CREEP PROPERTIES.pdf

7/27/2019 CREEP PROPERTIES.pdf

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ME 215

ENGINEERING MATERIALS I

CHP.8

HIGH TEMPERATURE

PROPERTIES OF MATERIALS



INTRODUCTIONWhile technolgy is advancing many engineering applications

require materials working at high temperatures.

High temperature means how close is the working temp to

melting temp of the material itself.

For example, could a material with 500C melting point be used

in an application where working temp is 300C for a relatively

long period of service?

The development of materials to resists high temperatures

therefore has been an important technological advence of

recent years.



Demands for alloys to resist high temperatures

come from many new applications, including

– space vehicles,

– rockets ,

– gas turbines,

– improvements in conventional power plants for increased thermodynamic effeciency,

– chemical processes,

etc.It can probably be said that high temperature

application represents one of the ultimates in

engineering desing.



• The corrosion and creep resistance limits of the avaliablematerials restricts the contemplated high pressurereactions and high temperatures conditions in chemical

processing.• In air craft and space vehicles, high temperatures arisefrom the heat generated in the engines and from theaerodynamic heating of airframes at supersonic speeds.



The point of choosing correct material for hightemp application is therefore very important.

Because there is the greatest variety of practicalconsiderations which determine the choice of material for high temperature applications .

These include: – Environment

– Stress system and range

– Permissible creep deformation

– Production requirement

– Weight and size restrictions

– Cost and availability

– Service temperature range



1. Environment;

it covers a wide range from an inert gas to completely

hostile environmental conditions. In the latter casethis becomes a prominent factor in the choice of materials.

2. Stress system and range;

this can vary from heavy loads in bulky structures tozero applied loads in covers and heat shields.

Fatigue,under fluctuating stresses or the thermalstresses is always of prime concern.

3. Permissible creep deformation;

this again is completely dependent upon the funtionalrequirements.



4. Production requirements; it should be noted thatsome of the high temperature materials are not

amenable to regular methods of production to producethe desired shapes.

5. Weight and size restriction; these are of importancein aircraft and,especially, space like applications.

6. Cost and availability; there are many excellent hightemperature materials but their cost limits their use inmost conventional applications

7. Service temperature range; probably the mostimportant factor in deciding the essantial properties.”



STATIC CREEP PROPERTIES

• Creep is a progressive deformation of a metarial under

stress.

• It is a time dependant permanent deformation

• In creep, at constant stress, strain continues to

increase by time

Time

Stress

Strain



There are three main kinds of creep:

• Logarithmic creep

• Diffusion creep• High-temperature creep

The logarithmic creep

occurs at low

temperatures where norecovery of mechanical

properties is possible.

It is given the name

“logaritmic creep”because the

deformation is

proportional to the

logarithm of time.

•The diffusion creep is observed

at very high temperatures.

•High rates of diffusion permitreshaping of crystals to relieve

stress

•Diffusion is significant at both

grain boundaries and in the bulk

•High energy and weak bonds

allow dislocations to “climb”

around structures that pin them

at lower temperature



The Critical Temperature

• Contrary to the general belief, creep is not

exclusive to high temperatures.

• Creep may occur from very low temperatures,

probably from 0oK (-273oC) ,to the melting point.

• The Critical Temperature for Creep is usually

40% of the Melting Temperature in Kelvin.

• If T > 0.40 TM ; Then creep is likely



Example for creep risk at room temp.

Will products made from Lead Creep at Room Temperature?

• TM = 327 C = 600 K (melting point for lead in Kelvin)

• TROOM = 23 C = 296 K (room temp in Kelvin)

• TR/TM = ?

• (296 / 600)*100 = 49.3 % > 40% Yes They Will Creep



• Creep is important in the engineering sense becauseit causes faiulure by excessive of deformation or loss

of strength.• Excessive creep deformations may also lead to

buckling of structural members or complete distortionof structural components which requires withdrawal

from service.• For most metals and alloys creep occurs at stressesbelow the yield point only when the temperature iselevated.

• Some nonferrous alloys and metals such as lead

exhibit creep at low stresses and at roomtemperatures.

• For nonmetallic materials,such as plastics andelastomers,creep may also be produced at lowstresses and room temperatures



CREEP TESTING OF

MATERIALS

The basic creep testingarrangement consist of three pieces of major equipment;

1-An electric furnace withsuitable temperatureregulation

2-a loading device toapply stress

3-an extensometer tomeasure elongationagainst time



Time Extension

0 ?

1 ?

2 ?



In making a creep test at a specified temperature, the

unloaded tensile specimen is first heated to the

required temperature.

When the temperature of the specimen is steady, the

gauge length is observed and the predetermined load

is applied quickly without shock.

The resulting instantaneousextension under load is largely

an elastic strain.

Measuremets of the

subsequent creep (elongation)

are taken at sufficiently

frequent intervals to define the

strain – time curve till rupture.



Creep time curves have four stages:

• İnitial instantaneous stage in which total deformation may be fullyelastic,or partly elastic and partly plastic;

• A transient stage where the rate of creep deformation decreaseswith time as a result of strain hardening (primary creep )

• A stage of constant minimum creep

rate where the effect of strain-hardening

is balanced by an annealing influence

(secondary creep )

• A final stage where the creep rate

accelerates. The acceleration can have

many causes.It is attiributed to the

progressive increase of the true stress

acting on the cross-sectional area of the

specimen,and to metallurgical changes

and the formation of internal cavities

(tertiary creep).



Sample deformation at a constant stress (s) vs. time

Primary Creep: slope (creep rate)

decreases with time.

Secondary Creep: steady-state

i.e., constant slope.

Tertiary Creep: slope (creep rate)

increases with time, i.e. acceleration of rate.

s

s,e

0 t



Creep usually occurs at elevated temperature, T > 0.4 Tm

And effect of temperature incerase on the curve is seen infigure for constant stress condition

Adapted from Figs. 8.29,

Callister 7e.

elastic

primary secondary

tertiary



Creep tests can be done under different stresses too.

When tension-creep tests are conducted at various stressvalues (and a constant temperature), a fam ily of creep

curves is obtained as shown in figure



Both temperature & applied stress adversely affect thecreep strains.

Usually under the same temperature different stresslevels are applied & the creep strain vs time curves

are determined.

Creep

Strain

T1 or σ1

Time

T2 or σ2

T3 or σ3T4 or σ4 T1<T2<T3<T4

σ1<σ2<σ3<σ4



Creep can be a big problem in connection of parts like bolt.

And creep in constant load tensile specimen and a bolted

connection are not the same.

While the stress is constant in tensile test specimen due to

constant load, stress decreases (relaxes) in a bolt since

there is no external load to keep a constant stress

The figure shows that the initial rate of relaxation is high, but

it levels off as the stress level decreases creep rate

increases.

time

e

ep

eel

s

time



Stress relaxation

time

e

ep

eel

s

time

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