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 (<i>Fagus Sylvatica</i> 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 <i>G</i>_c with 0.80 N/mm and fracture energy <i>G</i>_f 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 (<i>G</i>_c = 0.50 N/mm and <i>G</i>_f = 620 N/mm) and PUR (<i>G</i>_c = 0.25 N/mm and <i>G</i>_f = 290.9 N/mm), respectively. PUR was least effected by elevated temperature, and no significant differences for <i>G</i>_c and <i>G</i>_f 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.

本研究采用I型单边缘缺口三点弯曲试验（Single edge-notched three-point bending tests，SEN-TPB），对欧洲山毛榉（Fagus Sylvatica L.）胶粘剂胶层的断裂性能开展实验评估。试验分别针对TR、RT两种取向的解剖学切面，以及对照组与两个高温工况（70℃、140℃）下的胶层进行测试。在环氧树脂（EPI）、密胺-脲醛树脂（MUF）及聚氨酯（PUR）三类胶粘剂中，环氧树脂在TR切面与标准气候条件（20℃/65%相对湿度）下表现出最优的断裂性能，其平均临界断裂能G_c达0.80 N/mm，断裂能G_f达1079.4 N/mm；其次为密胺-脲醛树脂（G_c=0.50 N/mm，G_f=620 N/mm），聚氨酯则位列第三（G_c=0.25 N/mm，G_f=290.9 N/mm）。聚氨酯受高温的影响程度最小；在20℃/65%相对湿度下处理的密胺-脲醛树脂与聚氨酯试样中，其胶层TR与RT取向间的G_c和G_f均无显著差异，而其余因素间的对比则存在显著差异。无论木纹取向或胶粘剂体系如何，高温处理试样均会导致其断裂性能下降。