Validation of Free-Free Vibration Test Method for Fatigue and Damping Characterization of Thin Structures

Abstract

A modular vibration-based fatigue test capability that significantly minimizes the effects of boundary conditions has been developed. The system utilizes a specimen with boundary conditions on its node lines, which isolates the gage section for deflection as a free-free beam in its first bending mode. The thin specimen is suspended inside an electromagnet by 6 lbs. monofilament fishing line, and permanent magnets are bolted to the bottom of the specimen. The alternating current inside of an electromagnet attracts and repels the permanent magnets rapidly (~55 Hz for a 0.016? thick specimen), causing the beam to cycle in first bend. This testing capability is ideal for generating and assessing fatigue life of thin specimens requiring large deflections for failure. Understanding and characterizing fatigue behavior of thin components is important, especially since the emergence of additive manufacturing (AM) for small, fatigue susceptible components. An alternate solution is to suppress the vibratory susceptibility of the component. Therefore, the importance of thin coatings capable of providing damping to components is on par with fatigue characterization. In this study, 0.4 mm cold-rolled Titanium (Ti) 6Al-4 V specimen were fatigued and compared to published data. Also, a thin damping coating (titanium nitride, or TiN) was applied to a few Ti 6Al-4 V specimens to assess the performance. Both the fatigue and damping assessments are necessary to validate the free-free test method.