THE INFLUENCE OF THE ADHESIVE LAYER THICKNESS ON THE FRACTURE BEHAVIOUR OF STRUCTURAL BONDED JOINTS

This paper investigates the fracture behaviour of structural adhesive joints with respect to the thickness of the adhesive layer. While the fracture behaviour under Mode I loading is evaluated in Tapered Double Cantilever Beam (TDCB) tests, the recently developed End-Loaded Shear Joint (ELSJ) test is used to analyze the Mode II fracture behaviour. The evaluation of TDCB and ELSJ tests by an integration method gives, beside the critical energy release rates, also some further information about the crack initiation energy, which is the energy dissipated inside the adhesive layer prior to crack propagation. Five adhesive layer thicknesses, ranging from 0.25 to 4.0 mm, were investigated for both fracture modes. Furthermore, TDCB bulk specimens were fabricated and tested to approximate the limit case of an adhesive layer of infinite thickness. For both, the critical energy release rates as well as the crack initiation energies, a correlation with the adhesive layer thickness is found in both modes. All experiments were done using the structural adhesive SikaPower498 (TM). This adhesive is mainly used in automotive application cases, where the fracture behaviour of an adhesive joint with varying adhesive layer thickness is of prime interest.