Fatigue & Surface Finish Gc-08
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Transcript of 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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ORWANS THEORY ON FATIGUE Contd..
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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ORWANS THEORY ON FATIGUE Contd..
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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STRESS CONCENTRATION FACTOR
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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STRESS CONCENTRATION FACTOR
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 FACTOR
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 FACTOR
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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FATIGUE REDUCTION FACTOR
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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FATIGUE REDUCTION FACTOR
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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SURFACE FINISH AND FATIGUE LIFE
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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SURFACE FINISH AND FATIGUE LIFE
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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ON IMPROVEMENT OF FATIGUE LIFE
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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ON IMPROVEMENT OF FATIGUE LIFE
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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