DISTRIBUTED FIBER-OPTIC SENSORS FOR PRESSURE AND STRAIN MEASUREMENTS DURING SLAMMING OF A FLEXIBLE PLATE

Abstract

The investigation of fluid-structure interaction during the impact of a flexible plate on a water surface has received much attention. Measurement of highly transient, distributed strain and pressure of the plate during the slamming event is of great interest. Multiplexed fiber Bragg grating (FBG) strain sensors provide a promising solution for such measurement since these sensors are inherently waterproof and are immune to electromagnetic interference. However, in order to monitor the highly transient, distributed strain and pressure responses (up to 20 kHz), high-speed simultaneous interrogation of multiplexed sensors is required, which is challenging by using commercial optical interrogators. Furthermore, a it is challenging to use conventional pressure piezoelectric sensors is unable to deploy onfor pressure measurement on a flexible plate due to the intrusiveness of their its size. A miniature fiber-optics sensor is desirable for such application. In this dissertation work, a distributed fiber optic sensor system is explored for strain and pressure measurement on a flexible plate during slamming. First, a a high-speed optical interrogation system for the interrogation of multiplexed FBG strain sensors and Fabry-Perot pressure sensors is developed. A tunable-wavelength-filter-based optical interrogation system for high-speed simultaneous interrogation of multiplexed FBG strain sensors is investigated. The interrogation system employs a piezoelectric-transducer-controlled Fabry-Perot tunable filter. By operating the tunable filter at its resonant frequency and demodulating the sensor signal based on a peak tracing method, the system can operate at the interrogation speed of 100 kHz, an interrogation range of 98 nm, and an interrogation resolution of 5 pm. To demonstrate the its capability of the interrogation system, the interrogation system is used to monitor the vibrational responses of a cantilever plate under impact loading and the measurement of vibration modes up to 6.785 kHz. was demonstrated. The system is also used demonstrated to be able to interrogate Fabry-Perot acoustic pressure sensors forto measure the acoustic wave up to 20 kHz. Furthermore, miniatureSecond, miniature Fabry-Perot pressure sensors with temperature compensation is areare designed and fabricated developed based on the additive manufacturmanufacturinge technique. Two types of miniature Fabry-Perot pressure sensors (a single cavity FP sensor and a dual cavity FP sensor) were designed and developed. Due to the large coefficient of thermal expansion of the polymer material, the change of the optical path length induced by the temperature can result in a large error in the pressure measurement. By characterizing the pressure and temperature sensitivity of the sensor, the experimental result shows the temperature compensated pressure response of the FP sensor agreed well with the reference sensor. Finally, the experimental study of the impact of a flexible plate on a water surface is carried out by using the distributed fiber optic strain and pressure measurement system. With multiplexed FBG strain sensors and FP pressure sensors mounted on the flexible plate, the dynamic strain and pressure responses occurred on the plate during the slamming event were successfully monitored. The maximum strain increased with increasing impact speeds, which was in good agreement with the behavior of the measured maximum deflection. The high-speed spectral domain optical interrogation system with FBG strain sensors and FP sensors can serve as a useful measurement tool for a better understanding of the fluid-structure interaction.