods. As a case study, bladed disk systems with integrally inserted shrouds are studied. Using the developed structural modification method and the nonlinear forced response prediction method, the effect of change of shroud angle is studied.