Flying-spot lock-in thermography and its application to thickness measurement and crack detection

Abstract A heating laser beam was scanned periodically along a testing path on the surface of a test object. The thermal response of the beam was recorded by an infrared camera. Using a lock-in thermography algorithm, amplitude and phase images were generated. The phase image is corrected for effects due to the beam movement. A first application shows the contact-free determination of the steel sheet thickness at forming edges. Calibration of phase values to thickness was achieved by using an analytical model of thermal wave transmission. A second application is the detection of a perpendicular surface crack in steel