On the high fracture toughness of wood and polymer-filled wood composites - Crack deflection analysis for materials design

Cracks oriented in the toughest direction across the grain of wood (0 degrees) tend to deflect at 90 degrees to the precrack rather than extending in 0 degrees direction. Fracture toughness data across the grain are therefore difficult to interpret. Crack growth mechanisms and effects from replacing wood pore space with a polymer are investigated. Crack growth is analyzed in four-point bending fracture mechanics specimens of birch and two different polymer-filled birch composites using strain-field measurements and finite element analysis (FEA). Calibrated cohesive zone models in both precrack and 90 degrees-directions describe fracture process zone properties in orthotropic FEA-models. Conditions for 0 degrees crack penetration versus 90 degrees crack deflection are analyzed based on cohesive zone properties. Stable, subcritical crack deflection takes place at low load, reduces crack tip stress concentration, and contributes to high structural toughness, provided the 90 degrees toughness is not too low. Polymer-filled neat birch composites have the best structural toughness properties in the present investigation, since 90 degrees toughness is not compromised by any chemical treatment.