Retrieving A0-Lamb Wave Or Surface Wave Dispersion Curve Form The Reassigned Spectrogram - Case Study On Layered Concrete Plate

• Main Authors: TAO,HUNG-YU, 陶宏育
• Other Authors: Cheng,Chia-Chi
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/2q97um

碩士 === 朝陽科技大學 === 營建工程系 === 105 === 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.