Mild wear modeling in the boundary lubrication regime


Bosman, Rob (2011) Mild wear modeling in the boundary lubrication regime. thesis.

Abstract:Currently, the increasing demand for smaller and more efficient systems is increasing
the stress put on interacting components. This forces components to operate in the
boundary lubrication regime. In this lubrication regime, the normal load put on the
components is no longer carried by the lubricant but rather by the interacting asperities,
and by doing so solid-solid contact is inevitable. This increases the specific wear seen
in these types of systems shortening the lifetime of components and increasing
maintenance intervals. This decreases the operational times significantly. Therefore, it
is of great importance to get a clear understanding of the concept of corrosive wear
under these specific conditions.
In this thesis three different aspects of wear are discussed namely: the transition from
mild to severe wear, running-in and the steady state mild wear. The first is modeled
using a thermal threshold originating from Blok’s hypothesis that the transition to
adhesive wear is caused by transcending a predefined critical temperature. The model
discussed in the current work is based on a numerical thermal model combined with an
elastic-plastic contact solver, which are both using the DC-FFT algorithms combined
with CGM iterative schemes. In this way the model is able to incorporate mild wear
into the thermal and contact calculations while keeping the computational times within
a reasonable range. The model is validated through an experimentally determined
transition diagram.
Running-in of surfaces is modeled using the hypothesis that an additive rich oil is able
to protect the contacting elements from metal to metal contact therefore, the growth
rate should be the same or greater than the layer removal rate. This hypothesis is
combined with a wear model based on a maximum equivalent strain assumption. This
states that for material to be removed both an equivalent plastic strain threshold should
be met and that the volume including this strain should reach the surface. To be able to
compute the plastic strain, a Semi-Analytical-Contact solver is developed based on a
local friction model.

The mild wear model is based on the dynamic chemical balance at the surface.
Through mechanical removal the balance is disturbed and the system will restore the
balance through chemical reactions between the base material and additives present in
the oil. Since the chemical reaction layers are very thin compared to contact regions, it
can be assumed that it has only a limited effect on contact conditions. Using this
hypothesis, a model is presented to determine the removal rate of the chemical
reaction layer and thus the intensity of corrosive wear. The validation of this model is
done using model systems.
This thesis is divided into two parts: the first part is a summary of theory presented in
the appended papers presented in the second part. This way the reader is able to keep a
clear view on the overall goal of the research by reading the first part while the details
are discussed in the second part.
Item Type:Thesis
Engineering Technology (CTW)
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