Characterization of Defects in Polymeric Material using Step Heating Thermography: Estimation of Geometric Parameters and Uncertainty

Abstract

Polyethylene Terephthalate Glycol (PETG) is increasingly adopted in Fused Deposition Modeling (FDM) for manufacturing functional and mechanical components. However, current literature predominantly focuses on printing optimization, leaving a critical gap in postproduction non-destructive testing (NDT). This work investigates the quantitative capabilities of Step Heating (SH) active thermography for identifying subsurface anomalies in FDM printed PETG, using a sample containing artificial defects of varying depths and aspect ratios (AR). SH thermography, utilizing halogen lamps, was selected as a cost-effective alternative to traditional flash-pulsed techniques, providing the prolonged thermal stimulation inherently required to overcome the polymer’s low thermal diffusivity. The analytical estimations for defect depth (Z) and transverse size (X) were cross-validated through destructive micrographic analysis, establishing a physical Ground Truth in place of nominal CAD geometries. To metrologically assess the reliability of the measurements, a combined uncertainty budget was developed in accordance with the ISO GUM framework. The results demonstrate that the analytical model successfully retrieves the actual defect dimensions for AR ≥ 1.33. For narrow defects (AR < 1), 3D thermal blurring drops the signal below the noise floor. In these cases, the high combined uncertainty clearly marks the physical detection limit of the technique. Finally, the micrographic cross-validation confirmed that spatial deviations from the nominal size are primarily attributable to inherent FDM manufacturing phenomena, such as thermal shrinkage and filament sagging.