Characterisation of the hygro-thermo-mechanical behaviour of organic matrix composites instrumented with optical fibres: A study of interfacial bonding

Moisture diffusion can decrease the mechanical stiffness and strength of organic matrix composites. Recently, Fibre Bragg Grating (FBG) sensors have been used in order to study the hygroscopic ageing of polyester/glass composites at room temperature. In the present study, the hygroscopic strain measurement of polyester/glass composite has been achieved at room temperature. Furthermore, measurements have been done at higher temperatures in order to better understand the combination of both hygroscopic ageing and varying temperatures on the mechanical properties of these composite samples. The Bragg wavelength (λb) was found to shift linearly over a temperature range from room temperature to 35 °C. Beyond 35 °C, the Bragg wavelength does not linearly vary as a function of the temperature. A strong variation of the Bragg wavelength above a specific temperature threshold was found. This could be explained according to two mechanisms. Firstly, hygroscopic ageing could result in a decrease of the glass transition temperature (Tg) of the polymer matrix. Therefore viscoelastic behavior may appear beyond this temperature. Hygroscopic ageing could also degrade the interfacial shear strength between the fibre and the resin. Differential Scanning Calorimetry (DSC) analyses showed that hygroscopic aging does not affect the Tg of the polyester resin. Furthermore, it has been found that the hygroscopic aging degrades the interfacial adhesion of the optical fibre/polyester according to the considerable decreases of the interfacial shear strength observed in practice.