Abstract-Nondestructive imaging of hidden defects in aircraft sandwich composites using terahertz time-domain spectroscopy

Qiang Wang, Xinyi Li, Tianying Chang, Jin Zhang, Lingyu Liu, Hongbin Zhou, Jinpeng Bai,



https://www.sciencedirect.com/science/article/pii/S1350449518306091

Aircraft sandwich composites are superior to ordinary composites and have been largely used in the manufacture of military aircraft. This study attempts to investigate the efficacy of terahertz (THz) time-domain spectroscopy (TDS) imaging technology in detecting hidden defects in aircraft glass fiber (GF) sandwich composites. The defects of various sizes, such as debonding, delamination, and multi-delamination, at different depths in GF A-sandwich structure composites with foam core, C-sandwich structure composites with honeycomb core, and sheet-to-sheet cementing structure composites were evaluated. The THz frequency-domain amplitude and time-domain amplitude imaging algorithms were used to visualize the defects simulated by ultrathin double-layer Teflon inserts. And the suitable image processing methods which include wavelet-based fusion and multiscale edge representation were employed. With a combination of high-resolution THz C-scan and B-scan imaging, both the horizontal size and location, and the vertical depth and thickness of the defects were obtained in three dimensions. This study experimentally demonstrated that THz imaging technology can clearly detect various hidden defects in aircraft GF sandwich composites through reflection or transmission imaging mode.

Abstract-Terahertz spectroscopic study of aeronautical composite matrix resins with different dielectric properties

Qiang Wang, Xinyi Li,  Tianying Chang,, Qiuping Hu, Xiaohui Yang,

https://www.sciencedirect.com/science/article/pii/S0030402618305047

This study was to investigate the spectral characteristics of aeronautical composite matrix resins (ACMRs) that differ in dielectric properties in terahertz frequency range. Terahertz time-domain spectroscopy was employed to examine the spectral absorption and dielectric dispersion characteristics of five ACMRs of three major types, namely, epoxy resins 3238A and BA9916, cyanate ester (CE) resin 9915, and bismaleimide (BMI) resins QY8911 and QY9611, between 0.2 THz to 1.6 THz. The refractive index n$$, the absorption coefficient α$$ and the real part ε^′${}^{}$ and imaginary part ε^″${}^{}$ of the dielectric constant of each resin were calculated. On this basis, the relaxation process of dipoles was theoretically analyzed using the Debye model. The results showed that, ε^″${}^{}$ of each epoxy resin was higher than that of each BMI resin, while ε^″${}^{}$ of the CE resin was the lowest. The CE resins had the lowest ε^′${}^{}$ and exhibited the highest and most stable dielectric performance, followed by the BMI resins, whereas the epoxy resins exhibited fluctuating and slightly less stable dielectric performance. This study gives, for the first time, the basic parameters of five ACMRs (epoxy, CE and BMI resins) in terahertz frequency range, and provides an important reference for THz nondestructive testing of aeronautical composites.