- 學位論文
再分配時頻譜內擷取A0藍姆波及表面群速頻 散曲線 -以混凝土分層劣化深度問題為例
Retrieving A0-Lamb Wave Or Surface Wave Dispersion Curve Form The Reassigned Spectrogram - Case Study On Layered Concrete Plate
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摘要
研究中主要以混凝土板表面經敲擊後激發出板波的傳遞特性，檢測混凝土板結構中是否有缺陷問題。將針對混凝土板結構，以一個敲擊源及一個接收器的配置進行實驗，以短時傅立葉轉換以及再分配時頻譜求得頻散曲線。 利用有限元素模擬可感測敲擊源與接收器擷取接收間距的反應位移波形，得到數值解之波慢-頻率圖並擷取出A0反對稱模態，與Disperse程式得到之理論解進行比較分析。A0模態擷取研究以分層劣化混凝土為例，以數值模擬及實驗分層劣化混凝土來驗證此程式之可行性，並進行分析比較。 數值模擬顯示，敲接距離過短對於整體波速的影響甚大，但若把敲接距離拉長約至所要量測板厚的3倍以上即可將干擾降低。程式對於擷取出A0模態之波速轉折的波長位置約上層厚度的1.6倍，三層劣化混擬土對於中間層的波速難以觀測只能看出最低與最高波速。 實驗結果得知，邊界反應的影響甚大，使得其它模態在波慢譜中的訊號更加明顯以致A0模態擷取困難，上層3 cm與上層5 cm板厚的分層厚度與數值模擬結果相互比對，可發現上層厚度約1.6-2倍的波長可以清楚發現波速轉折的趨勢，藉此判別分層的厚度。
並列摘要
The purpose of this research is on detecting flaw inside concrete slab structure using the characteristics of Lamb waves generated by an impacting force applying on the surface. The experiments were conducted by one excitation source and one receiver configuration. The dispersion curves of the test results were obtained by short-time Fourier transform spectrum and amplitude reassigned method. Using the finite-element simulation, the response displacement waveform between the impact source and the receiver can be obtained. The slowness dispersion diagrams of the numerical solution is obtained and the antisymmetric mode of A0 is extracted. That is compared with the theoretical solution obtained by the program Disperse. The methodology of extracting A0 modal dispersion curve is demonstrated by the example of concrete plate with layered deterioration. FEM simulation and experiments of layering deteriorated concrete plate were conducted to verify the feasibility of using this program for analysis and comparison. Finite-element simulations show that short impactor-receiver distance has a great effect on the overall wave velocity, but the interference is reduced if the impactor-receiver distance is more than three times longer than the interested thickness. For A0 mode, the turning point of wave speed from lower speed to higher one is at the wavelength about 1.6 times the thickness of the upper deteriorated concrete layer. For three-layered case, the wave velocity of the middle layer is difficult to observe, and only the lowest layer wave speed and the one of bottom sound concrete can be seen. The experimental results show that the effect of boundary reflection is very strong, which makes the signal of the other modes in the slowness spectrum more obvious and makes the recognition of A0 mode difficult. The cases with the thicknesses of the upper layer 3cm and the upper layer 5cm are compared with the numerical simulation results. It was found that there is a trend of wave velocity transition for the wave length in the range about 1.6-2 times of the upper layer thickness. This trend can be utilized to discriminate the thickness of the weak top concrete layer.
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