Detection of Surface Cracks in Metals Using Microwave and Millimeter-Wave Nondestructive Testing Techniques—A Review

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 $5~\mu \text{m}$ using noncontact microwave-based inspection methods was demonstrated in the 1970s. Since their introduction, these methods have evolved considerably toward 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 noncontact 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 article 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.