Strain-based approach to characterize mode I crack propagation in Norway Spruce directly from optical data

The paper focuses on assessment and utilization of strain-based criterion obtained using the digital image correlation in characterization of fracture behavior of Norway spruce wood. The study employed a single-edge notched beam loaded in three-point bending (SEN-TPB) to examine mode I at three anatomical directions of crack propagation (radial, tangential, tangential-radial - R, T and TR). The criterion is evaluated at the maximal load (F max ), where the compliance-based beam method (CBBM) provides critical strain energy (G c ), which ensures the proper criteria representing equivalent crack length growth is described. The novel approach also enables one to determine the fracture process zone (FPZ) length using an algorithm which finds the onset of the nonlinear region. Uniqueness of the approach lies in processing a big set of optical data and simultaneous tracking of crack length on both sides of medium-size specimens. Results indicate that crack length is dependent on the anatomical direction, for instance in T direction the criterion ε 1crit is 2.5e-3 producing crack length equal to a c =23.9 mm, whilst in R direction, the ε 1crit is least and equals 1.3e-3 producing crack length of 22.1 mm. The highest ε 1crit is attained in TR (on average ε 1crit = 3.4e-3) and distance from the place where the crack started is 19.4 mm. Size of the non-linear region here attributed to FPZ length reaches the value of 38.4 mm in T, 30.1 mm in R and 36.3 mm in TR directions, respectively. The study presents a novel approach in characterization of fracture properties by coupling optical and energetical data and may find its usage in evaluation of other fracture modes.