Developing a Consistent Resonance-Induced Fatigue Testing Method on Novel Freely Supported Specimens

A fatigue test employing vibration-based fatigue known as the “Free-Free” has been in use inside the Turbine Engine Fatigue Facility (TEFF) of the Air Force Research Laboratory. The vibration-setup utilizes free boundary conditions by suspending a specimen on its nodal line that when excited, allows for deflection in its first bending mode. The excitation source is an electromagnet coil that attracts and repels a permanent magnet located on the specimen. The experimental specimens, 0.4 mm cold-rolled Ti6Al-4V, are thin specimens representative of geometries that undergo large amounts of cyclic loading during system operations expected for supersonic and hypersonic aerospace technologies. The Free-Free offers an ideal solution to addressing key structural assessment needs for these critical technologies. In the past, the Free-Frees’ usage in the TEFF has resulted in inconsistent experimental setups that lead to increased risk in data repeatability. Diagnosing the known and unknown inconsistencies in the vibration test setup and strategizing solutions to fix the issues are of the utmost concern. Complications in the test setup parameters are artifacts of the uniqueness of the Free-Free test itself. Therefore, a greater understanding of how to achieve reliable and consistent Free-Free fatigue results is crucial for the advancement of aerospace structures and systems.