A theoretical investigation into material characterisation using pulsed thermography in the reflection and transmission modes

Abstract

Recent years have seen substantial growth of pulsed thermography as a popular non-destructive testing tool to characterise surface and near-surface defects for both metals and composites. The current research focus is on material characterisation to determine material thermal properties such as thermal diffusivity. However, most of these studies have focused on using the reflection mode of pulsed thermography where data is captured from the front wall of the specimen at which the heat flux is applied. The transmission mode, where the heat flux is applied on the front wall and data is acquired at the back wall, has not been investigated as comprehensively as the reflection mode. Research has indicated that the transmission mode is able to detect defects deeper into the specimen when compared to the reflection mode however, it has mainly been used as an indicator to detect defects and not to quantify the depth of subsurface defects within the specimen. This study develops a finite element model using the commercially available software COMSOL to investigate material characterisation using both the reflection and transmission modes of pulsed thermography. A finite element model of a thin steel plate was first created followed by the application of a heat flux to the front surface for the model. The solvers from the software were used to record the temperature variation on both the front and back surfaces. Results show that the thermal contrast curves on the backwall were less sensitive to changes in depth compared to changes in defect size. Furthermore, this study provides motivation for conducting a more in-depth study of the through transmission thermography to better understand its capabilities.