Fracture process zone and crack migration in pure mode II bonded composite joints: influence of pre-crack, stacking sequence, normalized crack-geometry

• G IIC is highly process-dependent, influencing the crack growth and migration. • Mode II pre-crack creates diffuse FPZ, delays crack migration in QI laminates. • Mode I pre-cracks trigger earlier migration into QI composite layers. • Crack migration angles are sensitive to the initial a 0 /L ratio • Scaling span length does not change mechanisms if a 0 /L is preserved. This study examines how loading mode during pre-cracking, stacking sequence, and initial delamination ratio ( a 0 /L ) influence Mode II fracture characterization ( G IIC ) of bonded composite joints. 3-point End-Notched Flexural tests were performed on Unidirectional (UD) and Quasi-Isotropic (QI) carbon fibre/epoxy laminates bonded with AF163-2 K adhesive. Results reveal that fracture toughness and crack migration are governed by the morphology of the Fracture Process Zone (FPZ). In UD laminates, Mode I pre-cracking forms localized FPZ, requiring intense plastic deformation to transition into shear-dominated FPZ, capturing the upper-bound fracture resistance. Conversely, Mode II pre-cracked specimens exhibited diffused shear FPZ, resulting in lower G IIC . In QI laminates, diffused FPZ by Mode II pre-cracking delays crack migration into the weaker interlaminar, promoting growth within the bond-layer. However, localized FPZ from Mode I pre-cracks requires intense plastic deformation and shear cracks for the crack to grow in the bond-layer, triggering earlier migration. The crack migration was sensitive to the “ a 0 /L” ratio : a ratio of 0.4 induces independent interlaminar delamination, while 0.6 displays angular crack-migration. These mechanisms remained invariant when the span-length was scaled, provided the normalized crack length was preserved. This study demonstrates that G IIC is process-dependent, underscoring the need to characterize fractures based on FPZ evolution.