Approximating Damping Coefficient of Bolted Joints using the Finite Element Method

Abstract: Bolted joints are important due to their energy dissipation property in structures,but the damping mechanism is also highly nonlinear and localized. The goal ofthis thesis is to develop an accurate method for modeling bolted joint dampingin large structures using finite element (FE) software. To model bolted jointdamping, the first step is to study the mechanism and define the terms like slip,micro-slip, and macro-slip. An extensive literature review identified the necessarymethods: detailed contact model, thin-layer elements, and connector elements.These methods are compared based on parameters such as computation time,modeling time, etc. The thin-layer method was used for modeling bolted jointdamping in large structures. To evaluate parameters for thin-layer element modeling, a local joint model wasbuilt using contact formulation of an engine housing and ladder frame assembly.The computed parameters include normal stiffness, tangential stiffness, and lossfactor. Analysis reveals that the loss factor depends on pre-load and amplitudeload. The micro-slip is the region of interest where the loss factor was computed. Using curve-fitting, a range of amplitude-dependent loss factors was calculated. Finally, the thin-layer elements are used in the engine housing and ladder frameassembly to model bolted joint damping. The parameters estimated using the localjoint model are used to define the properties of the thin-layer elements such thatthe elements are a phenomenological representation of bolted joint. A mode-basedsteady-state analysis has been performed to estimate the loss factor on a systemlevel. The frequency response of such an analysis accurately captures the frequencyresponse curves of structures with bolted joints. The two important behaviors thathave been captured are the shifting of the resonance peak to a lower value and thewidening of the frequency response curve as the applied load increases. However,the resonance frequency shifting to a lower frequency (softening) has not beencaptured due to modeling limitations in the FE software. A substructure couplingmodel using the Craig-Bampton formulation of the engine housing and ladderframe assembly has been analyzed using a constant loss factor. The frequencyresponse of such a system appears to give an approximate behavior of a structurewith bolted joint damping.