Influence of absorptivity of the material surface in crack detection using laser spot thermography

Laser thermography (LT) is one of the novel NDT methods for detecting surface-breaking cracks in metals. LT technique is well known for its non-contact nature and relatively fast-detection of surface defects for objects that are at room temperature. In LT, a continuous-wave (CW) laser is used to make a laser spot/line/area, which excites the sample under inspection to create local heating. Using optical mirrors, the spot is then rapidly made to move over the surface of the test sample. The temperature distribution over the sample due to the laser excitation is then monitored using an infrared (IR) thermal camera. This paper investigates the influence of the absorptivity of the material surface during defect detection using LT and its influence in detecting surface-breaking cracks. Six different materials were used under this study, i.e., Aluminum, Brass, Copper, mild Steel, Stainless Steel, and Titanium and with two surface conditions i.e., with and without black paint. 3D FEM numerical models were developed and validated with experiments for the different case studies for surface crack detection. The term Crack Thermal Contrast (CTC) is introduced to indicate the detectability of the surface crack and found to increase linearly with absorptivity of the material.