Considerations for Measurements of Jointed Structures

The nonlinearity inherent in jointed structures is invariably affected by the boundary conditions, loading conditions, and measurement techniques. Small changes in the experimental setup can significantly affect the measured damping and stiffness of a joint. In this chapter, a series of experiments are reported that are conducted on both a monolithic beam and a jointed 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 even though each accelerometer is approximately 5 g (or about 0.1% of the mass of the beam). 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.