Microwave S-Parameter Analysis of Outer Structural Layer in Hydrogen Storage Plastic-Liner Cylinders^*

The outer structural layer of hydrogen storage plastic-liner cylinders demands materials with exceptional chemical and physical properties, for which Carbon Fiber Reinforced Polymer (CFRP) composite materials are typically selected. As an emerging non-destructive testing (NDT) technology, microwave NDT technology offers broad-spectrum detection capabilities, deep penetration, high sensitivity, and non-contact operation—advantages that demonstrate particular efficacy in CFRP inspection. To investigate the feasibility of applying microwave NDT to CFRP characterization, this study first employed the Wave Optics Module in COMSOL Multiphysics for electromagnetic modeling, simulation, and calculation of S-parameters and electric field modulus distributions; subsequently, experimental microwave NDT scans and permittivity measurements were conducted. Comparative analysis of simulated and experimental outcomes validated the utility of S-parameters for characterizing CFRP properties. Results reveal that within the X-band, S-parameters exhibit a distinct negative correlation with increasing frequency during CFRP assessment, with angular variation imposing significant effects on responses. Notably, in the low-frequency sub-band (8.2-9.2 GHz), this inverse frequency relationship intensifies while angular influences become markedly more pronounced; furthermore, permittivity measurements demonstrate a parallel negative correlation with rising frequency, substantiating dielectric variation as a primary contributor to observed discrepancies between simulation and experimental data.