Research Of Lamb Waves-based Active Structure Health Monitoring System

With paying more and more attention to that the integrity of in-service structures needs to be inspected to determine their physical condition throughout their lifetime, we have discussed active Structural Health Monitoring System (SHMS) using interrogative Lamb waves in this thesis. SHMS can be either passive or active. Passive SHMS infers the state of the structure using passive sensors (loading, stress, environmental condition, performance indicators, acoustic emission from cracks, etc.) that are monitored over time and fed back into a structural model. Passive SHMS only "listens"to the structure but does not interact with it. Active SHMS is different from passive SHMS because it uses sensors to interrogate the structure in order to detect damage presence, extent, and intensity. Active sensors act upon the structure in much the same way that conventional nondestructive evaluation (NDE) transducers do. Unlike passive sensors (which only listen to the structure), active sensors interact directly with the structure and find its state of health and reliability. Active SHMS using interrogative Lamb waves would be able to cover large areas from one single location. Such systems would be cost-effective and efficient. Furthermore, we have discussed the transducers using in SHMS. Conventional ultrasonic transducers are inappropriate for SHMS applications due to their cost, weight, and size. Conventional ultrasonic transducers could not be embedded in large numbers into a structure without incurring important cost and weight penalties. For SHMS applications, new types of Lamb-wave transducers such as piezoelectric wafer transducers must be developed, they must be small, light weight, unobtrusive, and low cost. On the other hand, the key requirements of an advanced SHMS are the ability to detect damaging events, characterize the nature, extent and seriousness of the damages. In this thesis through discussing the propagation of Lamb waves with Mindlin Plate Theory (MPT) and Finite Difference Algorithm, a damage localization concept using ellipses method is proposed based on the propagation of Lamb waves, which are excited and received by surface-mounted or embedded piezoelectric transducers. Using the method can provide explicit solutions and can draw a conclusion on the damage area. Numeric and theory analysis basing on the waves'difference arrival time of transducers that has been interacted by damages, it is proved that the method basing TDOA also can be easy implemented in real-time active SHMS and can get the damage location which maybe exist in the structure exactly and promptly.