A new method for surface crack detection by laser thermography based on Thermal Barrier effect

The aim of this work is to detect open surface cracks on metallic welds using laser thermography and posterior processing algorithms. In the last years the trend on welding processes has been to extend their use to applications with higher stress conditions. As a consequence, obtained welded regions are more prone to cracks. The types of cracks originated from welding processes are, in general, superficial and their detection is not straightforward since it detection is affected by their width. Traditional crack detection Non Destructive Techniques, such as Dye Penetrant or Magnetic Particles, are time consuming and require an experienced technician to correctly interpret the results. Furthermore, they foul the surface which needs to be cleaned after the test. Laser Thermography [5] [10] has several advantages over these traditional techniques: it is fast, non-contact and clean. In this paper, dynamic tests have been performed by scanning the weld maintaining the laser and camera motionless, and moving the piece linearly across them, so that the weld is scanned. The use of a scanning laser line to heat the surface allows the detection of the characteristic disruption that cracks generate in the natural diffusion of heat on the surface. Briefly, cracks block the heat front. This makes the temperature just before the crack to increase, while keeping the temperature of the region just after it at similar values for larger times than for other nearby regions. This thermal effect is known as "Thermal barrier" [11]. Here, a new method to detect the cracks based on this effect has been developed starting from two different approaches.