An array of impact-echo array technique for the two dimensional imaging of defects in thick structures such as concrete. A Finite Difference Time Difference Model was employed for the forward model of the impact-echo method. The use of absorbing boundary conditions allow for efficient modeling of the technique. The signals predicted using the FDTD models from a linear array of impact-echo transmitter-receiver system in a multiplexed configuration was used for 2-D image reconstruction of the cross-sectional region of the structure. The reconstruction algorithm uses time-shifting of the signals based point-source assumption for the impact sources. The images are represented in a typical “B-scan” representation. The data reconstruction algorithm uses phased addition in the time domain to reconstruct the position of reflectors present in the structure such as the back wall, defects, voids. The pixel based approach is used to perform phased addition reconstruction in the domain of interest. The scanning area is sampled in Cartesian co-ordinates and the phased addition is done point by point in this domain. Wherever there are defects or other features like the back wall etc., constructive addition takes place and the signal is reinforced. However when there is no particular feature in the point considered, then a very weak resultant is produced as a result of both constructive and destructive interference taking place. Thus, the signal at the defect location is reinforced and the relative noise level is reduced. The phased reconstruction algorithm can be both applied to the single transmitter case as well as the multiple transmitter case.
The B-scan images were successfully reconstructed from the surface displacements along the points on the surface using a phased addition reconstruction algorithm. The algorithm was tested for different pulse-width of the impact and it was found, that smaller the pulse width, greater is the resolution of the reconstructed image. The algorithm was also tested for different types of defects, laminations, and crack. A parameter study, involving (a) different spacing of the transmitters and receivers, and (b) different number of transducers, was also conducted. From the results so obtained, an optimal combination of different parameters in the experimental study was taken, to obtain effective results from the real specimen.
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Phase Addition Algorithm Domain with Transmitters and Receivers.
Phase Addition Reconstructed Results on a concrete block with a defect.
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The prototype impact array
prototype with a typical signal.
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