Impact of mode shapes on experimental loss factor estimation in automotive joints

This paper presents the experimental work carried out on single-lap joints fastened together with bolts and nuts to investigate the contribution of mode shapes, and the effect that bolt sizes has in dissipating energy in built-up structures. Five different bolt sizes are chosen to assemble five single-bolted single-lap joints using aluminum plates. An analogous monolithic solid piece carved from the same aluminum material is used to determine the material damping and compare it against the damping from bolted joints. The dynamic response of all structures is captured under free-free boundary conditions, and the common modes are analyzed to understand the contribution and primary source of damping in the same range of the sampling frequency. This investigation has revealed that the source of damping in the joints is heavily linked to the mode shapes of the structure and structural damping (also referred to as material damping) contributes more during specific mode shapes compared to the joint damping itself. The findings allow a more accurate implementation of energy loss in automotive structures which contain bolted joints, allowing an implementation of both material and joint loss factor, respectively.