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Study on Moire phenomenon in ultrasonic C-scan ImageGuo Guangping
Beijing Institute of Aeronautical Materials, P. R. China.
A strange phenomenon was found when we inspected a C sandwich (3 layers of skin and 2 layers of honeycomb) composite structure with curved surface using through transmission ultrasonic method. Dark fringes - high attenuation - appeared in C-scan image which looks like optical moiré phenomenon. At the segments corresponding to moiré in C-scan image, two layers of honeycomb were placed in parallel direction. The honeycomb cell's walls of the two layers superpose with each other in different way at different location. It could cause different attenuation in ultrasonic.
First, computer simulation was used to simulate the superposition of two layers of honeycomb. Optical moiré phenomenon caused by superposition of honeycomb cell's wall can be found if two layers place in parallel direction. Then, a specimen of C sandwich structure was made, whose two lays of honeycomb were placed in the parallel direction. Moiré phenomenon was found again in its TTU C-scan image. X-ray radiographic inspection of the specimen show good agreement between ultrasonic attenuation and superposition of honeycomb cell's wall. In conclusion, different superposition of the honeycomb cells' wall is the reason of moiré phenomenon in C-scan image.
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