Dynamic Characterization of Aircraft Shock Cords Used for Free-Free Boundary Conditions for Ground Vibration Testing

In ground vibration tests (GVT), it is often critical to separate rigid body modes from flexible body modes to simulate a free-free boundary condition. This is often done by chaining several aircraft shock cords in series and parallel to achieve a stiffness that results in a sufficiently isolated rigid body bounce mode for the test article. Thus, the stiffness values of these aircraft shock cords are critical in estimating the rigid body bounce frequency. However, the stiffness of these aircraft shock cords are often uncharacterized by the manufacturer or are presented as an elongation percentage at a given static load. One approach to characterizing the stiffness of these shock cords is to measure deflection as a function of an applied static load. Using this approach has typically resulted in significant underestimation of the measured rigid body bounce frequency, potentially leading to less than desirable separation between rigid body modes and flexible body modes. In response to this discrepancy, ATA performed an in-depth study to better characterize the dynamic stiffness of these aircraft shock cords to better estimate rigid body isolation frequencies. This chapter presents the methodology used to characterize the dynamic stiffness of these shock cords and the results of this study, which has significantly reduced bounce mode frequency estimation error.