In-situ strain measurements in composites during fatigue testing using optical fibre Bragg gratings and a portable CCD detection system

Abstract

An optical fibre Bragg grating (FBG) sensor was surface mounted onto a carbon fibre reinforced epoxy composite and subjected to static and dynamic loading. The FBG sensor was found to operate satisfactorily up to 700,000 cycles when the fatigue test was terminated. The fatigue test was conducted at a peak stress of 210 MPa and a stress ratio of -0.5. The FBG sensor was also found to operate satisfactorily over the cyclic loading frequency range of 0.1 - 6 Hz used in this study. The feasibility of using the sensor system for monitoring the stiffness ofthe composite during the fatigue test was demonstrated. The signal processing for the sensor system was based on a CCD spectrometer. The sensitivity of the static strain measurements was found to be approximately 80 jtc. A broadening ofthe FBG reflective spectrum was seen to develop as a function of fatigue cycles, and this phenomenon may be due to the delamination ofthe sensor from the surface ofthe composite.