Influence of intermediate deformation rates in softwoods characterized with fracture resistance R-curves

Fracture resistance curves of Norway spruce were evaluated over a range of crosshead speeds between 0.05 and 200 mm/min using compact tension specimens. The fracture resistance curves were determined via J-integral K (R) versus Delta a (crack extension) using hybrid experimental-finite element method with stepwise quasi-static and transient dynamic analysis. Coupling experiments and high-speed camera with a rate of 0.5-135 frames/s capture enabled determination of crack kinetics and velocities. Digital image correlation method was employed to evaluate strain distribution maps of the fracture process zone. Deformation measurements suggested strong strain localization and significant influence of the loading rate. The most distinct strain map was evaluated for the loading rate of 200 mm/min, suggesting inertial effects. Rising fracture resistance develops for smaller crack lengths (0.15-0.20 a/w) for all loading rates, with an increase of up to 20-50% compared to initiation values. Fracture resistance R-curves were evaluated for single individual specimens as well as for averaged load-deformation responses, which hide strong localisations in single responses. Influence of deformation rate on the shape and magnitude of fracture resistance curves is significant. Inertial effects in transient dynamic response give rise to twofold higher fracture resistance of softwoods compared to quasi-static resistance.