Nondestructive assessment of glued joints in timber applying vibration-based methods

In this study, three different shapes and glue coverage rates of joints were evaluated nondestructively using longitudinal and flexural vibration-based methods. Rectangular specimens of 20 × 40 × 360 mm³ (R × T × L) dimensions were prepared from clear eastern beech (Fagus orientalis; Lipsky) wood. Joints were located at the middle of the beams. The selected joint shapes were 45° scarf joint, 30° scarf joint and finger joint with 10 mm length and 3 mm pitch. Longitudinal vibration test proved a better assessment for gluing than flexural efficiency. Some weaker jointed timbers could demonstrate identical Young’s moduli by both longitudinal and flexural vibration-based methods. In the presence of any significant difference between Young’s moduli evaluated by two methods, it was realized that the joints were weak and required to be rejected or assigned to re-manufacturing. The R² of Timoshenko’s linear model significantly decreased when there was any un-integrity in the joints. This factor was suggested to be used as a potential subsidiary indicator. The vibration-based methods were recommended to be applied for joint assessment of old jointed timbers and for grading newly constructed jointed lumbers.