敲擊回音相位於鋼筋與裂縫檢測之應用

Abstract

敲擊回音法已廣泛應用於混凝土內部缺陷檢測，該方法在試體表面敲擊激發應力波，並量取試體表面之位移響應以進行傅立葉轉換，再由頻譜中回音尖峰之頻率判讀缺陷位置。然而回音尖峰並無法顯示內部反應界面究竟是鋼筋或裂縫所造成。 本研究之目的即在提出一個可分辨混凝土內部裂縫與鋼筋回音訊號的方法。首先對待測試體進行敲擊回音試驗，將訊號之時間原點延後以移除表面波，接著進行傅立葉轉換，並找出頻譜之回音尖峰之相位，再以回音相位之數值與斜率研判回音係由裂縫或鋼筋造成。 本研究先以動態有限元素法模擬試體內含鋼筋與裂縫之波傳行為，並分析此兩類訊號之相位特徵與差異，接著再以模型實驗進行驗證。從數值與實驗結果可看出裂縫回音之相位較趨近於0且相位斜率較大，而鋼筋回音之相位則較趨近於 且相位斜率較小。 相較於一般敲擊回音分析，本研究提出之方法除了可以計算內部反射面之位置，亦可辨識該反射面為鋼筋或裂縫。

The impact-echo test is a nondestructive testing technique for civil structures. In the test, the surface response of the target structure due to an impact is measured. Then, the Fourier transform is adopted to transform the signal from the time domain to the frequency domain. The frequencies of the peaks can be used to determine the size of the structure or the location of the defect. In practice, the inclusion of target structure is unknown. Therefore, it cannot infer the interfaces is crack or reinforcing steel bar only by observing the frequencies of peaks. The purpose of this study is to analyze the phase characteristic of stress wave that comes from crack and reinforcing steel bar. Frist, progress the impact-echo test to obtain the impact-echo signal and remove surface wave of impact-echo signal by delaying the time origin. Then, use the Fourier transform to transform the impact-echo signal to frequency domain and find the phase of echo peak. Finally, identify the echo caused by crack or reinforcing steel bar by the phase and phase slope. The explicit finite element methods was firstly adopt to simulate the stress wave propagation when stress wave encounter crack and reinforcing steel bar. Then, the model tests were used to verify the numerical results. From the numerical and experimental results, one can conclude that the phase of crack echo approximate to zero and the phase slope is high. The phase of reinforcing steel bar echo approximate to and the phase slope is small. Compared to the conventional impact-echo data analysis, the proposed methods can not only determine the defect depth but also identify the defect that is crack or reinforcing steel bar.