Modeling of geometry effects in fretting fatigue
The stress field that results from two bodies in contact is an important aspect that governs the fretting fatigue behavior of materials. Applied loads as well as contact geometries influence the contact stresses. The profile of an indenter and the boundary conditions provide sufficient information from which the surface tractions and the corresponding subsurface stresses have been calculated in a semi-infinite halfspace using singular integral equations. In this investigation, a semi-analytical, Fast Fourier Transform (FFT) based numerical subroutine is developed to calculate the surface tractions and the corresponding surface and subsurface stresses of an arbitrary finite thickness infinite plate subjected to loading through a random indenter. The numerical subroutine is verified with existing halfspace solutions and with Finite Element Methods (FEM). A fretting fatigue fixture is designed and built to conduct fatigue experiments on relatively thin specimens in contact with arbitrary pad geometries. The numerical subroutine is used to calculate the stress field of each experiment. The results from the detailed stress analysis of the contact region are required by both an initiation and fracture mechanics approach. While initiation criteria involving stress gradient fields, such as sharp notches and edges of contact in fretting fatigue, are not well established or agreed upon, stress intensity factor calculations using tools such as weight functions are more reliable. The stress intensity analysis, which is used to determine whether an initiated crack will continue to grow if it is above the threshold, depends on many variables in the stress analysis such as pad and specimen geometry, loading configuration and friction coefficient. The contact stress analysis has been used to determine equivalent stress parameters that are related to the initiation of a crack. Similarly the numerical subroutine for the contact stresses is used in conjunction with the stress intensity analysis to determine the influence of the geometry, loading configuration and friction coefficient on the stress intensity factor. Results from high-cycle fretting fatigue experiments are used to determine the threshold stress intensity factor for a given configuration. The combination of the numerical and experimental analysis is then used to develop a tool for high-cycle fretting fatigue based on a threshold approach involving a go-no go criterion.
Farris, Purdue University.
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