Excitation-invariant pre-processing of thermographic data

There is a large number of industries that make extensive use of composite materials in their respective sectors. This rise in composites’ use has necessitated the development of new non-destructive inspection techniques that focus on manufacturing quality assurance, as well as in-service damage testing. Active infrared thermography is now a popular nondestructive testing method for detecting defects in composite structures. Non-uniform emissivity, uneven heating of the test surface, and variation in thermal properties of the test material are some of the crucial factors in experimental thermography. These unwanted thermal effects are typically coped with the application of a number of well-established thermographic techniques including pulse phase thermography and thermographic signal reconstruction. This article addresses this problem of the induced uneven heating at the pre-processing phase prior to the application of the thermographic processing techniques. To accomplish this, a number of excitation invariant pre-processing techniques were developed and tested in this article addressing the unwanted effect of non-uniform excitation in the collected thermographic data. Various fitting approaches were validated in light of modeling the non-uniform heating effect, and new normalization approaches were proposed following a time-dependent framework. The proposed pre-processing techniques were validated on a testing composite sample with pre-determined defects. The results demonstrated the effectiveness of the proposed processing algorithms in terms of removing the unwanted heat distribution effect along with the signal-to-noise ratio of the produced infrared images.