fatigue & surface finish gc-08

8/6/2019 Fatigue & Surface Finish Gc-08

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FATIGUE & SURFACE FINISH

G. CHOWDHURY Prof. (Met)


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INTRODUCTION

Design load is always kept much below UTS

In fact, with Safety Factor it is below YS

But the fact of life remains that even with thatsafe load, failure do occur

On post mortem of failure, the reasons are

normally found to be Wear, Corrosion,Sudden fracture or Fatigue fracture

Interestingly, all originate from the SURFACE


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INTRODUCTION

MODES OF FAILURE IN SERVICE

Mode Contribution of

Surface

Contribution of

InteriorWear 100 NIL

Corrosion 100 NIL

Sudden

fracture

95 5

Fatigue

fracture

90 10


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INTRODUCTION

While the first two can presumably be

related to loss of surface area (not really),

and the third one to overloading The last one is most intriguing

Why a material should fail without any loss

of surface area or overloading?

We will try to address this issue here


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INTRODUCTION

WHAT IS FATIGUE FAILURE?

Failure of a metal subjected to repetitive

or fluctuating stress at a level muchlower than its YS is known as FATIGUE

The key points are

Load level much lower than YS Nature of loading- Repetitive or Fluctuating


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INTRODUCTION

What is the implication?

A design load, often kept lower than theYS with safety factor, may not be reallysafe if subjected to dynamic loading

Two immediate Questions pop up

Why it is so?

What is the safe design load undercondition of dynamic loading?

We will rather try to answer the last one first


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INTRODUCTION

A fatigue failure is often characterized by

One or more fatigue initiation points

Progress of crack in multiple smooth steps ofcircle centering the initiation point (normallyknown as Beach Marks)

This zone appears old and lustureless,

occasionally with rust on the surface! Sudden failure or break apart zone,

lusturous and often associated with ChevronMarks


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FIG-1 FATIGUE SRESS CYCLES.

DIFFERENT TYPES OF DYNAMIC LOAD


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DIFFERENT KEY TERMS

The Range of stress (r) = Max Min

The Alternating stress (a) = r/2

The Mean or Steady stress (m) = (Max + Min)/2

R= Max

/ Min

A= a/ m


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THE IDEAL S-N CURVE

If a material is subjected to dynamic loading

of Type-1 under laboratory condition, it is

observed that Generally, materials fail after some time

Higher the max, lower the number of cycle

For some materials like Steel and Titanium,at some max number of cycle is infinity

For others, no such thresh hold value ofmax


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FIG-2 FATIGUE CURVES

THE S-N CURVE


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SAFE DESIGN LOAD

So, what a Designer can do?

For steel, (fortunately) he can take theEndurance Limit as the safe load

For others, he can fix a reasonably highnumber of cycle, normally 108, and take thecorresponding max as the safe load

The obvious question which arises is whetherthis is a material property or not?

The answer is both YES AND NO!


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SAFE DESIGN LOAD

YES The basic shape and nature of thecurve cant be altered

NO The value grossly depends on surface

condition and metallurgical conditions Moreover, it is dependant on size of

component, type or nature of load, point ofapplication of load, degree of freedom etc.

It is often necessary to carry out acomponent fatigue testing to arrive at adesign data


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ORWANS THEORY ON FATIGUE

Metal contains small weak regions of highstress concentration due to surfaceroughness or metallurgical notches such as

inclusions or other imperfections even GB This small region is treated as plastic

regions in an elastic matrix

With repeated cycles of constant stress

amplitude, the plastic region will experiencea built-up of stress & decrease in strain as aresult of progressive localized strainhardening


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ORWANS THEORY ON FATIGUE Contd..

Total plastic strain converges towards

a finite value as the number of cycles

increases towards infinity

Once the Critical value of Strain and

corresponding Built-up Stress is

reached, the region opens up to releasethe stress

This creates a micro fissure or crack


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This crack itself creates a stress

concentration & forms a new localized

plastic region in which process is

repeated

This process is repeated over & over

until the crack becomes large enough

to cause sudden fracture on applicationof the full tensile stress of the cycles


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This limiting value of total plastic strainapproaches faster with increase in thestress applied to the specimen

The Fatigue limit or Endurance limithinges upon the fact that below acertain stress, the total plastic strain

can not reach the critical value The essence of the theory Localizedstrain hardening uses up the plasticityof metal to cause fracture


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WOODS CONCEPT OF FATIGUE

It basically refines Orwans Theory on theissue of plastic strain

Dislocations play a major role in thefatigue crack initiation phase.

It has been observed in laboratorytesting that after a large number of

loading cycles dislocations pile up andform structures called persistent slipbands (PSB)


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Movement of an Edge Dislocation

Movement of an edge dislocation across the crystal lattice under a shear stress.


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WOODS CONCEPT OF FATIGUE Contd

PSBs are areas that rise above (extrusion)or fall below (intrusion) on the surface ofthe component due to movement ofmaterial along slip planes

This leaves tiny steps or notches in thesurface that serve as stress raisers wherefatigue cracks can initiate

This mechanism is in agreement with factsthat fatigue cracks start at the surface &cracks have been found to initiate at theslip band intrusion & extrusion


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WOODS CONCEPT OF FATIGUE Contd


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WOODS Concept Contd..WOODS Concept Contd.. Demonstration of CrackPropagation Due to Fatigue

The figure right illustrates the variousways in which cracks are initiated and

the stages that occur after they start.

This is extremely important since thesecracks will ultimately lead to failure ofthe material if not detected andrecognized.

The material shown is pulled in tensionwith a cyclic stress in the y direction.

Cracks can be initiated by severaldifferent causes, the most three

important causes are

Nucleating slip planes,

Notches.

Internal flaws.


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MECHANISM OF FATIGUE

So, now we have detected Stress Raisersas the culprit to initiate fatigue crack

Metallurgical stress raisers are Inclusion,Opened up seam, Internal flaws, GrainBoundaries, Slip Planes etc.

Mechanical stress raisers are many; Holes,

Key ways, Surface roughness & notches,Machine marks, Undercut, Abrupt changein diameter, all are potential stress raisers.

However, the stress intensity factor varies


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STRESS CONCENTRATION FACTOR

The Stress Concentration factor, known as kt, is

the ratio of stress due to stress concentration

and the nominal stress

For a circular hole in the component

Maximum stress Wmax = 3 W

where W is the nominal stress

So kt = 3 The factor kt strongly depends on shape and

rises abruptly for Sharp notches, Tool marks etc.


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A shaft with varying diameter having fillet

of radius r and big and small diameters as

D and d

for D/d r/d kt

1.1 0 3.5

0.06 1.20.12 1.16


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A shaft with varying diameter having fillet

of radius r and big and small diameters as

D and d

for D/d r/d kt

2.0 0 >10

0.06 1.620.12 1.40


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FATIGUE REDUCTION FACTOR

Fatigue reduction factor, kf is given by ratio offatigue strength without notch and fatigue strengthwith notch

This ratio depends on kt Brittle and Ductile materials behave differently in

the face of same type of stress concentration

At high kt values the reduction in fatigue strength

of ductile material is less, for harder materials thereduction is more

This means that at times use of lower strengthmaterials can give better fatigue resistance


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Fatigue Reduction factors of different materials dueto surface obtained from various surface treatments

Treatment kf(for 400Mpa UTS)

kf(for 1800Mpa UTS)

Mirror Polish 1 1

Fine Ground 0.9 0.7

Machined 0.8 0.5

Hot Rolled 0.75 0.22


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Fatigue Reduction factors of different materials dueto surface obtained from various surface treatments

Treatment kf(for 400Mpa UTS)

kf(for 1800Mpa UTS)

Corroded in

water

0.65 0.17

As forged 0.60 0.14

Corroded in

salt water

0.47 0.10


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Case of high pressure tube

High pressure tube of diesel loco was

earlier made of low alloy steel withstrength of around 650 Mpa. There were

large scale failures.

Changed to St 52 plane carbon steel with

strength of 520 Mpa. It worked.


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Sharp notches

Sharp notches have very high kt However, the reduction in k

fmay be much

less compared to increase in kt This is even more so for ductile materials.

However, the sharp notches have another

great effect on failure of components. Thisis failure by impact. Here the chances offailure may be directly related to kt


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SURFACE FINISH AND FATIGUE LIFE

Different surface finishes produced bydifferent material removal mechanical

processes can appreciably affect fatigue

performance. Preferably the last machining operation

should leave marks parallel to the direction

of principal stress.

In this case, the ridges lie parallel to the

principal stress


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Fatigue life of SAE 3130 Steel

Reversible stress 655Mpa

Type of finish Surface roughness

(in m)

Fatigue Life

(in cycle)

Lathe formed 2.67 24000

Part Hand Polished 1.50 91000

Hand Polished 1.30 1,37000

Ground 0.18 2,17000

Ground and Polished 0.05 2,34000


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Case of crank shaft breakage

HEC crank shafts were breaking in large

numbers. So, material had to be importedfrom National Forge

The main difference in quality was in the

finish of fillets which had machine marks

left in case of HEC crank shafts while

National Forge supplies had excellent finish


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ON IMPROVEMENT OF FATIGUE LIFE

Surface hardening

Surface hardening generally improves fatigue life

After surface hardening, ground finish and magnetic

particle check for grinding cracks is almost mandatory

Selective quenching

Rim quenching in wheel is very effective

Shot peening

very effective method of introducing compressive

stresses on surface which improves fatigue strength

of components


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Electro Plating

Plating should be compatible with substrate. Crplating on steel is notorious for reducing

fatigue strength Shot peening after electro plating gives very

good results

Heat Treatment

Decarburized surface of heat treated steeldetrimental to fatigue performance

Hence, care to avoid decarburization


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Sharp Notch Sharp notch, accidentally formed, are to be

grounded to flatten it It will reduce kt

Heat Affected Zone Welding is not advisable on dynamically

loaded safety components

Grinding Care to be taken to avoid formation of HAZdue to overheating during grinding


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COMPONENT SIZE & FATIGUE LIFE

Size effect Fatigue strength of large members are lower

than that of small members

Surface area increases with increasing

diameter. Amount of surface area is of significance-

fatigue failure usually starts at the surface.

For loading in bending or torsion, an increase in

diameter usually decreases the stress gradientacross the diameter & increases the volume ofmaterial which is highly stressed


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FATIGUEFRACTURE


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Fig.1Photograph showing

the cracked pieces with

deep punch marks on

front rim side and fractureends

Fig.2 Photograph showing

the fatigue fracture initiated

from deep punch mark.

Fig.3 Photograph showing

the close view of the fatigue

zone.


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fatigue & surface finish gc-08

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