Measuring and modeling fiber bridging: Application to wood and wood composites exposed to moisture cycling

We propose a new method for determining fiber-bridging, cohesive laws in fiber-reinforced composites and in natural fibrous materials. In brief, the method requires direct measurement of energy released during crack growth, known as the R curve, followed by a new approach to extracting a cohesive law. We claim that some previous attempts at determining cohesive laws have used inappropriate, and potentially inaccurate, methods. This new approach was applied to finding fiber bridging tractions in laminated veneer lumber (LVL) made from Douglas-fir veneer and four different adhesives. In addition, the LVL specimens were subjected to moisture exposure cycles and observations of changes in the bridging cohesive laws were used to rank the adhesives for their durability. Finally, we developed both analytical and numerical models for fiber bridging materials. The numerical modeling was a material point method (MPM) simulation of crack propagation that includes crack tip propagation, fiber bridging zone development, and steady state crack growth. The simulated R curves agreed with experimental results.