布拉格光纖光柵感測器應用於三維結構物邊點之暫態應變量測

Abstract

布拉格光纖光柵（Fiber Bragg Grating）由於具有優良的傳輸特性和機械性質，且無論是在單純拉伸的靜態量測能力或是配合能量調變法的動態量測能力皆已受到肯定，故成為近年來各界致力於開發與應用的感測器。不同於大部分的感測器，如應變規、壓電薄膜等，光纖本身細長型的幾何特性，也有助於應用在一些其他感測器無法量測的位置。 本文主要是利用光纖光柵感測器細長型的幾何特性來量測三維固體邊點上的動態應變特性。量測分為固體邊上的一維暫態應變與固體邊點上的三維暫態應變，首先，將受鋼珠落擊激振結構體的長時間暫態訊號利用快速傅立葉轉換獲得共振頻率，並與FEM有限元素法模擬分析加以比對。在確認頻率域下的解之後，利用壓電薄膜（PVDF）量測鋼珠落擊的波源歷時，有了波源歷時就可應用FEM有限元素法計算暫態時域下的暫態波傳特性，並與光纖光柵感測器實驗量測的結果做比較。在有了固體結構物在空氣中的動態特性之後，進一步的分析固液耦合的問題。將固體放入水中後，改變水位的高低並分析其暫態訊號與固體自然共振頻率的改變。

Recently, Fiber Bragg Grating has become a popular sensor to all walks of life for its excellent transmission characteristics and mechanical properties. And its static and dynamic, especially with power modulation, measurement ability has been recognized as well. Different from other sensors such as strain gauge and piezoelectric file, optical fiber has slender geometric characteristic that can apply to some tricky positions, which other sensors failed to achieve. The thesis is to discuss the result by using FBG’s slender geometric characteristic to measure the dynamic strain behavior on edge and vertex of a three-dimensional solid. The experiments will be divided into two parts, the measurement of one-dimensional transient strain on solid edge and the measurement of three-dimensional transient strain on solid vertex. First, by using free fall steel ball to impact the structure, a long time response will be generated. Transferring the long time response to get the resonant frequency by using Fast Fourier Transform, then compare with the simulate result by using finite element method. After confirming the resonant frequency is correct, I use PVDF sensor to measure the force history of the impact of free fall steel ball. With the force history, we can simulate the transient wave propagation result by using finite element method and compare with the measurement result by using Fiber Bragg Grating sensor. After getting the dynamic characteristic of solid in the air, the next step is to analysis the problem of the solid-liquid coupling. Put the solid into the water and measure the change of time response and resonant frequency under different water levels.