PhD Defence by Anela Bajric-Hodzic - "Identification of Damping from Structural Vibrations"

Reliable predictions of the dynamic loads and the lifetime of structures are influenced by the limited accuracy concerning the inherent level of damping. The mechanisms of damping in structures cannot be derived analytically from first principles, and in the design of structures, the damping is therefore based on experience or estimated from measurements. This thesis focuses on enhanced methods for time-domain output-only system identification of damping from random structural vibrations.

The first method presents an automated identification method for the natural frequency and modal damping ratio. The method eliminates the necessity of manual selection of the number of time lags in the correlation function and the model order of the system, through a proposed data fitting routine. The procedure is applied to stochastic simulations and vibration measurements of the tower response of an 8 MW offshore wind turbine generator during downtime. This is a scenario in which a limited amount of damping can be expected. Therefore, it may be significant in the design of the next generation of wind turbines.

The second developed method, demonstrates how the spatial location of damping can be obtained by a derived explicit expression of the non-classical damping matrix. The modal parameters without a specific scaling are required in the expression as well as the mass distribution. The identified damping matrix is of high accuracy and yields a real-valued symmetric matrix. Measurements of a model-scale five-story shear building show that the estimated complex-valued mode shapes are reproducible and their convergence concerning the measurement duration validates that the non-classical damping matrix can be reconstructed robustly.

In the last part of the thesis a method for identification of damping in hysteretic systems is presented. Hysteresis is modeled by the Bouc-Wen model which is represented by an equivalent linear relaxation model. The linear relaxation model is related to the Bouc-Wen model by explicit expressions of the relation between the model parameters obtained by harmonic averaging. These expressions are incorporated in the identification procedure and they depend on the identification of a cluster of non-oscillatory poles, the root-mean-square of the response amplitude and the resonance frequency. The displacement response from random excitation of a hysteretic system is contained in the validation data set, by which it is shown that the model parameters identified by the method can predict the response at both low and high-levels of excitation amplitudes.

The identification methods in this thesis offer an improved insight into the damping mechanisms and their magnitude. They can therefore be helpful for design validation and optimization of both existing and future dynamic structures.

Supervisor:
Associate Professor Jan Becker Høgsberg, DTU Mechanical Engineering

Examiners:
Associate Professor Jon Juel Thomsen, DTU Mechanical Engineering
Professor Dr. Eleni Chatzi, Institute of Structural Engineering, ETH Zürich, Switzerland
Senior Researcher, Dr. ir. Bart Peeters, Siemens Industry Software Nv, Leuven, Belgium

Chairman:
Researcher Konstantinos Poulios, DTU Mechanical Engineering

Tidspunkt

fre 15 dec 17
12:00 - 16:00

Arrangør

DTU Mekanik

Hvor

Auditorium 074, Building 421
The Technical University of Denmark