Mode II fracture behavior of Norway spruce: CBBM and DIC analysis at small and structural scales

Fracture characteristics of spruce timber in shear mode II are crucial for fracture analysis of timber beams, either separately in pure mode II or as factors involved in mixed-mode scenarios. Gaining insight into this phenomenon at both small and large scales, including the influence of imperfections, is essential. This study investigates the fracture performance of sawn spruce timber in mode II at two scales: small (5 × 6 cm cross-section, n = 47) and large (24 ×20 cm cross-section, n = 14). The small-scale specimens were further categorized by their anatomical direction relative to the crack plane: radial–longitudinal (RL) and tangential–longitudinal (TL). Sawn spruce beams were tested using end-notched flexure in three-point4 bending (ENF-3PB). Crack initiation and propagation were monitored using compliance-based beam method (CBBM) and digital image correlation (DIC) uniquely incorporating optical data from both sides of the beam. Results showed that the critical strain-energy release rate ( G c ) and total fracture toughness ( G f ) were significantly (α=0.05) higher in the larger cross-sections, by approximately 41 % and 85 %, respectively. Anatomical direction (RL vs. TL) had a negligible effect on shear behaviour for small-scale, though some differences in fracture properties were observed using optical measurement. Notably, large-scale specimens exhibited considerably greater effective and total crack lengths, as measured by CBBM (75 mm vs. 16 mm for effective crack length; 383 mm vs. 218 mm for total crack length measured optically). Optical measurements of crack and fracture process zone (FPZ) lengths prior to rupture consistently showed a difference of approximately 20 mm across all criteria and scales which shows that existing drying cracks did not propagate extensively in mode II or caused immediate damage; however, they notably impacted total crack length. Knots were found to significantly influence total fracture toughness, as crack propagation frequently halted and reinitiated at these locations. The findings of this study suggest that mode II fracture criteria for structural-scale elements are possible and can be formulated multiple ways; nevertheless, when taking into account also the crack propagation asymmetry at each side, a practical criterion can be considered: the critical crack length sum of both sides to be 24 mm when 0.3 mm relative displacement is used. This threshold allows testing to be stopped at this point, enabling the remaining timber to be repurposed, reducing material waste among others. • Large-scale specimens show higher fracture toughness (Gc, Gf) and longer cracks than small-scale samples. • Anatomical direction (RL vs. TL) has a minor effect on small-scale fracture properties but aligns with large-scale trends. • Drying cracks have minimal influence on crack initiation and propagation. • Uniquely captures crack propagation from both sides of the beam, offering a more comprehensive fracture analysis. • Optical strain-based measurement provides reliable fracture assessment.