Towards pure mode I loading in dissimilar adhesively bonded double cantilever beams

Nowadays, dissimilar adhesive joints are used widely in different industrial sectors. For this type of joints, the investigation of the fracture behaviour under pure mode I loading is a vital issue. In order to study mode I fracture parameters of adhesive joints, scientists have proposed standard double cantilever beam (DCB) specimens. In dissimilar DCB adhesive joints, because of material asymmetry, obtaining pure mode I is challenging. To obtain pure mode I loading in dissimilar DCB joints, scientists have proposed two criteria based on (1) the flexural stiffness and (2) the distribution of the longitudinal strain along the bondline. However, the achievement of pure mode I in dissimilar DCB joints needs more research. In this paper, the mode-mixity in dissimilar DCB joints designed based on the two mentioned criteria is investigated numerically and experimentally. Metal-composite DCBs were fabricated and tested. During the tensile tests, the horizontal and vertical displacement fields of the adhesive layer were calculated using digital image correlation (DIC) and the finite element method (FEM). Based on the obtained displacement fields and using Williams’ series expansion, the mode-mixities were investigated. The results show that in order to achieve pure mode I loading in steel-composite joints, the criterion based on longitudinal strain is more suitable and in aluminium-composite joints the mode-mixities given by two criteria are almost the same.