Download - Surface Roughness Modeling Fatigue Failure Results

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April 1, 2019Mechanical Engineering Tribology Laboratory (METL)

Motivation & Background

• Rougher surface finishes have lower fatiguestrength

• Machine components having rough surfacefinish have lower endurance limit Se’

– �′� = �� where �� is surface finishfactor

• Stress concentrations introduced due toroughness accelerate crack initiation atsurface

• In Rolling Contact Fatigue, the failuremode of surface originated failures is oftenlinked to surface finish– Interferes with development of EHL film

– Increased stress level of contacting bodies

= � �, ��

= � ��, ��

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Objectives

• The objectives of this study is to experimentally and

analytically investigate the effects of surface roughness on

fatigue of tensile specimen

– Conduct fully reversed fatigue on tensile specimens

– 3 levels of roughness

– Model experimentally observed grain morphology using

Voronoi Tessellation

– Model surface roughness using actual Ra obtained from

surface profilometry

– Corroborate experimental and analytical results

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Surface Roughness Modeling

• Smooth surface: Exact reflection at boundary

• Rough surface: Perturbed reflection

∆� = �� − ��

For smooth:

rnew (��) = �� - ∆�For Rough:

rnew(��) = �� - (∆�, !)! ~ Ra parameter

A B C

Higher σ ~ Higher Ra

Low Ra

0.7 um

High Ra

2.5 um

O

A B C

Boundary generation by reflection: RCF domain

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Fatigue Failure Results

• As milled specimens (Ra ~ 0.5 um) have

lower life than smooth specimen,

simulations corroborate with experiments

• Crack initiation, propagation and final

fracture can be observed in SEM

S-N diagram for Ra = 0.5 um vs Smooth

1- Initiation

2- Propagation

3- Fracture

Full view