Abstract

Many assembled structures have bolted or riveted joints. Aside from providing a convenient method of assembly, mechanical joints have an important effect on the structure's dynamic response to its environment. Friction and microslip within the joints help to damp out unwanted vibration. However, the joints also introduce nonlinearity into the response. Most notably, the natural vibrational modes of the structure no longer have constant natural frequencies and damping ratios. Rather, they shift as a function of vibration amplitude. Several methods exist to characterize the dynamics of nonlinear systems, yet they are generally validated on simple benchmark structures, and many still struggle to characterize the complicated, hysteretic behavior that is displayed by mechanical joints. Existing methods need to be applied in more practical applications to determine what improvements are needed before they can be used widely in industry. Chapter 2 of this thesis presents measurements of a slender riveted beam which has similar features to a component on an aircraft engine. Although much research has focused on the nonlinear behavior of bolted joints, riveted joints have not been studied as widely. This work uses the Hilbert Transform, which has proved successful in several previous works, to explore the complex amplitude-dependent behavior of the modes of this riveted beam. It demonstrates how the Hilbert transform can be used in the presence of strong nonlinear features that push the limits of this method. This study results in a set of measurements complete enough to successfully update a finite element model of this structure [1]. Finite element modeling of jointed structures also poses significant challenges. Modeling the joints in detail is often much too computationally expensive, so it is common to simplify the model by replacing the bolts with springs, friction sliders, or joint models. One such joint model, the 4-parameter Iwan element, is able to capture the nonlinear shift in damping and frequency that is observed in jointed structures. The work in Chapter 3 helps to extend this modeling approach by accounting for variations in bolt preload. Preload is known to have a significant effect on the dynamics of a bolted joint. This study first verifies that this modeling approach can capture the response of a bolted structure at several different preloads and then examines the values of the Iwan parameters used at each preload to determine how these parameters evolve as preload changes. This is first done computationally with a 2D model of a stacked cantilever beam, and then the results are validated experimentally using measurements from the S4 Beam [2] to update a reduced-order model. This is seen as a first step toward identifying functional relationships between the Iwan parameters and bolt preload.

Degree

College and Department

Ira A. Fulton College of Engineering; Mechanical Engineering

Rights

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