Effect of growth rings on fracture toughness of wood

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.