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