Lateral thermal flows for defect detection
Final Report Abstract
The methods of active thermography are widely used in the inspection of hidden defects with significant lateral sizes and in the detection of surfaces cracks. At the same time, these methods demonstrate a strong limitation in the detection of hidden defects with narrow sizes. The empirical rule for the single side inspection procedure (reflective scheme) says that the radius of the smallest detectable defect should be at least one to two times larger than its depth under the surface. The practice shows that the detection of defects with a diameter/depth ratio less than 0.8 is almost impossible. The main reasons are strong thermal diffusion and variations in surface emissivity. In this project a new thermal inspection approach has been proposed and studied. The approach is based on exciting thermal flows in lateral directions in the sample volume and analyzing the thermal response in surrounding areas. The procedure involves the inspection of a reference (defect-free) sample for acquiring a reference thermal response. This method allows to reveal the defects/cracks which are not detectable by the conventional inspection methods. The experimental investigations were conducted on steel samples with the artificial defects using laser excitation in 2D and 3D cases. The sample thickness was 6 mm, the residual wall thickness was 3 mm and the lateral defect size was 1 mm. The excitation power of the laser was 25 - 50 Watt. The inspection in the 2D case allowed to study the approach and develop a reconstruction method. The single inspection time was in the range 50 - 100 seconds. As the proposed technique is a time consuming procedure, for an acceptance in industrial applications, the total inspection time (the number of individual inspections) has to be minimized. So two procedures based on line and point excitations are suggested and investigated. The approach was experimentally studied in 3D case using a set with 4 steel samples: one reference sample and three samples with various defect arrangements. The defects were especially arranged to study the inspection at blind zones in two cases, with respect to excitation of the lateral thermal flows and with respect to propagation of these flows. The line inspection procedure demonstrated the best performance. All defects were successfully revealed. This is explained by the facts that the line excitation allows to better direct the lateral flows and use the applied energy mostly in one direction. The results of point inspections are noisy and more difficult to interpret. The lateral thermal flows are generated in all directions normal to the heated spot, so the applied energy is equally distributed over 360°. The line inspection procedure with inductive excitation is suggested for industrial applications.
Publications
- Detection of hidden defects by lateral thermal flows. NDT&E International, 56:48- 55, 2013
S. Lugin
(See online at https://doi.org/10.1016/j.ndteint.2013.02.003) - Detection of hidden defects by lateral thermal flows. NDT&E International, Volume 56, June 2013, Pages 48-55
S. Lugin
(See online at https://doi.org/10.1016/j.ndteint.2013.02.003) - Laterale Wärmeflüsse als Werkzeug für die Fehlerprüfung. Jahrestagung der Deutschen Gesellschaft für Zerstörungsfreie Prüfung (DGZfP), 2013
- Prüfung von verdeckten Fehlern mit lateralen Wärmeflüssen. Thermografie Kolloquium / Deutsche Gesellschaft für Zerstörungsfreie Prüfung (DGZfP), 2013
S. Lugin
