Rough Surface Contacts, Third-Body Layers and the Wheel-Rail Interface

Understanding and managing adhesion at the wheel-rail interface is paramount to
the safe running of a railway. Anything that increases or decreases the level of
traction/adhesion is therefore of interest to the industry. Various third-body layers
(3BLs) contribute to the levels of traction, be it leaves in autumn or lubricants ap-
plied at curves. Surface roughness is known to affect friction, however it is poorly
understood when paired with 3BLs. High surface roughness occurs on wheels and
rails following rail grinding and wheel turning procedures. How this surface rough-
ness evolves after these reprofiling procedures is not yet known.

The aim of this thesis is to better understand surface roughness at the wheel-rail
interface with and without 3BLs present. To do this the high pressure torsion (HPT)
rig was employed to simulate the the tribology at the wheel-rail interface. Surface
roughness was applied to the HPT specimens and measured using surface roughness
replicas and an Alicona microscope. Water, grease and leaves were applied to the
interface to better understand what effect these 3BLs would have on the traction.
In addition to this, the HPT specimens were instrumented with ultrasonic sensors
which would measure the interfacial stiffness in-situ during a test. Following this
the extended creep-force (ECF) model was parameterised with the outputs from
the HPT tests to predict full-scale outcomes when surfaces are both rough and
contaminated with 3BLs. These predictions were then validated using the full-scale
wheel-rail rig at the University of Sheffeld.

A link between surface roughness and the traction coecient when running-in was
found for dry contacts. Post grinding roughness levels were found to reduce down to
near run-in after just a few equivalent train passes. High levels of roughness reduced
the impact of friction lowering 3BLs. Leaf layers slowed the rate at which roughness
decreases. The ECF model parameterisation was successful and good agreement
was found between the ECF and the HPT results. The ECF was able to predict
full-scale outcomes with a fair degree of accuracy especially for the dry, water and
leaf cases. A link was found between interfacial stiffness and surface roughness for
dry contacts, meaning that surface roughness may be remotely measurable in real-
time using ultrasonics. The ultrasonic sensors were also able to detect the presence
of 3BLs, but not the distinguish the type of 3BL.