Precise depth detection of EdM notches lock-in thermography

Abstract

Lock in thermography is an advanced NDE technique that has been successfully applied for a wide variety of inspection applications such as detection of lical heat sources in electronic components, shunt detection in solar cells, NDE of aircraft structures and coating characterization. The main advantage of the lock-in thermography is that it is invariant to emissivity variations, inhomogenities of optical surface absorption and distribution of optical illumination. In this paper, we explore the possibility of using lick-in thermography to quantitatively estimate the depth of defect from surface, in stainless stell AISI type 316. This steel is widely used in the nuclear and other process industries as a structural material due to its excellent mechanical properties. No detailed investigation has been done on defect detectability in this material. Specimens with artificial defects of varying depths were subjected to lick in using a state of art system at the authors lab. Experimental investigations reveal that defects located at a depth of about 30 % from the surface van be clearly resolved in AISI type 316 stainless steel. It is also observed that the defect depth can be estimated with an overall error of less than 10 %. Lock in thermography can thus be an ideal choice to complement conventional techniques such as ultrosonics and radiography especially for the detection of defects located close to the surface.