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Determining the Fatigue Life of Composite Aircraft Structures Using Life and Load-Enhancement Factors

Over the past 25 years, the use Of advanced composite materials in aircraft primary structures has 
increased significantly. In 1994, with the Advanced General Aviation Transport Experiments 
program, the National Aeronautics and Space Administration and the Federal Aviation 
Administration revitalized the use Of composites in general and commercial aviation. Driven by 
the demand for fuel-efficient, light-weight, and high-stiffness Structures that have fatigue 
durability and corrosion resistance, modern large commercial aircraft are designed with more 
than 50 percent composite materials. Although there are key differences between metal and 
composite damage mechanics and durability concerns, the certification philosophy for 
composites must meet the same structural integrity, safety, and durability requirements as that Of 
metals. Despite the many advantages, composite structural certification becomes challenging 
due to the lack Of experience in large-scale structures, complex interactive failure mechanisms, 
sensitivity to temperature and moisture, and scatter in the data, especially in fatigue. The overall 
objective Of this research was to provide guidance into structural substantiation Of composite 
airframe structures under repeated loads through an efficient approach that weighs both the 
economic aspects of certification and the timeframe required for testing, while ensuring safety. 
The research methodology reported here consisted Of combining existing certification 
approaches used by various aircraft manufacturers with protocols for applying these 
methodologies. This will permit extension Of the methodologies to new material systems and 
construction techniques. 
This study included data for materials commonly used in aircraft applications, including 
adhesives and sandwich construction. Testing consisted Of various element-type tests and 
concentrated on tests that were generic in nature and were representative Of various loading 
modes and construction techniques. In addition, the database available at the National Institute 
Of Aviation Research was included to expand the data for the scatter analysis. Three different 
techniques were used for scatter analysis Of fatigue data: individual Weibull, joint Weibull, and 
the Sendeckyj wearout model. Procedures to generate reliable and economical scatter and load- 
enhancement factors necessary for a particular structural test by selecting the design details 
representing the critical areas Of the structure is outlined with several examples and case studies. 
The effects Of laminate stacking sequence, test environment, stress ratios, and several design 
features, such as sandwich and bonded joints on the static-strength and fatigue-life shape 
parameters, are discussed with detailed examples. Furthermore, several analytical techniques for 
obtaining these shape parameters are discussed with examples. Finally, the application Of load- 
enhancement factors and life factors for a full-scale test spectrum without adversely affecting the 
fatigue life and the damage mechanism Of the composite Structure is discussed.