Simulation of mechanical loss factor in automotive joints

Mechanical joints have been widely used in the automotive industries to connect structural components due to their secure assembly method and low cost. In contrast to material damping, joint damping appears only at the mechanical joint interface. Numerous studies have investigated the material damping in structures in the past. However, the impact of damping on mechanical joints in engineering structures has not been comprehensively researched, particularly the practical and cost-effective modelling of mechanical joints.

Numerous techniques have been developed and presented over the last few decades to predict the mechanical characteristics of structures under dynamic loading. However, developing a model which is capable of determining the dynamic characteristics and accurately predicting the energy losses of mechanical joints is still a longstanding issue.

Deterministic finite element methods capable of multibody dynamics are widely used in commercial software packages, and the principal intentions are to predict only the eigenvalues, eigenmodes, responses and failure of the systems. Due to the non-linearities and uncertainties in the mechanical joints, these algorithms struggle to address the dynamic characteristics of these joints and hence unable to accurately predict the local energy losses in engineering structures.

This report explains the importance of modelling damping in mechanical joints and outlines the existing work done on mechanical joints. Finite element modelling and experimental work carried out on mechanical joints are explained and the methods used to extract the modal parameters such as damping loss factor are discussed.

Single-lap joints are assembled from aluminium beams, and five different bolt sizes are used to investigate the energy dissipation in engineering structures. To isolate the joint effects and directly compare the gathered data of the bolted single-lap joint structures, an analogous monolithic solid piece beam is prepared in the laboratory, with exact geometric and mass distribution as the assembled structure. The dynamic response of the assembled structures and the analogous monolithic solid piece beam exposed to forced excitation is captured under free-free boundary conditions. A finite element analysis is carried out to identify the motion of the assembled structures and the monolithic beam.

The effect of the bolt size on the source of damping, the change in bolted interface region and the influence of mode shapes on modal damping in assembled jointed structures during external dynamic loading are investigated as the wavelength of resonant vibration changes the pressure distribution in the joint. The introduction of the loss factor ratio enables to understand the source of damping in assembled structures and the influence of the fastener size used to build the jointed structures. Furthermore, the interface regions are increased by using washers during the assembly of jointed structures to determine the effect of the increase in interface regions on modal damping in assembled engineering structures during external dynamic loading. Also investigated is the contribution of mode shapes of jointed structures in dissipating energy during forced excitation is studied in detail.