Sweep frequency microwave NDT for subsurface defect detection in GFRP

The use of glass fibre-reinforced plastic (GFRP) has been rapidly growing because of the stiffness, strength and low cost of the material. Due to the presence of subsurface defects, such as debonding and delaminations, the service lifetime of composite structures is shorter than expected. Moreover, such defects can lead to catastrophic results, which means that subsurface defect detection and assessment are required. This paper proposes an open-ended waveguide-based sweep frequency X-band microwave non-destructive testing (NDT) method to detect subsurface defects in GFRP components. Due to changes in the magnitude of the reflection coefficient, a raster-like relative motion of the waveguide probe with respect to the sample can be used to visualise subsurface damage. Simulation results indicate that the magnitude of the reflection coefficient is capable of subsurface defect detection. Experimental studies have been performed in order to estimate the depth of subsurface defects where principal component analysis (PCA) has been applied to eliminate the influence of noise and enhance detection results. Information concerning the location, size and depth of the subsurface defects has been provided.