On the application of an optimized Frequency-Phase Modulated waveform for enhanced infrared thermal wave radar imaging of composites

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.