Innovative Millimeter-Wave Imaging System for Detecting Metal Defects through Polyurethane Coatings

A continuing theme in shipboard monitoring is the need to inspect hulls of boats and submarines for signs of metal fatigue, typically manifesting as weld breaks, corrosion, and for submarines, metal-polymer debonding. Visual inspection remains the most reliable diagnostic, but is cumbersome if paint or plastic must be removed to reach the metal. Non-invasive diagnostics have problems too: eddy-current monitoring is reliable for weld fatigue, but not for corrosion onset or debonding, and polymers are opaque to ultrasound. Because missing a defect is unacceptable, existing non-visual methods are inherently slow. SARA proposes a millimeter-wave (MMW) imaging system, based on the science used for airport body scanners, to inspect welds without needing to remove the insulating layer. Because structural polymers typically have windows of transparency in the MMW domain, this technology provides rapid “see-thru” imaging of metal and associated defects in steel (such as those in butt/fillet welds, and corrosion onset). Stand-off scanning of a MMW imager will enable the Navy to localize weld joints and corrosion on quarter-wave scales (1mm or less), and will detect hairline cracks of even smaller scale. Debonding will be measured by phase rotation of multipath reflection, which only occurs when an air/water gap is present.