In situ near-field microwave characterization and quantitative evaluation of phase change inclusion in honeycomb composites

Inclusions are common threats for honeycomb composites. Conventional non-destructive testing (NDT) methods mainly focused on inclusions in a specific phase state, while little attention has been paid to phase change inclusions under different service conditions. Herein, we conducted in situ detections of a phase change paraffin inclusion in honeycomb sandwich panels at different temperatures with a near-field microwave method. The scanning amplitude and phase images of subwavelength core cells and inclusions were obtained with super-resolution. The paraffin inclusion (melting point, 58°C ) was characterized with a large amplitude decrease and phase delay in both the solid and liquid phases. Besides, the liquid paraffin inclusion at 100°C presented a whole shift of amplitude increase and phase decrease relative to the solid phase at 20°C. The boundaries of the phase change inclusion were accurately characterized using a characteristic peak method (CPM) with relative errors less than 0.5%. In summary, the near-field microwave method is feasible and effective for super-resolution detection and quantitative evaluation of phase change inclusions under varied service conditions.