A modified four-point-bend end-notched flexure test to study mixed-mode II+III fracture of adhesive joints

A novel experimental setup to determine the fracture behavior of structural adhesively bonded joints in mixed-mode II+III loading is presented. The setup combines the established four-point end-notched flexure (4ENF) test and the recently published (Geisel and Marzi, 2024) four-point-bend out-of-plane loaded single-leg bending (4OSLB) test. The proposed setup introduces mixed-mode loading by mounting the sample in a position that is arbitrarily rotated around its longitudinal axis compared to the mentioned setups. Experiments were performed on adhesive joints bonded by the structural adhesive SikaPower®-498. Test results over the entire range of mixed-mode ratios between single modes II and III are presented and discussed. Besides an evaluation of the critical energy release rate by means of linear-elastic fracture mechanics, the crack opening displacement is additionally evaluated through the use of digital image correlation (DIC) in selected tests. The main conclusion from the data obtained is that, within the limits of the evaluation methods, it is not necessary to distinguish between modes II and III for isotropic materials. • Novel 4RENF test setup for mixed mode II + III is presented. • Only force measurement is required for G-analysis in the 4REFN test. • Obtained critical energy release rates were independent of mode-mixity. • Modes II and III can be reduced to a single shear fracture mode for adhesives.