Improvement of EFPI sensor system for damage detection of composites

Abstract

A gold-deposited EFPI (G-EFPI) sensor was developed in order to measure the dynamic strain of composites and to improve the sensitivity for detecting ultrasonic waves. Since stabilization of the sensitivity is very important to improve the performance of the sensor system for the detection of ultrasonic wave, a phase-stabilization controlling sensor system was thus developed as well. The phase-stabilization controlling sensor system was composed of an EDFA (erbium-doped fiber amplifier) source, a fiber Fabry-Perot (F-P) tunable filter and a control-circuit board. The control-circuit board controlled the F-P tunable filter to lock the sensitivity at the quadrature-point where the sensitivity reaches to the maximum value. In addition, a multi-parameter sensing system was developed using a WDM to measure the strain and AE signals simultaneously. Using the aforementioned sensor and system, the damage evaluation of composites was successfully conducted. The characteristics between matrix cracks and delamination were investigated though the spectrum analyses of AE signals detected by G-EFPI during the fracture test of composites. We also evaluated successfully the status of the damaged composites through the investigation of AE rate in an accumulated AE graph. Moreover, the strain and the AE signal could be measured simultaneously by the multi-parameter sensing system. Finally, the directivity of FOS (fiber optic sensor) was investigated, regarding on the directivity as a special property of the FOS. Inherently, FOS has the directivity to be aware of ultrasonic waves because the geometric figure is like the shape of a string; that is, the amplitude of the detected ultrasonic wave is dependent on the direction of the propagation of the wave. Therefore, the directivity should be investigated for the exact analysis of the ultrasonic wave detected by FOS. In this thesis, the directivity of G-EFPI sensor was investigated performing two analyses - theoretical and experimen...