Nondestructive Evaluation of Cryofoam with Uneven Surface by Continuous Wave Terahertz Imaging Using Dynamic Depth Focusing Technique

• Published in: Journal of nondestructive evaluation 2023-12, Vol.42 (4), Article 103
• Main Authors: Kumar, B Nidheesh, Kumar, M C Santhosh, Nallaperumal, M, Latha, A Mercy, Moideenkutty, K K, Remakanthan, S, Kumar, A Suresh, Kumar, L Mohan, Anandapadmanabhan, E N, Kabilan, K
• Format: Article
• Language: English
• Subjects: CAD; Characterization and Evaluation of Materials; Classical Mechanics; Computer aided design; Continuous radiation; Control; Cryogenic fluid storage; Cryogenic fluids; Debonding; Depth profiling; Dynamical Systems; Engineering; Frequency ranges; Imaging; Multilayers; Nondestructive testing; Polyurethane foam; Raster scanning; Software; Solid Mechanics; Stepping motors; Storage tanks; Substrates; Surface roughness; Terahertz frequencies; Ultrasonic attenuation; Unevenness; Vibration; Wave attenuation
• ISSN: 0195-9298; 1573-4862
• DOI: 10.1007/s10921-023-01015-y

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.