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-Hilbert-Transform-Based Accurate Determination of Ultrashort-Time Delays in Terahertz Time-Domain Spectroscopy

Tianying Chang ;  Qijia Guo ;  Lingyu Liu ;  Hong-Liang Cui

http://ieeexplore.ieee.org/document/7951024/

Accurate determination of time delays is crucial for distinguishing a sample's different interfaces in terahertz time-domain spectroscopy (THz-TDS), especially for ultrathin samples or interfacial gaps. In this paper, Hilbert transform (HT) is invoked, along with posttransform signal spectral estimation of several varieties, including the conventional, that based on multiple-signal classification spectrum estimation (HT-MUSIC), and that based on autoregressive spectrum estimation using Yule–Walker law (HT-AR), to determine the sample's different interfaces. The results are compared with the traditional method of obtaining time delays in THz-TDS, along with analysis of the resolution and accuracy, as well as advantages and deficiencies of the various approaches. It is demonstrated that HT is an effective method for determining different interfaces of an ultrathin sample by simulation and experiments. The simulation results show that MUSIC-HT has the best resolution, at about 0.35 ps and AR-HT has the best accuracy, whose error is less than 0.075 ps. Experimental results for ultrathin air gaps and polymer films confirmed that MUSIC-HT can distinguish time delays as short as 0.44 ps from the THz time-domain spectra.