Available under a Creative Commons Attribution Non-Commercial Share Alike 4.0 International Licence
Electrical and electronic engineering
There has been a rapid growth in the use of advanced composite materials in a variety of load-bearing structures, for example in aviation for structures such as rotor blades, aircraft fuselage and wing structures. Composite materials embedded with fiber-optic sensors (FOS) have been recognized as one of the prominent enabling technologies for smart materials and structures. The rapid increase in the interest in composite materials embedded with FOS has been driven by numerous applications, such as intelligent composite manufacturing/processing, and safety-related areas in aircrafts. Research has been focused recently on using several optical sensor types working together to form so called “hybrid optical fiber sensors” in order to overcome the limitations of the individual sensor technologies.
The main aim of the research described in this thesis is to investigate a hybrid sensing scheme that utilizes polarimetric sensors and FBG sensors working in a complimentary fashion to measure multiple physical parameters in a composite material, with a particular focus on measuring the complex indirect parameters thermal expansion and vibration. The research described in this thesis investigates the performance of a hybrid sensing scheme based on polarimetric sensors and FBG sensors after embedding in a composite material. It is shown that the influence of thermal expansion within a composite material on embedded polarimetric sensors is the main source of errors for embedded fiber sensor strain measurements and that for practical strain sensing applications buffer coated PM-PCF are more suitable for embedding in composite. Further, using a buffer stripped PM-PCF polarimetric sensor, a measurement scheme to measure a composite material's thermal elongation induced strain is proposed. A novel hybrid sensor for simultaneous measurement of strain, temperature and thermal strain is demonstrated by integrating polarimetric sensors based on acrylate coated high bi-refringent polarization maintaining photonic crystal fiber (HB-PM-PCF), and a coating stripped HB-PM-PCF sensor together with an FBG sensor. Flexible demodulation modules that can be embedded or surface attached is a challenge for composite materials containing fiber-optic sensors. In this thesis an interrogation method that allows intensity domain operation of hybrid sensor is demonstrated. Further focusing towards the miniaturization of the hybrid sensor interrogator, a miniaturized flexible interrogator for the demonstrated hybrid sensing scheme embedded in a composite material is also designed.
Low frequency vibration measurements are performed for glass fibre-reinforced composite material samples with two different strain-sensitive polarimetric sensor types embedded. It is shown that the strain sensitivity of polarimetric sensors limits the vibration measurements to a certain range of vibration amplitudes. A polarimetric sensor based buffer stripped HB-PM-PCF is demonstrated for monitoring the different stages of the curing process for a Mageneto-Rheological composite material. By providing information about multiple parameters such as strain, temperature, thermal strain, vibration amplitude and vibration frequency the proposed and demonstrated hybrid sensing approach has a high potential to change the paradigm for smart material design in the future.
Ramakrishnan, M. (2016) Hybrid optical fiber sensors for smart materials and structures. Doctoral thesis, 2016.