Prediction of lifetime and failure mechanisms of rolling contacts in DOT Direct Drive Pump

Abstract

In recent years, the increased need for renewable energy has
put more interest in wind energy and therefore in making wind turbines more
efficient and building them as efficiently and cheap as possible. Delft
Offshore Turbines (DOT) is developing a drivetrain that replaces the heavy and
expensive parts in the nacelle of a wind turbine and makes these parts more
accessible and aims to improve the reliability of the wind turbines by
replacing the parts that are most prone to failure.  Due to the innovative design of their
drivetrain, several unknowns still have to be answered since this has never
been tried before. One of the most important parts of this drivetrain requires
rolling contacts that endure high contact stresses and have to operate for a
long time in harsh conditions. These rolling contacts have to be tested and for
this, DOT created the Roller Test Bench (RTB) to test different materials and
material combinations until failure and to see what type of failure is
occurring and what the lifetime of these rollers is. The rollers are a lot
smaller compared to the rollers found in the drivetrain to reduce cost and
testing time. The scaling of the dimensions has been done several times in
different fields of research but it is unclear how this affects the results and
how the results translate to the original rollers. In literature some papers
have stated that the results scale in a certain way but this has not been
proven or the statements have not been argumented in a clear way such that
these statements hold no ground.  A
rolling contact has several ways that it can fail; wear, deformation, corrosion
or fatigue are most commonly found. From the literature study performed in this
thesis it has been noted that the main mode of failure in the RTB is rolling
contact fatigue since the rollers are smooth, have no lubrication applied, are
free of external contaminants and have no significant amount of friction on the
surface. With the stress applied and the material used it is also noted that
the fatigue will occur after a high amount of cycles and in literature this is
also called high-cycle rolling contact fatigue. Experimental testing of such
problems takes a very long time, even if the rollers are smaller and are run
faster through the load cycles.  With
both problems stated, the lack of information on the influence of scaling on
fatigue life and the testing of high cycle fatigue, the goal of this thesis was
found and it was to find a method to predict the lifetime and wear mechanisms
of rolling contacts in the DOT Direct Drive Pump. This method could then be
used to figure out how the scaling affects the fatigue life for rolling
contacts and make a possible statement of its influence.  From the literature study a numerical
modelling method was found that incorporated the microstructure and included
both crack initiation and propagation and was able to model high cycle fatigue.
This model suited the problem the best and was proven to provide results that
agree well with previously performed experiments and their experimental
results. This method implemented a Voronoi tessellation to reproduce the
microstructure and applied continuum damage mechanics to calculate the damage
evolution and apply the damage to the microstructure. A jump-in-cycle method
was applied to improve the efficiency of the model and greatly reduce the time
to model until failure. The created model is able to reproduce subsurface
initiated fatigue and with that produce a scatter and L10 life comparable with
previous models and experimental results. The model was validated by comparing
the stress distribution, crack formation and the initiation and total lives
with other models and data. The model was off the required range since the
models uses an homogeneous isotropic material which causes the scatter to be
lower.  With the model created and
validated, it was applied to several different half-widths to test how the
scaling affects the fatigue life. From the results it was noted that for a
larger roller size the fatigue life decreases but it has to be noted that
further tests, experimental and numerical, have to be done and improvements to
the model have to be made such that at some point a correlation can be made
between the scaling and the fatigue life. Further recommendations were provided
on improving the model and with that creating more reliable results such that a
better prediction can be made on how the fatigue life is affected but also to
predict the life of a rolling contact.