Updating of Local Non-Linear Stiffness- and Damping Parameters in Large Order Finite Element Models by Using Vibration Test Data

Experimental modal data is extensively used to validate finite element models. These models are typically linear, even though real structures often exhibit amplitude dependent non-linear behaviour which is disregarded in the finite element models. The reason for non-linear behaviour can frequently be found at joints between otherwise linear substructures of overall assembled structures. Hence, finite element models can only accurately predict the dynamic behaviour of the underlying linear system if joint non-linearities are disregarded. The accuracy of response predictions for large excitation force levels can be inaccurate. This report is concerned with improving the prediction capabilities of finite element models by taking into account local non-linear stiffness and damping effects at the joints between substructures of overall assembled structures. This can be achieved by applying a computational model updating approach which minimizes the differences between measured non-linear frequency response functions and those simulated with a non-linear finite element model according to the Harmonic Balance method. In the first part of this report, a modelling approach for bolted flange joints is proposed, which aims at a more realistic modelling of the load path. In the second part of this report, different methods for non-linear frequency response analysis are described, which are based on the Harmonic Balance method. The theory of a non-linear computational model updating approach is presented in the third part of the report. This non-linear model updating approach uses the differences between experimental and analytical non-linear frequency response functions to improve the initial parameters of non-linear spring and damper elements introduced at the joints of the finite element model. Applications to analytical systems are presented, together with the application to an aero-engine subassembly with the objective to identify non-linear stiffness- and damping parameters of a bolted flange casing joint from measured non-linear frequency response functions. This report is supplemented by a literature review, which gives an extensive overview about computational model updating methods, non-linear parameter identification, non-linear response analysis, and model reduction techniques. Furthermore, additional chapters describe different approaches for the generation of damping matrices and the application of order reduction techniques to finite element models with concentrated non-linearities