The Effect of Periodic Overloads on Flight-by-Flight Fatigue Crack Growth Rates
Final rept. Jul 1977-May 1979
AIR FORCE WRIGHT AERONAUTICAL LABS WRIGHT-PATTERSON AFB OH
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The life prediction of aerospace structures subjected to service loading conditions can be complex and expensive. Fortunately, many flight-by- flight loadings can be analyzed as constant amplitude loading if the variability in crack growth rate is low. A test program was designed such that the stress intensity factor Kmax was held constant by uniformly reducing the load as the fatigue crack grew. With a constant Kmax, the crack growth rate remains constant throughout the life of the test. The baseline fatigue crack growth rate variability was established for the 7075-T6 center-cracked panels under a flight-by-flight load history. The effect of introducing overloads to the baseline flight history on the crack growth rate variability was reported. The results indicate that the delay region for the applied 114 overloads was predicted by Irwins plastic zone model and was equal to 0.0382 inches which was equal to 225 flights. The delay behavior was consistent for the several overload conditions that were studied. When the occurrence of overload increased, the fatigue crack growth decreased. The standard error of estimate remained relatively constant, suggesting that a block approach to life prediction is feasible for flight-by-flight load history.