Strength properties of juvenile and mature wood in black locust (Robinia pseudoacacia L.)

This study was undertaken to investigate the strength properties of juvenile and mature wood in black locust (Robinia pseudoacacia L.). Wood specimens were prepared from various stem heights, up to 9 in, of five naturally-grown black locust trees, 21-37 years old. Mechanical properties tested included moduli of rupture (MOR) and elasticity (MOE), and pure modulus of elasticity (PMOE) in static bending, stress wave modulus of elasticity (SWMOE) in dynamic strength, axial compression, and toughness. Comparisons between juvenile and mature wood specimens of similar densities (0.667-0.894 g/cm(3) and 0.682-0.892 g/cm(3), respectively) showed that juvenile wood had a statistically significant lower mean MOR (138.78 N/mm(2)), MOE (13,936 N/mm(2)), PMOE (18,125 N/mm(2)), SWMOE (16,813 N/mm(2)) and toughness strength (155.25 KJ/m(2)) than the mature wood (148.29 N/mm(2), 14,747 N/mm(2), 19,498 N/mm(2), 17,635 N/mm(2) and 181.27 KJ/m(2), respectively). On the contrary, no statistically significant differences were found for the mean strength in axial compression among juvenile (63.75 N/mm(2)) and mature wood (66.65 N/mm(2)). Fractured surfaces of juvenile and mature wood specimens in static bending and toughness were classified into the "splintering tension" type of failure, while compression failures were of the "shearing type" according to ASTM D 143-83 standards. Lower strength of juvenile wood in most of the properties examined may be attributed to anatomical and chemical properties rather than density of wood specimens. The adverse influence of juvenile wood on strength properties should be considered for effective management (e.g. longer rotation age and other genetic and forest or plantation management measures that reduce juvenile wood content) and utilization of the species.