Fracture Toughness of Wood and Wood Composites during Crack Propagation

The mode I fracture toughness as a function of crack length of medium density fiberboard (MDF), particle board (PB), and Douglas-fir (DF) were all measured using a new energy-based method. PB and MDF are examples of composites that develop fiber bridging during crack propagation, which causes their toughness to increase with crack length. Longitudinal cracks in DF also displayed fiber bridging behavior, but only when the crack plane was normal to the tangential direction (TL cracks). MDF and PB experiments were done for both in-plane and out-ofplane cracks. The toughness of the former was much higher than the later. The in-plane crack toughness of MDF was higher then PB, but its out-of-plane toughness was lower. PB made using a new soy based resin had an in-plane toughness similar to commercial PB, but an out-ofplane toughness three times higher. Out-of-plane crack propagation is suggested as an improved method for measuring internal bond properties. When the fracture method was compared conventional internal bond (IB) tests, both methods showed that the soy PB is better, but the fracture method provided a clearer distinction.