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

Abstract

Non-homogeneous, bi-directional glass fabric reinforced thermoplastic (GFRT) composites are used in the automobile industry to produce economic, environmentally friendly and recyclable 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 3D non-destructive testing (NDT) technology to detect and identify such hidden defects. This research investigates the ability of 3D 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 3D-THz cross-sectional images. Non-defective and defective samples with a thickness of 1.5 mm have been measured with two 3D 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 µm 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.

Defect detection in glass fabric reinforced thermoplastics by laboratory-based X-ray scattering

“…Therefore, various non-destructive testing (NDT) techniques are investigated for the understanding of defect formation and evolution as well as for the application in production monitoring of GFRT components [7]. Examples for NDT techniques applied to composite materials include ultrasonic imaging [8,9], THz imaging [10,11,12], X-ray computed tomography [11] or infrared thermography [13].…”

“…Defects in composite materials include delamination (i.e., separation of fiber layers), consolidation (i.e., variations of the polymer matrix thickness), local fiber shifts (i.e., local agglomeration of fibers in one layer leaving behind a hole in the fabric of this layer), global fiber shift (i.e., lateral offset of one fabric layer with respect to the others) or cracks (i.e., broken fibers in one or more fabric layer). For GFRT it has been demonstrated that delamination and consolidation can be detected by THz imaging [12]. In this study, we are aiming to provide a proof of concept for the detection of local fiber shifts in GFRT by means of X-ray scattering with EI.…”