Understanding moisture durability of wood-based composites using crack propagation fracture toughness.

A persistent issue in adhesively bonded wood-based composites is their moisture durability. A key question, therefore, is: what is the best approach to assessing moisture durability of wood composites? Previous work has focused on failure initiation (e.g. internal bond strength) and therefore, could not consider effects of moisture on subsequent damage propagation. This research addressed that knowledge gap by using crack propagation fracture experiments to study moisture effects on medium-density fiberboard (MDF), oriented strand board (OSB), and particleboard (PB). Fracture toughness vs. crack growth curves (i.e. material R curves) were measured for MDF, OSB, and PB after immersing them in water for immersion times up to one week. Experiments were done immediately after immersion to record crack propagation in states of increasing moisture content. Trends in experimental results could be characterized by a three-point model for MDF and PB or a linear model for OSB. Experiments and modeling showed that all composites degraded in both initiation toughness and in their ability to inhibit crack growth by fiber bridging mechanisms. Summary plots compare how R curves for MDF, OSB, and PB changed from control conditions to water saturation conditions and compare their moisture degradation rates. [ABSTRACT FROM AUTHOR]