On the experimental mixed-mode failure of adhesively bonded metallic joints

The use of adhesively bonded structures is widespread in various engineering fields, as they provide many advantages over other conventional types of mechanical joints. In this study, we use a crash optimized, single-component epoxy adhesive (SikaPower®-498 made of a rigid epoxy matrix containing soft, tough polymer inclusions that provide additional ductility to the adhesive layer) at a constant layer thickness of 0.5 mm to bond metallic substrates. We investigate its fracture properties under mode I and mixed-mode I/II loadings, in order to obtain the full fracture envelope. Mode I loading has been performed using the ISO 25217 standard: the substrates were designed according to the TDCB (Tapered Double Cantilever Beam) geometry, and the fracture toughness GIC has been calculated by means of the ECM (Experimental Compliance Method). Mixed-mode I/II loading has been applied using the MMB (Mixed Mode Bending) experimental fixture described in the ASTM D6671 standard. The fracture toughness GC has been calculated via Finite Element Analysis and mode partitioning has been determined according to the methodology described in the standard. The mixed mode fracture behavior measured using the previous two methodologies shows that the adhesive seems to follow the Benzeggagh–Kenane failure criterion (expressed in 2D).