A Fiber-Optics Based Technique for Delamination Detection at the Web/Flange Junction of GFRP I-Beams

Fiber reinforced polymeric composites can be employed for the construction of structures that are highly durable and easy to fabricate. At EMPA, a prototype footbridge with girders and deck made of pultruded composite sections has been designed and constructed. For thin-wall pultruded sections such as I-beam or T-beam, delaminations can form easily at the web/flange junction. If the delamination is left unnoticed, its continuous growth will result in failure of the composite members. The present work aims at developing a fiber optic based technique to detect delamination at the web/flange junction of a GFRP I-beam. A PZT transmitter was employed to create stress waves in the beam element, and an optical fiber interferometric sensor was attached longitudinally along the web of the beam to serve as an ultrasonic receiver. During the test, the PZT transmitter was placed at different locations along a beam with artificial delamination. At each PZT position, a series of stress pulse was generated. The interferometer output was then analyzed in the frequency domain. When the PZT was close to the delaminated region, the frequency spectrum was found to be significantly different from situations when the PZT and delamination were far apart. Based on this finding, the feasibility of delamination detection with the proposed technique is demonstrated.