Keywords: 
• nondestructive testing /
• LFMITWI /
• image sequence /
• cross-correlation /
• FFT scanning

Abstract: Linear frequency modulation infrared thermal-wave imaging(LFMITWI) is an active infrared thermography based on linear frequency modulation thermal-wave signal processing, which makes up for the disadvantage of traditional lock-in thermography that only detects the limited depth of subsurface defects by using a single frequency modulation thermal-wave. The theory, mathematics analysis and experiments in support of the LFMITWI for nondestructive subsurface defects detection in solids were described. The finite element modeling was used to analyze the thermal-wave behavior generated by linear frequency modulation chirp heat flux. The cross correlation peak value and its time were calculated by the use of cross correlation between surface temperature signal and reference chirp signal, and the peak value image and its time image were obtained. In experimental study, the law of chirp modulated halogen lamp was used as active heat source into metal flat-bottom holes sample, and the thermal-waveimage sequences were collected with Jade MWIR 550 FPA infrared camera. The cross correlation processing of time field and Fourier transform scanning of frequency field were used to obtain the peak value image, peak value time image and phase image respectively. The experimental results show that the peak value image and phase image are helpful to identify the geometric characteristic of different depths defects under the condition of given chirp thermal stimulation.