Air-coupled ultrasound wave propagation in glued laminated timber structures applied to bonding quality assessment

Non-destructive assessment of glulam laminations is necessary in order to prevent security hazards. Air-coupled ultrasound (ACU) is sensitive to delamination, reproducible and allows for high scanning resolution. In this work we developed a novel ACU imaging system in normal transmission for inspection of up to 280 mm high glulam. The coupling and propagation of ACU waves in glulam were investigated with plane wave theory applied to an orthotropic model of wood as a function of grain and ring angles. Finite Difference Time Domain (FDTD) simulations were also carried out. The systematic amplitude heterogeneity in defect-free regions was linked to energy flux shifts and mode conversion phenomena, and compensated for in the images. Specific amplitude tracking and difference imaging algorithms allowed successful imaging of bonding and saw cut defects.