Fatigue Life Analysis of Rail-Welds Based on Linear Fracture Mechanics

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Date

2016-02

Authors

Yalelet, Endalemaw

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Publisher

Addis Ababa University

Abstract

Engineering Structures can fail in various ways including yielding, buckling and brittle fracture. While all materials suffer fatigue, the effect is particularly pronounced for ductile materials, such as metals and glassy polymers. For such a ductile material, fracture may occur after a small number of cycles, when cyclic loads repeatedly cause large-scale plastic deformation. In most engineering structures, however, loads are kept small so that the structures deform elastically. Even so, a crack may initiate somewhere in such a structure, and extend by a small amount during each cycle of the load. Rail welds are the weak link in continuously welded rails i.e. Small imperfection in welds can cause cracks to initiate. Detection and rectification of rail-weld defects are major issues for all rail players around the world. If undetected and/or untreated these defects can lead to rail breaks and derailments. These are challenges to perform effective inspection and cost-effective maintenance decisions. If these issues are addressed properly, inspection and maintenance decisions can reduce potential risk of rail breaks and derailments. This thesis work was majorly aimed at determining probability of failure as a result of cyclic fatigue loading of the wheel on the Weldments so as to arrive at an inspection frequency. Primarily, a finite element model (Abaqus model) was used to estimate the contact stress between the wheel section and the rail section at the weld and at the base rail, as well. Second, flaw (i. e the presence and size of cracks) was determined using an Ultrasonic flaw detector at different locations. Third, the model was updated to fracture mechanics-based fatigue models (FRANC3D model), based on Paris law with a flaw inserted inside. There by, the critical crack size and the number of cycles to failure was determined as 0.146955mm and 20,957,747 cycles of wheel passage, respectively. Then, probabilistic reliability model was developed with uncertainties in the detected crack size and final crack size. Thus, using the LEFM approach the reliability to failure was determined using Advanced First Order Reliability Method, Hasofer-Lind Reliability Index, as 96.69%. Finally, addressing some of the issues associated with probability of failure, an inspection frequency was suggested to be in five months interval based on reducing the long-term life cycle cost. Last, a recommendation is set to future researchers for a development of a detailed maintenance strategy.

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Keywords

Rail-Weld, Ultrasonic Test, Paris Law, Linear Fracture Mechanics, Reliability Analysis, Inspection Interval

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