A differential temperature contour map to characterize the detection limit of pulsed thermography of materials

Abstract

Pulsed thermography has been widely used in the inspection of composite materials. The detection limit of pulsed thermography of the materials is often a question being concerned by end users. However, the detection limit cannot be described only by a simple parameter, and is affected by many factors associated with the defect, material, excitation, infrared camera and data processing algorithm. The current concept of detection limit defined with the minimum detectable size, the maximum detectable depth or the minimum aspect ratio depends on the specific experimental conditions, and it lacks universality when the extrinsic experimental conditions change. The aim of this paper is to establish a unified standard model of the detection limit which can be applied to various experimental conditions. The solution was to decouple the material’s intrinsic thermal property and extrinsic experimental condition dependencies and then allow them to be described independently. A new expression of the detection limit of the PT was firstly proposed by creating a differential temperature contour map. Taking delaminations in a C/SiC composite as detecting defects, an experiment was conducted to assess the feasibility and the detection limit, and to evaluate the noise level in practice. The influences of defect size and defect depth on the defect informative parameters were analyzed by using the finite element method based on 2D modeling. Eventually, the differential temperature contour map was calculated for the pulsed thermography of delaminations of C/SiC composites. The results show that the differential temperature contour map proposed can strictly describe the detection limit due to its differential temperature invariance property, and it provides a comprehensive tool to consider many experimental parameters including thermal excitation energy intensity, noise level, and SNR threshold by using a criterion equation in practice.