On the behaviour of in-fibre Bragg grating sensors for strain measurements on plane and curved surfaces

When compared with traditional electrical strain gauges used for strain monitoring, Fibre Bragg Grating (FBG) sensors have several distinguishing advantages that make them very attractive for many applications in different fields. Nevertheless, for practical applications, their metrological performance needs to be assessed under different operative conditions. In this paper attention has been focused on the response of FBGs when glued on a metallic surface that is not flat. Hollow-tube specimens with two different curvatures have been considered, together with a plane specimen used as reference. FBG signals were compared with those produced by electrical strain gauges. In addition, for the two hollow tube specimens, shadow moiré images of the specimen area between the FBG sensor and the strain gauge have been recorded in order to check the displacement field induced on the specimen. Preliminary results obtained show that the optical signal is markedly affected by the radius of curvature of the surface when the FBG sensor is bonded on a curved surface.