Feasibility of integrated optical fibre sensors for condition monitoring of composite structures Part I: Comparison of Bragg-sensors and strain gauges

This first paper discusses the possibility of continuously monitoring the mechanical behaviour of composite structures by means of optical fibre sensors. In the introduction, some inherent advantages of optical fibre sensors, which make them ideally suited for this purpose, are discussed. Optical fibres with an intracore Bragg-grating were chosen as an alternative for classical electrical-resistance strain gauges. The main principles of Bragg-sensors are briefly discussed and some previously obtained results are summarized. Filament-wound pressure vessels have been used as a representative composite structural element. These vessels have been equipped with surface-mounted Bragg-sensors and classical strain gauges. Comparative measurements between these two sensor types are discussed in this first of two papers. The results of these experiments lead to the conclusion that optical fibre Bragg-sensors are more reliable than classical strain gauges. In the second paper 'fully monitored' experiments will be discussed: strain is monitored by means of Bragg-sensors and damage is monitored through acoustic emission detection. With additional research effort, and provided that adequate tools for the simulation of a degrading fibre-reinforced structure become available, permanent condition monitoring of critical structures could become realistic in the near future. The Bragg-sensor type optical fibre sensor can be considered a key instrument for monitoring applications of composite structures providing that they become available at economically interesting prices.