Fatigue Life Analysis of Rail-Welds Based on Linear Fracture Mechanics
No Thumbnail Available
Date
2016-02
Authors
Journal Title
Journal ISSN
Volume Title
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.
Description
Keywords
Rail-Weld, Ultrasonic Test, Paris Law, Linear Fracture Mechanics, Reliability Analysis, Inspection Interval