In the scope of developing a bonded iron-based shape memory alloy (Fe-SMA) strengthening solution, nonlinear deformation in the adherends upon joint failure due to phase transformation and high adhesive toughness is unavoidable. The effect of this nonlinear deformation has not been studied in the case of Mode I failure. In this study, the first experimental and theoretical investigation on Mode I failure of Fe-SMA bonded joints is presented. A new analytical model is proposed and validated using experimental results to gain an in-depth understanding of the influence of Fe-SMA nonlinear material deformation on the joint failure process. The proposed model is shown to be significantly faster than traditional elasto-plastic finite elements using cohesive zone modeling with a minimum compromise to the accuracy. These are observed to result in lower bond strength and a shorter fracture process zone than that of linear elastic adherends using the same adhesive. Neglecting the nonlinear behavior of the Fe-SMA bonded joints can lead to an unconservative joint design, jeopardizing safety. The developed model is aimed at facilitating the development of adhesively bonded Fe-SMA strengthening systems.