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Fatigue Life Determination of a Damage Tolerant Composite Airframe - Dec. 2008

The methodology proposed in this research extends the current full-scale test approach 
based on the life factor and the load enhancement factor, and provides information necessary to 
define inspection intervals for composite structures by studying the effects Of extremely 
improbable, high-energy impact damage. This methodology further extend the current practice 
during damage-tolerance certification to focus on the most critical damage locations Of the 
structure and interpret the structural and loads details into the most representative repeated load 
testing in element level to gain information on the residual strength, fatigue sensitivity, 
inspection methods and inspection intervals during full-scale test substantiation. A reliability 
approach to determine the inspection intervals to mitigate risks Of unexpected failure during the 
damage tolerance phase, especially with large impact damages, was discussed. This 
methodology was validated with several full-scale test examples Of the Beechcraft Starship 
forward wings with large impact damages on the front and aft spars. 
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.