Integrity assessment of metallic structures requires inspection tools capable of detecting and evaluating cracks reliably. To this end, many microwave and millimeter wave nondestructive testing and evaluation (NDT&E) methods have been developed and applied successfully in the past. Detection of fatigue cracks with widths less than 5 micrometers using non-contact microwave-based inspection methods was demonstrated in the 1970s. Since their introduction, these methods have evolved considerably towards enhancing the detection sensitivity and resolution. Undertaking key application challenges has attracted considerable attention in the past three decades and led to the development of the near-field techniques for crack detection. To address a need that cannot be fulfilled by other NDT&E modalities, innovative non-contact microwave and millimeter wave NDT&E methods were devised recently to detect cracks of arbitrary orientations under thick dielectric structures. While the reported methods share the same underlying physical principles, they vary considerably in terms of the devised probes/sensors and the application procedure. Consequently, their sensitivity and resolution as well as their limitations vary. This paper reviews the various crack detection methods developed to-date and compares them in terms of common performance metrics. This comprehensive review is augmented with experimental comparisons and benchmarking aimed to benefit NDT&E practitioners and researchers alike.

Terahertz (THz) metasurfaces based on high Q-factor electromagnetically induced transparency-like (EIT-like) resonances are promising for biological sensing. Despite this potential, they have not often been investigated for practical differentiation between cancerous and healthy cells. The present methodology relies mainly on refractive index sensing, while factors of transmission magnitude and Q-factor offer significant information about the tumors. To address this limitation and improve sensitivity, we fabricated a THz EIT-like metasurface based on asymmetric resonators on an ultra-thin and flexible dielectric substrate. Bright-dark modes coupling at 1.96 THz was experimentally verified, and numerical results and theoretical analysis were presented. An enhanced theoretical sensitivity of 550 GHz/RIU was achieved for a sample with a thickness of $13\mu m$ due to the ultra-thin substrate and novel design. A two-layer skin model was generated whereby keratinocyte cell lines were cultured on a base of collagen. When NEB1-shPTCH (basal cell carcinoma (BCC)) were switched out for NEB1-shCON cell lines (healthy) and when BCC's density was raised from $1\times 10^{5}$ to $2.5\times 10^{5}$ , a frequency shift of 40 and 20 GHz were observed, respectively. A combined sensing analysis characterizes different cell lines. The findings may open new opportunities for early cancer detection with a fast, less-complicated, and inexpensive method.

Various near-field microwave imaging probes were developed previously for nondestructive testing of material structures. Sensitivity and resolution are key parameters for quantifying the efficacy of a given imaging probe. For polarized targets like cracks, the sensitivity of conventional aperture probes depends on the relative orientation between the probe’s electric-field vector and the target. In practice, target orientation is typically unknown. Furthermore, the imaging resolution of conventional aperture probes is limited to aperture dimensions. Obtaining imaging resolution in the order of few millimeters using conventional aperture probes requires operation at high frequencies, (above 30 GHz). In contrast, planar probes consisting of loops loaded with spiral resonators could provide resolution in the order of few millimeters while working at relatively low frequencies (below 1 GHz). Unlike conventional aperture probes, the radiated near-fields of these probes are characterized by two significant components oriented along orthogonal axes. Therefore, the probe intrinsically provides a polarization-independent response. In this paper, the near-field imaging resolution of the planar resonator probe is further enhanced, and its polarization-independent response is demonstrated for the first time. The response of three planar probe prototypes of different dimensions is analyzed based on numerical simulation and experimental results. The performance of the probes is extensively evaluated by acquiring line-scans and 2D-images of metal and dielectric samples. The near-field images of the metal sample obtained using the planar probes working below 1 GHz are compared to the images obtained using aperture probes working at 24 GHz, 33 GHz, and 70 GHz. Furthermore, the probe imaging performance for a polarized target (i.e., narrow cuts) is compared to dual-polarized circular probe working at 24 GHz and open-ended coaxial probe working at 10 GHz. Overall, the proposed probe yielded more than twofold increase in the image signal-to-noise ratio (SNR) compared to all the probes utilized herein.