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.

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