Adaptive segmentation-based evaluation of material properties of dielectric sheets using microwave NDE

Composite materials/structures are rapidly advancing, offering improved product efficiency, cost-effectiveness, and superior specific properties. These materials find increasing applications in corrosion resistance and load-carrying structures across industries such as aerospace, wind turbines, transportation, and medical equipment. Therefore, ensuring reliable non-destructive evaluation of composites is crucial to address safety concerns and minimise maintenance expenses. The microwave non-destructive evaluation (NDE) method is particularly well-suited for characterising the material properties of composites. In our study, we utilise a large flange open-ended rectangular waveguide (OERW) probe, measuring 152 mm x 152 mm, to assess the complex reflection coefficient of dielectric materials. This measurement enables the extraction of essential material parameters using a forward electromagnetic model combined with an optimised method. To enhance accuracy, we introduce an iterative conjugate gradient descent (CGD) optimiser with adaptive segmentation. This approach effectively avoids local minima and reduces disparities in integrand values, resulting in precise measurements of material properties. Our proposed method provides estimations of the dielectric constant within a range of 1 to 10 and accurately determines sample thickness with a controlled limit of 3% error across all samples. Furthermore, we observe that as the loss-factor value decreases, the error in calculating the loss-factor of samples increases, which is supported by numerical analysis.