Mode I fracture of beech-adhesive bondline at three different temperatures

Single edge-notched three-point bending tests (SEN-TPB) for mode I were utilized to experimentally evaluate fracture properties of adhesive bondlines in European beech (Fagus Sylvatica L.). The bondline was examined at two anatomical planes with TR and RT orientation and at control and two elevated temperatures (70°C and 140°C). Among epoxy (EPI), melamine-urea formaldehyde (MUF), and polyurethane (PUR) adhesives, the highest average critical energy Gc with 0.80 N/mm and fracture energy Gf with 1079.4 N/mm were obtained for EPI in the TR plane and under standard climate conditions (20°C/65% relative humidity), followed by MUF (Gc = 0.50 N/mm and Gf = 620 N/mm) and PUR (Gc = 0.25 N/mm and Gf = 290.9 N/mm), respectively. PUR was least effected by elevated temperature, and no significant differences for Gc and Gf between TR and RT bondline orientations were found for MUF and PUR treated at 20°C/65% relative humidity while comparisons between other factors varied significantly. Treatment of specimens at elevated temperatures resulted in reduced fracture performance regardless of wood grain orientation or the adhesive system.