creep introduction

8/11/2019 Creep Introduction

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CREEP

FAILURE


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Definition:

When materials under severe

service conditions are

required to sustain steadyloadsfor long periodsof time,

they undergoa time dependent deformation.

This is known as creep


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CREEP

can also be defined as

the slow and progressive

deformation of a material

with time at constant stress.


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Creep is found to occur at higher

temperature than at lower temp.

Therefore the study of creep is veryimportant for those materials which

are used at high temp like

components of gas turbines,furnaces, rockets, missiles etc.


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Creep curve:

The creep curve is obtained

by applying a constanttensile loadbelow the yieldpointto a specimen

maintained at constanttemp.


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CREEP CURVE


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As soon as the specimen

is loaded, there will be aninstantaneous strain

which is denoted by o

on the creep curve.


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Further deformation of the

metal only after theinstantaneous strain is

considered as creep deformation.


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Creep deformation of

materials up to failure aredivided into 3 stages

i) primary creep ii) secondary creep

iii) tertiary creep.


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CREEP CURVE


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Primary creep: OR TRANSIENT CREEP

This is the first stage of thecreep which represents a

region of decreasing creep

rate.


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In this region the rate at

which the material deformsdecreases with time until it

reaches a constant value.


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The creep rate goes on

reducing because as the

metal deforms it undergoes

strain hardeningand offers

more and more resistance tofurther elongation.


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Transient creep:

The princ iple character ist ic of

trans ien t creep is the decreasingrate in deformat ion .

Deformat ion is rapid at f i rs t but

gradual ly becomes slower andslower as the rate approaches

some fixed value.

Transient c reep in metals isobserved at al l temp, even near

abso lute zero . Hence it is some

times referred to as cold creep


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Secondary creep: [steady statecreep]

Nearly constantcreep rate,

because strain-hardening and

recovery effectsbalanceeach other.

Creep in this region takesplace by the viscous flowin the materials.


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Viscous creep: SECONDARY CREEP] It is characterized by the

viscous flow of the material

means that there is a constant

or a steady increase in

deformation at constant stress.


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Although strain hardening is

present, its effect is just

balanced by the recoveryprocess which has the

opposite effect i.e softening the metal.


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viscous creep is stopped when

there is considerable reduction

in cross sectional area andenters the tertiary stage .

The rate of deformation

increases rapidly in this 3rd

stage and fracture occurs atthe end of this stage.


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Viscous creep also knownas hot creep', since it is

observed only at higher

temperature.


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Tertiary creep :

This stage is period ofincreasing strain rate.

Tertiary creep occurs when

there is an effectivereduction in cross-sectional area due tonecking or internal voidformation.


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If the stress is kept constantof the load or if true strain is

taken into considerationthen the resulting fracture

due to creep would be at B.


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Effect of low temperature :

Temperature below Tm/4

are called as LOWER TEMP.

lower temp have an effect of

decreasing the creep rate.

This is because strain

hardening effects will bemore and recovery process

is negligible.


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STRAIN

STRESS


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Creep occurring at lower temp

is known as logarithmic creep

= ln twhere - strain ,

a constantttime .


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lowtemp. logarithmic creep

obeys a mechanical

equation of state i.e the rate

of strainat a given time

dependsonly on theinstantaneous values of

stress and strain and notonthe previous strain history.


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Effect of hightemperature:

At Higher temp. the creep rate

increases.

structural changes takes

place.


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Mobility of atomsincreases with temp and

occupy lower energypositions.

Mobility of dislocation alsoincreases and theyovercome the obstacles bythe mechanism of climb.


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The concentration of vacancies

increases with temp and the rate of diffusion

increases.

Recrystallization takes place as

a result of increased rate ofdiffusion.


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Mechanism of creep:The chief

mechanism responsible for creep

deformation are , Dislocation glide

Dislocation creep ordislocation climb

Diffusion creep

Grain boundary sliding.


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1) Dislocation glide :

The creep rate is established

by the ease with which the

dislocation move across

obstacles such as

precipitates etc.


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Dislocation glide

Dislocation

Grasin Boundary


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This may include cross-slip

of dislocations with the aid

of thermal energy.

Dislocation glide results in

increase in plastic strain

during creep deformation

Di l ti


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2) Dislocation creep or

dislocation climb:

This is caused due to mutualmovement of dislocations &

vacancies.


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At high temp the diffusion rate of

vacancies is more which make the

dislocations to glide & climb, edge dislocation can move end of

a slip plane

OR to a plane above or below the

slip Plane.

This is called dislocation climb.


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.

Vacancy

Dislocation

Mutual movement of

dislocations & vacancies.


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This occurs at high temperature

(T>0.4Tm) &

When dislocation cause across

obstacles, they use vacancies/or interstitial

atoms to climb and go around them

for slip & hence plastic deformation continues.


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3) Diffusion creep: -

It occurs when temp. is high &

at relatively low stresses.

diffusion of vacancies controlsthe creep rate.

But edge dislocation is notinvolved in them.


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vacancies move from the surface of

the specimen towards the stress

axis

Stress axis


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i.e when load is applied &

kept constant ,the vertical

boundaries are subjected tocompression& the horizontal

grain boundaries aresubjected to tensilestress.

so vacancies migrate from


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so vacancies migrate from

tensile region to compression

region &atoms migrate in opposite

direction to vacancies.

i.e causing the creep strain

along tensile axis,


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the mechanism is called

Nabarro-herring creepbecause the flux of atoms is

through the bulk of atoms.


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4) Grain boundary sliding:-

At low tempthe Grain

boundaries will notflow

viscously & provide obstacles

to dislocation motion.


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At elevated temp, the grains inpolycrystalline materials areable to move relative to each

other, this is called grainboundary slidingand is anshear process which occurs in

the direction of grain boundary[GB] .


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a large no. of grains sliding with

each other results in plastic

deformation due to creep .

GB sliding is promoted by

increases the temp. T & or

decreasingthe strain rate.

C


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Creep properties :-

1) creep strength/ or creep

limit:

It is defined as the higheststress that material can

withstand without excessivedeformation for a specified

length of time.


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Ex :- creep strength for a steam

turbine blade may be that

stress which will produce just

0.2% creep for 10,000 hours of

working at 800o

C.


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Creep strength can also be

defined as the stress at given

temperature which produces

steady state creep rate of fixed

amount say 10-11

to 10-8

/sec.OR

It is the stress to cause a creep

strain of 1% in fixed time say

105 hours.

C t t th it i


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2) Creep rupture strength:- it is

defined as the highest stress that

a material can withstand withoutrupture for a specified length of

time.

Ex : For the same turbine blade . the

rupture strength is that stress whichproduces a fracture in 1000HR OR

10000HR OR 100 000 hrs at 800oC.

Thus creep rupture strength is


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Thus creep rupture strength is

also defined as the limiting

stress below which creep is soslow that will not result in

fracture within any finite length

of time.

It is also called as stress ruptures

strength.


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3) Creep life:

it is defined as the

TIME to fracture under a given

static load.

0

C

0

ff C


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Factors affecting Creep:

Load :

Temperature: Composition:

Grain size Heat treatment

L d C t i


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Load : Creep rate increases as

load increases.

Temperature: Creep rate

increases as T increases.

Composition: Pure metals aresofter than alloys , the

different phases present stops

the dislocation glide .

Hence creep is more in pure

metals.

G i i S ll th i


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Grain size: Smaller the grain,

stronger the material.

But above Equicohessive temp.

this effect will be reversed one.

Equicohessive temp. (Kelvin) Te >Tm/2

Heat Treatment: This changes the

structure,Obviously the materials

property changes, creep resistance

also changes.


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.


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Creep curve equations:


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Strain Rate = increment of strain / increment in time

creep introduction

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