Terahertz cross-correlation spectroscopy for non-destructive testing of thick glass fiber epoxy composites

Non-destructive testing (NDT) is a crucial tool in the manufacturing and quality control of wind turbine blades. The current state-of-the-art method, ultrasonic inspection (UT), suffers from limitations such as loss of depth resolution when scanning in-depth and shadowing effects, making it unable to detect and characterize multiple layered cracks or delaminations. To address these issues, we employed a novel cross-correlation spectroscopy (THz-CCS) terahertz device developed by GLAZE Technologies. 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. We used the THz-CCS system 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. Initial experiments indicate that THz-CCS provides results comparable to or exceeding those of UT. Given the relative maturity of UT compared to THz-CCS, there is enormous potential for further improvement in THz-based inspection technology.