| The following dissertation describes research work evaluating the effects that frequency of loading and material anisotropy have on the corrosion fatigue crack propagation behavior of aluminum alloy 2224-T3511. This information is particularly valuable to the commercial aircraft industry, as much of the modern commercial airframe structures are manufactured from aluminum alloys.; Chapters 1 through 3 describe the nature of the problem and what research has been accomplished in this area of corrosion fatigue. A discussion of damage tolerance, the epistemology of corrosion fatigue, and a description of the mechanisms of corrosion fatigue crack are given. A brief review of the metallurgy of aluminum alloys is also discussed.; Chapters 4 through 6 discuss the test methods used in this research. Fatigue crack growth rate tests were conducted to ASTM standard E 647-93 on side-grooved compact tension C(T) specimens at loading frequencies of 0.1, 1.0, and 10.0 Hz. in a 3.5% NaCl aqueous solution, and at 1.0 and 10.0 Hz. in a 10% relative humidity (RH) dry air environment. Two orientations, the L-T and the T-L orientations, were used to evaluate material anisotropy.; Chapters 7 and 8 include the results of scanning electron microscopy (SEM) work, and a final review of this research. Pitting, inclusions, and fatigue striations were all observed in the SEM work, and show strong environmental and material anisotropy effects on the fracture surface of the C(T) specimens. The fatigue crack growth rate tests, in general, showed that decreasing the frequency of loading in the 10% RH dry air environment accelerated fatigue crack growth rates in both the L-T and T-L orientations. In the 3.5% NaCl aqueous solution, frequency of loading had inconclusive effects on the fatigue crack growth rates. The 3.5% NaCl aqueous environment accelerated fatigue crack growth rates at both 10.0 and 1.0 Hz., as opposed to the 10% RH dry air environment. The effects of material anisotropy on fatigue crack growth rates, evaluating the L-T against the T-L orientations, were inconclusive. |
