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The plane-strain fracture toughness of precipitation-hardening aluminum alloys of 7000 and 2000 series and a strain-hardening alloy 5456 have been determined at both room temperature and −320 F using circumferentially notched rounds. These results show that the plane-strain fracture toughness is an inverse function of the yield strength and that at equivalent yield-strength levels the 7000 series of alloys is tougher than the 2000 series of alloys. Plane strain-fracture toughness values were determined using “pop-in” technique employing both the center crack and single-edge-notch specimens. Comparable values were obtained in all examples tested. The effects of impurity elements, iron and silicon, on the fracture toughness of 7075-T6 aluminum alloy were investigated using a special low iron-and-silicon melt of 7075-T6 material. Reduction of these impurity elements resulted in a 30 to 45 percent upgrading of the plane-strain fracture toughness of this alloy. These data have been interpreted in terms of the process zone size, dT, using electron microfractography as an indication of this parameter. The plane strain-fracture toughness values have been used to calculate the breaking stress of part-through-cracked panel. These calculations have been confirmed experimentally for two alloys. Such data have direct applicability in the design of structures.

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.

Criteria for predicting pressure vessel performance based on fracture toughness are reviewed in general terms. Experimental studies of small pressure vessels fabricated of high toughness, high strength steel 4330V (Mod + Si) are described. Data presented include fatigue life in presence of a small part-through-crack and burst properties of the fatigue cracked cylinders. Interpretation of the fatigue data is based on Paris’ relationship dadN=K4M. The failure stresses are discussed in relation to the stress elevating effect of local bulging on the apparent fracture toughness. The behavior observed in testing full scale high strength pressure vessels fabricated from materials having intermediate fracture toughness, namely, D6A steel at 200,000-psi yield strength and 300M steel at 230,000-psi yield strength and materials having limited fracture toughness, namely, twenty percent nickel maraging steel at 280,000-psi yield strength, are discussed in relation to the ratio of fracture toughness to plane-strain fracture toughness based on the part-through-crack model. Precautions necessary for fabrication and inspection to insure reliable performance are discussed.