Plane-Strain Fracture Toughness of High-Strength Aluminum Alloys

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, d_T, 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.