Abstract: To meet the integrity requirements of advanced lightweight bonded structures, this study designed conventional (titanium alloy-titanium alloy, Ti-Ti) and bi-material (carbon fiber-reinforced polymer-titanium alloy, CFRP-Ti) double cantilever beam (DCB) bonded joints capable of achieving pure Mode I fracture based on the longitudinal strain consistency criterion, focusing on the fracture behavior of CFRP-Ti bi-material bonded joints (a key load-bearing component in lightweight structures) under Mode I loading; meanwhile, integrating distributed optical fiber sensing (DOFS) technology with Euler-Bernoulli beam theory, a method for deriving the interface traction-separation relationship (TSR) and fracture toughness ( ) based on surface strain measurements was proposed. Through systematic investigation of the fracture behavior of the two types of joints under Mode I loading, the influence mechanisms of different adherend combinations on the interfacial TSR, , and fracture process zone (FPZ) were clarified. Results show that the proposed method can effectively obtain key parameters throughout the fracture process, including TSR, fracture toughness, crack propagation length, and cohesive zone size, and inputting the measured experimental data into finite element simulations enables accurate reproduction of the experimental process. Under pure Mode I loading, the fracture toughness values of the conventional and bi-material joints are 0.45±0.04 N/mm and 0.49±0.08 N/mm, respectively, indicating that the macroscopic fracture toughness and cohesive constitutive relationship are mainly dominated by the adhesive layer's own properties with no significant influence from the adherend materials; however, the stiffness mismatch of adherends in the bi-material joint significantly affects the mechanical behavior of the FPZ, which exhibits a higher peak traction (20±1 MPa) and distinct traction distribution characteristics. By extracting the traction distribution at the interface, precise quantification of the crack length and FPZ is achieved, providing important theoretical basis and data support for the failure analysis and performance evaluation of advanced lightweight bonded structures.

Key words: TSR, DCB, Mode I Fracture, Adherends, Surface Strain, DOFS