| Description |
Damage tolerance analysis is used by the United States Air Force to maintain the airworthiness of its aircraft through inspection, fatigue crack growth prediction, and residual strength analysis. Cold expansion is commonly used to impart compressive residual stress around fastener holes to slow the rate of fatigue crack propagation and reduce the frequency of inspections. Previous research has shown that the fatigue life benefit of cold expansion is dependent upon hole edge margin and applied loading, resulting in unconservative predictions of fatigue life in some cases. Spectrum loading fatigue experiments were performed on cold-expanded 2024- T351 aluminum alloy specimens with holes at varying short edge margins, which were precracked prior to cold expansion. These tests used a fighter aircraft wing root bending spectrum with 33,000 pounds per square inch maximum stress. Results of fatigue testing were compared various fatigue crack growth prediction tools used by the United States Air Force to evaluate the conservatism of the prediction methods. Results were consistent with previous research and showed that cold expansion increased fatigue life by a factor between 3.44 to 8.22 based on edge margin. This research also found that crack growth analysis using a 0.005 inch initial flaw size did not conservatively predict the growth of cracks at cold-expanded holes with edge margins of 1.30. |
