The fracture toughness of medium density fiberboard (MDF) as a function of crack length was measured. Fracture toughness was determined from force–displacement and crack length data using a new energy analysis procedure that avoids the scatter of prior discrete analysis methods. Because crack lengths were difficult to observe, they were measured using digital image correlation (DIC). The R curves for two different densities of MDF, two thicknesses, and for both in-plane and through-the-thickness cracks all increased linearly with crack length. The increase was interpreted as the development of a fiber-bridging process zone. Numerical modeling methods were used to determine the cohesive stress of the fiber-bridging zone.
