Effects of test fixture connections and interference of the BARC structure on its dynamical responses

Abstract

The Box Assembly with Removable Component (BARC) structure has been recently introduced as a challenge problem for the study of the effects of boundary conditions on vibrational testing and modal analysis. Current efforts in studying shaker input excitations on the BARC structure have focused on either varying the degrees of freedom, varying the input signal, or various substructuring methods. This study presents numerical and experimental investigations on the influences of accelerometer location, test fixture connections, fixture interference, and varied excitation methods on the dynamical responses of the BARC system for the purpose of establishing a standard fixture connection and excitation for general testing and test replication. Likewise, this study investigates the previous non-uniformities in the literature between numerical simulations and experimental testing for the dynamical system and fixture interface, both of which are critical for ensuring accuracy between computational methods and the lab environment. Specifically, this investigation is done by varying the distance between bolted connections to compare the effects of stiffness from the fixture connection on the dynamic response. Results from this study show a strong case for using the widest possible connection geometry between the shaker and BARC system in reducing nonlinearities and producing high-quality data. In addition to modal analysis, experimental random and forced vibrations are carried out to determine the dominant frequencies in the system. The experimental measurements are compared to finite element simulations to demonstrate the variability due to fixture interference. The finite element simulations show that the reduction in separation of geometry connections lead to a softening effect in natural frequencies of the system. On the other hand, the experimental measurements indicate that the effects of the distance between the fixture connections have an often-negligible impact on the system's dominant frequencies due to the contact interface for a common experimental setup. The traditional experimental setup is carried out by attaching the system directly to the fixture, allowing for heavy contact between the system and the fixture that leads to discrepancies in the computational and experimental results. In this work, the experimental results are also carried out by adding a novel spacer (nut) between these contact points to provide tests that better match numerical simulations.