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.

Fast sizing of the width of infinite vertical cracks using constant velocity Flying-Spot thermography

“…This technique allows detecting cracks a few micrometers wide [17]. Recently, several approaches to characterize the geometrical parameters of the crack (depth, length, width, orientation…) have been introduced by several research groups [21][22][23][24][25][26][27]. They take advantage of the asymmetry of the temperature field at both sides of the crack arising from the thermal resistance produced by the crack, which partially blocks heat flux when the laser spot approaches the crack.…”

Open crack depth sizing by multi-speed continuous laser stimulated lock-in thermography

“…The active infrared thermography is a non-contact and non-destructive promising technique for perpendicular crack detection. It uses a local heating solicitation which can be continuous [1][2][3], pulsed [4][5][6][7][8] or modulated [9][10]. The disturbances of the heat diffusion produced by a crack which acts as a thermal barrier are then analyzed.…”