M. J. Caton, Air Force Research Laboratory, Wright-Patterson AFB, OH; R. John, US Air Force Research Laboratory, Wright-Patterson AFB, OH
In the aerospace industry, rotating engine components are subject to resonant vibratory loads under various operating conditions. These vibratory loads are typically experienced for only brief periods as the rotational speed of the engine traverses critical modes. These relatively brief bursts of resonant vibration, referred to as “mini-sweeps,” can contribute to the initiation of fatigue cracks, or the propagation of existing fatigue damage. This study investigates the propagation of long and small fatigue cracks in Ti-6Al-4V under mini-sweep loading conditions. Bursts of variable amplitude loading were applied to cylindrical, dog-bone specimens and a compact-tension (C(T)) specimen under a frequency of 20 Hz at room temperature. The loading profile is consistent with a typical mini-sweep experienced in rotating hardware during service. Long crack growth rates under the mini-sweep loading profile were monitored in the C(T) specimen using direct current potential drop (DCPD) and optical observation. The growth of small fatigue cracks, initiated from artificial notches, was monitored using a standard replication technique. The fatigue crack growth behavior under these loading conditions will be presented and the implications for improved lifing methods of critical rotating components will be discussed.
Summary: The Engine Systems Prognosis program, funded by the Defense Advanced Research Projects Agency (DARPA), is aimed at developing technology that will enable real-time state awareness and asset capability assessment for turbine engines. This vision involves the marriage of enhanced sensing capability to detect cracks within structures in real time and models for predicting the progression of damage under usage conditions. This presentation will focus on improvements in predicting how fatigue cracks propagate under bursts of variable amplitude loading experienced in airfoils as the rotational speed of the engine traverses critical modes.
