Abstract Thermal Wave Radar (TWR) imaging employs the concept of pulse compression in order to obtain an increased probing depth and depth resolution in infrared thermographic testing of materials. The efficiency of the TWR imaging is highly dependent on the nature of the employed excitation signal. Most studies exploit the use of an excitation signal with an analogue frequency modulation (e.g. sweep signal) or a discrete phase modulation (e.g. Barker coded signal). Recently, a novel frequency-phase modulated (FPM) waveform was introduced, and computationally verified by the current authors, which couples the concept of frequency- and phase modulation to each other in view of obtaining an optimized excitation signal for improved TWR imaging. This paper experimentally investigates the performance of the novel optimized FPM waveform for the inspection of glass and carbon fiber reinforced polymer (GFRP and CFRP) composites, using an optical infrared thermography set-up in reflection mode. The response of the halogen lamps to the FPM waveform is measured, and further the influence of the electro-thermal latency of excitation lamps on the applicability of the novel FPM excitation signal is analytically investigated. Then, the performance of the FPM waveform is experimentally investigated for both glass- and carbon fiber reinforced polymers with defects of different depths and sizes. A comparative analysis is performed with amplitude modulated (classical lock-in), frequency modulated (sweep) and phase modulated (Barker coded) excitation, each with the same time duration as the FPM waveform. The novel FPM waveform outperforms these existing waveforms in terms of defect detectability and contrast-to-noise ratio, especially for the deeper defects. Different central frequencies are examined and the improved performance of the FPM waveform in TWR imaging is demonstrated in all cases.

中文翻译：

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一种优化的频相调制波形在复合材料增强红外热波雷达成像中的应用

摘要 热波雷达 (TWR) 成像采用脉冲压缩的概念，以便在材料的红外热成像测试中获得更高的探测深度和深度分辨率。TWR 成像的效率高度依赖于所采用的激励信号的性质。大多数研究利用具有模拟频率调制（例如扫描信号）或离散相位调制（例如巴克编码信号）的激励信号的使用。最近，引入了一种新颖的频率相位调制 (FPM) 波形，并由当前作者进行了计算验证，该波形将频率和相位调制的概念相互结合，以获得用于改进 TWR 成像的优化激励信号。本文使用反射模式下的光学红外热成像装置，通过实验研究了新型优化 FPM 波形用于检测玻璃和碳纤维增强聚合物（GFRP 和 CFRP）复合材料的性能。测量卤素灯对 FPM 波形的响应，并进一步分析研究了激发灯的电热延迟对新型 FPM 激发信号的适用性的影响。然后，对具有不同深度和尺寸缺陷的玻璃纤维和碳纤维增强聚合物的 FPM 波形的性能进行了实验研究。使用调幅（经典锁定）、调频（扫描）和调相（巴克编码）激励进行比较分析，每个激励具有与 FPM 波形相同的持续时间。新的 FPM 波形在缺陷可检测性和对比度噪声比方面优于这些现有波形，尤其是对于更深的缺陷。检查了不同的中心频率，并在所有情况下证明了 FPM 波形在 TWR 成像中的改进性能。