Nonlinear Vibration Behaviour And Crack Detection Of Cracked Beam Bridge

Cracks often exist in concrete structure. For this reason, vibration-based methods allowing early detection of cracks have been the subject of intensive investigation the last two decades. Most of the researchers assumed in their work that the crack in a structural element remains always open during vibration. Practically, compared with stiffness reduction and variation of damping, more important effect on structure aroused by cracks is that they will change the vibration mode of structure on local scale. There is more or less nonlinear vibration before the occurrence of cracks. Once cracks appeared on structure, there will be obvious nonlinear vibration aroused by effect of breathing crack.Firstly, different approaches to crack modeling, that fall into three main categories: local stiffness reduction, discrete spring models, and complex models in two or three dimensions are compared. The most suitable one for practical crack modeling is found and its reliability is demonstrated by experimental findings of former literature.Then, the phenomena of nonlinear vibration aroused by effect of breathing crack is studied by a simple single-degree-of-freedom system and a cracked cantilever beam. The bilinear stiffness depending on whether the crack is open or closed is modelled using nonlinear elements in finite element model. It is shown that a breathing crack shows nonlinear dynamic behaviour. The main distinctive feature of this behaviour is the presence of higher harmonic components. A breathing crack gives rise to the IF oscillates between the frequencies corresponding to open and closed states under free vibration. The variation of the IF increases with increasing crack depth. Nonlinear vibration aroused by effect of breathing crack is more clearly under harmonic exitation. The dynamic response data is analysed by applying EMD. The ratio of energy of IMF components which is the ratio of energy distoration C_disturb^E is extracted as an indicator of crack size. C_disturb^E has the potential to localize cracks by comparing the ratio of energy distoration on different position of structure.Lastly, relating to practical mode of damage of bridges, the most commonly damage of multiple T-beam bridge is modelled in detail, which is diaphragm crack and rust-eaten even desquamation of steel plate for transverse connection. The variation of static and dynamic performance introduced by damage is investigated by finite element analysis. Indicator of damage C_disturb^E is computed under different degree of local damage on diaphragm using breathing crack model.