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

Abstract

A crack located in the thermal diffusion zone of a heat source behaves like a thermal barrier modifying the heat diffusion. For a moving continuous source, the sample surface is heated on a little area near the crack for a duration which depends on the speed of the thermal source. A lock-in process synchronized by the displacement of the continuous heat source along the crack is studied. The thermal signature of the crack is extracted via a space operator applied to the amplitude and the phase of surface temperature images for various speeds of the thermal source. With the technical solution presented in this article, the thermal signature images are analysed according to a length representative of the thermal diffusion length to give a local evaluation of the crack depth (around 3 mm at the maximum) for crack lengths of about few centimetres long. The multi-speed lock-in thermography approach is initially studied with Finite Element Method (FEM) simulations. Experimental tests using an infra-red camera validate the method in a second part. The results do not depend on the heating source if its power is sufficient to produce a temperature rise detectable by an infra-red camera. The depth estimations are obtained independently of the crack width and heat source trajectory. The multispeed lock-in thermography is a method without contact, without sample preparation, non-polluting, non-destructive and with simple optical adjustments.

Remote measurement and shape reconstruction of surface-breaking fatigue cracks by laser-line thermography

“…The study objects of LIT contain the laser heating method [5][6][7] , thermal image processing [8][9][10] , and the defect characterizations [11][12][13] . Until now, several studies have been conducted on characterization of surface cracks by using the LIT method [14][15][16][17][18] . The main idea of these studies is to evaluate the crack-caused asymmetries of the surface temperature distribution based on image processing.…”

Sizing the Depth and Width of Narrow Cracks in Real Parts by Laser-Spot Lock-In Thermography

“…If the depth of the crack is finite, the temperature distribution in the material cannot be written analytically and the solution has to be sought numerically. Several research groups have applied finite element methods (FEM) to solve the heat diffusion equation to simulate the surface temperature of a sample containing a finite crack when it is illuminated by a focused laser beam [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. However, in these works, only cracks wider than 40 µm were studied because in classical continuous FEM, the cracked sample is divided into two domains: the bulk and the air filling the crack [ 33 ].…”

Measurement of in-plane thermal diffusivity of solids moving at constant velocity using laser spot infrared thermography

“…It is worth noting that these methods can be directly applied when the sample remains at rest and the laser spot scans the sample at constant velocity. This configuration, known as Flying Spot infrared thermography, was proposed by Kubiak [3] and has been developed by several research groups to detect cracks in a fast manner [4][5][6][7][8][9][10][11].…”