Landes and Begley’s definition of J has been applied to each group of specimens analyzed enabling the fracture toughness variation to be known depending on the level of
pre-strain reached in the material. It was found that is not advisable to take the maximum load point as a reference point to apply this definition. However, if the 70% of maximum load point is used, the obtained results reveal a linear decreasing fracture toughness tendency under the influence of biaxial pre-strain. These results also reveal that the value of toughness for the base material (without biaxial pre-strain) is in the environment of CT10 and CT15 specimen behavior (plane stress). So, applying the methodology presented in this paper it is possible to estimate the fracture toughness of the material in plane stress conditions.