CONTACT STRESS FATIGUE

Presented by

Purvesh K. NanavatyME (Mat.Tech.)

Sem II

Guided byDr.Vandana Rao

Contents

Introduction Fundamentals of CSF Subsurface –origin fatigue Surface –origin fatigue Subcase-origin fatigue (“Spalling”

fatigue) Cavitation fatigue Conclusion References

introduction Fatigue :- phenomenon that results from

cyclic slip under repetitive load applications for many thousands & millions of cycles.

Certain type of metal removal –or wear- that are not caused directly by sliding action

Fatigue that produces cavities, or pits , in either of two surfaces in contact primarily by rolling and/or sliding action, or –---- in case of cavitation fatigue---in a metal surface in contact with liquid.

Other name is “pitting wear”

The difference… Instead of causing gross fractures of

parts, only fragments of the surface are removed at least initially. Then lost gradually results in pits & cavities in the surfaces.

Behaviour of pits / cavities

Frequently acts as a stress concentrations If metal removed from the cavities is very

hard & brittle –readily crushed-fragmented cause abrasive wear

Some start as microscopic in size & remain as throughout the life of the part

Some start as microscopic but gradually becomes large.

Start as large & then rapidly become even larger.

Contact stress fatigue / pitting fatigue are observed with.

Various Mechanisms involved with rolling and/ or sliding type. as Pure rolling is never possible, in fact there is some degree of sliding due to elastic deflection –of the part under load

Bearings, gears, cams, pumps, impeller, propellers ….

One or both mating surfaces under compressive load on point or line contact.

Instantaneous contact area bearing high amount of compressive & shear stresses

Subsurface-origin Fatigue Most common in antifriction, or rolling

element bearings –ball & roller bearings, needle bearings, roller cams

Various types of inclusions inherent in the steels randomly distributed.

Hard, angular, brittle often “stress raisers” Max shear stress is located at short distance

below the surface Result in a high stress conc. Crack initiation,

propagation, surrounding metal removal Re occurrence at various points of surface.

Stress distribution in contacting surfaces due to rolling, sliding, and combined effect

Gear tooth section

Gear-tooth section. Rolling-contact fatigue. Crack origin subsurface. Progression was parallel tosurface and inward away from surface. Not etched. 60×

Gear-tooth section. Rolling-contact fatigue. Crack origin subsurface. Progression was parallelwith surface, inward, and finally to the surface to form a large pit or spall. Not etched. 60×

Fracture surface of a hardened steel connecting rod. Arrows indicate large

inclusions from which fatigue cracking initiated

Fracture surface of a hardened steel valve spring that failed in torsional fatigue.

Arrow indicates fracture origin at a subsurface nonmetallic inclusion.

Fracture surface of a carburized-and-hardened steel roller. As a result of banded

alloy segregation, circumferential fatigue fracture initiated at a subsurface origin near the

case/core interface (arrow).

Surface -origin fatigue

Pure rolling / meshing (In case of gears) is never possible because of difference in the driver/driven relative surface velocities.

Sliding at the interface. Difference in the sliding causing “drag”

usually of slow surface speed driven member clockwise or counter clockwise.

Negative sliding- rolling & sliding are in opposite directions

Positive sliding- rolling & sliding are in same directions

Rolling /sliding action in gear teeth.

Dedendum (the region below the pitch line) of all gear teeth are in negative sliding. -origin of surface –potential source of surface origin pitting.

Gear with arrow-shaped surface-origin pit

Macropitting

Macro pitting is a general term that includes spalling and other forms of macro scale damage caused by contact fatigue. Macro pitting of bearing raceways or gear teeth is generally due to contact fatigue, which occurs from localized plastic deformation, crack initiation, and finally macro pitting from crack propagation in and near the contact surface

Fractograph showing the advanced stages of point-surface-origin macropitting

fatigue of a bearing raceway in which the origin is still visible

Micropitting

Micropitting is the preferred term for peeling fatigue, which is defined as microscale spalling fatigue. It is damage of….

rolling/sliding contact metal surfaces by the formation of a Micropitting damage is a gradual type of surface fatigue damage that is a complex function of surface topography and its interaction with the lubricant.

Gear with micropitting (frosting) failure

Subcase –origin Fatigue Damage to case-hardened rolling /sliding

surfaces - gear teeth and roller mechanism

Very large pieces are suddenly lost from the surface & extensive damage may result.

Sub case –origin fatigue

Sub case fatigue cracking. The micrographs show etched cross sections of

(a) a carburized cylindrical test specimen (1.9×) and (b) a carburized gear tooth

Sub case fatigue, also known as case crushing

(c ) how the stress and strength gradients combine to cause the weakest condition at the sub case location

Pinion with several very large cavities where metal from the surface down to the

depth of the case has fallen out due to sub case fatigue

Cavitation pitting Fatigue Serious problem in Marine propellers,

diesel engine cylinder liners, pump impellers, hydraulic pumps & equipments, turbines.

Those parts that vibrate & come in contact with liquids.

Liquid metal interface. Pit size range from pin head to golf ball

size

Applied shear stress and material shear strength as a function of depthrepresenting types of fatigue damage

(a) No damage. (b) Subsurface-

origin, macro pitting fatigue.

(c) Micropitting or surface-origin macropitting fatigue. (d) Subcase fatigue.

SUMMARY

REFERANCES

ASM HAND BOOK VOL. 11.FAILURE ANALYSIS & PREVENTION

UNDERSTANDING HOW COMPONENTS FAIL- DONALD J.WULPI

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