敲擊回音相位法適用性之研究

Abstract

敲擊回音法為最廣泛應用於檢測混凝土結構之非破壞性檢測技術。傳統之敲擊回音法是透過敲擊源於待測物表面進行敲擊以產生應力波波源，並在敲擊源附近以位移感測器接收應力波造成之位移，再將位移訊號經傅立葉轉換得到頻率域訊號，即可由此判讀試體內缺陷大小與位置。然而，傳統之敲擊回音法雖能偵測到反射面可能位置，但卻無法辨識反射面之種類，以鋼筋與裂縫為例，鋼筋能有效加強混凝土結構，而裂縫卻會危害混凝土結構。因此，判別反射面之種類相當重要。 敲擊回音相位法則透過當應力波遇到不同聲阻抗之反射面時，回波相位會改變之特性，將鋼筋與裂縫之表面位移函數經由傅立葉轉換分析，導出鋼筋與裂縫之相位解，並經由參數分析得知當回波頻率( )與鋼珠頻率中心( )之比值小於1.6時，鋼筋與裂縫之回波相位可用 區分，當相位大於 時，可推測反射面為鋼筋，而當相位小於 時，可推測反射面為裂縫。 本文之主旨為探討敲擊回音相位法之適用範圍，首先探討反射面聲阻抗之影響，並分析敲擊源、反射面深度對相位之影響。而為了快速觀察各個參數對相位之影響，將三維簡化為一維問題，由Green’s function與D’Alembert solution推導一維之表面位移函數，並透過傅立葉轉換求得相位解，再由數值軟體模擬平行層狀之兩層試體，驗證一維相位解。由分析結果可知，一維之相位解與三維模擬結果相近，確認一維相位解之可行性。 而反射面聲阻抗之研究中，發現聲阻抗主要會影響回波頻率尖峰，當反射面聲阻抗( )與上層介質聲阻抗( )比值達到3倍以上時才能有效找到頻率尖峰並找到對應之相位值。接著於敲擊源對相位影響之分析過程中，發現不論 或是 ，其所得之相位十分接近，並無法以 區分。而我們探討其中與鋼筋裂縫之差異，一維波傳或平行層狀結構與鋼筋裂縫反射面大小有明顯的不同，推測反射面大小亦會影響相位值，因此於三維數值模擬中進一步改變反射面之大小，探討其對相位之影響。 綜合反射面大小、反射面深度與敲擊源對相位之影響，我們可得以下幾點: 1. 當 且 時，不論其寬度(W)與深度(D)之大小，所得之相位值皆會大於 。 2. 當 且 時，不論其 之大小，所得之相位值皆會小於 。 3. 當 且 時，不論其 之大小，所得之相位值皆會大於 。 4. 當 且 時，不論其 之大小，所得之相位值皆會小於 。 若考慮以敲擊回音相位法判別鋼筋與裂縫之情形，在鋼筋之直徑與深度比不超過1.6之情況下，檢測人員可調整敲擊源使回波頻率與敲擊源之頻率中心之比值大於0.73且不超過1.6，即可以 作為判別標準區分鋼筋與裂縫。

The impact-echo test can be used to detect the inclusions or defects 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 traditional method can detect the possible location of the reflected surface, it is unable to distinguish the type. Take rebars and cracks as an example, concrete can be enhanced by rebars, but it can be damaged by cracks. Therefore, it is essential to determine the type of reflective surface. The impact-echo phase method is based on the change phase when stress wave is encountering the reflective surface with different acoustic impedance, so we can use the characteristic to derive the phase of rebar and crack by Fourier transform. It is found that if the ratio of echo frequency ( ) and the center frequency of the steel ball ( ) is less than 1.6, then phases of rebar echoes and the phases of crack echoes can be divided by . When phase is greater than , the reflective surface can be speculated as rebar, and when the phase is less than , the reflective surface can be speculated as crack. The objective of this study is to explore the applicability of the impact-echo phase method. Firstly, we investigated the influences of acoustic impedance, impact source and the depth of the reflective surface on the phase. To obtain quick estimate on the influence of each parameter, the three-dimensional wave propagation was simplified into a normal incident plane wave. The surface displacement due to an impact was obtained by the Green function solution and the D’Alembert solution. Then, the Fourier transform was applied to the displacement response to determine the phase at the echo frequency. The results showed that the phase obtained by one-dimensional approximation was close to the three-dimensional simulations. That confirmed the feasibility of using the one-dimensional approximation. While studying of the influence of acoustic impedance, it was found that the acoustic impedance could affect the peak of echo frequency. In a layered medium, only when there is a 3 times difference between the acoustic impedance of the upper layer ( ) and the bottom layer ( ) can the echo frequency be identified in the Fourier spectrum effectively. Moreover, the phase at the echo frequency in both and cases were close and could not divide by . This is different from the rebar/crack case, in which can serve as a divider. The major difference between a medium with a rebar or crack inclusion and a layered medium lies in the size of the reflector. Therefore, the influence of the size of the reflecting surface was further studied using three-dimensional numerical simulations. Based on the numerous numerical examples, we drew the following conclusions: 1. When and , regardless of the width (W) and depth (D) ratio, , the phase is greater than . 2. When and , regardless of the ratio, the phase is less than . 3. When and , regardless of the ratio, the phase is greater than . 4. When and , regardless of the ratio, the phase is less than . In the distinction of rebar and crack inclusions, if for the rebar, the investigator could adjust the impact source such that , then uses as a decision line to judge the type of inclusion.