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The effect of specimen thickness on retardation behavior was investigated by using 12.7, 6.4, and 1.6-mm thick single-edge notched specimens of 7075 alloy in the T6 and T73 conditions and 2024 alloy in the T3 and T8 conditions. Single overload cycles were used with two different overload ratios, and interferometry was used to measure the surface plastic-zone size. The cyclic hardening exponents for all four materials were determined by obtaining cyclic stress-strain curves using incremental strain-cycling tests on 6.4-mm diameter cylindrical specimens. Test results indicated that the amount of retardation decreases with increasing specimen thickness. However, the relative decrease was more predominant in the 7075 alloy and at the higher overload ratio, 2.0. The effect of thickness on the retardation behavior of the 2024-T8 alloy was not conclusive. It appeared that thickness changes have only a minor effect on the retardation behavior of the 2024-T8 alloy. Fractography was used to study the micromechanisms of crack growth. Well-defined striations and changes in the striation spacings after an overload cycle were not seen in all specimens studied, and no striations were observed immediately following the overload cycle. The lack of discernable striations could be due to: abrasion, difficulty in resolving striations associated with relatively low crack-growth rates, change in crack-propagation mode, or a combination of all of these factors. However, the agreement between the measured da/dN and the striation spacings was good for all the specimens in which striations were discernable.
crack propagation, fracture (materials), aluminum alloys
Engineering specialist, Metallics Research Department, Northrop Corporation, Hawthorne, Calif.