Developing the through-transmission technique in pulsed thermography for material characterisation

Abstract

Pulsed Thermography (PT) is a reliable, non-contact, and non-intrusive non- destructive testing (NDT) technique for assessing the structural health of materials. Based on the relative positioning of the thermal excitation source and the infrared radiometer, measurements can be conducted in either reflection or transmission mode. While reflection mode is widely adopted due to its single- sided accessibility, transmission mode offers superior lateral resolution but remains limited in use due to the lack of reliable depth quantification methods. In the context of thermal diffusivity evaluation, the transmission mode has demonstrated greater reliability; however, the existing literature lacks a deterministic approach to systematically assess this in laboratory settings. This research investigates the current state-of-the-art in through-transmission thermography and identifies key knowledge gaps. A transparent and repeatable methodology is developed to evaluate thermal diffusivity using both finite element models (FEM) and controlled laboratory experiments. The FEM is also used to assess the temporal behaviour of a sample containing subsurface defects, and a physical sample is fabricated to validate the simulation results. A novel method for defect depth quantification is then proposed by establishing a relationship with the Fourier number. This approach demonstrated a 63% improvement in depth estimation accuracy (from a 29.3% measurement error to 10.75%) compared to the Log Second Derivative (LSD) method derived from thermographic signal reconstruction (TSR) in the simulation environment across all defect sizes and depths. Additionally, the technique shows potential for estimating impact damage in carbon fibre-reinforced polymer (CFRP) samples subjected to varying impact energy levels. By addressing the challenges of thermal property measurement and depth quantification within the transmission mode, this thesis provides a foundation for improved material characterisation and supports renewed research interest in through-transmission pulsed thermography.