Non-destructive damage assessment using non-linear vibration

The dynamic properties like natural frequencies and damping values of linear behaving structures can be identified by means of frequency response functions. However, in many cases, structures do not behave strictly linear, e.g. for concrete structures a non-linear material behaviour has to be considered. Furthermore, defects, as for instance cracks in reinforced concrete, lead to additional non-linearities that increase with the level of damage. Exciting the structure harmonically, a force dependency of all the modal parameters and the occurrence of higher harmonics can be observed. Hence, the use of dynamic methods to assess the state of damage has to take care of these nonlinearities to avoid misinterpretation. Moreover, the nonlinearities themselves can be explicitly used as damage indicators, as they are dependent on the damage state. This paper presents the results of an experimental analysis with three reinforced concrete beams at different damage states. Furthermore, the first results of forced excitation tests on a gradually damaged bridge concerning non-linear-behaviour are shown.