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Finite element and fatigue analysis of welded joints with application to disc filter core pipes

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Title: Finite element and fatigue analysis of welded joints with application to disc filter core pipes
Author: Penner, Kerry Lee
Degree: Master of Applied Science - MASc
Program: Mechanical Engineering
Copyright Date: 1997
Issue Date: 2009-04-28
Series/Report no. UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/]
Abstract: In the pulp and paper industry, rotary vacuum disc filters are commonly used as save-all devices to reclaim fine, reusable fibers from mill white water systems while simultaneously clarifying white water for the purpose of recycle. A complete failure analysis was performed on one of Ingersoll-Rand's type 316L stainless steel disc filters which suffered severe cracking in the outer shell of the center support frame or core pipe. Based on a thorough visual examination of the fracture surfaces, aided by low power and scanning electron microscopy, the cracking problem was found to be caused by corrosion fatigue emanating from lack of penetration root defects in the circumferential weld seams. The ensuing research program was focused on the development of a numerical procedure (i.e. finite elements) to evaluate the crack propagation rate in a welded joint. A new finite element sub-modeling technique was first developed to facilitate the transfer of boundary conditions from a coarse finite element model to a refined sub-model. The proposed technique resolves a number of frequently encountered boundary condition incompatibilities and provides an alternative approach to finite element sub-modeling or sub-structuring. Various concepts relevant to the study of fracture and fatigue in welded joints were then considered. A method for predicting the fatigue crack growth rate in a welded joint with residual stress was used in conjunction with the proposed sub-modeling technique to predict the fatigue life for a new core pipe design. The original Ingersoll-Rand design was also modeled to assess the accuracy of the procedure since the actual equipment service life was known. The fatigue lives were determined in both air and simulated white water (i.e. NaCl) environments. The calculated fatigue life of the original design in air was much longer than the actual equipment service life; therefore, it was concluded that pure mechanical fatigue does not accurately describe the failure mechanism. Moreover, the calculated fatigue life in NaCl is much closer to the actual service life of the equipment. The new design's superiority is also evident since its fatigue life in NaCl is more than eight times that of the original design.
Affiliation: Applied Science, Faculty of
URI: http://hdl.handle.net/2429/7653
Scholarly Level: Graduate

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