Simultaneous measurement of strain and damage signal of composite structures using a fiber Bragg grating sensor

For the simultaneous measurement of strain and a damage signal, a fiber Bragg grating sensor system with a dual demodulator is proposed. The dual demodulator is composed of a demodulator with a tunable Fabry-Perot filter measuring the low-frequency signal with large magnitude such as strain and another with a passive Mach-Zehnder interferometer measuring the high-frequency signal with small amplitude such as a damage or impact signal. Using the proposed fiber Bragg grating sensor system, both the strain and the damage signal of a cross-ply laminated composite beam under tensile loading were simultaneously measured. The strain and the damage signal measured by the single fiber Bragg grating sensor showed that sudden strain shifts were accompanied with vibration at a maximum frequency of several hundred kilohertz at the instant of transverse crack propagation in the 90 degrees layer of the composite beam.