Terahertz Nondestructive Imaging for Foreign Object Detection in Glass Fibre-Reinforced Polymer Composite Panels

Terahertz time-domain spectroscopy (THz-TDS) technology, a new powerful nondestructive testing (NDT) method, can provide noninvasive, noncontact, nonionizing radiation to penetrate common nonmetallic materials. In this study, we have employed THz-TDS technology to detect hidden inclusions with various shapes and materials in glass fiber reinforced polymer (GFRP) composite solid panels. Traditional transmission and reflection THz-TDS imaging are performed for detecting inclusions in GFRP solid panels. Next, the spectral difference between defect area and non-defect area has been analyzed, and different parameters for imaging are selected according to defect characteristics, which greatly improves defect detection. The THz-TDS imaging results are compared with those employing X-ray computed tomography (CT) and ultrasonic C-scan, particularly in terms of spatial resolution and overall image quality. It is found that THz-TDS technology can successfully detect hidden inclusions with relatively high image quality, and has an acceptable precision for measuring inclusion size. As such, THz-TDS technology has certain advantages in evaluating the reliability of GFRP composites.