A novel experimental approach to determine mode I traction–separation relationship for bonded composites-to-metal DCB joints: A back face strain derived method

This paper proposes a novel experimental approach to determine the mode I traction–separation relationship (TSR) and fracture toughness for bonded composite-to-metal double cantilever beam (DCB) joints. CFRP-to-Ti DCB joints, equipped with distributed optical fiber sensors (DOFS) on their back faces, were deliberately designed and tested to validate the proposed method. The back face strain distributions measured using DOFS were utilized to derive the bonded interface TSR based on Euler–Bernoulli beam theory. The TSR derived from the proposed methodology demonstrated strong agreement with that from the established direct method. In addition to determining the TSR, this approach provides extensive insights into fracture behavior, including crack length measurement, cohesive length, and cohesive stress distributions. Moreover, the method is easy to implement in laboratory settings and holds promise for applications under extreme loading conditions.