Embedded optical fibre sensors for the permanent monitoring of filament wound pressure vessels

摘要

Worldwide a lot of research effort is carried out concerning new inspection techniques for the in-service monitoring of composite structures. This paper focuses on the use of optical fibre sensors. The sensor part of the optical fibre is a Bragg-grating. This is a periodic perturbation in the refractive index of the fibre core. When broadband light is coupled into the optical fibre sensor, a reflection peak will be obtained centered around a wavelength called Bragg-wavelength. The Bragg-wavelength depends on the refractive index and the period of the grating, which both change due to mechanical and thermal strain applied to the sensor. Optical fibres with Bragg-ser.sor have been embedded into filament wound pressure vessels. A test setup has been implemented to demonstrate the ability of remote sensing. This is of great importance for the applicability of Bragg-sensors to mechanical and civil structures. The controlling computer and the test set-up are connected to an optical spectrum-analyser by means of independent optical links of 200m. In this way the relative positions of the different parts of the test set-up are irrelevant. The vessels have been subjected to static loading cycles as well as to slowly varying dynamic loadings. The measured shifts in Bragg-wavelength show excellent linear agreement with the pressure applied to the vessel. The combination of this measuring method and the test set-up is an essential step towards a continuous remote monitoring system for composite structures. It is also an important part in the total design of 'smart' structures.