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On August 27, 2015, Sanne Sulejmani successfully defended her PhD thesis entitled "Microstructured optical fiber Bragg grating-based sensors for shear strain measurements" in a public session. In her PhD research work she successfully demonstrated shear strain detection in various materials and structures based on fiber Bragg grating sensors fabricated in specialty microstructured optical fibers (‘MOFBG sensor’).
Her research first focused on the use of a so-called ‘butterfly’ MOFBG sensor that was specifically designed for transverse strain sensing in smart materials. By aligning the angular orientation of its transverse strain sensing axes with the direction of principal stress in a shear loaded material, shear stress can be detected. She demonstrated this by embedding butterfly MOFBG sensors in the adhesive layer of a single lap joint structure and she achieved an unprecedented shear strain resolution of 50 µε when this joint is loaded in tension. This is a 4-fold improvement of shear strain sensitivity when compared to conventional highly birefringent FBG sensors. In a similar manner, she demonstrated that shear strain in laminated composite can be detected materials with a resolution of 10-20 µε with this sensor.
Second and owing to the high shear strain resolution of this sensor, she developed a sensing system for adhesive bond quality that allows detecting the initiation of adhesive disbonds as short as 0.4% of the bond overlap length, and monitoring the growth of these disbonds.
Third and finally, she has designed a novel dedicated MOFBG sensor using numerical modelling techniques that can further improve the shear strain resolution of the butterfly MOFBG sensor by 75%. The results achieved during her PhD research work demonstrate the potential of microstructured optical fiber Bragg grating-based sensors for shear strain sensing, and emphasize the added value of dedicated microstructured optical fiber sensors for smart material applications.
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