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Asperities and surface initiated rolling contact fatigueBo Alfredsson Solid Mechanics at KTH2015-10-07 – 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 Rollingcontact
Sub-surface initiation
3 mm
Tallian 1992
Surface initiation
Surface initiated spalling
0.2 mm
0.1 mm
b
= 20 – 24º
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
Þ Always pressure!Þ Stress free surface
r
Pr
cos2 (J 2.15)
sr
rq
Line load – 2D
P
The spall is local,asperity is local – local point contact
Tensile radial stress!
3
21max,
or
p
z
r
sr
Sphere
Asperity mechanism for surface initiated spalling
Inside – compression
Together
Line
Cmpressive stressesStress free
Entry – compression and tension
Asperity point contact
Tensile surface stress
Gear surface roughness profile
• 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
d h /mm
200 2100 150 2
Yes, asperities exist! How can they be responsible for spalling?
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: sR / 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 influenceo Well, more to followo On-going research
Mechanism explains why and how • Design guidelines
Asperities and surface initiated rolling contact fatigue
Bo Alfredsson