Detecting deeper defects using pulse phase thermography

Detectable defect depth by pulse phase thermography (PPT) is reportedly improved when using phase at low frequency. This study was conducted to identify mechanisms detecting deeper defects by the PPT, and to determine the optimum frequencies for detecting defects with various depths and sizes. One-dimensional and finite element analyses reveal that the optimum frequency decreases continuously with increasing defect depth, and that the amplitude of noise appearing in phase data decreases with decreased frequency. These engender a large signal-to-noise ratio for deep defects in a lower-frequency range. The analytical results were verified by experiments for a polymethylmethacrylate specimen having artificial defects. The experimental results at the optimum frequency demonstrated that defects with up to 5–6 mm depth were detected, which is a significant improvement compared with the reported detectable defect depth of 3.5 mm.

► Detectable defect depth depends on the phase image frequency.
► Analytical calculations were performed to evaluate the phase data.
► We reveal a mechanism to detect deeper defects in lower-frequency phase images.
► Optimum frequencies were determined for various defect depths and sizes.
► Detectable depths of defects were improved by considering the optimum frequency.