Dynamic strain distribution monitoring of 5.5 m CFRP blade using 5 m FBGs interrogated by optical frequency domain reflectometry

We have developed an optical frequency domain reflectometry (OFDR) system which is capable of conducting distributed strain measurements with a sampling rate of 150 Hz, a spatial resolution of less than 1 mm and a sensing range of 20 m. In this study, we applied this technique to monitor dynamic strain distributions along a 5.5 m helicopter blade structure with carbon fiber reinforced plastics (CFRP) skins and shear webs. We used 5 m fiber Bragg grating (FBG) which were bonded on the CFRP blade, and measured the strain distributions along the FBGs. We hung up and released the blade at several points and applied several cases of vibrations. The time variations of strain distributions were monitored by the OFDR-FBG system. In order to validate the results, we attached 16 strain gauges along the FBGs, and compared time variations of measured strains at individual locations. The comparison results validated the accuracy of the results, and demonstrated a promising capability of the optical fiber distributed sensing system to monitor vibrations of the CFRP blade.