This study investigated the influence of growth rings (earlywood, earlywood and latewood interface, and latewood) on the mode I fracture toughness of solid wood of the two species southern yellow pine (Pinus taeda L.) and red oak (Quercus falcata) subjected to cracking loads in their radial-longitudinal crack propagation system. Experimental results indicated that southern yellow pine latewood had a significantly greater fracture toughness than its earlywood as well as its earlywood and latewood interface. The earlywood and latewood interface of southern yellow pine had a greater fracture toughness than its earlywood, but this difference was not significant. Red oak latewood had a greater fracture toughness than its earlywood and latewood interface, however this difference was not significant. Red oak earlywood had a significantly lower fracture toughness than its latewood and earlywood and latewood interface. In general, red oak had a significantly greater fracture toughness than southern yellow pine. The differences in specific gravity among latewood, earlywood and latewood interface, and earlywood for each of the two wood species evaluated were significant. The mode I fracture toughness of a tested wood block in its radial-longitudinal crack propagation system can be affected by not only its specific gravity but also by its microstructure such as ray cell size and quantity. The regression analyses indicated a strong positive linear relationship existed between the fracture toughness and specific gravity for the two wood species evaluated. The rate of increase in fracture toughness of red oak was about three times of southern yellow pine as their specific gravity increased.

本研究调查了年轮（早材、早材和晚材界面以及晚材）对南方黄松（Pinus taeda L.）和红橡（Quercus falcata）两种实木的I型断裂韧性的影响) 在径向-纵向裂纹扩展系统中承受裂纹载荷。试验结果表明，南方黄松晚材的断裂韧性明显高于早材，以及早材和晚材界面。南方黄松早材和晚材界面的断裂韧性比早材大，但差异不显着。红橡木晚材的断裂韧性高于早材和晚材界面，但这种差异并不显着。红橡早材的断裂韧性明显低于其晚材以及早材和晚材界面。一般来说，红橡木的断裂韧性明显高于南方黄松。晚材、早材和晚材界面之间的比重差异，所评估的两种木材种类中的每一种的早材和早材均显着。受试木块在其径向-纵向裂纹扩展系统中的 I 型断裂韧性不仅受其比重的影响，还受其微观结构的影响，例如射线单元的大小和数量。回归分析表明，所评估的两种木材的断裂韧性和比重之间存在很强的正线性关系。随着比重的增加，红橡的断裂韧性增加率约为南方黄松的三倍。受试木块在其径向-纵向裂纹扩展系统中的 I 型断裂韧性不仅受其比重的影响，还受其微观结构的影响，例如射线单元的大小和数量。回归分析表明，所评估的两种木材的断裂韧性和比重之间存在很强的正线性关系。随着比重的增加，红橡的断裂韧性增加率约为南方黄松的三倍。受试木块在其径向-纵向裂纹扩展系统中的 I 型断裂韧性不仅受其比重的影响，还受其微观结构的影响，例如射线单元的大小和数量。回归分析表明，所评估的两种木材的断裂韧性和比重之间存在很强的正线性关系。随着比重的增加，红橡的断裂韧性增加率约为南方黄松的三倍。