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Spectrum Fatigue of 7075-T651 Aluminum Alloy under Overloading and Underloading

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Technical Report

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Naval Air Warfare Center Aircraft Division Patuxent River United States

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A study was conducted to clarify the effects of overload, underload, stress ratio, and environment on fatigue crack growth. Fatigue crack growth tests were conducted with a 7075-T651 aluminum alloy under constant amplitude, and over- and under-loading of two different stress ratios, 0.1 and 0.85, in vacuum, air and 1 percent NaCl solution. Under constant amplitude loading, dadN of R0.1 and 0.85 was mostly greatest in 1 percent NaCl solution, intermediate in air and lowest in vacuum, and it was greater at R0.85 than at R0.1. Furthermore, the dadN under constant amplitude loading was similar to that of underloading at high K. On the other hand, dadN was greater under underloading than under overloading, greater in 1 percent NaCl solution, and greater at R0.85 than at R0.1.To account for the load spectrum sequence effects, the recently developed cycle-by-cycle fatigue crack growth FCG model, UniGrow, was chosen in this study. This model regards the FCG as a process of successive crack re-initiation in the crack tip region, controlled by a two- parameter driving force. Employing the UniGrow equation, the variation of crack length with number of loading cycle was predicted. The prediction and the fatigue test life were found to agree fairly closely.

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