Non-destructive testing of laminated paper products by active infra-red thermography

Active infra-red (IA) thermography has been applied to the NDT (non-destructive testing) of paper products, and has been shown to be successful in identifying hidden air-filled defects within the thickness of laminated paperboard samples. In a thermally non-stationary state, the region over a defect can be visually distinguished from neighbouring defect-free areas by its thermal contrast. For quantitative determination, the pulsed thermography method was used and temperature history curves were carefully studied in order to characterise the unknown defects. It was found that a defect can be quantitatively evaluated in a reasonable time at a depth of less than 2 mm beneath the surface. Compared with many other materials, paper has a higher specific heat and lower thermal conductivity, and therefore the thermal contrast persists for a longer time. The work suggests that significant defects can be usefully detected in practical situations in the paper industry by active IA thermography.