Defect Detection in Bidirectional Glass Fabric Reinforced Thermoplastics Based on 3-D-THz Imaging

Nonhomogeneous, bidirectional glass fabric reinforced thermoplastic (GFRT) composites are used in the automobile industry to produce economic and environment friendly vehicles. Despite the fact that GFRT materials have superior mechanical characteristics and lower manufacturing cost compared with traditional materials, they are prone to significant deformation and defect emergence during the forming/manufacturing of parts from prefabricated laminates. Some defects, especially hidden ones like delamination and consolidation defects, can significantly reduce the mechanical properties of the final parts. It is essential to use 3-D nondestructive testing technology to detect and identify such hidden defects. This research investigates the ability of 3-D-THz imaging to detect hidden defects (i.e., delamination and consolidation) in GFRT composite materials. Frequency modulated continuous wave (FMCW) technique is used to generate 3-D-THz cross-sectional images. Nondefective and defective samples with a thickness of 1.5 mm have been measured with two 3-D THz imaging systems operating in the frequency range from 238 to 316 GHz and from 499 to 733 GHz. In addition, a simulation of the reflected FMCW spectrum is presented to analyze the measurements and intercompare the performance of the two imaging systems. A systematic analysis in a large sample set demonstrates that a delamination defect with a thickness of 125
$\mu$
m as well as a consolidation defect with a difference in thickness less than 0.4 mm can be successfully detected by the 700-GHz imaging system.