Development of innovative techniques for lamb wave based structural health monitoring systems in aeronautics

Resumen

Structural Health Monitoring (SHM) is one of the fields which is getting the most attention within Non-Destructive Testing (NDT). NDT pursues structural evaluation through the identification of damages, location of their position, and analysis of their influence on the operational conditions of the structure. However, limitation in terms of offline evaluation (i.e., out of service) of the data, generally obtained manually by operators, makes it necessary to integrate autonomous systems that generate and process information efficiently. Moreover, the SHM perspective adds a component for evaluating the structure online (i.e., during service), by analyzing the acquired data using actuators and sensors directly embedded in the structure, and their subsequent processing to obtain indicators that determine the structure’s operational status at any time. Physical and geometrical characteristics of certain structures allow the propagation of so-called ultrasonic guided waves (UGW) and, particularly, Lamb waves. These elastic waves propagate through plate-like structures in such a way that their interaction with the possible existing damages modifies the physical characteristics of the wave, making it possible to extract damage indicators to be subsequently evaluated. One of the most extended technologies for the generation and acquisition of UGW is the one based on piezoelectric transducers which, being permanently attached to the structure with an adhesive, propagate the elastic vibration to the structure, as well as record the structural response. Although there are numerous fields where SHM is applied (e.g., rotating machinery, civil engineering, or automotive industry), this thesis has been focused on materials used in the aeronautical field. Traditionally, the use of metallic materials has been the most widespread. However, in recent decades, the utilization of composite materials has increased notably due to their exceptional mechanical properties and lower weight, reaching more than half the percentage by weight of certain commercial aircraft. Besides, the information processing capability has increased dramatically in recent years, thanks to technologies that allow the analysis of large amounts of data in reduced times, allowing the reduction of both operational and maintenance costs due to the fast detection of faults in the structures. The integration of all the above in a single platform, in addition to the consideration of external factors such as changes in operating temperature, would make possible an SHM system that allows the detection, localization, evaluation, and even a repair proposal of the structural damage. Summarizing the above, the main topics addressed in this doctoral thesis are the following: • A study of the physical characteristics of the UGW, which will provide robust indicators of the existence or appearance of damages in aeronautical structures. • Analysis of the existing technologies for the generation and acquisition of electrical signals transmitted by piezoelectric transducers, and definition of the optimal characteristics for obtaining data to perform the extraction of reliable indicators. • Application of compensation techniques for the effects of temperature on the propagation of UGW, through the databases’ analysis that allow extracting the variables to be processed to avoid false indicators in the signals. • Development of damage detection and localization algorithms that allow obtaining, through the processing of the signals acquired by the transducers, images of the structures to be monitored, containing the damage positions (i.e., heatmaps). • Experimental evaluation of the proposed solutions, focusing on plates of aeronautics materials such as aluminum and, mainly, carbon fiber composites, largely used in the aeronautical industry.