A Study On Structural Damage Severity Assessment Based On The Nonlinear Vibro-Acoustic Modulation

The cantilever beam structure has been widely applied in the field of industrial engineering, such as large mechanical equipments, intelligent wind turbine, energy harviesting, etc. And the micro structural defects have gradually become a security risk during the period of engineering structures’ serving. Therefore, it is of great significant to propose a feasible way which can effectively detect the defects and accurately estimate the damage severity for the cantilever. In recent years, the nonlinear theory-based ultrasonic non-destructive evaluation (NDE) techniques have been regarded as a more effective approach method for structure damage detection, especially for detection of nonlinear contact-type microcracks. Vibro-acoustic modulation(VAM) is one of the nonlinear non-destructive testing technique based on the above theory, and its typical manifestation is that the high frequency ultrasonic signal is modulated by low frequency vibration.Firstly, in order to realize the weak signal enhancement in the early period of signal detection, this paper built a nonlinear bistable system based on the theory of stochastic resonance by introducing the magnet system to the conventional linear cantilever oscillator. From the experimental results, this model will be rather stable on estimating the damage severity. Besides, it effectively enhances the vibration response of the structure even when the excitation frequency is out of resonance and dramatically increases the bandwidth of operation, which enables this model suitable for various dimensions of cantilever beams.Secondly, this thesis introduced the nonlinear hysteretic properties to the study of structural damage assessment. In the experiments we measured and analyzed the dynamic response of each cantilever beams under different damage severity. From the results we found that there is a connection between the damage severity and hysteretic nonlinearities. That is to say, the increase of damage size will enhance the performance of hysteretic phenomena in nonlinear vibration system.Third, this thesis combined the hysteretic phenomena with the conventional VAM technique. According to different hysteretic frequency response phenomena (jumping phenomena or hysteretic region, etc.) in each modulation index curves which can be observed in our experiments for different damaged beams, this thesis provided an effective fatigue crack severity assessment approach based on a more concise criterion:the difference of hysteretic region in the VAM frequency response will become larger with the increase of the cracking severity. The thesis verified this criterion and provided an interpretation in terms of physical considerations through experiments and simulations studies. The results showed that, owing to the damage in cantilever, the beam’s effective stiffness and ability to resist deformation gradually decrease, which is the root cause of the hysteresis phenomena observed in the experiment. The approach has been proved in this thesis to be able to overcome the limitations that simply using the modulation intensity is not reliable to estimate damage degree.