Application of optical fibre sensors for structural health and usage monitoring
Structural Health and Usage Monitoring has gained considerable interest throughout the engineering technologies. Especially for the aircraft industry, where damage can lead to catastrophic and expensive failures, and the vehicles involved undergo regular cost intensive inspections, a Health and Usage Monitoring System (HUMS) has one of the highest payoffs. Furthermore, HUMS allow new design principles for the realisation of lightweight aircraft structures. Different approaches towards a HUMS can be found in the literature. A system based on load monitoring and damage detection could provide highest potential for implementation in future aircrafts. This thesis investigates the use of multifunctional fibre Bragg grating (FBG) sensors for structural health and usage monitoring. It is shown, how FBG sensors can be used simultaneously for both, a strain sensing based load monitoring system and a Lamb wave based damage detection system. Several fundamental areas are addressed analytically and experimentally. This work adds new approaches towards the implementation of large area FBG sensor networks using fibre optical rosettes and temperature compensated strain sensors. A miniaturised build-up technique for a FBG temperature sensor is demonstrated which allows multiplexing of several strain and temperature sensors within a single fibre network. The use of a backing patch for FBG sensors is studied numerically and experimentally. In this thesis, surface mounted and structural integrated FBG sensors are used to detect Lamb waves. The theoretical approach that leads to the development of an appropriate ultrasonic interrogation system for FBG sensors is introduced. Numerical simulations on the influence of the grating dimensions on its ability to detect ultrasonic strain fields and their experimental validation are presented. Three different tasks of damage identification based on Lamb waves are considered: detection of damage, localisation of damage and severity of damage. Experimental results on all three tasks show that FBG sensors can compete with existing technologies. As part of the experimental work, a reliable, temperature independent damage index is introduced and a novel detection scheme using fibre grating rosettes and Genetic Algorithms for the localisation of damage is developed. The results of a simple fatigue test experiment on which the same FBG sensors were used to measure the load parameters and the crack size agree very well with the results using standard technologies.