Non-linear feature based damage localization in riveted plates using FBG sensors for guided wave sensing utilizing instantaneous baseline

Structural health monitoring (SHM) systems ensure safety and reliability of structures. They may be used for maintenance planning which reduces the maintenance cost and potentially can extend lifetime. As a result, there is a lot of active research in the area for SHM of structures. The SHM system should be low cost, suitable for continuous monitoring, able to detect small levels of damage. Guided waves (GW) based SHM techniques allow monitoring of large plate-like structures with a few sensors and have been identified as the most promising of techniques for SHM. One of the challenges of GW based techniques is that most of the developed techniques depend on comparison of the current measurements with a known baseline. Often the baseline measurement is not available or is acquired under different operating conditions. As a result, the applicability of these techniques although excellent in laboratory testing is not satisfactory in real applications. Few works utilizing non-linear features as a damage sensitive feature for reference-free SHM have been proposed with varying degrees of success. The current work builds on this initial idea for utilizing non-linear features and pairs it up with instantaneous baseline approach to develop a technique for reference-free damage detection and localization. Through the utilization of appropriate time windows and the inherent directional sensitivity of the fiber Bragg grating (FBG), it has been shown that damage can be satisfactorily detected and localized. In order to ascertain the presence of damage, a novel metric based on the spectral energy is developed. The robustness of the metric is further enhanced through multi-frequency data fusion. The developed metric and the utilization of FBG sensors for measurement of non-linear features is the main novelty of this work. In addition, a systematic approach for the threshold determination which takes into consideration systematic non-linearities is proposed. The methodology is applied on two aluminium plates riveted together. Damage is simulated through removal of the particular rivet. Various damage scenarios have been studied and the performance of the technique is compared with existing state-of-the art methods.