Measuring corrosion thinning by thermal-wave imaging

We describe an infrared (IR) thermal wave imaging technique for making corrosion thinning determinations on aging aircraft skins. The technique uses pulsed surface heating (with flashlamps) and fast, synchronous IR imaging of the surface temperature to form rapid (two or three seconds), wide-area (more than a square foot) images of subsurface structure, such as skin corrosion and disbonded doublers or tear straps. Sensitivity to corrosion thinning of less than two percent is demonstrated. Practical implementation of a simplified numerical measurement algorithm is presented, and the results are compared with profilometry and ultrasonic measurements of calibration standards. Examples are presented of thermal wave imaging of fuselage skin corrosion on a B737 testbed aircraft in a hangar environment at the FAA's Aging Aircraft NDI Validation Center (AANC).