A Novel Microwave NDT Technique for Precise Defect Evaluation in Non-Metallic Materials

Abstract Microwave non-destructive testing (NDT) is particularly suitable for detecting defects in non-metallic materials due to its ability to penetrate and interact with internal structures. This paper proposes a microwave near-field NDT technique based on a sensor grid scanning method, enhanced by a standard open-ended rectangular waveguide(OERW) equipped with a gradient-index(GRIN) metasurface(MS) lens for beam focusing. The detection algorithm integrates frequency-domain and time-domain analyses of S-parameters to achieve comprehensive defect quantification. Frequency-domain analysis facilitates rapid and efficient measurement of defect lateral dimensions using only the amplitude of S11 at the center frequency, without requiring complex algorithms. To overcome the challenge of extracting longitudinal information, a novel feature extraction algorithm for time-domain S-parameters is developed, enabling precise estimation of defect burial depth. In addition to depth characterization, both simulation and experimental results demonstrate that the loaded lens significantly enhances detection sensitivity and lateral resolution, improving the distinguishability of small or closely spaced defects. In simulations, two groups of defects with 12 different burial depths demonstrated the effectiveness of the proposed feature extraction algorithm for estimating burial depth. To further validate the innovation in time-domain S-parameter analysis, experimental tests were performed using 13 specimens with varying burial depths (5 in Group-I and 8 in Group-II). Linear regression analysis yielded R² values of 96 for Group-I and 0.95 for Group-II, confirming the robustness and reliability of the proposed method in quantitatively detecting defect burial depth. Overall, both simulation and experimental results demonstrate that this technique offers high computational efficiency, eliminates the need for complex imaging algorithms, and represents a meaningful advancement in the detection and evaluation of defects in non-metallic materials.