Millimeter Wave Detection of Surface-Breaking Cracks in Steel and Aluminum under Severe Corrosion and Paint

Many components of critical and heavily utilized infrastructure, such as ships, planes, bridges, and so on, are operating at or beyond their designed lifetime. Replacement is no longer an option, and "retirement for cause" is the current approach to maintenance and replacement. Consequently, there is an ever-increasing demand for efficient and robust nondestructive testing (NDT) methods and techniques that can determine the physical health of these structures. Large structures, which are primarily made of metals, either steel or aluminum, are susceptible to in-service corrosion and stress-induced cracking. Stress-induced cracks, particularly in heavily corroded steel members used in bridges, railroads, storage tanks, and similar structures, are difficult to detect using many of the standard NDT methods, increasing the risk of not being able to detect an existing crack. Microwave signals readily penetrate through dielectric materials such as paint and corrosion byproducts (such as rust) and can interact with the underlying metal, rendering cracks detectable. In this paper, the implementation of a microwave imaging system that utilizes the synthetic aperture radar (SAR) approach to detect surface-breaking cracks (in the form of manufactured narrow notches) in steel and aluminum samples under heavy corrosion and corrosion-preventive paints is investigated. In addition, the influence of signal polarization direction relative to crack direction and the cyclical severity of corrosion inducement are investigated. The resulting SAR images are analyzed and compared to numerical simulations to identify the real-world capabilities and theoretical limitations of the technique.