Influence of process zone evolution on apparent mode II fracture characterization: Role of pre-crack and compliance in bonded joints

This study investigates the influence of pre-crack conditions (introduced under Mode I and Mode II loading prior to fracture testing) and specimen compliance on the Mode II fracture characterization ( G IIC ) of adhesively bonded composite joints. Calibrated End-Loaded Split (CELS) and 3-Point-Bending ENF tests were performed using structural AF163-2 K adhesive. Various data reduction schemes were employed to account for pre-crack morphology and compliance in the development of the R-curve. The data reduction schemes showed significant scatter, ranging from 8.07 ± 0.17 to 17.3 ± 1.19 N/mm, depending on the pre-cracking conditions and compliance effect. Mode I pre-cracked specimens consistently exhibit higher G IIC values compared to Mode II pre-cracked specimens, a difference governed by the morphology and extent of the fracture process zone (FPZ). Mode I pre-cracking forms a localized FPZ that subsequently transitions into a shear-dominated FPZ for G IIC evaluation during the subsequent Mode II fracture test. In contrast, Mode II pre-cracked specimens contain an already-developed shear FPZ that is broader and more diffuse, resulting in lower strain-energy release rates and lower G IIC values. High compliance effects cause significant bending, additionally introducing high derogatory energy deformation from the test fixtures, obscuring the actual crack tip. The apparent crack length methods demonstrated reliable estimates of fracture energy and R-curve behavior by accounting for the effects of large FPZ, thereby capturing both crack-tip and distributed dissipation mechanisms. The experimental findings correlate with computational results, displaying stable cohesive disbond growth in the adhesive layer. This study indicates that pre-cracking and compliance effects significantly influence Mode II fracture characterization and, therefore, need to be properly addressed.