Henrik Wentzel

Numerical prediction of damping in structures with frictional joints

In many vehicle and space structures, friction in bolted joints is the primary source of energy dissipation during vibrations. Several simplified finite element models have proven their capacity to describe the energy dissipation owing to micro-slip in joints. However, these simplified models require extensive physical testing in order to extract model parameters. This paper proposes a methodology based on non-linear FE-analysis of the loading on joints for numerical evaluation of the energy dissipation due to micro-slip. The methodology is extended to allow for computation of modal damping in vibrating structures. Presented applications include studies of an isolated joint as well as a more complex structure subjected to dynamic excitation.

Influence from Contact Pressure Distribution on Energy Dissipation in Bolted Joints

Energy dissipation due to micro-slip in joints is the primary source of damping in many vehicle and space structures. This paper presents results on how the surface topology may be modified to increase the energy dissipation in joints. An analytical solution for general forms of contact pressure of a one-dimensional micro-slip problem is presented. The solution indicates how the contact pressure should be distributed to maximize the energy dissipation. Two dimensional contact pressures are optimized using finite element methods in combination with numerical optimization methods and the results are used to modify the surface topology in bolted joints in order to increase the energy dissipation during loading. The predicted increase of energy dissipation is validated with physical testing. A direct result of the study is a washer with varying thickness increasing the energy dissipation in joints and hence the structural damping of joined structures.

Metallic inserts as a tool to alter the structural damping of joined structures

The dynamic response of joined structures, notably the amplification of vibrations, is strongly influenced by the characteristics of the joints. Bolted joints are non-linear both in stiffness and magnitude of energy dissipation, and structures assembled with bolted joints inherit these non-linearities. This largely experimental study shows how thin metallic inserts in the contact region may serve as a tool to alter the non-linear properties of the joint and in this way increase or decrease the level of equivalent viscous damping in the structure. Both quasi-static and dynamic measurements have been performed and a mathematical model trimmed to the static measurements has been shown to produce valid results also for dynamic simulations.