Enhancing defect detection in solid materials using low-duty cycle square wave lock-in thermography: a time-efficient and quantitative approach

Abstract

Lock-in thermography has been used to detect material defects but has faced challenges in quantitatively identifying defects. Traditional methods with a sine wave or symmetrical square wave heat source require time-consuming frequency adjustments with multiple experiments. This study introduces a novel technique using a heat source of a low-duty cycle square wave in lock-in thermography. This harmonic-rich heat source provides information on the blind frequency and the depth of defects, significantly reducing testing time. This method was validated by finite element simulations, which generated phase maps and phase difference-frequency curves at multiple frequencies that located defects. The study explored the correlation between thermal diffusion length at the blind frequency and defect depth and the impact of transverse thermal diffusion on depth measurement. The findings indicate that phase maps at the optimal frequency are most suitable for measuring defect edges. This approach offers a promising, time-efficient, and quantitative solution for defect detection.