Reducing inspection time of pulse phase thermography by using phase data at higher frequency range

The defect detection capability of pulse phase thermographic (PPT) non-destructive inspection depends on the frequency of Fourier transformed phase images. Conventionally, phase images at lower frequencies are required to detect deeper defects or defects in objects with lower thermal diffusivity. However, in order to obtain phase images at a lower frequency, a longer inspection time is required. In this paper, we suggest using the phase contrast between defective and non-defective areas that appears in a higher frequency range in order to reduce the inspection time of the PPT method. The numerical calculations and experimental results revealed that the phase data have a small peak (P-H) in the high frequency range and that defects can be detected in phase images at the P-H frequency because a discernible phase contrast is observed there. We derived a relationship among the frequency of the phase peak, defect depth, and thermal diffusivity of the test object through numerical calculations and dimensional analysis. In addition, the phase contrast at the peak was revealed to vary depending on the data point of the thermal data before Fourier transform. By using the phase images at the phase peak in a high frequency range, the inspection time can be reduced by approximately one order of magnitude.