Analysis of White Etching Cracks on Wind Turbine Systems' Performance

This engineering project report addresses the critical issue of white etching cracks (WECs) and their detrimental impact on wind turbine systems. The report begins by highlighting the relative youth of the wind energy industry and the prevalence of WECs in turbine gearboxes. It then delves into a comprehensive literature review, analyzing the effects of material properties, operating conditions, and environmental factors on the formation of WECs. The report identifies key drivers, including sliding kinematics, lubricant additives, and water contamination, and explores potential manufacturing solutions to mitigate premature failures. The analysis covers the operating conditions affecting the gearboxes of wind turbine systems including load, speed, and lubrication, and discusses the advantages and disadvantages of the available solutions, including the impact of dynamic loads, structural deformation, and misalignment. The report also emphasizes the need for further research to develop a consistent theory regarding WECs, the unavailability of heavily loaded systems which have a significantly innovative lifecycle design and sufficient experiences about machines' endurance, and the correlation between influencing factors such as material, loading, and environment. The report concludes by emphasizing the need for further investigation to quantify the role played by hydrogen generation and corrosion fatigue etching cracking in the context of moderate bearing load conditions. The report aims to provide an in-depth understanding of the effects of white etching cracks on the performance of wind turbine systems.

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Effect of White Etching Cracks on the performance of Wind Turbine System
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Abstract
The report handles the issue of white etching cracks as being caused by turbine conditions that
lead to altered bearing kinematics namely friction and vibrations. It starts by highlighting the
industry of wind energy as young in development as reflected in sizes of gearboxes. The
literature review puts across an in-depth analysis is put across on the material, environment, load,
and situations caused by operations of mounting and transportation. The report further identifies
the possible solutions in manufacturing that are viable to the problem of premature failure. The
conclusion gives a recap of the report from the background, the literature review, the current
issues and the available remedies at the early stage of development of the industry because
evolution in the necessity of wind energy will drive to increase in gearbox sizes to produce a
higher strength to weight ratio.

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1. Introduction
The wind turbine industry is still less developed. White etching cracks have identified to be
among the tribological failures prevalently affecting the rolling elements of the wind turbine
system. White etching cracks bring about unconventional rolling that results in contact fatigue
mode is caused by eccentric rolling. A partial microstructure borders of white etching
corresponds to cracks branching to three-dimensional networks leading to premature failure that
is always unpredictable. Recent research proposes that such failure mode could be associated
with numerous combinations of operating circumstances either depending on test rigs or
application. Both relations with test rigs and systems converge to comparable tribological drivers
relating to the evolution of hydrogen at asperity measures whose surfaces gets affected and
embrittle the bearing steel. The white etching cracks keep on being delicate to continue
reproducing regardless of synthetic hydrogen charging, and the underlying formation
mechanisms remain unsettled.
This report aims to give an in-depth understanding of the effects of white etching cracks on the
performance of wind turbine systems. It identifies the drivers including sliding kinematics,
lubricant additives, electrical potential and water contamination are endlessly swapped through
twin-disc machines which provide the enhanced control of the contact conditions.
2. Literature Review
Research topic
White etching is defined as the appearance of the transformed steel microstructure while etching
and polishing micro-sections (Benjamin et al. 2015). The affected sections are made up of ultra-
fine and nano-recrystallized ferrite which is carbide free. It is seen as white on a light
photosensitive micrograph because of the small etching reaction of the material (Doll, 2010).

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The rate of premature failures in wind turbine systems is higher and could be related to a larger
number of the machines which are installed (Ramasmawi, 2016). Other industrial systems which
include the paper mills, marine propulsion, constant variable drives, crusher mills and lifting
gears have a lower frequency of premature failure caused by white itching cracks (Dwyer-Joyce,
2015).
The rate of failure in mechanical components in wind turbine systems has remained to be lower
than in electrical components. Despite this, engineering downtimes due to driving train failures
create high loses on revenue and high repair costs (Munn, 2012). The service lives of gearboxes
of turbines are always not more than the deliberate 20 years. Gearboxes are used to upsurge rotor
speed to that of the generator. Locations that have bearings namely the intermediate shaft, the
high-speed shaft and the planet bearings (Ramaswami, 2013). Classic mechanisms of rolling
initiate fatigue in the subsurface as well as on the surface.
Early cracks have occurred commonly during the first years of operational life from the first year
to the third year at 5%-10% of the rating life. The first cracks appear visually as vertical axial
cracks and cracks combining both the small spalls and the large spalls. Early cracks are not
related to the particular bearing. The white cracks at an early life are also not linked to standard
treatment of heat which is particular (Mann, 2016).
However, the white appearance of failure is allied with treatment of heat like the normal residual
field stress. The progress stage of failure, operating conditions and the bearing position such as
the stress field loading have high likelihood cause of failure appearance. Early white cracks in
martensite rings as a particular application tends to grow into the material hence signifying the
axial appearance (Erdemir, 2012). On the other hand, the concrete bainitic rings and the
carburized casing hardened rings tend to have white etching cracks growing circumferentially

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just below the raceway indicating spalls and the flaking types of appearance. When going to the
inner-ring raceways at an advanced stage of failure are at all times highly spalled regardless of
heat treatment (Igba, 2015).
Operating conditions
Gearboxes of wind turbine systems are exposed to various operating conditions. The operating
conditions subjected to the operation of the systems include load, speed, lubrication either as
single elements or in combination (Ramaswami, 2014). The segment of wind energy faces tough
challenges to extend bearing life and reduce the occurrence of etching cracks while at the same
time try to control the overall costs of energy (Wood, 2013).
Common indicators of worsening operating conditions about premature failures caused by white
etching of wind turbine systems have attracted various descriptions in the available literature.
i. Occasions of dynamic and cumbersome loads lead to vibrations and rapid changes in
pressure.
ii. The rotor-axial motion of the primary shaft created additional axial and radial forces
depending on the type of turbine which leads to more stresses of components, that is,
dynamic loading (Wood, 2014).
iii. Occasional connection and disconnection of the generator from the power network result
in rotational reversal and bouncing effects building impact loads.
iv. Failures have also been linked to rapid acceleration, rapid deceleration and quick motions
of the shafts in the gearbox.
v. Structural deformation and misalignments of housing and the nacelle hub.

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vi. Compromise in lubrication between the requirements of the bearings and gears as well as
between high-speed, and low-speed stages result from insufficient oil refill intervals and
drain.
vii. At some times, the environmental conditions may be harsh coming from high changes in
temperature and subsequently cause strong changes in temperature than expected
between the inner ring of the bearing and the housing. Moisture, cold climate, dust and
offshore winds are the other adverse external conditions.
viii. Surprisingly, idling leads to low conditions of the load as adhesive wear risking damage
by skidding.
ix. Some requirements in design can be conflicting, for example, increasing the size of the
rolling element will consequently increase the carrying capacity of load and thereby
increase the risk of roller slip, sliding and cage damage.
Advantages
A clear indication of theses available theories shows a correlation between wind speed, failure
rate, and heavy fluctuating loads. Toward turbines of larger sizes which have a higher power-to-
weight ratio, the trend should invariably lead greater flexibility of the supporting structures
which in turn influences sharing and distribution of load over the rolling bearings and to the
other drive components (Wood, 2013).
Disadvantages of the available solutions
Failure of a bearing is not always attributed to which are not connected to white etching cracks.
A case of combined effects of the mechanical itching cracks and some electrical failures are
difficult to measure their individual effects statistically to come up with an appropriate solution
(Wood, 2013). The available evaluation in statistics apply to a partial number of wind turbines

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which are offshore to give a clear correlation between the rate of failure, the speed of wind and
load which is heavy and fluctuating but are not applicable to all wind turbines (Vegter, 2015).
Presence of cracks on bearings is has been at sometimes interpreted as an indication of
uncontrolled kinematic behavior (Ruellan, 2014).
Research gap
A consistent theory is not available regarding premature failures caused by white etching cracks
on wind turbines even though universities, manufacturers, independent institutions and suppliers
investigating the issue. The studies have kept around the local change in the microstructure of the
bearing material. Industrial experiences are at some points significant causes of failure of
turbines. There is the unavailability of heavily loaded systems which have a significantly
innovative lifecycle design. In other terms, sufficient experiences are absent about machines'
endurance (Solano-Alvarez, 2015).
3. Evaluation
Analysis
Influencing factors
 Material
Include heat treatment, microstructure residual stresses, cleanliness, and hydrogen
content.
 Loading
It comprises of overloads, impact loads, vibration, peak loads, torque reversals, structural
stresses, slip and electrical currents.
 Environment

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Lubricant, corrosion, additives, tribochemical effects, temperature gradients,
contamination and hydrogen generation.
 Others
The other aspects are mounting which may lead to scratches, quality issues, and transport.
Evaluation
A lot of the influencing factors are found to be correlated. The factors that drive the failures will
either operate as single elements or a combination of the factors. The bearings will fail by
spalling as a consequence of nucleation sites of white etching cracks. The hypotheses are divided
up to hydrogen enhanced white etching cracks development, load and stress-related white
etching crack development and the overlapping ones. The damages identified above influences
generator bearings and alternator applications by damaging current and corrective actions using
special greases bearings and special steels (Ščepanskis, 2015).
Most early failures in bearings are related to issues of the surfaces mostly lubrication. Most white
etching cracking failures in the wind turbine gearbox is positioned to originate at the surface or
close to the surface and then propagate into the steel or case material corrosion fatigue. Wind
turbine gearboxes bearings are always large. The initiation and spread mechanism differ as
likened to small bearings. Deeper radial cracks have been reported for larger bearings with
restrained loads because of residual stress and that of a higher loop.
For the case of premature bearing failure suggests a fast propagation of the crack. The high-
speed branching and spread will be explained by the existence of chemical elements such as
oxygen and aging lubricant products at the face or tips of the cracks.

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A system with subsurface cracks experiences vacuum conditions and hence has a slower rate of
crack growth from mechanical fatigue wholly. At earlier stages, the cracks and crack systems
connected to the surface to permit entry of lubricant and oxygen.
The hydrogen-enabled fatigue has same effects and also the classic rolling fatigue from contact.
Conversely, it requires aggressive and corrosive environment or continuous passage of electric
currents of high frequency. The presence of free water facilitates the creation of a highly erosive
situation. Turbine manufacturers have claimed to have control over contents of elevated water in
the lubricants. The investigation does not identify moisture corrosion in wind turbine gearboxes.
If all factors are considered and humidity ignored, the growing back passivating tribolayers
provide a barricade layer to preventing corrosion and the absorption of hydrogen into steel
remains to be intact and continuous. Absorption of hydrogen into steel is detrimental. Severely
mixed contacts of friction cause local generation of hydrogen. The maintenance of a constant
production of hydrogen, the interacting metal surfaces must be fresh and could lead to a
weakening effect on the surface thus facilitating generation of surface cracks.
Failed raceways hardly reveal severe wear that could allow permeation of hydrogen. Therefore,
diffusion of hydrogen through the raceway is not likely to occur without the addition of any other
factor. The potential additional factor is the relatively corrosive oils combined with contaminants
eventually. Operation of grease initiated failure is therefore distinguished from failure
mechanisms which are initiated by the surface like distress. Further investigations are required
for relevance quantification. Currently, the role played by hydrogen generation can be seen to be
a local effect which is generated by the system of cracks because of entry of lubricant leading to
the mechanism of corrosion fatigue etching cracking.

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Furthermore, conditions of moderate bearing load in wind turbine gearboxes, there is a build-up
of residual stress together with a decrease in the line of X-rays diffraction in failed bearings
which broadens close to the raceways. The mixed friction of vibrations and shear stress as shown
by the material is also an indication supporting initiation of failure on the surface and near the
surface. Inadequate conditions of lubrication and some vibrations affects with higher frequency,
reduce the thickness of the film and therefore raise the risk for the local mixed friction.
Viable solutions
The discussed conditions roots to the following to counter the challenge of white etching cracks.
 Special passivation.
 Stabilization of structures near surfaces.
 Increasing resistance of bearings to hydrogen and chemical attacks.
 Improvement of running-in.
 Lowering the levels of micro-friction at in peak loading.
 Use of special clean steel.
 Extensive strengthening processes for the surfaces.
 Allowing the conditioning of the components.
 Raising resistance against the surface and subsurface crack initiation and propagation.
 Reduction of some additions that act as surface and material stress raisers.
4. Conclusions
The high growth in wind industry goes hand in hand with increasing the sizes of the turbine, and
the turbulent conditions give unique challenges on rolling bearings. The industry is young and is

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faced with the problem of premature failures. In addition to bearing and heat treatment, wind
conditions interfere with bearing kinematics, lubrication, and loading. The phenomenon of white
etching cracks has been observed. Due to the high wind of the appliance, different locations of
the turbine are affected, and the conditions that lead to differentiated kinematics should be
avoided to reduce tensile stresses and micro-wear.
In general, bearing manufacturers are supposed to shift their focus to modifying the applications
in a way that will aim to lower the risk of premature failures and increase robustness under the
specific conditions of wind turbine gearboxes.

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5. References
Benjamin Gould, Greco Aaron, 2015. The influence of sliding and contact severity on the
generation of white etching cracks. Tribology Letters, 60(2), p. 29.
Doll, G., 2010. Tribological advancements for reliable wind turbine performance. Mathematical,
Physical and Engineering Sciences, 369(1929), pp. 4829-4850.
Dwyer-Joyce, R., 2015. Characterisation of white etching crack damage in wind turbine gearbox
bearings. Wear, 33(8), pp. 164-177.
Erdemir, K., 2012. Material wear and fatigue in wind turbine systems. Wear, 302(1), pp. 1583-
1591.
Igba, J., 2015. Performance assessment of wind turbine gearboxes using in-service data: Current
approaches and future trends. Renewable and Sustainable Energy, 50(1), pp. 144-159.
Mann, E., 2016. An updated review: white etching cracks (WECs) and axial cracks in wind
turbine gearbox bearings. Materials Science and Technology, 32(11), pp. 1133-1169.
Munn, E., 2012. White structure flaking (WSF) in wind turbine gearbox bearings: effects of
‘butterflies’ and white etching cracks (WECs). Materials Science and Technology, 1(28), pp. 3-
22.
Ramaswami, G., 2016. Multiphysics computational analysis of white-etch cracking failure mode
in wind turbine gearbox bearings." Proceedings of the Institution of Mechanical Engineers.
Journal of Materials: Design and Applications, 230(1), pp. 43-63.
Ramaswami, G., 2013. Computational investigation of roller-bearing premature-failure in
horizontal-axis wind-turbine gearboxes. Solids and Structures, 2(4), pp. 46-55.

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Ramaswami, G., 2014. Wind-turbine gear-box roller-bearing premature-failure caused by grain-
boundary hydrogen embrittlement: A multi-physics computational investigation. Journal of
materials engineering and performance, 32(11), pp. 3984-4001.
Ruellan, A., 2014. Understanding white etching cracks in rolling element bearings. Journal of
Engineering Tribology, 228(11), pp. 1252-1265.
Ščepanskis, M., 2015. The numerical model of electrothermal deformations of carbides in
bearing steel as the possible cause of white etching cracks initiation. Tribology Letters, 59(2), p.
37.
Solano-Alvarez, B., 2015. Critical assessment 13: elimination of white etching matter in bearing
steels. Materials Science and Technology, 31(9), pp. 1011-1015.
Vegter, R., 2015. A review: the dilemma with premature white etching crack (WEC) bearing
failures." Bearing Steel Technologies. Advances in Steel Technologies for Rolling Bearings,
10(1), pp. 1-83.
Wood, W., 2013. Effect of hydrogen on butterfly and white etching crack (WEC) formation
under rolling contact fatigue (RCF). Wear, 306(1), pp. 226-241.
Wood, W., 2013. Serial sectioning investigation of butterfly and white etching crack (WEC)
formation in wind turbine gearbox bearings. Wear, 302(1), pp. 1573-1582.
Wood, W., 2013. White etching crack (WEC) investigation by serial sectioning, focused ion
beam and 3-D crack modelling. Tribology International, 65(1), pp. 146-160.
Wood, W., 2014. Confirming subsurface initiation at non-metallic inclusions as one mechanism
for white etching crack (WEC) formation. Tribology International, 75(1), pp. 87-97.

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