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.

中文翻译：

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材料（振动）热波成像中的相位反转：提取交流分量和过滤非线性

在材料的主动红外热成像检测中，热波是通过热源的激活来激发的，例如，通过光热辐射或振动引起的散热。因此，激发的单极（即仅加热）性质在测量的热响应中引入了不可避免的上升趋势。为了获得改进的热波成像质量，消除这种上升趋势并分析热响应的去耦双极（即交流）分量至关重要。本研究在热成像检测中引入了相位反转的概念，作为一种确定性方法，用于 (i) 将交流分量去耦和 (ii) 从热响应中滤除显着的二阶非线性。第一的，这种“反相热成像 (PIT)”在理论上通过分析通过受耗散边界条件的固体材料厚度的热扩散得到证实。然后，通过对各向异性复合试样进行光学红外热成像的有限元模拟，验证了 PIT 在将交流响应与各种激励波形精确解耦方面的性能。结果表明，通过正确选择信号的初始相位和波形持续时间，坑产生了零平均幅度的交流响应。最后，针对两种不同的测试案例评估了 PIT 的实验适用性：（1）背面强化碳纤维增强聚合物（CFRP）飞机面板上的光学热成像，具有复杂的生产缺陷集群；（2）低功耗受影响的 CFRP 试样上的振动热成像。结果表明，PIT 作为一种基于物理的信号处理技术，可以稳健地解决激发的强瞬态起始，并系统地解耦相当于理想线性和双极激发的热响应的 AC 响应。记录的热响应通过傅里叶变换进行后处理，证明了增强的热成像质量和改进的去耦交流组件的缺陷检测能力。