Experiment and fractional derivative rheological modeling of the shear creep of HFRP overlap joints under dry and moist environments

The long-term performance of overlap adhesion, one of the major joint technologies of FRP, is key to its practical engineering application. Experiments on the creep behaviors of the double lap joint of a hybrid FRP (HFRP) were conducted. Obvious creeps were observed and measured under various environments reflecting different levels of shear stress and humidity. Then, a fractional derivative rheological model was proposed to characterize the creep deformation of the overlap joints. By applying the Mittag-Leffler function to the proposed model, an improved Powell optimization algorithm with an initial-value calculation method was utilized to determine the values of the fitting parameters on the basis of the experimental data obtained. By introducing stress influence functions to the conventional fractional calculus, a modified formula of fractional creep compliance was considered to reflect the influence of stress level on the creep property of the overlap joint. Results indicate that the model is capable of predicting the creep behavior of overlap joints up to a sustained stress level of 30%-70% of the shear strength with relative simple mathematical expressions and only a few fitting parameters.