The Effects of Boundary Conditions, Measurement Techniques, and Excitation Type on Measurements of the Properties of Mechanical Joints

Abstract

This paper investigates how the responses of mechanical joints are influenced by using different experimental setups. The experiments are conducted on both a monolithic beam and a bolted beam, and the beams are excited by hammer tests and a shaker. Multiple boundary conditions are also studied. It is found that the hammer tests performed on the “free” boundary condition monolithic beam (for multiple bungee lengths and positions) had a negligible influence on the system in terms of damping ratio and frequency variation. Multiple sensors attached to the monolithic beam are studied; the effect of multiple accelerometers manifests as a significant shift of frequency and damping due to the additional mass. In the case of the jointed beam, both mirror-like and rough interfaces are used. Several sets of different interface pairs, bolt torques, bolt preloads, excitation frequency sweep rates and bolt tightening orders are considered in this study. The time varying changes in stiffening and damping are measured by testing multiple combinations of the experimental setup at different levels of excitation. The results showed that the mirror-like surface finish for the interface has higher damping values compared to the rough surface across multiple bolt torque scenarios (such as preload and tightening order) and modes of vibration. Guidelines for a more reliable measurement of the properties of a mechanical joint are made based on the results of this research.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U. S. Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000