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Existing methods for predicting the shear stresses led to calculation results of great differences. In this paper, the shear stress was primarily evaluated by incorporating typical bond-slip (δ-τ) constitutive models and some test results obtained by the authors and other researchers. Then a new simplified and reliable finite element method (FEM) model named "beam segment model" is proposed and validated. Based on this model, the maximal value and distributions of interfacial shear stress in FRP strengthened beams under the influence of various parameters are calculated. The calculation results show a basic agreement with the existed experimental studies. It is indicated that the shear stress may reach the ultimate value τu even when the FRP tensile stress is at 30% of tensile strength. Based on the research results of both experimental studies and calculated analyses, it is consequently concluded that the interface shear stress should not be used to judge the debonding failures. It is also suggested that maximal relative slip between FRP and concrete may be better to be used as the debonding failure criterion.