Influence of pinned joints on damping and dynamic behavior of a truss

This paper discusses measured damping characteristics of a three-bay truss that uses pinned joints. A clevis/tang joint design was used. Pinned joints normally have a small amount of clearance between the pins and the clevis/tang fittings. The small gap can cause very significant changes in the dynamics and damping of a truss structure, particularly when the joint can traverse the deadband region. If the gravity induced loads prevent a joint from entering the deadband region, one might expect damping to be effected. Test results demonstrate that structures using pinned joints can have damping that is dependent on gravity. Damping rates can change by a factor of up to five due to variation of gravity induced loads. This variation could be very significant when characterizing space structures in a 1-g environment. The data also suggest that impact, as the joint gaps open and close, transfers vibration energy from low-frequency global modes to high-frequency local modes, increasing the rate of energy dissipation through material damping.