Effects of heating duration on pulse phase thermographic non-destructive testing

•Effects of long-duration heating on pulse phase thermography were investigated.•Larger amounts of heat can be input easily using long-duration heating.•The greater heat input engenders lower amplitude of noise in phase images.•For detection of deep defects, long-duration heating enhances the phase contrast.•Detectable defect depth was improved by application of long-duration heating.

To enhance the detectable defect depth when using pulse phase thermographic non-destructive testing, the effect of applying long-duration heating was studied. Analytical calculations revealed that long-duration heating improves the phase difference between defective and non-defective areas especially for deep defects, and that phase noise decreases concomitantly with increasing heating duration. These results mean that the long-duration heating is effective to detect deep defects. Experiments using a polymethylmethacrylate specimen having artificial defects demonstrated that defects with up to 8–9 mm depth were detected in a phase image obtained after applying 30 s heating, which is a significant improvement compared with results obtained by instantaneous pulse heating (detectable defect depth was 5–6 mm).