Study of inspection on metal sheet with subsurface defects using linear frequency modulated ultrasound excitation thermal-wave imaging (LFM-UTWI)

The linear frequency modulated ultrasound excitation thermal wave imaging (LFM-UTWI) was investigated on detection of subsurface defects of metal sheet. A numerical finite element analysis is carried out to calculate thermal wave signal dependence of time by linear frequency modulated ultrasonic wave excitation. Cross-correlation operation in time domain and frequency domain are used to extract the main peak value and the corresponding delay time, respectively. Fourier transform (FT) is applied to calculate the amplitude and phase angle of harmonic component of thermal wave. Experimental results show that various deep subsurface defects are readily detected using LFM-UTWI with once excitation, and LFM-UTWI has an advantage of better defect detectability compared to ultrasound lock-in thermography (ULIT).