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The factors that affect the fatigue performance of shape memory alloys (SMAs), including fatigue crack growth (FCG) response, is far from being well-understood. In this study, we point to a mechanism that degrades the FCG performance considerably. We introduce the notion of FCG being affected by shielding and deshielding mechanisms, the former enhancing the resistance while the latter reducing the mate- rials’ resistance. We show that the deshielding mechanism creates additional driving forces (positive K contribution) of both Mode II and Mode I types (as much as 5e10 MPa m1/2) which accelerates the crack advance. The origin of the positive K component is associated with the localized martensite variant formation that is highly asymmetric with respect to the crack tip. We derive a resultant DK in excellent agreement with that measured based on experimental displacement measurements. Overall, this study represents an advancement of our understanding in FCG of SMAs by quantifying the deshielding mechanism.