Non-destructive testing (NDT) is a crucial technique in the manufacturing and quality control of wind turbine rotor blades. The current state-of-the-art method, ultrasonic inspection (UT), suffers from limitations such as loss of depth resolution when scanning through the thickness/height and shadowing effects, making it unable to detect and characterize multiple-layered cracks or delaminations, damages in sandwich structures, or wrinkle defects in composite manufacturing. Terahertz (THz) radiation is non-ionizing, and can easily penetrate most nonconductive materials, such as glass fiber, resin, and balsa wood, making it an ideal candidate for wind turbine blade NDT techniques. However, current terahertz time domain spectroscopy (THz-TDS) systems are costly and limited to coupon specimens. To address these issues, this study investigates terahertz cross-correlation spectroscopy (THz-CCS) as a novel non-destructive testing method for composite materials, aiming to enhance image quality and overcome the penetration limitations of conventional ultrasonic testing. A GLAZE Technologies Carmen THz-CCS system was used with two distinct sensor head configurations to detect and characterize common composite damage types and geometries, including
delamination cracks, wrinkles, and sandwich structures. The THz-CCS system was used to identify and characterize the delamination crack within a 16.9 mm thick unidirectional glass/epoxy double cantilever beam sample. The specimen was held open so that the opening displacements gradually decreased from 1.5 mm to a closed crack. Depending on the scanning parameters, the THz-CCS device identified the crack with a crack opening resolution of 0.07 - 0.28 mm. It was also used to examine wrinkle and sandwich specimens. The system successfully traced the surface displacement of a wrinkle specimen and estimated the critical angle of the wrinkle within 1 degree. THz-CCS scans were also able to estimate the refractive index of the core material and measure the thickness of the top and bottom skins of a sandwich specimen. Given the relative maturity of UT compared to THz-CCS, there is enormous potential for further improvement in THz-based inspection technology. The results demonstrate the feasibility of using THz-CCS to detect subsurface damage at significant depths, underscoring its potential and current limitations as an emerging quality control and detailed inspection technology for thick composite wind turbine blades.