Analysing the Probability of Detection of Shallow Spherical Defects by Means of Pulsed Thermography

Abstract

The capability of Active Thermography (AT) techniques in detecting shallow defects has been proved by many works in the last years, both on metals and composites. However, there are few works in which these techniques have been used adopting simulated defects more representative of the real ones. The aim of this work is to investigate the capability of Pulsed Thermography of detecting shallow spherical defects in metal specimens produced with laser powder bed fusion (L-PBF) process and characterized by a thermal behaviour very far from the flat bottom hole and so near to the real one. In particular, the quantitative characterization of defects has been carried out to obtain the Probability of Detection (PoD) curves. In fact, it is very common in non-destructive controls to define the limits of defect detectability by referring to PoD curves based on the analysis of flat bottom holes with a more generous estimation and therefore not true to real defect conditions. For this purpose, a series of specimens, made by means of Laser-Powder Bed Fusion technology (L-PBF) in AISI 316L, were inspected using Pulsed Thermography (PT), adopting two flash lamps and a cooled infrared sensor. To improve the quality of the raw thermal data, different post-processing algorithms were adopted. The results provide indications about the advantages and limitations of Active Thermography (AT) for the non-destructive offline controls of the structural integrity of metallic components.