Creep characteristics and rheological mechanical model of CFRP-steel interface

Objective The study investigated the creep characteristics of adhesive at CFRP-steel interface, and obtained the viscoelastic constitutive model characterized by creep characteristics. The study objects were twelve CFRP-steel double-lap specimens. A self-developed loading device based on the lever principle was employed to carry out sustained loading tests over 2 160 hours. The influence of three different interfacial shear stresses on viscoelastic properties of CFRP-steel interface adhesive was investigated. Method The normalized creep compliance of the generalized Hook-Kelvin model was derived based on viscoelastic theory. The nonlinear regression analysis was performed on the experimental data by using the least square method, yielding an expression for the normalized creep compliance parameters. The relation between each parameter and shear stress was clarified. A comparative evaluation was conducted between the predicted creep compliance values and the experimental data. Result CFRP strain distribution exhibited the nonlinear relation with high strain values observed in the area near the loading end. Stress was transmitted only within a partial area near the loading end. The strain at each measuring point increased with loading time, but the rate of increase gradually decreased. When the ratios of preloaded nominal shear stress to ultimate shear stress were 0.40, 0.55, and 0.70, the ratios of creep strain to elastic strain at the first strain measuring point of each group of specimens were 0.115, 0.201, and 0.246 respectively. The corresponding normalized creep compliances were 4.81, 5.27, and 5.94 times their initial values. The shear elastic modulus indicated a linear relation with shear stress by using H-K-K model, while the viscous coefficient followed a quadratic polynomial relation with shear stress. The creep compliance predicted with H-K-K model deviated only slightly from the experimental values, with RMSE of 0.103. The five-element H-K-K model is recommended for creep stress analysis on bonded interface in steel structures strengthened with externally bonded CFRP under sustained loading. Conclusion The adhesive exhibits creep behavior under sustained loading. The higher the applied stress, the greater the creep deformation. The H-K-K rheological model could be employed as a viscoelastic constitutive model to analyze the creep stress at bonding interface of CFRP-strengthened steel structures.