Evaluation of Vertical Fatigue Cracks by Means of Flying Laser Thermography

Abstract

The present paper proposes a new procedure to analyze the temperature field distribution during Flying Laser Spot and Laser Line Thermographic scanning (FLST, FLLT) of metallic components, in order to detect vertical surface cracks. The methodology exploits the changes in the temperature field produced by a vertical crack, acting as a barrier towards heat diffusion, when the laser approaches the defect. A number of small regions of interests (ROIs) is placed nearby and around the laser source. The average temperature from each ROI is then monitored during the laser scanning. Vertical cracks can be detected by analyzing and comparing the temperature fluctuations from each ROI when the laser crosses a crack. The paper, in particular, illustrates how the use of multiple ROIs, placed at different locations, may provide additional information that can be used to characterize the defect, and to identify the crack tip location. The approach is validated on plates made of steel and aluminum alloy, where natural cracks have been introduced by fatigue loading, and whose surface has been painted to enhance emissivity. Scratches in the paint have been artificially made in order to analyze their influence on the defect signature. The proposed experimental setup is further simplified by moving the plate samples, mounted on slits, in front of a still laser source and camera head.