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For the case of an elliptical notch in an infinite solid, a relationship between the stress concentration factor and the fracture toughness parameter was examined. Edge-notched specimens from three high-strength aluminum alloys were tensile loaded to failure. The resulting data were analyzed in the light of this relationship. It was indicated that a predicted proportionality between the fracture toughness parameter and the square root of the notch root radius exists. Further examination of the relationship based upon the proportionality showed that fracture occurs at a fixed state of strain within a plastic zone having a size proportional to the original root radius. However, a departure from the predicted behavior was evident with the introduction of plane strain components at the notch root. It was also demonstrated that the use of specimens with intermediate root radii for either the evaluation of a single material or as a basis of comparison between materials can lead to invalid conclusions. The reversion of fracture toughness data from blunt notch specimens to stress concentration factors is shown for one alloy. Due to a constancy in the ratio of the fracture parameter to the nominal stress, the resulting factor lacks sensitivity.

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