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TECHNICAL ARTICLE
AS PUBLISHED IN
The Journal January 2018 Volume 136 Part 1
If you would like to reproduce this article,
please contact:
Alison Stansfield
MARKETING DIRECTOR
Permanent Way Institution
alison.stansfield@thepwi.org
PLEASE NOTE THE OPINIONS EXPRESSED IN THIS JOURNAL
ARE NOT NECESSARILY THOSE OF THE EDITOR OR OF THE
INSTITUTION AS A BODY.
TECHNICAL
Studs and squats: AUTHOR:
Stuart L Grassie
BE, MEngSc, PhD, CEng, MIMechE, FPWI
A best practice approach Stuart Grassie Engineering Ltd
This paper was previously published in This paper is based on field work undertaken work, and accordingly also no significant
“Studs and squats: the evolving story”, primarily in NSW to reveal some of the depth of compressive residual stress. These
Wear, vol 366-367, pp194-199, 2016 (also characteristics of stud defects, in particular characteristics are very different from classical
Procs of 10th Intnl Conference on Contact in contrast to rolling contact fatigue (RCF) of squats.
Mechanics and Wear of Rail/Wheel which squats are a classical example. Studs
Systems, Colorado Springs, USA, August are associated with sites where there is high Although some transverse defects (TDs) have
2015). traction, such as the exit from stations. In been associated with studs, both gauge corner
NSW, they commonly initiate at about 10 0 -20 0 cracking (GCC) and studs have coincided in
The paper provides some guidance on to the vertical towards gauge on the high rail all of the cases examined. In these cases, the
treatment and maintenance of studs as well in curves, then grow into the rail at an angle TD has clearly developed in the conventional
as updating a hypothesis that still rests of about 20 0 to the surface. Studs can at first and well understood manner from the GCC.
substantially on circumstantial evidence. develop very quickly e.g. to a depth of 2.2mm The stud has given rise to a dynamic load
in 6MGT. The stud fans out across the rail from that accelerates growth of the TD. But if the
ABSTRACT the initial surface crack, developing across the GCC had not existed, the TD would not have
rail at a substantially constant depth of 3-6mm. developed.
Several railways suffer from a defect that If the stud is left, it may rise to the surface at
has been christened a “stud” which appears the opposite side of the railhead, giving rise to INTRODUCTION
superficially similar to a squat but is very an ugly spall with a fracture surface typical of a
different in character. Although both conventional fatigue crack. This paper follows a pair of papers[1, 2] that
initiation and propagation of studs are poorly examined the current state of understanding
understood, at least a couple of railways have There is no evidence whatsoever that studs of “squat-type defects” in rails and proposed
already benefitted from exploiting the less become transverse defects, nor should this that there existed a defect that was
malevolent nature of studs, particularly the fact occur with a crack that develops across rather commonly confused with a squat but was in
that these do not themselves initiate transverse than along the rail as there is an absence of fact significantly different. In order to avoid
defects. flexure to drive the crack. Studs have grown confusion, this defect was given the less
in rails in which there is no significant plastic appealing name of a “stud”. The original work[2]
was based primarily on analysis of defects
from London Underground (LU), where studs
on some lines were the most prevalent type of
rail defect. These defects have subsequently
been given a lower priority for treatment on
at least some of LU with significant savings in
maintenance and apparently, no increase in
risk.
Studs are apparently also widespread in
Australia, where these have been examined,
for example, as part of the CRC for Rail
Innovation[3]. A considerable body of work on
these defects exists from research groups in
the Netherlands and France[4, 5]. Although the
defects [2 - 5] appear substantially identical, the
author and his colleagues have differed from
others working in this area in concluding that
these are not RCF defects.
A substantial body of work has been
undertaken by staff of RailCorp and successor
organisations, primarily Sydney Trains and the
Asset Standards Authority (ASA), on defects
that they have called “lamination defects”
(LDs)[6]. This has included, amongst other
things, very thorough monitoring of individual
defects over a period of years, documentation
Figure 1: Stud that has spalled out
16
TECHNICAL
Monitoring is described in more detail in
reference 6. The development of the crack
front for a typical stud at Chatswood is shown
in figure 3. The green dotted line in this Figure
shows the visible crack mouth. Evidently the
crack has initiated at the rail surface towards
the gauge corner then “fanned out” towards the
field side of the rail.
This is remarkably similar to the pattern shown
for supposed “normal squat development” in
reference 7.
Figure 2: Multiple studs with unusual development The author was a co-author of reference 7,
albeit not of this section of the document, and
would suggest that the document exemplifies
of the occurrence of defects and factors Beach marks that are typical of conventional
the confusion of studs and squats, and the
that appear to affect their development, and fatigue are apparent on the fracture surface,
need for a clear differentiation of two very
examination of the extent to which defects can and the defect extends from the gauge corner
different rail defects, see figure 3.
be detected using the ultrasonic test train that right across to the field side of the rail. There is
is used on the NSW system. no sign of the defect having turned down, see
The average depth of the crack front has been
figure 2.
calculated for 4 studs at Chatswood and 7 at
The paper provides some guidance to
Erskineville, and is shown as a function of time
differentiate studs from squats and proposes Although the features shown in figure 1 are
in figure 4. There is a consistent difference in
measures regarding treatment of studs. The by far the most common, studs do develop in
depth between the sites (a range of 2.06-
evidence in NSW, as in London, has been other ways. For example, the defects shown in
2.62mm and 3.00-3.37mm in mean depth for
that studs do not initiate broken rails. This figure 2 were in the high rail of a curve on the
Chatswood and Erskineville respectively) but
remains the case. Some progress is made uphill approach to signals where many trains
no significant change in depth with time. See
on a hypothesis for studs that is as yet poorly would stop. These appear to have initiated
figure 4.
developed. towards the field side of the rail and developed
towards gauge. The line along which defects
Overall the measurements suggest that the
This paper is based on work done by the initiate is probably the preferred line along
defects develop at a roughly constant depth,
author as part of a project, initially for Railcorp, which the wheels run that initiate the defects in
substantially across (not along) the rail. This
to review previous work on LDs and to advise these conditions of e.g. high traction, high cant
is very different from a squat. The rate of
in certain areas. It includes work undertaken excess.
growth of depth is also very much greater than
by his colleagues in NSW that was part of that
that of an RCF defect: a not atypical defect at
review. Nevertheless, the views contained here Similar behaviour of a preferred line of initiation
Chatswood developed from 0mm to 2mm depth
are those of the author alone. of studs has been noted on rails from at least
in 6MGT while one at Erskineville developed
one, very different, system. Evidently the
from 0.9mm to 3.5mm depth in 7MGT.
MONITORING OF DEFECTS detailed tangential contact stresses, arising
probably from both curving and traction,
Metallurgical examinations have shown that
Some characteristics of studs and differences influence crack growth.
cracks grow initially at about 20 to the rail
between these studs and GCC are highlighted
surface towards the field side of the rail from
here, see figure 1. A well-developed stud that Comprehensive monitoring of individual stud
gauge. A possible explanation of this behaviour
has spalled out is shown in figure 1, in which defects had been undertaken over a period of
is that the crack grows under the relatively
traffic is towards the left and gauge corner to about 18 months by RailCorp staff at sites at
shallow sub- surface layer of compressive
the bottom. Some general characteristics of Chatswood (14MGT p.a., 1010m radius) and
residual stress. Although plastic working of
studs are apparent here, including the inverted Erskineville (13MGT p.a., 402m radius) on the
the material is required to develop residual
V towards the gauge corner, where the stud suburban network[6]. Studs at both sites were
stress, and some studs develop in almost virgin
initiates. In NSW this is typically at 10 0 -20 0 to in 60kg/m head-hardened rail in the high leg of
rail, in older rails (such as those shown here),
vertical towards gauge. the curves. Monitoring included photography
there would be a layer of compressive residual
and detailed measurements of depth using a
stress.
hand-held ultrasonic gauge.
Figure 4: Variation of average defect depth with time
Figure 3: Growth of individual stud (13-14MGT p.a. of traffic)
17
TECHNICAL
an indentation that increases dynamic loading.
The localised dynamic loading has increased
flexural stresses, thereby accelerating
propagation of the fatigue crack from RCF.
The character of studs and typical GCC is such
that confusion between the two is unsurprising.
For example, a rail with a couple of studs and
coincident, light GCC is shown in figure 6.
One “leg” of the surface-breaking crack on
each stud is at almost the same angle as
the GCC, and the dark spot is clear from the
depressed layer over each stud. It would be
easy to conclude from casual observation that
there was a single problem here, whereas
Figure 5: Transverse defect associated with stud: note coincident RCF in there are in fact two problems that appear
running band to have completely different origins, which
develop at very different rates, in substantially
different directions and with different
TRANSVERSE DEFECTS, RCF cracks. Coincidentally the running band has consequences.
broadened where a crack has grown sub-
AND STUDS
surface towards the field side of the rail. The author has examined several TDs that
It had been noted in both Australia[8] and the have been associated with studs from two
What appears to have happened at this different railway systems. In every case these
UK [2] that transverse defects (TDs) were not
location is that an RCF crack has initiated share the features shown in figure 5.
associated with studs, although TDs are a
at the rail surface, developed longitudinally
common occurrence with RCF that is not
(doubtless under the influence of water), turned The stud is significant because it has
maintained or eliminated. In the last few years
down on reaching the edge of the layer of accelerated development of the TD. However,
there has been some association of TDs with
compressive residual stress and developed if there had not been RCF in the rail or if RCF
studs that may call the apparently benign
into a TD under the influence of flexural had been controlled e.g. by routine reprofiling
nature of studs into question. To examine this,
stresses in the rail. This mechanism is well and a modified profile that relieved the area
a TD that has been associated with a stud is
understood. At the same location, a stud has in which RCF is initiated, the TD would not
shown in figure 5.
developed (for reasons that are as yet poorly have occurred. Growth of a stud across the
understood). rail would not and does not result in a TD.
The fracture surface of this defect has the
characteristic appearance of a break that It nevertheless presents a risk because a
This has developed substantially across (rather stud could conceal a conventional TD from
has occurred from an RCF crack that has
than along) the rail. The large sub-surface ultrasonic inspection. RailCorp and their
developed longitudinally and then turned
crack has resulted in a thin, poorly supported contractor for ultrasonic inspection have made
down into the rail. Moreover, the rail surface
surface layer that has deformed and caused significant progress in identifying studs[6].
is covered in relatively closely-spaced RCF
Further progress appears likely in this area
even if relatively little is known about the
detailed development of these defects. It is
particularly important to develop a routine
method of distinguishing studs from genuine
TDs, whose signature is very similar, see
figure 6.
RAIL TYPE, REPROFILING AND
STUD DEVELOPMENT
It has been noted that studs are less prevalent
in standard carbon than in head-hardened rail
on the NSW system [6]. Evidence for this was
also noted during this project. Photographs
in figures 7 are from a curve that was being
rerailed because of defects. The SC rail
in this curve, from 1995, is free of defects
whereas HH rail only 11m away in the same
high rail of the curve, rolled in 2002, is
riddled with defects. This behaviour is quite
contrary to what would be expected for RCF
defects, provided routine reprofiling had been
undertaken to a profile that did not increase
contact stresses, see figures 7a and 7b.
A possible explanation for the different
propensity for defects in different types of rail
steel is that rail wear is higher in the softer rail.
This could also act as a guide to the extent
Figure 6: Rail with light GCC and coincident studs of reprofiling required to keep defects at bay.
18
TECHNICAL
propagate across the rail and often rise to
the rail surface beyond the running band to
cause an ugly spall suggests that compressive
residual stress influences crack growth and
that the crack may grow under the layer of
compressive residual stress. Unfortunately,
this hypothesis remains both tentative and
poorly developed.
Illustrations of different propensity of defects to occur in HH rail ACKNOWLEDGEMENTS
Figure 7a: Quasi-continuous defects in 2002 HH rail at 1.741km, high rail
The author is grateful to RailCorp for funding
the study and for their tentative permission to
publish the paper. He is indebted to Malcolm
Kerr, Andrew Wilson and David Cooper,
whose enthusiasm, openness and generosity
contributed greatly to this project. Others
contributed to this work, but the opinions and
omissions are his alone.
REFERENCES
[1] SL Grassie, “Squats and squat-type defects
Figure 7b: Defect-free 1996 SC rail at 1.730km, high rail in rails: the understanding to date”, Journal
of Rail and Rapid Transit, Procs of I mech E,
To this end a very preliminary examination understood and has been studied exhaustively. 2012, 226F, 235-242.
was made to correlate metal removal with the On the other hand, the stud develops across
presence of defects in head-hardened rail. the rail, slightly below the surface, thereby [2] SL Grassie, DI Fletcher, AE Gallardo-
Notwithstanding the limitations of the data causing a depression that exacerbates Hernandez and P Summers, “’Studs’: a
there was a tendency for defects to have dynamic loading. The high dynamic loading squat-type defect in rails”, Journal of Rail and
developed where MR was low and for there to accelerates development of fracture from the Rapid Transit, Procs of I mech E, 2012, 226F,
be no defects where MR was relatively high. RCF. However, in the absence of RCF the 243-256.
If an attempt were made to implement these stud itself would not develop into a rail break.
findings in a reprofiling programme to control This problem is treated most successfully by [3] WJT Daniel, S Pal and M Farjoo, “Rail
studs, the indication was that MR rates of at reprofiling to control the development of RCF. squats: progress in understanding the
least 0.025mm/MGT would be required and Australian experience”, Journal of Rail and
less than 0.020mm/MGT would be inadequate. The common morphology of a stud is for it to Rapid Transit, Procs of I mech E, 2013, 227F,
Routine preventative reprofiling may in practice initiate towards the gauge side of a rail, at an 481-492.
be extremely difficult given the experience of angle of about 10 0 -20 0 to gauge. The crack
rapid stud growth e.g. at Chatswood from 0mm grows initially into the rail at about 20 0 to the [4] Z Li, R Dollevoet, M Molodova, X Zhao,
to 2mm depth in 6MGT. A second possible rail surface to a depth of 3-6mm, then fans “Squat growth – some observations and the
explanation of the behaviour is given by the out from the point at which it initiates, mainly validation of numerical predictions”, Wear,
work of Joerg and his co-authors[9]. This work across but to some extent also along the rail in 2011, 271, 148-157.
tentatively suggests that a narrow band of both directions. The depth of the crack remains
high tensile residual stress may exist in some substantially constant. A well- developed [5] S Simon, A Saulot, C Dayot, X Quost, Y
circumstances at the edge of the running band. stud may rise to the surface of the field side of Berthier, “Tribological characterisation of rail
This band would tend to be more concentrated the rail, perhaps when it extends beyond the squat defects”, Wear, 2013, 297, 926- 942.
on a harder rail as the contact would broaden layer of compressive residual stress. Different
less as a result of plastic deformation. The characteristics, in particular a different initiation [6] M Kerr, A Wilson, S Marich and S
high tensile residual stresses could initiate the point across the rail and different growth Kaewunruen, “Wheel/rail conditions and squat
cracks from which studs propagate. pattern, are associated with different wheel/rail development on moderately curved tracks”,
contact conditions and tangential tractions. CORE 2012, Brisbane, 10-12 September 2012.
CONCLUSIONS
Studs are less common in standard carbon [7] “Rolling contact fatigue in rails: a guide to
Observations are recorded from field than in head hardened rail. A possible reason current understanding and practice”, Railtrack
investigations undertaken on the NSW railway for this is a higher wear rate of the softer rail. plc, RT/PWG/001 Issue 1, February 2001.
system of rail defects that have been called There is evidence that studs do not develop
“studs”. It is clear from these observations that in head hardened rails if the wear rate is [8] M Kerr, A Wilson and S Marich, “The
studs are not rolling contact fatigue and that sufficiently high. A very simplistic analysis epidemiology of squats and related rail
they differ significantly from RCF. suggests that few defects exist if the metal defects”, CORE 2008, Perth, 7-10 September
removal rate is more than 0.025mm/MGT 2008.
Of critical importance for the safety and whereas studs are prevalent at sites with a
integrity of the railway, there is as yet no metal removal rate of less than 0.020mm/MGT. [9] A Joerg, R Stock, S Scheriau, HP Brantner,
evidence that transverse defects propagate B Knoll, M Mach, W Daves, “The squat
from studs. Although TDs have been The evidence remains that these defects condition of rail materials – a novel approach to
associated with studs, the evidence in all initiate from wheelslip, and possibly from squat prevention”, Procs of 10th International
cases is that TDs have occurred where thermally transformed material that is the Conference on Contact Mechanics and Wear
RCF and studs coincide. The TD develops result of wheelslip. The different morphology of Rail/Wheel Systems, Colorado Springs,
from propagation of RCF, which is initially of defects in areas in which tangential USA, 2015.
substantially longitudinal. Once the RCF turns tractions differ significantly suggests that the
down into the rail, it develops as a result of defects grow under the influence of tangential
flexural stresses. This mechanism is well tractions. The fact that defects usually
19
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