Thickness effect in wood — statistical or structural?
Publication Name: Proceedings of the 11th International Conference on Fracture
Testing and modelling was carried out for Compact Tension (CT) specimens in an attempt to clarify the importance of size of specimen effects on opening fracture mode I properties of spruce. The experimental part of the investigation was carried out on wood in green and 12 percent Moisture Content (MC) conditions in two wood weakest directions RL and TL. Here R denotes radial, T tangential and L longitudinal directions relative to the tree stem. Standard CT specimen dimensions were modified to avoid possible ligament and width size effects on the measured properties. Subsequent to the experimental part, finite element computations were performed to estimate fracture toughness and energy release rates, while the total fracture energy was computed separately. Testing results revealed no clear evidence of size effects. If size effects exist they are masked by inherent variability in fracture properties. Therefore, the question of a statistical thickness effect needs to be considered as opposed to a structural size effect. The authors found that discrete finite element lattices were capable of accurately mimicking experimental load-deformation responses and damage patterns in CT specimens. Lattice fracture models take into account the statistical variation in element stiffness and strength properties using Monte Carlo simulation, and thereby describe the local heterogeneities inherent to wood. The analysis is performed in 3-D with the removal of failed elements as they reach randomly prescribed strength values. It is thereby possible to numerically follow progressive evolution of fracture as well as damage patterns. The lattice fracture model has proven superior to other non-linear fracture models due to its theoretical postulates having physical validity.