Non-destructive evaluation of GFRP-wood sandwich structure composite using terahertz imaging

Engineering of novel structures with high strength to weight ratio for applications in aerospace, renewable energy and naval industries has resulted in an increased popularity of sandwich structured composites. A sandwich-structured composite is fabricated by bonding a thick lightweight core between two stiff, thin skins such as Glass Fiber Reinforced Plastic (GFRP). Balsawood is a type of homogeneous core which is widely used for renewable energy structures, such as wind turbine blades. In this paper, a GFRP-balsawood sandwich structure is evaluated non-destructively for internal defects such as holes, using a CW Terahertz system in transmission mode. Internal defects will give rise to differential THz transmission and hence can be identified using THz imaging. The imaging studies are carried out with a central frequency of 0.35 THz and the sample is raster scanned using 2-D translational stages controlled by high precision stepper motors in x-y directions to obtain the THz image. The image acquired using CW THz system clearly identifies the defects in the GFRP-balsawood composite structure with good contrast demonstrating the potential of THz imaging for non-destructive testing of sandwich composite structures.