EFFECTS OF PURITY ON FATIGUE AND FRACTURE OF 7XXX-T76511 ALUMINUM EXTRUSION

The 7050 and 7049 aluminum alloys were developed by Alcoa and Kaiser Aluminum as improvements over the currently used stress corrosion susceptible high-strength 7075-T6 and 7079-T6 compositions, and over the stress corrosion resistant but lower strength 7075-T73. As part of Lockheed's continuing program to correlate fatigue crack growth with fracture toughness, tests were conducted to compare the effects of various impurity levels (iron and silicon) on the plane strain fracture toughness and fatigue crack propagation rates of these alloys. The lower impurity compositions showed significantly higher toughness. Fatigue crack growth rates at stress intensity values below 20 MPa √m (18 ksi √in.) showed little variation; however, for stress intensity values greater than 20 MPa √m (18 ksi √m.), differences were significant. For example, the crack growth rate for 7075-T76511 at ΔK=30MPa √m(27ksi √in.) for the high impurity level was approximately 3.6 times the rate for the low impurity material. Stress corrosion and exfoliation corrosion properties were not affected by the impurity level within the scope of the test. The microstructures and fracture surfaces were examined to correlate differences in fracture and crack growth behavior with constituent particle size and distribution and typical examples are shown in the paper. © 1979 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.