Using crack propagation fracture toughness to characterize the durability of wood and wood composites

• Published in: Materials & design Vol. 87; pp. 586 - 592
• Main Authors: Mirzaei, B., Sinha, A., Nairn, J.A.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.12.2015
• Subjects: Adhesives; Bridging; Durability; Fibers; Fracture; Fracture mechanics; Fracture toughness; Polymers; Resins; Wood; Wood based composites; Wood based composites; Durability; Fracture; Adhesives
• ISSN: 0264-1275; 1873-4197
• DOI: 10.1016/j.matdes.2015.08.010

We measured fracture resistance curves (or R curves) for laminated veneer lumber (LVL) made with Douglas-fir veneer and polyvinyl acetate resin and for solid wood Douglas-fir. The LVL and solid wood R curves were the same for initiation of fracture, but the LVL toughness rose much higher than solid wood. Because a rising R curve is caused by fiber bridging effects, these differences show that the LVL resin has a large effect on the fiber bridging process. We exploited this resin effect to develop a test method for characterizing the ability of a resin to provide wood composites that are durable to moisture exposure. The test method exposed LVL specimens to vacuum pressure soaking and drying (VPSD) cycles and then monitored the rising portion of the LVL R curves as a function of treatment cycles. Douglas-fir/polyvinyl acetate LVL lost about 30% of its toughness after 16cycles. In characterizing toughness changes, it was important to focus on the magnitude and rate of the toughness increase attributed to fiber bridging. We suggest that these properties are much preferred over other fracture or mechanical properties of wood that might be used when characterizing durability. [Display omitted] •Toughness increase as a function of crack growth was measured and shown to be significant in solid wood and LVL.•The increased rise in LVL toughness compared to solid wood can be attributed to the contribution of resin to fiber bridging.•Differences in the rising R curve for LVL vs. solid wood emphasizes the importance of monitoring crack propagation in tests.•Scatter can high, but monitoring crack propagation enabled us to use results at a crack length that minimizes scatter.•Toughness characteristics, excluding initiation toughness, properly reflect the moisture durability of solid wood and LVL.