On It was noted that the stress caused

 

On the
morning of June 3, 1998, ICE train 884, the “Wilhelm Conrad Rontgen” (WCR),
consisted of a single locomotive pulling 12 cars, including passenger coaches,
buffet car, service car and a rear locomotive derailed about 6km outside of
Eschede, in Lower Saxony. A total of 101 people lost their lives and 200
sustained injuries. The cause of the accident was attributed to fatigue. It was
noted that the stress caused by the wheel rims being flattened into an ellipse
with each revolution up to (500000 times a day) resulted in development of
cracks inside the wheel rim leading to failure. The thinning of the rim
exaggerated dynamic forces causing micro cracks to grow larger.

After the
accident, all ICE operations were suspended pending the outcome of the full
investigation. Consequently, the wheel-tire design was completely discontinued
throughout Germany and was replaced by a mono-block wheel design. Charges of
manslaughter were brought against two Deutsche Bahn officials and one engineer
in August 2002. Financial settlements were made in April 2003 after court
battles which lasted at least 53 days. Aerial view of the derailed train is
shown in Appendix C.

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Preliminary Literature
Survey

Adhesion
Contact Theories

Whenever
we travel, the vehicle we are riding in will speed up and slow down several
times during the course of the journey. Manoeuvres like these require force and
trains in particular need to generate a lot of tractive force in order to
overcome rolling resistant. A rail wagon typically carries a load of about 10
tonnes and this load rests on a small area roughly 10mm across. The pressures
at this interface are very high and considerably in excess of the normal yield
stress of the material. Generally, there are two types of contacts namely
Conforming and Non-Conforming contacts. When surfaces of two bodies fit exactly
or even closely together without deforming they are regarded as conforming.
Typical examples of conforming bodies include flat slider bearings and journal
bearings. On the other hand, bodies which have dissimilar profiles are said to
be non-conforming. A schematic representation of non-conforming bodies is shown
in Appendix D.

Hertz

Surfaces
of Engineering components are frequently subjected to contact loading where
large stresses are applied over highly localised areas. According to (N. P.
Padture) these kinds of loading configurations in the elastic limit are typified
by Hertizian contacts. Between 1886 and 1896 Hertz published two articles which
formed the basis of what is now known as the field of contact mechanics. Hertz
investigated the case of deformation of elastic bodies of different curvatures.
These non-conforming bodies, such as a sphere, indenting a flat plane or a pair
of cylinders in cross contact configuration make an idealised point contact
when brought to touch each other (Vasilis Votsios, 2003). As load is applied
to the target surface the point of contact grows, with the increase in
deformation on the surfaces in contact.  Hertz theory enables one to calculate the
shape of the area of contact and magnitude of distribution of normal and
tangential stresses on the surface of contacting bodies. There are five types
of commonly used solutions, and these shall be discussed in more detail in the
body of the main report. For the purposes of this submission the model of
contact between two spheres from which contact mechanics between the rail and
the wheel is based shall be discussed.

For contact between two spheres of radii R1 and R2,
the area of contact is a circle of radius a, as showed in Appendix D.

Rolling
contact defects

Wear

Squats,

Squats are surface or near surface initiated
rolling contact fatigue defects. They are sometimes referred to as “dark
spots”. These types of defects occur in areas of unsteady rolling contact.
These typically include areas with transition curves, turnarounds, and welds,
regions of braking and acceleration and areas with unsupported slippers. These
type of defects are characterised by cracking which initiates on the rail surface
growing to a depth of about 3-6mm below the surface. The rail surface becomes
depressed and a dark patch as seen in Appendix E.

Wheel burns,

These are defects that form on the running
surface of the rails and generally occur in pairs directly opposite one
another. They are caused by continuous slipping of the locomotive or traction
wheels on the rails, which occur when longitudinal creepage between contacts
points of the wheels and the rails reaches saturation. The slipping action of
the wheels has the effect of increasing the surface temperature of the wheel
beyond the transition point and on cooling the rail material undergoes a
brittle transition into a martensitic phase resulting in crack formations as
deep as 6mm.

Rolling Contact Fatigue.

Rolling contact fatigue is considered one of
the leading causes of failures of rail track and train wheels. As mentioned
earlier in the report, fatigue had led to a heavy loss in life and companies
responsible have suffered heavy penalties. Fatigue in rail terms occurs on
rails or wheels due to exposure to repeated dynamic loads which cause micro
cracks or defects to grow to a critical size over time and consequently leading
to failure.  Repeated loads cause an
increase in internal stress which gradually work harden steel until it’s
behaviour is purely plastic until 
leading to damage. An example of fatigue wear is shown in Figure 1.

Figure 1. Fatigue Failure on rail head and Fatigue cracks on wheel (Anders Ekberg, 2014)

A lot work has been carried out in the past
few decades to try and combat fatigue related problems on permeant ways.
Despite this, catalogued field observations of the extent, development and rate
of fatigue occurrences and wear rates on rail lines are still sparse. A better
understanding of both wear and fatigue phenomena is required in order that
improved ways of eliminating  the issue
can be achieved. In situations where crack initiation is unavoidable, the
progress of cracking need to monitored carefully using appropriate engineering
techniques and early intervention methods such as grinding and welding need to
carried out before critical growth levels are reached.

Corrugations.

Corrugations are type of defects which occur
on the running surface of the rail in form of wave-like patterns which show up
as long pitch and short pitch form. Long pitch corrugations form about 300mm
long wave lengths and generally develop under high axel load above 20 tons. The
depth of the waves can range from 0.1mm to above 2.0mm. Plastic flow of
material with long pitch corrugation occurs on the rail head due to excessive
contact stresses and vertical resonance of unsprung mass on the track. This can
be seen in Appendix F. Short pitch corrugations generally develop under lighter
axel load below 20 tons synonymous with passenger operations and are usually of
depth below 0.3mm and exhibit 30mmm to 90mm wave lengths. It is
thought that short pitch corrugations form from the differential wear caused by a
cyclic longitudinal sliding action of the wheel on the rail, either during the
acceleration, braking phase and lateral motion across the rail.

Proposed Approach and
Methodology

It is proposed that the delivery of the project shall be
undertaken using approaches highlighted in following gateways:

Stage 1: Review literature on the following
aspects

•     
History of rail accidents due to fatigue failure

•     
Non-linear steady state FEA analysis