Research and Development: Applications

Research objective- The aim of this project is detailed study of phenomenon of elastic guided wave sensing using fibre optic strain sensors based on Bragg gratings (FBGs). Attention in this research will be focused on possibility of symmetric and antisymmetric guided wave mode separation and registration effects of mode conversion S0/A0 and A0/S0. Moreover sensitivity of FBG sensor to elastic guided wave will be compared with piezoelectric transducers (PT) which are very popular in this application. Research will be related to investigation of hybrid method of elastic wave sensing based on combination of piezoelectric and fibre optic sensors (PT-FBG). Elastic guided waves are type of mechanical waves that propagate between parallel surface of solid body. Such a waves are guided by the parallel surfaces and propagate as symmetric (S) and anti-symmetric (A) modes. Depending on the product of frequency and thickness of the structure different number of symmetric and antisymmetric modes could propagate which are named (S0, S1, …) and (A0, A1, …). Modes A0 and S0 are the fundamental modes that could propagate bellow certain cut-off excitation frequency of wave for given material and its thickness. Above this frequency higher modes also propagate in the structure. In the case of symmetric and anti-symmetric modes material particles follows the elliptical trajectories including in-plane and out-of-plane displacements The ratio of in-plane to out-of-plane displacements depends of the mode. In the case of symmetric modes dominant are in-plane displacements of material particles while as for the antisymmetric modes dominant are out-of-plane displacement of material particles. Therefore both modes could be excited and sensed using piezoelectric transducer bonded on the surface of the plate-like structure. The registered modes could be distinguished based on difference in velocity of propagation for symmetric and anti-symmetric modes. However multi-mode propagation is very hard in interpretation (signal processing) in the field of damage detection and localisation. Guided waves can propagate at large distances in metallic structures (even hundreds of meters) therefore are utilised for structural health monitoring (SHM) of different structures. In SHM network of transducers that are permanently attached to the surface of structures on large area is utilised for structural monitoring.
Guided wave based SHM utilises fact that damage of the structure is source of changes in wave propagation phenomenon. Elastic waves interact with the damage and such effects like wave reflection, wave scattering and mode conversion could be observed. Based on these effects there is possibility of damage detection and localisation. Multi-mode character causes process of signal processing for the damage detection and localisation very complex. In case of signals gathered by transducers modes could be distinguished in registered signal based on the velocity of propagation but due to modes overlapping due to many internal wave reflections it is very complex task, especially in case when more than two modes propagate. Very often only one fundamental mode is excited effectively (large amplitude in comparison to second mode). This could be done on two ways: 1) using certain frequency of transducer where the amplitude of desired mode is much higher than the second – not so effective, very often second mode still has large amplitude. 2) using pair of piezoelectric transducer bonded on the top and bottom surface of sample. When both are excited in phase symmetrical mode is excited and when the transducers are excited out-of-phase the antisymmetric mode is excited. The similar idea could be utilised for wave sensing. If signals from both sensors are in phase - symmetrical mode was registered in second case - antisymmetric mode. This is more effective approach. 3) using specially designed transducers. Both approaches were realised in literature based on piezoelectric transducers (PT). In the proposed research authors would like to verify the application of FBG optic sensors in similar approach – for elastic wave mode separation and guided wave sensing taking into account effects of damage induced mode conversions. In this case following research will be conducted:
- study of sensitivity of FBG sensors for elastic wave mode (symmetric and anti-symmetric) and mode conversions effect sensing
- study of possibility of mode separation using FBG sensors (bonded on top and bottom surface)
- utilisation of FBG sensor directivity for damage detection
- study of dimensions of FBG and piezoelectric transducers on sensitivity for modes sensing also for mode conversion effects
- Study of resonant effect of FBG on mode conversion effect sensing
- study of FBG based registration of damage caused wave propagation changes (wave reflation, mode conversion)
- study of optical spectra deterioration for bonded FBGs and embedded in bond line
- detailed comp