Consolidated Presentation of Fatigue Data for Design Applications

Analytical methods have been developed to consolidate fatigue data for use in design of aerospace metallic structures. Fatigue data from constant-amplitude, uniaxially loaded specimens of 2024 and 7075 aluminum alloys, Ti-6A1-4V alloy, and 300M steel were consolidated and statistically analyzed. An equivalent strain parameter was used to account for the influence of stress ratio on fatigue life. Notched-specimen data were consolidated by using a local cyclic stress-strain approach to account for fatigue damage at the notch root. An arc-hyperbolic tangent function was used to relate the equivalent strain parameter to the common logarithm of fatigue life. After computing mean fatigue curves by least-squares regression, 90 and 99% lower-level tolerance curves were determined for a 95% level of confidence. These curves were constructed to represent a statistically based lower bound on fatigue life for any combination of strain amplitude, maximum stress, and stress-concentration factor within the defined boundaries of the function.