How pure mode I can be obtained in bi-material bonded DCB joints: A longitudinal strain-based criterion

An essential question to predict the structural integrity of bi-material bonded joints is how to obtain their fracture properties under pure mode I. From open literature, it is found that the most commonly used design criterion to test mode I fracture is matching the flexural stiffnesses of the two adherents in a DCB coupon. However, the material asymmetry in such designed joints results in mode II fracture as well. In this paper, a new design criterion is proposed to obtain pure mode I fracture in adhesively bonded bi-material DCB joints by matching the longitudinal strain distributions of the two adherends at the bondline - longitudinal strain based criterion. A test program and Finite Element modelling have been carried out to verify the proposed design criterion using composite-metal bonded DCB joints. Both the experimental and numerical results show that pure mode I can be achieved in bi-material joints designed with the proposed criterion. G_II/G_I ratio is reduced by a factor of 5 when using the proposed longitudinal strain based criterion in comparison with the flexural stiffness based criterion.