book - chap 05 creep test

8/12/2019 Book - Chap 05 Creep Test

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CHAPTER 5CHAPTER 5

Dr. Talaat El- Benawy

CREEP TEST


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Several metallic components, which are used in heat

exchangers or boilers, are subjected to static

external loading under high temperature serviceconditions which lead to a continuous deformation

with time.

Introduction

This phenomenon is so-called creepcreep.


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In many designs these creep deformations must be maintained smallmust be maintained small

for the sake of avoiding failure or because of the principle offor the sake of avoiding failure or because of the principle of thethe

manufacturing designmanufacturing design ..

Importance of CreepImportance of Creep

Excessive creep deformations may also lead toExcessive creep deformations may also lead to premature bucklingpremature buckling ofofstructural members subjected to compressive loads.structural members subjected to compressive loads. PermanentPermanent

creepcreep deformations under repetitive flight conditions are additive andeformations under repetitive flight conditions are additive andd

may reach such large values as to make an aircraft aerodynamicalmay reach such large values as to make an aircraft aerodynamical lyly

unsatisfactoryunsatisfactory.

For instance, if the creep deformations are not limited to small values,if the creep deformations are not limited to small values,

necessary clearances between moving and stationary parts, as innecessary clearances between moving and stationary parts, as in

steam and gas turbines, are not maintained.steam and gas turbines, are not maintained.

Furthermore, for certain stress values, the creep deformations mFurthermore, for certain stress values, the creep deformations mayay

become so large that fracture is produced during the lifetime ofbecome so large that fracture is produced during the lifetime of thethe

partpart..


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Creep Definition

Metals Handbook defined the creep as the slowMetals Handbook defined the creep as the slowdeformation of adeformation of a material under amaterial under a stress that results in astress that results in a

permanent change in shapepermanent change in shape..

In other words, creep can be defined as the plasticIn other words, creep can be defined as the plasticdeformation timedeformation time--dependent strain of adependent strain of a material that ismaterial that is

subjected to a stress below its yield stress for asubjected to a stress below its yield stress for a

prolonged period of time.prolonged period of time.

Creep occurs for stresses below the yield point only when

the temperature is exceeding about 0.3is exceeding about 0.3--0.4 of the melting0.4 of the melting

point of the material.point of the material. However, some nonferrous alloysHowever, some nonferrous alloys

and metals such as lead and tin and their alloys exhibitand metals such as lead and tin and their alloys exhibitcreep at low stresses and also at room temperatures.creep at low stresses and also at room temperatures. ForFor

a number of nonmetallic materials, including variousa number of nonmetallic materials, including various

types of plastics, concrete, and wood, creep may also betypes of plastics, concrete, and wood, creep may also beproduced at low stresses and roomproduced at low stresses and room temperatures.temperatures.


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Creep Testing Experiment ApparatusCreep Testing Experiment ApparatusCreep testing machine is consisting of three main pieces:Creep testing machine is consisting of three main pieces:

(1)(1) an electric furnace with suitable temperature controlling devicean electric furnace with suitable temperature controlling device;;

(2) an extensometer to measure the creep deformation and(2) an extensometer to measure the creep deformation and

(3) a loading setup system.(3) a loading setup system.


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Creep Data and Their InterpretationCreep Data and Their InterpretationCreep behavior of a material is determined by uniaxial loading of test

specimen under constant stress (()) heated at temperature (T).(T). The obtainedresult data from such experiment is a creep (creep ())--time (t)time (t) curve as shown:

Elastic strain OAOA will occur upon

application of the load. Atelevated temperature strain OCOC

occurs upon the application of

the same stress . Strain underStrain underthis condition will be representedthis condition will be representedby the curveby the curve

OABOAB. Strain OCOC

may be entirely elastic, or elasticelastic, or elastic

plus plastic, depending upon theplus plastic, depending upon the

material, temp. and stress.material, temp. and stress.Strain increases as time of load

application extends as illustrated

by curve CDEFCDEF. This continued

increase in with time, underconstant stress, is defined as

creep.


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In this stage the rate of strain hardening by dislocations is baIn this stage the rate of strain hardening by dislocations is balancedlanced

by the rate of recovery.by the rate of recovery.

In initial stageinitial stage the total

deformation is elastic due

to application of the load.

In first stage (Primaryfirst stage (PrimaryCreep),Creep), creep continues at

a decreasing rate as the

strain hardening is the

dominating factor on thecreep deformation.

In second stagesecond stage, creep

rate becomes constant

and this stage is therebycalled steady state regionsteady state region.

Creep sequence mainly

consists of three stagesthree stages as

well as the init ial stageinitial stage.

In third (final) stagethird (final) stage, a rapid increase in creep rate can be observed,

which continues unti l fracture occurs.


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InIn third stagethird stage, high strains, high strains

produced will start to causeproduced will start to cause

necking in specimen, which willnecking in specimen, which willcausecause increase in the local stressincrease in the local stress

and eventually the material willand eventually the material will

pull apart in apull apart in a ductile fractureductile fracture

around the defects which couldaround the defects which couldbe prebe pre--existed in the material.existed in the material.

These defects could beThese defects could be

precipitates at high temperaturesprecipitates at high temperatures

or grain boundaries.or grain boundaries.

In fact, this region is not important to normal operation and crIn fact, this region is not important to normal operation and creepeep

design criteria since it is usually to end the serviceable lifedesign criteria since it is usually to end the serviceable life of anyof any

component at the end of the steady state creep regioncomponent at the end of the steady state creep region (second(secondstage)stage)..

It has to be mentioned that for the lower stresses andIt has to be mentioned that for the lower stresses and

temperatures, the final stage of creep is not observable duringtemperatures, the final stage of creep is not observable during

the usual times covered by creep tests.the usual times covered by creep tests.


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Given figure shows the effect of stress and temperature on theGiven figure shows the effect of stress and temperature on the

creepcreep--time curve.time curve.

>>22>>ss33 and Tand T11>T>T22>T>T33


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Creep Testing MachineCreep Testing Machine

Single lever creep testing machine Multiple lever creep testing machine


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Different loading method machinesDifferent loading method machines


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Explanation of Creep DataExplanation of Creep Data

Generally, all creep and stress rupture related data areGenerally, all creep and stress rupture related data areanalyzed in terms of three variables: time, stress, andanalyzed in terms of three variables: time, stress, and

temperature. There are two important parameter shouldtemperature. There are two important parameter should

always be considered and calculate from creep testingalways be considered and calculate from creep testingdata: time to fracture (data: time to fracture (ttRR), which is sometimes called), which is sometimes called

creep l ifetime and steadycreep l ifetime and steady--state creep ratestate creep rateIt is possible to calculate theIt is possible to calculate the

creep lifetimecreep lifetime ((ttRR) by using the) by using the

stressstress--rupture time curverupture time curve

shown. This curve permits toshown. This curve permits to

expectexpect ttRR of the component forof the component forparticular combination ofparticular combination of

stress (stress () and temperature (T)) and temperature (T)as the time to ruptureas the time to rupture ttRR isis

proportionally in linearproportionally in linearrelationship on logrelationship on log--log scalelog scale

with the stress (with the stress () as shown) as shown


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As shown in the figure for

Cr-Ni steel alloy. The

following equation can be

deduced from the figure:

Rt m

= Kwhere tR= creep rupturetime in hours,

= rupture

stress in MPa and K and mconstants can be found

from the figure .


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In the second stage of thecreep-time curve, the

creep rate becomesapproximately constantand this stage is thereby

called steady state regionas shown in the figure.

The figure also indicates

that this stage has thelongest duration and the

steady-state creep rate can

be easily calculated

through the following

equation:

t


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In fact, the creep rate can often be represented by the

Arrhenius relationship of the form:

n' Qexp

RT

whereA and n are constants; is usually in the range of 3

to 7 for metals and 1 to 2 for polymers; T is the

temperature in [K] and Q is called the activation energyfor creep in [J.mol-1] and it is mainly depending on the

material. The value of Q can be determined by plotting the

natural logarithm of the creep rate vs. (1/T). The datashould fall along a straight line of slopeof slope --Q/RQ/R where R is

the gas constant that equals to 8.314 [J.mol-1 k-1] and. isthe applied stress in [MPa].


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The shown figure

indicates the relationshipbetween creep stress andthe creep rate for carbon

steel at differenttemperature. The figurealso suggests that thecreep strain rate wouldcorrelate with the creepstress through thefollowing equation:

n

t

where B and n are constants


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The given equations give complete scope for the designThe given equations give complete scope for the design

based on the creep properties.based on the creep properties.

In some application, it is required to define when theIn some application, it is required to define when thecomponent, which is subjected to creep, should becomponent, which is subjected to creep, should be

replaced before failure. In this case the creep deformationreplaced before failure. In this case the creep deformation

itself is not important and then equation ofitself is not important and then equation of ttRR

can be usedcan be used

to calculate the creep lifetime for particular combinationto calculate the creep lifetime for particular combination

of stress and temperature for specific material, at whichof stress and temperature for specific material, at which

the component should be replaced before creep failure.the component should be replaced before creep failure.

On the other hand, in some other applications, there is aOn the other hand, in some other applications, there is a

certain creep deformation valuecertain creep deformation value should not beshould not be

exceeded. In this case, equation of creep rate can be usedexceeded. In this case, equation of creep rate can be used

to decide when the component should be replaced beforeto decide when the component should be replaced beforethe creep strain reaches to the critical value.the creep strain reaches to the critical value.

Rt m

= K

n


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For the same propose of calculating the creep lifetime, aFor the same propose of calculating the creep lifetime, aparameter calledparameter called Larson and MillerLarson and Miller parameterparameter ((LMLM) have) have

been given in the literature to facilities the calculation.been given in the literature to facilities the calculation.

This parameter is in the following form:This parameter is in the following form:

RLM T C log twhere T is the temperature in [K]; C is material constantwhere T is the temperature in [K]; C is material constant

usually in the range ofusually in the range of 1818--22 and in general is taken to22 and in general is taken toequal 20.equal 20.


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In order to understand

LM equation, let uslet usestimate the creepestimate the creep

rupture time for arupture time for a

NimonicNimonic 80A at stress80A at stress

of 300of 300 MPaMPa andand

temperature of 800temperature of 800 C.C.

The given figure showsthe relationship

between LM parameter

and logarithmic scale of

the applied creep stressfor Nimonic 80A and

Nimonic 105

From figure LM =22.8x10LM =22.8x1033 at =300=300 MPaMPa for NimonicNimonic 80A80A


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By using LM parameter equation:

R3

R

3

R

R

LM T C logt22.8 10 800 273 20 logt

22.8 10logt 20

800 273

t 17.74 Hours


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Exercise 5


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5. From uniaxial creep tests of a canvas-base laminatedplastic the following creep strains were found


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a) Plot the creep strain (e)-time (t) curves for each of

the stresses using the foregoing data


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b)

Obtain the approximate values of the creep rates and deduce the

constantsB andn of the equation: n


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6.

Using the Larson-Miller parameter for ductile cast

iron shown in Figure 2, determine the time requiredbefore the metal fails at:a)

an applied stress of 40 MPa

and temperature of 400 C and 600 C

b) an applied stress of 80 MPa and temperature of 400 C and 600 C


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a)


and

temperature of 400 C and 600 C

R

R

R

R

40 MPa

LM T log t

T

logt

t

logt

t

T

-3

-3

10 5R

-3

at from the figure LM =20.35

20 10

at 400 C

20.35 400+273 20 10

=1.7310 Hours 2010 Years

20.35 20 10

204

at 600

4 Hours

C

600+273

85 Days


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b)


and

temperature of 400 C and 600 C

R

R

R

R

LM T log t

T

logt

t

logt

t

T

-3

-3

8 3R

-3

at = MPa from the figure LM =19

20 10

at 400 C

19 400+273 20 10

=1.7110 Hours 2010 Years

19 20 10

at 600

58 Hour

C

600

80

s 2.

+ 3

s

27

5 Day

book - chap 05 creep test

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