敲擊回音相位之參數分析

Parametric Study on Impact-Echo Phase

徐穎彥(Ying-Yan Hsu)

指導教授 ： 劉佩玲

國立臺灣大學/工學院/應用力學研究所/碩士(2016年)

https://doi.org/10.6342/NTU201602378

摘要

敲擊回音法為非破壞檢測技術中，最廣為應用於混凝土結構之檢測。傳統敲擊回音法為藉由結構表面受敲擊後產生表面位移，再經由傅立葉轉換得到振幅頻譜，並以振幅頻譜判讀結構內之結構尺寸或缺陷位置。然而，傳統敲擊回音分析僅能偵測混凝土內部反射面可能位置，卻無法得知反射面係由裂縫或鋼筋形成，鋼筋屬補強構件，但裂縫卻會危害混凝土結構完整性，因此，反射面類型的判別相當重要。
本研究之主旨為探討敲擊回音相位法用於辨識裂縫與鋼筋之可行性，此想法是由於反射波相位會改變而來。當應力波所遇到反射面為鋼筋時，回波相位會改變π ；而當應力波所遇到反射面為裂縫時，其回波相位沒有變化。因此，推測由裂縫或鋼筋所造成之回波相位會不相等。
首先，將理想化表面位移函數藉由傅立葉轉換得到回波相位之解析解，即可發現裂縫之回波相位趨近於0，而鋼筋之回波相位會趨近於π/2。而本研究經由數值模擬與模型試驗驗證後，得知裂縫或鋼筋之回波相位值會分別落在(-π,π/4)與(π/4, π)間。此外，上述結果亦發現裂縫之回波相位會靠近0，而鋼筋之回波相位會靠近π/2。
本研究亦探討敲擊源、反射面深度與內含物尺寸三種參數對回波相位之影響，期使量測結果能更易於定量分析與討論，以提供非破壞檢測人員檢測時之建議。由敲擊源之影響來看，數值模擬結果顯示當回波頻率與敲擊源頻率中心之比值超過2時，相位結果會變差，故建議選擇之鋼珠為使比值小於2之範圍，尤其當敲擊源頻率中心與回波頻率接近時，相位值會較準確；由反射面深度之影響來討論，當深度改變時，若鋼珠直徑愈大，會使得相位結果導致誤判，故建議須慎選鋼珠直徑以作為敲擊源；而由內含物尺寸之影響來分析，若裂縫邊長與裂縫深度之比值大於0.2，裂縫回波相位會愈靠近0；而若鋼筋半徑與鋼筋深度之比值大於0.4，則鋼筋回波相位會靠近π/2 。因此，推測裂縫愈大或鋼筋愈粗，能使得回波相位結果愈佳。
根據上述結果，本研究建議於敲擊回音試驗時，檢測人員須選擇合適之鋼珠作為敲擊源。待將試驗之位移訊號進行傅立葉轉換後，應同時建立振幅頻譜與相位頻譜，並由振幅頻譜定出回波頻率，由相位頻譜找出對應之回波相位，再以π/4 作為相位值之分界線，以判斷反射面為裂縫或鋼筋。最後，將回波頻率帶入正確之敲擊回音公式，以得到正確的深度。

關鍵字

敲擊回音法 ； 非破壞檢測 ； 相位 ； 裂縫 ； 鋼筋混凝土

並列摘要

The impact-echo test can be adopted to detect the defects or inclusions in concrete structures. The conventional impact-echo analysis applies the Fourier transform to the surface response of the target structure due to an impact of a steel ball. Then, the magnitude spectrum is used to determine the frequency of the echo signals. Although the magnitude spectrum may disclose the existence of an interface in the structure, it cannot tell whether the interface comes from a crack or a rebar. Such information is crucial in the safety assessment of the structure. It is also necessary in the determination of interface depth because the impact-echo equations for crack and rebar differ.
This research examines the feasibility of the impact-echo phase analysis in the recognition of rebars and cracks. The idea is based on the phase change of the reflected wave. As a wave encounters a hard interface, it reflects with a phase change of π, but as it encounters a soft interface, there is no phase change. Hence, the phases of the echo signals from a crack and a rebar should disagree.
Firstly, Fourier transform was applied to the idealized signals. It is found that the phases of crack echoes are indeed close to 0, while the phases of rebar echoes are close toπ/2 . Then, numerical and model tests were conducted to verify the results. The influence of the impact source, depth of interface, and the size of inclusion on the impact-echo phase are also studied. It is found that the phase offsets of crack and rebar echoes mostly fall within two separated ranges, ( -π,π/4) and ( π/4 ,π), respectively.
By the parametric study, it is found that the impact-echo phase method works under the following conditions. Firstly, when the echo frequency equals to central frequency of the impact source, one would get satisfactory results. However, if the echo frequency exceeds twice central frequency of the impact source, the phase offsets would get worse results. Secondly, as the length-depth ratio of the crack exceeds 0.2 or the radius-depth ratio exceeds 0.4, one would get worse phase offsets.
Hence, it is suggested that the impact-echo test be conducted using proper impact ball. Construct both the magnitude and phase spectra in the Fourier analysis. Use the magnitude spectrum to determine the echo frequency and use the phase spectrum to determine the echo phase. Then, useπ/4 as the decision line to judge the type of inclusion. As such, one can get both the type and depth of the inclusion correctly.

並列關鍵字

Impact echo method ； Nondestructive test ； Phase ； Crack ； Reinforcing Bars

參考文獻

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