Detection of glue deficiency in laminated wood with pulse thermography

Abstract

Adhesion problems sometimes occur during the production of laminated wood products. To minimize such quality problems, there is a need for a nondestructive test that can provide continuous control of the process and the product. This study presents results from measurements performed to evaluate the potential of pulse thermography as a method to detect glue deficiency in laminated wood. Defect depth, defect size, and degree of glue deficiency have been varied. The surface layer was made of merbau (Intsia bijuga) and the substrate of Scots pine (Pinus silvestris). The results showed that pulse thermography is a promising tool for detecting glue deficiency underneath the thin laminated wood surface layers, mainly because of the short inspection time. Lack of glue with a minimum thermal defect size of 3 was detectable (thermal defect size is defined as the quotient of defect size and defect depth). The penetration depth was 1.0mm and the highest contrast, 0.62°C, was achieved for one of the largest defects (24mm) below the thinnest (0.5mm) surface layer after 1 second. Starved glue joints showed about half the contrast compared to areas with total lack of glue.