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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The effect of unidirectional solidification on the mechanical properties of high-strength steels was investigated by means of split heats of AISI 4340 and 25 per cent nickel maraging steel. One half of these heats were unidirectionally solidified and the other half of these melts were allowed to solidify to produce an equiaxed structure. These ingots were forged and rolled to a light gage sheet 0.040 to 0.060 in. thick. The materials were evaluated by means of standard tension tests and fracture toughness studies. By means of compliance measurements, it is possible to determine the crack resistance as a function of absolute crack extension. This investigative technique was used to study the AISI 4340 steel. Unidirectional solidification did not alter the crack resistance properties of this steel. However, a 10 per cent improvement in plane-strain fracture toughness as determined by the pop-in technique was observed for the unidirectionally solidified material. In order to obtain very high strength levels, the total content of hardener elements was increased in the 25 per cent nickel maraging steels. Consequently these materials were quite brittle and the results not definitive. However, it was demonstrated that a homogeneous structure is necessary for high values of fracture toughness in this material. The data were interpreted in terms of critical crack size for instability at yield stress. Both through-cracks and part-through-cracks were considered in this analysis.

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