Asymptotic Description of Forced Response Vibration Saturation by Friction Forces

The estimation of the final vibration amplitude of a turbomachinery bladed disk is of extreme practical importance; it is an essential information for the prediction of the level of high cycle fatigue of the blades and for the subsequent estimation of its operative life span. The forced response vibration is saturated by the nonlinear damping introduced by the friction forces at the interfaces between blade and disk (and/or at the included dampers). The computation of the final amplitude of the limit cycle oscillation requires to solve a quite complicated nonlinear problem. In the case of a tuned bladed disk, this problem can be reduced to a single sector calculation with phase lag boundary conditions. The solution of this one-sector problem requires to consider many harmonics in order to capture the details of the nonlinear time periodic oscillation that sets in. If the small unavoidable differences among blades (mistuning) are also taken into account, then the situation becomes even more complicated because the solution of the mistuned vibration problem requires to consider not only a single sector but also the complete bladed disk. The possibility of applying multiple scales techniques to drastically simplify this problem is explored in this paper. The idea is to exploit the fact that all relevant effects present (forcing, nonlinear friction, and mistuning) are, in most practical situations, small effects that develop in a time scale that is much longer than that associated with the natural elastic vibration frequency of the tuned system. A mass-spring model with microslip nonlinear friction is used to represent the forced bladed disk. The multiple scales method is used to asymptotically derive simplified models for both tuned and mistuned configurations. The results of the asymptotic model are compared with those from the mass-spring system and used to analyze the particular characteristics of the nonlinear friction effects on the final vibration states.