FATIGUE STRENGTH DESIGN AND ANALYSIS OF AIRCRAFT STRUCTURES. PART I. SCATTER FACTORS AND DESIGN CHARTS
Final rept. Dec 1965-Sep 1966
DOUGLAS AIRCRAFT CO LONG BEACH CA AIRCRAFT DIV
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Aircraft fatigue strength design and analysis concepts were investigated in the areas of fatigue life scatter factors and fatigue strength design-analysis charts. A fatigue scatter factor is defined as the ratio of the mean life to the life for a specified probability of failure and confidence level. For design purposes, operational life scatter factor are defined in terms of the joint probability distribution of the applied loads spectra variation in a fleet of aircraft and the basic fatigue life scatter represented by fatigue test data. Basic fatigue life scatter properties for aluminum alloy materials and structures were statistically derived from a fatigue test data survey of over 6,000 specimens. The basic scatter derived frequency and probability distributions greatly deviate from the log Normal distribution beyond micron or -2 sigma. Several joint probability distribution models illustrate the procedure of calculating operational life scatter factors. An actual aircraft service failure history is accurately predicted by the joint probability distribution concept. A procedure for the development of fatigue strength design-analysis charts is outlined and illustrated by several examples. The charts, in the form of damage rate curves, are defined by generalized loads spectra parameters and the fatigue quality of the structural element.
- Metallurgy and Metallography