A new technique to measure shear fracture toughness of adhesives using tensile load

The end notched flexure (ENF) test is the most common method for measuring the mode II fracture energy of adhesives which provides reliable results but has some inherent problems. The shear fracture energy calculated using ENF for brittle adhesives is often based on a single data point (corresponding to the fracture load) obtained from the test where the crack grows catastrophically and the results are significantly sensitive to pre-crack tip conditions. The aim of the current study is to introduce a new pure mode II fracture test suitable for measuring the shear fracture energy of adhesives. A new technique for measuring the mode II critical strain energy release rate (SERR) of brittle adhesives is proposed based on the central cut ply (CCP) configuration. Fracture tests were carried out on adhesively bonded steel CCP specimens. It is shown that the crack growth in CCP joints is more stable, and the failure develops in multiple steps that generates more experimental data point during the crack propagation. The mode II fracture energy of a brittle epoxy-based adhesive is extracted using CCP method and is compared with the shear fracture energy obtained from the ENF tests. The effect of adhesive thickness on the obtained fracture energies is also analyzed. The CCP technique can be used to find both the crack initiation and crack propagation toughness values whereas the ENF method only provides the crack initiation fracture energy.