A correlation between plastic deformation at crack tips in sodium chloride and the measured value of the fracture surface energy is presented. Plastic deformation can either aid or hinder crack growth, depending on the mode of deformation at the crack tip. If plane‐stress deformation occurs, crack motion is hindered by step formation, dislocation generation, and plastic blunting of the crack tip. If plane‐strain deformation occurs, crack motion is aided by stress fields that arise from the deformation. The specific surface free energy of sodium chloride, {100} plane, is estimated to be less than 0.37 J