Abstract
Cryogenic fluid storage tanks of launch vehicles use polyurethane-based foam with multi-layered coating as an insulation system. Detection of metal-to-foam debonds in cryofoam with the multi-layered coating is challenging due to the high attenuation of ultrasonic waves in cryofoam, the porous nature of the foam, varying substrate and foam thickness, etc. In this study a novel approach using dynamic depth focusing technique with Terahertz waves has been employed. Cryofoam sample with artificial metal-to-foam debonds is scanned with a continuous wave terahertz imaging system in reflection mode with a frequency range of 0.1 to 0.4 THz. The sample is raster scanned point by point in X-Y directions using a stepper motor. The reflected terahertz signal is received with a photomixer and Schottky detector to obtain a complete C-scan image. The debonds are not detectable by continuous-wave terahertz imaging due to the surface unevenness since the focal distance of the operating system is not variable. This report proposes a three-step procedure to perform dynamic depth focusing on multi-layer coated cryofoam samples. The first step involves the quantification of surface roughness at distinct points using a digital depth gauge to acquire the depth profile of the sample initially with a step size of 2 mm. A depth profile has been generated by interpolating the intermediate depth values as a second step. Finally, the focus of imaging is dynamically varied using a motorized stage at each imaging point based on the initially generated depth profile. This approach gives a better resolution at all depths without using geometry description tools like computer-aided design or other software tools.
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Nidheesh Kumar B and Santhosh Kumar M C wrote the main manuscript text. Nidheesh Kumar B, K Kabilan and A. Mercy Latha are carried out the experimental studies. A Suresh Kumar and Nallaperumal M gave the guidance and the technical inputs regarding sample preparation and process. Santhosh Kumar M C supervised the research work and provided his constant guidance. And all authors reviewed the manuscript.
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Kumar, B.N., Kumar, M.C.S., Nallaperumal, M. et al. Nondestructive Evaluation of Cryofoam with Uneven Surface by Continuous Wave Terahertz Imaging Using Dynamic Depth Focusing Technique. J Nondestruct Eval 42, 103 (2023). https://doi.org/10.1007/s10921-023-01015-y
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DOI: https://doi.org/10.1007/s10921-023-01015-y