Modal Iwan Models for Structures with Bolted Joints

Structures with mechanical joints are difficult to model accurately; even when the natural frequencies of the system remain essentially constant, the damping introduced by the joints is often observed to depend nonlinearly on amplitude. Although models for individual joints have been employed with some success, the modeling of a structure with many joints remains a significant obstacle. This chapter explores whether nonlinear damping can be applied in a modal framework, where instead of modeling each discrete joint within a structure, a nonlinear damping model is used for each mode of interest. This approach assumes that the mode shapes of the structure do not change significantly with amplitude and that there is negligible coupling between modes. The nonlinear Iwan joint model has had success in modeling the nonlinear damping of individual joints and is used as a modal damping model in this work. The proposed methodology is first evaluated by simulating a structure with a small number of discrete Iwan joints (bolted joints) in a finite element code. A modal Iwan model is fit to simulated measurements from this structure and the accuracy of the modal model is assessed. The same methodology is then applied to actual experimental hardware with a similar configuration. The proposed approach seems to capture the response of the system quite well in both cases, especially at up to moderate force levels when macroslip does not occur.