Asperities and surface initiated
rolling contact fatigue
Bo Alfredsson
Solid Mechanics at KTH
2015-10-07 – Tribodays Nynäshamn
Rolling contact fatigue
Rolling contact fatigue has been considered the default
failure mode of Hertzian rolling contact elements:
If all other failure modes could be avoided, then rolling
contact fatigue would eventually limit the component life.
Rolling Contact Fatigue – gears and bearings
Two types of
Rolling Contact Fatigue
w
v ≈ w · r Rolling
contact
3 mm
Tallian 1992
Sub-surface initiation
Surface initiation
Surface initiated spalling
b
b = 20 – 24º
0.2 mm
0.1 mm
General conditions for fatigue initiation
and fatigue crack growth
Initiation
 Sufficiently large shear stress amplitude
 Tensile maximum stress
Fatigue crack growth
 Sufficiently large DKI ( > Kth, which is R dependent)
 Open crack tip: KI,max – DKI > KI,cl
 Crack with mode II or mode III fatigue load will arrest or kink to mode I
 Crack will follow direction of maximum DKI
Whole contact: 2D line contact
Line load – 2D
P
r  
r

r
2 P cos 
 r
 Always pressure!
 Stress free surface
(J 2.15)
The spall is local,
asperity is local – local point contact
Sphere
r
r
z
 r ,max 
1  2  po
3
 Tensile radial stress!
Line
Asperity mechanism for surface initiated
spalling
Stress free
Cmpressive stresses
Asperity  point contact
Tensile surface stress
Together
Inside – compression
Entry – compression and tension
Gear surface roughness profile
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Asperity size from roughness measurements
Radius of curvature at roll circle – R = 13.6 mm
Nominal contact pressures: 2.10 – 2.27 Gpa
Model asperities
Yes, asperities exist! How can they be responsible for spalling?
d
h /mm
200
100
50
2
1
2
Stress in front of rolling contact entering asperity
d = 200 mm
h = 2 mm
d = 100 mm
h = 2 mm
d = 100 mm
h = 1 mm
d = 50 mm
h = 2 mm
Influence of asperity size: lasp = hasp/rasp
Influence of contact friction: m
Influence of residual surface stress: R / MPa
Asperity mechanism for surface RCF:
Questions to be answered
 Can a point contact give fatigue?
 Do the conditions exist in applications?
 Asperities?
 Stress levels?
 Is the crack behaviour predicted?



 Crack profile of spalls
 Spalling life
Is the parameter influence predicted?
 Residual surface stress
 Surface roughness
Lubrication film thickness and EHD influence
o
o
Well, more to follow
On-going research
Mechanism explains why and how
•
Design guidelines
Asperities and surface initiated
rolling contact fatigue
Bo Alfredsson