Towards a strain-based approach to mode I fracture characterization in Norway spruce considering three anatomical directions

Abstract

Fracture in wood is often evaluated using energy-based methods, while strain-based criteria have been less studied. Their ability to indicate the onset of crack propagation and fracture process zone (FPZ) extent across grain orientations remains unclear. This study examines whether strain thresholds can detect the onset of crack propagation in Norway spruce and estimate FPZ length. Single edge-notched three-point bending (SEN-TPB) tests were performed on specimens (h = 0.12 m, b = h/2) in TL, RL, and mixed “TRL” (40–50° ring inclination) directions. Strain fields from digital image correlation (DIC) were analyzed to identify thresholds ranging from those representing the crack tip to the FPZ end. Crack lengths derived from each threshold were compared with compliance-based beam method (CBBM) estimates, yielding characteristic threshold–crack length pairs. Mean values of ϵ1crit were 0.0023 (TL), 0.0013 (RL), and 0.0037 (TRL), with crack length acrit around 20 mm and FPZ lengths of 26.9, 20.2, and 25.4 mm, respectively. Strain thresholds provided relatively consistent indicators of crack propagation onset and FPZ extent across orientations. These findings support their potential as a complementary approach to established fracture assessment methods and as a tool for improving understanding of crack growth in timber.