Microwave open-ended waveguide for detection and characterisation of FBHs in coated GFRP pipes

Accessing to glass fibre reinforcement plastic (GFRP) pipes in the on-line process is often limited to one side. To detect an internal defect of coated GFRP pipe, common non-destructive testing (NDT) methods such as eddy current testing and thermography are not effective because GFRP pipes are not conducive and their coating prevents heat transfer to the inner section of the material. Moreover, the curvature of GFRP pipes introduces liftoff variation, which is the primary challenge for NDT methods using C-scan technique; e.g., ultrasonic. This work proposes a novel microwave NDT application to detect and characterise internal defect; i.e., flat-bottom hole (FBH) in coated GFRP pipes. Initially, responses of four FBH areas obtained by microwave open-ended waveguide probe are analysed using principal component analysis (PCA) to reveal FBH defect and eliminate the pipe curvature influence. Following that, synthetic aperture radar (SAR) tomography images are produced to select the corresponding principal component and verify the location of FBHs. Finally, FBHs are evaluated using PC feature and SAR tomography including time-of-flight feature. The results show that the proposed method can clearly reveal the area of FBHs. Moreover, depths of FBHs are characterised by analysing the time-of-flight feature of background-subtracted responses.