1 MHz high-sensitivity FBG sensor system to measure low energy impact in droplet experiment

Fiber Bragg Grating (FBG) sensor is widely used for Structural Health Monitoring (SHM) purpose. For dedicated health monitoring of critical composite parts in aircrafts, maritime constructions, wind turbine blades, etc., high speed measurement can provide vital information to assess impact damage. For example, low-energy droplet impact to wind turbine blade will cause in long term material degradation. A measurement system which can detect the impact accurately and with high reproducibility will contribute to the development of SHM strategy of composite structures. Currently, no commercial system has the required high sampling frequency and sensitivity. We developed a fiber interferometer based FBG interrogation system for high-speed impact measurements. In the droplet experiment on a composite sample, the feasibility of using the high-speed (1 MHz) system to measure low energy down to 2.5 mJ impact is demonstrated. The amplitude of the impact induced FBG wavelength shift decreases with larger distance between the FBG and the impact location. For a test with 5.5 mJ impact at 5 mm from the FBG, the amplitude of the FBG wavelength shift is measured to be about 25 pm and the impact pulse width is about 0.1 ms. The RMS wavelength noise of the interrogator is typically < 1 pm. This system can hence be used to detect even lower impact energy or be optimized to measure stronger impacts in other applications by tuning the sensitivity of the fiber interferometer. Great reproducibility of the impact pulse shape is demonstrated.