Dynamic analysis with fibre optic sensors for Structural Health Monitoring

Structural Health Monitoring (SHM) is a new frontier of non destructing testing. Often SHM is associated with fibre optic sensors whose signals can be used to identify the structure and consequently its damage. Two ways of performing such an identification is either through the modal properties or through a direct use of time domain responses. Time domain techniques allows through inverse problem procedures to retrieve mass and stiffness matrices. The most common approach followed to retrieve dynamic parameters of a structure is called experimental modal analysis. This approach determines modal parameters (i.e. mode shapes, natural frequencies and modal damping) of the structure through the responses induced by known and sometimes unknown excitations (output only techniques).Modal parameters not only are useful for structural identifications but also to determine the resonances of the structures so that design changes can be performed if excitation frequencies are too close to ones of those resonances. So one can determine the damage, looking directly at the modal parameter variation or at changes in the structural matrices that can be derived, at least in principle, from the modal model. In the paper both modal analysis and time domain techniques will be described: modal appropriation technique with Electronic Speckle Pattern Interferometry (ESPI), classical broad band, broad band and stepped sign with the use of fibre optic sensors and inverse problems for time domain responses. In the paper numerical results relevant to damage identification in time domain will be presented, but particular emphasis will be given to the use of fibre optic sensors (specifically the Fibre Bragg Gratings or FBGs and sensorless fibre in a Michelson interferometer). Since in Structural Health Monitoring embedded sensors are usually considered, in the paper some embedding procedures for metallic materials by casting, as well as for composite materials by the hand lay-up and vacuum bagging technique will also be shown. Finally a case in which the optical fibre has been attached in the same way as a conventional strain gage will be shown. In all cases experimental results relevant to dynamic tests will be presented. It is worth noting that differently from the accelerometers, FBGs work very well in dynamics as well as in statics.