導波檢測鐵軌缺陷的數值模擬

Abstract

現行超音波檢測軌道技術主要以徹體波偵測鐵軌頭部與腹板缺陷，自軌頭發射導波檢測軌腳缺陷對於鐵軌檢測仍是一項挑戰。本文目的在發展軌腳缺陷的導波檢測技術，採用雙維有限元素分析程式數值計算JIS60及UIC60鐵軌導波頻散曲線，鐵軌導波群速度最快的模態因頻帶不同而異。另以有限元素套裝軟體模擬49 kHz鐵軌導波傳遞最快及次快的暫態訊號，經二維快速傅立葉轉換獲得對應之頻散點，與數值頻散曲線比較以辨別導波模態。傳遞次快的導波包含數個群速度接近的模態，其中數個模態在軌頭與軌腳處具有強度相當的響應，可應用軌頭暫態響應訊號檢測軌腳缺陷。由平行軌道縱軸之軌腳開槽缺陷的數值模擬B掃瞄影像，可以清楚發現繞射訊號，鐵軌導波繞射訊號經方向濾波器處理後可以用於檢測軌腳缺陷。

The current ultrasonic inspections using bulk waves are able to detect flaws in rail heads and webs but not in rail feet. It remains a challenge for defect detection of rail foot using guided wave launched from the rail head. This thesis aims to develop a guided wave inspection technique for defects in rail feet. The dispersion curves of guided waves propagating in JIS60 and UIC60 rails are determined using bi-dimensional finite-element code. The fastest guided modes change in various frequency bands. A commercial finite-element code was used to simulate the first and second arrivals of transient guided waves at 49 kHz. Their corresponding guided modes were identified by comparing the two-dimensional fast Fourier transformed frequency spectra of transient responses captured on the rail head with the above-mentioned dispersion curves. The second arrival combines several guided modes with close group velocities. Some of them have relatively equal intensity of responses in both rail head and foot. They can be used to detect defects in the foot from the rail head response. The diffracted signal from a notch parallel to the longitudinal axis of rail was clearly observed in the simulated B-scan images. It is feasible to detect defects in the rail foot using the diffracted guided wave signal filtered by a directional filter.