Phase inversion in (vibro‐)thermal wave imaging of materials: Extracting the AC component and filtering nonlinearity.

Summary: In active infrared thermographic inspection of materials, heat wave is stimulated by activation of heat sources, e.g., through optical heat radiation or vibration‐induced heat dissipation. Therefore, the monopolar (i.e., heating only) nature of excitation introduces an inevitable ascending trend in the measured thermal response. To obtain an improved thermal wave imaging quality, it is crucial to remove this ascending trend and to analyze the decoupled bipolar (i.e., AC) component of the thermal response. This study introduces the concept of phase inversion in thermographic inspection, as a deterministic method for (i) decoupling the AC component and (ii) filtering the prominent second‐order nonlinearities from the thermal response. First, this "phase inversion thermography (PIT)" is theoretically substantiated by analysis of heat diffusion through the thickness of a solid material subjected to dissipative boundary conditions. Then, the performance of PIT in accurately decoupling the AC response from various excitation waveforms is verified by finite element simulation of optical infrared thermography on an anisotropic composite coupon. It is shown that by proper selection of the signal's initial phase and waveform duration, PIT yields the AC response with a zero‐mean amplitude. At last, the experimental applicability of PIT is evaluated for two different test cases: (1) optical thermography on a backside‐stiffened carbon fiber‐reinforced polymer (CFRP) aircraft panel with a complex cluster of production defects and (2) low‐power vibrothermography on an impacted CFRP coupon. It is shown that PIT, as a physics‐based signal processing technique, robustly resolves the strongly transient onset of the excitation and systematically decouples an AC response which is equivalent to the thermal response to an ideally linear and bipolar excitation. The recorded thermal responses are post‐processed through Fourier transform, and the enhanced thermal imaging quality and improved defect detectability of the decoupled AC component are demonstrated. [ABSTRACT FROM AUTHOR]