Transferfuctiemetingen in de aanwezigheid van niet-lineaire storingen.

The use of multisine excitations to characterize damage in structures

K. Vanhoenacker, J. Schoukens, P. Guillaume and S. Vanlanduit

Introduction:
A desire to monitor a structure and detect damage at the earliest possible stage is pervasive throughout engineering communities. From the early seventies on, different attempts were made to detect damage in structures from vibration measurements. The vast majority of the work has considered only linear types of structural damage, such as a reduction in stiffness, a non-closing notch, or a change in geometry like removal of a member in a truss structure. Because different damage scenarios result in a non-linear behavior, the modal parameters are not always suitable as a damage detection feature. Studies have illustrated that a crack in a structure may cause the structure to exhibit a non-linear behavior if the crack is alternately open during a part of the response and closed during the remaining time intervals. Recently, non-linear damage detection methods were introduced in the literature and it was demonstrated that the non-linear damage detection techniques are more sensitive to the presence of damage than the classical linear methods.
Description of the objectives and results:
The purpose of this project is to supply further contributions in the field of damage detection and succession.
In order to detect the presence of damage and imperfections in materials, a new, fast and promising method for non-destructive material testing has been developed and applied in the field of damage detection and succession. This new method for non-destructive material testing combines the advantages of the existing non-linear damage detection methods. The technique used in this method, focuses on the non-linear distortions that are present in the results of an FRF or transfer function measurement of the sample. The kernel idea is to use well-chosen periodic excitations where only some of the considered frequency components are excited. The non-excited frequency lines are used to detect, qualify and quantify the non-linear distortions. The use of this broadband excitation signal allows us to estimate immediately the best linear approximation of the overall system (also called related linear dynamic system), the even and the odd non-linear contributions and the noise contributions.
Experimental data from a slate beam, used in civil constructions, with an opening and a closing crack and a mechanical cyclic fatigue loading experiment on a beam have demonstrated that undamaged materials are essentially linear in their response. However, the non-linear behavior of the same material increases significantly when damage appears. The progressive damage/fatigue experiment clearly illustrates that the sensitivity of the odd non-linear contributions to the detection of damage features is superior to the even non-linear contributions and the FRF of the overall system.
The described multisine excitation technique is a very fast and efficient technique to assess and to follow the evolution of global damage in a material. This method is superior compared to the other non-linear methods due to the fact that it is possible to assess and to follow the evolution of the damage in a material in situ and in working conditions. The ability to use the compensation method minimizes the influence of the disturbing contributions originated from the measurement equipment.