Comparison between the ENF and 4ENF fracture characterization tests to evaluate GIIC of bonded aluminium joints

Adhesively bonded joints have been increasingly used in structural applications over mechanical joints. Cohesive Zone Modelling (CZM) is the most widespread technique to predict the strength of these joints, and it uses the tensile fracture toughness (G(IC)) and the shear fracture toughness (G(IIC)). Different fracture characterization methods are available for shear loadings, among which the End-Notched Flexure (ENF) is undoubtedly the most popular. The 4-Point End-Notched Flexure (4ENF) is also available. This work consists of a detailed comparison between the ENF and 4ENF tests for the experimental estimation of G(IIC) of bonded aluminium joints. Three adhesives were used: a strong and brittle (Araldite (R) AV138), a less strong but with intermediate ductility (Araldite (R) 2015) and a highly ductile (SikaForce (R) 7752). Different data reduction methods were tested, and the comparison included the load-displacement (P-) curves, resistance curves (R-curves) and measured G(IIC). It was found that the ENF test presents a simpler setup and has a higher availability of reliable data reduction methods, one of these not requiring measuring the crack length (a) during its growth. For the 4ENF test, only one test method proved to be accurate, and the test geometry revealed to be highly affected by friction effects.