Main research group: Prof. W. Ostachowicz, Dr L. Murawski, Dr M. Palacz, Dr S. Opoka, Ms K. Majewska, Ms M. Mieloszyk

One of the diagnostic methodologies considered as a Structural Health Monitoring procedure is the one utilising Fibre Optic Sensors (FOS). FOS have high sensitivity, good resistance against water, chemicals and immunity towards electromagnetic interference. FOS technologies can be applied to metal, composite or sandwich structures. Depending upon the type of the technology used it has also a potential advantage to be embedded within the material of a structure itself especially in the case of composite materials. The drawback of fibre embedding is that if the surrounding material fails due to high stresses the fibre attached to the material can possibly debond leading to sensor failure.

The FOS technology allows measurements of either tensile or comprehensive strains that are applied along a sensor length. There is a linear relationship between the change in wavelength of the reflected light and strains in a fibre caused through externally applied loads or thermal expansion. To operate multiply sensors along a single optical fibre should have different Bragg wavelengths in order to differentiate between them. The number of signals recorded with special sensors can be integrated into an optical sensing fibre and depends upon the multiplexing method used, the strain range to be measured by an optical fibre, and the wavelength budget available for the incident light. The conventional method used to join sections of optical fibres is through fusion splicing. However the bondline at the physical discontinuity may have a short life because of concentration of high stresses and then the life of a sensor may be compromised by the fusion splice if a sensor is subject to long-term stress loadings.

The research performed by our team related to the FOS is connected with building a Finite Element Model of analysed structural elements and establishing structural parameters. For the most often used optical sensors the very important filed is the displacement field. The family of special finite elements with different forms of damage developed in this group are successfully applied for modelling changes in the displacement field caused by the appearance of such damage and the correctness of the numerical tools developed has been proven in many previous publications. From numerical simulations strains in structures with or without failures can be observed and compared. This information is undoubtedly very important for Structural Health Monitoring purposes.

Localising alarming symptoms of failures in structural elements is the main goal of the research carried out. The damage identification procedures can be implemented in the case of real and working structural elements and at the same time can improve the safety of large structures, like offshore engineering structures, air-plane fuselages or wings. The application of this damage monitoring technology can potentially result in great benefits to operators through improved safety and reduced operational costs. Frequently, structural damage within erosive environments may be very difficult to locate using conventional Non Destructive Inspection (NDI) techniques. Lengthy structural down-time is often required to allow these inspections to take place and this can result in significant maintenance costs. The ability for rapidly evaluation of the integrity of structures, ideally during service, through integrated SHM systems would have a substantial, positive impact on the costs.

Schematic of a FBG sensor

Schematic of a FBG sensor with reflected and transmitted spectra

View of an offshore oil platform