2005 Volume 17 Issue 1+2 Pages 90-104
Two examples of in situ monitoring of ageing of materials with optical fibres are shown in this paper: thermal degradation of polyethylene and corrosion in aeronautic structures (aluminium) with an optical fibre corrosion sensor (OFCS).
The thermal degradation of polyethylene is monitored by inclusion of an uncladded optical fibre in the polymer. Through modelling of the angle distribution of the normalized light power transmitted with geometric optics, the refractive index of the polymer is determined at different temperatures (n=1.4594 - i0.0007 at 28°C and n=1.5162 - i0.0007 at 120°C). The variation of the refractive index is related to a variation of crystallinity of the polymer. The kinetics of degradation was directly followed at different temperatures.
The OFCS is tested by two methods: optical and electrochemical methods for different aluminum thickness deposited on the sensitive part of the optical fibre, in different acid concentration. The mechanisms of corrosion (uniform or in preferential places) can be deduced from the optical signal. The sensitivity of the OFCS is good for the low thickness. Corrosion rates for aluminum and copper are quite different, the effect of oxide layer on aluminum is important and induces corrosion in several stages.
