Desarrollo de sensores de fibra óptica para su aplicación a la monitorización de la integridad estructural en estructuras aeronáuticas de material compuesto

Resumen

The main objective of this thesis is the technical development of optical fibers sensors technologies, in particular fiber Bragg grating sensors and distributed fiber based on Rayleigh backscattering frequency, and the demonstration of their applicability in structural health monitoring applications for aeronautical composites structures in a suitable selection of scenarios. The development strategy is eminently realistic. It start firstly with the study of the monitoring needs in composite structures and the identification and concretion of the requirements. These requirements covers the fundamental aspects in relation to the performance, accuracy, reliability or sensor resistance but also the compatible integration, durability or mitigation of any impacts with relation to the global process, -production, assembly and in-service life. Secondly, the study of the sensor technologies with capability to match with the previous requirements, FBG sensors and distributed sensing fiber, the gradual analysis to determine capabilities and limitations and modifications proposals to fulfill the requirements. Thirdly, the definition and development of the tests at different scales, -Lab, panel, component and complete a/c- that enable to assess and demonstrate these capabilities and limitations, as well as any other necessary aspects for the acceptance of the technologies in the application scenario. Once the tests have been carried out, the analysis and discussion of the results take place and conclude with the drafting of the conclusions and future actions. The application of the mentioned strategy is the base of the content of this document and is organized into seven chapters: - The first chapter is basically an introduction of the structural health monitoring concept, the relation between the classical approach – nondestructive inspection methods to control the structural quality of the parts in the different stages of the product life-, and the connection with the global concept of structural health management. This chapter introduces briefly some of the most important and promising technologies today in development for structural health monitoring (SHM) applications and specially the two optical fiber sensor technologies that are developed in this thesis. Also, this first chapter describes in a conceptual way the general architecture of any SHM systems and the key aspects have to be considered on the development process of SHM technologies with regard to a Multidiscipline that encompasses Structures and Sensors. - The second chapter is expressly dedicated to the presentation and description of the physical principles behind the two optical fiber sensors technologies developed in this thesis. The scope is not to provide a rigorous analysis of the physical principles of the technologies, which can be found in the special references at the end of this chapter, but to know how these technologies work up to the adequate level to understand their main capabilities and limitations from performance and engineering point of view. - The third chapter is essentially devoted to the presentation of the possible use case scenarios, and the identification of the major drivers for the use of the two optical fiber sensor technologies presented in the second chapter. At the end of the chapter is also found the main technical reasons justifying the integration of their technologies into the selected scenarios and the implementation process proposal in order to minimize any impact in the different stages of the structure: Manufacturing & Final Assembly Line (FAL), Operation and Maintenance. - The fourth chapter is entirely dedicated to the development of each the requirements applicable to the application scenarios identified in the third chapter. This compilation constitutes an essential task for the correct development of the technologies. The requirements were identified and organized according to the following criteria: strain monitoring application, damage monitoring, installation in structural test platforms, data management end user, operation in the aircraft, maintenance and operators. The main sources and references for the preparation of this chapter were obviously, on the one hand aeronautical standards related to operational conditions of the aircraft, and, on the other hand, the fruit of the knowledge acquired through the professional experience. - The fifth chapter is entitled as leading aspects within the technology development of the optical fiber sensor technologies. It is divided in four sections each one dealing with essential topics in these technologies: a) the main elements of the optical fiber technologies: fibers, cables, connectors and interrogators, b) the installation process and the breakdown of the main steps, c) the temperature compensation methods in those scenarios where is necessary to do it, and e) the conectorisation of fibers when they are embedded inside the composite materials. - The sixth chapter is completely dedicated to the experimental area of the thesis and although it constitutes the most hard-working part of this document, it would run the risk to be misaligned with the needs without all the preparatory work done in the previous chapters. The tests presented and developed are considered by the author as possible examples of the validation tests in the development phase of these technologies. The first group of tests describes the durability tests relating the resistance of the technology and integration process against normal operational conditions. The procedure and methodology for these tests are presented in five representative immersion test mediums: skydrol, kerosene, cleaning agent and water. The second group of tests that are reported refers to the temperature compensation concept when measuring strain with optical fiber sensors and acting simultaneously strain and temperature. In the third group, a simple test performed in electromagnetic lab is described to demonstrate the non-interference of Electromagnetic Field (EMI) on measurement process with optical fiber sensors. The test consisted of applying a controlled electrical discharge simulating a lightning strike and meantime measuring the strain and temperature with bonded optical fiber sensors. In the fourth group are detailed lab tests on selected coupons with integrated distributed fibers in order to evaluate the strain range of the technologies in mechanical tests. In the fifth group it is depicted a structural test on a metallic panel instrumented with distributed optical fiber and classical strain gages as reference. In the sixth group, a large composite structural tests, instrumented and monitored during static and fatigue conditions is described. In the seventh group, composite structural test panels containing a bonded repair patch and tested in static and dynamic conditions are described. In the eighth group an structural tests panel instrumented with embedded and bonded fibers is disclosed. The embedded fibers were installed in the production phase and the ingress-egress issue was solved by special connectors designed and manufactured for this particular purpose. The last section of this chapter is dedicated to the preparation and set up of an optical fiber installation based on FBG sensors for load monitoring in flight tests. - Finally, the last chapter of this thesis, the seventh one, highlights the final and most important conclusions of the thesis and tests done, as well as the future activities for a possible industrial implementation. In summary, the methodology followed and results of the tests constitute a new evidence demonstrating the capability and unique advantages of the optical fiber sensors for structural monitoring in aeronautical structures. Undoubtedly, the time for the systematic use of these technologies is closer, firstly in aeronautical structural test platforms and then, once consensus of the benefit is reached, in serial a/c, in the latter case not only for SHM purposes but even for other applications such as for example temperature monitoring.