Enhanced microwave waveguide probe-based methods for damage detection of GFRP composites

The aim of this work is to enhance the microwave waveguide-based non-destructive detection of glass fibre-reinforced polymer (GFRP) composites by introducing new strategies for probe design and signal processing. The tapering geometry in the waveguide probe design improves the spatial resolution and sensitivity, and the additive manufacturing technique employed reduces the overall cost. In addition, a new approach is proposed for the optimal selection of the inspection frequency in the analysis of the raw frequency-domain data, which achieves high signal contrast. The spatial Fourier transform is introduced to eliminate the undesirable stand-off distance effect in the conventional waveguide-based inspection. Test samples with subsurface grooves and impact-induced damage were examined. It was found that a 1 mm wide groove at a depth of 9 mm and a 10 J barely visible impact damage were well detected and characterised. The results demonstrate the significant potential of microwave testing for the evaluation of composite structures.