Asperities and surface initiated rolling contact fatigue Bo Alfredsson Solid Mechanics at KTH – 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  v ≈  · r Rolling contact Sub-surface initiation 3 mm Tallian 1992 Surface initiation

Surface initiated spalling 0.2 mm 0.1 mm   = 20 – 24 º

General conditions for fatigue initiation and fatigue crack growth Initiation  Sufficiently large shear stress amplitude  Tensile maximum stress Fatigue crack growth  Sufficiently large  K I ( > K th, which is R dependent)  Open crack tip: K I,max –  K I > K I,cl  Crack with mode II or mode III fatigue load will arrest or kink to mode I  Crack will follow direction of maximum  K I

Whole contact: 2D line contact  Always pressure!  Stress free surface (J 2.15) rr r  Line load – 2D P

The spall is local, asperity is local – local point contact  Tensile radial stress! z r rr Sphere

Asperity mechanism for surface initiated spalling Inside – compression Together Line Cmpressive stresses Stress 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  dh /  m Yes, asperities exist! How can they be responsible for spalling?

Stress in front of rolling contact entering asperity  d = 200  m h = 2  m  d = 100  m h = 2  m  d = 100  m h = 1  m  d = 50  m h = 2  m

Influence of asperity size: asp = h asp /r asp

Influence of contact friction: 

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 Well, more to follow o On-going research  Mechanism explains why and how Design guidelines

Asperities and surface initiated rolling contact fatigue Bo Alfredsson