ABSTRACT
Aerospace structures experience a severe amount of stress over their life cycle. For example, structures located on the exterior surface of a space shuttle often undergo severe stress due to the presence of variable pressure and high gravity. Additionally, with a huge influx of small and micro-sized satellites, the probability of an impact in the outer-space orbits is now a concerning factor. There is also a presence of similar disturbing events for commercial flights. Only within the USA, the incidents of planes striking birds are more than 40 times on average per day, as per a report published by the Federal Aviation Administration (FAA) in 2019. Now, the skin of the planes and the rockets are usually made of various composite light-weight materials and multiple layers of specially engineered tempered glasses to withstand excessive stress due to the variation of pressure and temperature and unwanted impacts of a small scale. However, repetitive occurrences of impacts on the skin of the aerospace structures, even on the small-scale, may lead to the generation of severe mechanical stress which can lead to a catastrophic incident. Hence, it is imperative to identify, locate, analyse and monitor any accidental impact on aerospace structures in realtime. Various kinds of strain sensors are generally used for aerospace structures. However, the application of such sensors is restricted in places where visibility is enforced, such as the cockpits and windows. Such restriction can be overcome by incorporating a special kind of layout of optical fibre sensors that use fibre Bragg grating (FBG) principle. Specifically, FBG principle can be enforced in specially designed fibre optics. When light travels through such fibre optics, a specific wavelength of light (Bragg wavelength) reflects in the opposite direction. In the presence of any miniscule mechanical stress on such fibre optics, its physical deformation creates an axial strain. This then results the Bragg wavelength to be shifted. This proportional shift caused by the strain due to an event of impact can be physically measured using optical interrogators. Moreover, use of optical fibres on glasses does not significantly impede the visibility. The non-destructive damage assessment (NDDA) of impacts on aerospace structures by using the FBG optical sensors involves multiple steps, such as location estimation, time identification and magnitude quantification in correctly evaluating an event of impact. Furthermore, a remote and real-time scheme is usually preferred in such NDDA operations. This book chapter presents a real-time and wireless structural health monitoring method for analysing impact responses of aerospace structures using FBG optical sensors. A theoretical background is discussed, and a laboratory-scale experimental measurement is presented.
