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Definit

Creep

ions

Deformaticonstant sat elevateplastic ma

obtained istress anpoint of thdiagram,or primar(secondahas an acrupture. S

Time

n that octress at ctempera

terials an

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ee also St

epend

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tures. Creis called

est usuallure constatime for ruf a materirts at a raas a relaticreep rateress-Rela

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period operature.p at rooold flow

is present. Slopepture. Asal can bepid rate aely uniforand termiation.

aviou

time wheIn metals,temperatr deforma

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-Cree

a matericreep usre is morion under

lot of creee is creep

the accothree sta

ith time. Sird stage (ilure of m

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load. Dat

p vs. timerate andmpanyingges. Firstecond statertiary) craterial at t

ted tos onlyin

withnd

stage,e

eepime for

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Creep Limit

Alternate term for creep strength.

Creep Rate

Time rate of deformation of a material subject to stress at a constanttemperature. It is the slope of the creep vs. time diagram obtained in a creeptest. Units usually are in/in/hr or % of elongation/hr. Minimum creep rate is theslope of the portion of the creep vs. time diagram corresponding to secondarycreep.

Creep Recovery

Rate of decrease in deformation that occurs when load is removed after

prolonged application in a creep test. Constant temperature is maintained toeliminate effects of thermal expansion, and measurements are taken fromtime load is zero to eliminate elastic effects. Creep Limit.

Creep Rupture StrengthStress required to cause fracture in a creep test within a specified time.Alternate term is stress rupture strength.

Creep Strength

Maximum stress required to cause a specified amount of creep in a specifiedtime. Also used to describe maximum stress that can be generated in amaterial at constant temperature under which creep rate decreases with time.An alternate term is creep limit.

Creep Test

Method for determining creep or stress relaxation behaviour. To determinecreep properties, material is subjected to prolonged constant tension or

compression loading at constant temperature. Deformation is recorded atspecified time intervals and a creep vs. time diagram is plotted. Slope of curveat any point is creep rate. If failure occurs, it terminates test and time forrupture is recorded. If specimen does not fracture within test period, creeprecovery may be measured. To determine stress relaxation of material,specimen is deformed a given amount and decrease in stress over prolongedperiod of exposure at constant temperature is recorded. Standard creeptesting procedures are detailed in ASTM E-139, ASTM D-2990 and D-2991(plastics) and ASTM D-2294 (adhesives).

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The Bi

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It is important to note that atomic mobility is related to diffusion which can bedescribed using Ficks Law:

D = DO exp( Q / RT)

where D is the diffusion rate, Do is a constant, Q is the activation energy for atomicmotion, R is the universal gas constant (8.314J /mole K) and T is the absolutetemperature. Thus, diffusion-controlled mechanisms will have significant effect onhigh temperature mechanical properties and performances. For example, dislocationclimb, concentration of vacancies, new slip systems, and grain boundary sliding allare diffusion-controlled and will affect the behaviour of materials at hightemperatures. In addition, corrosion or oxidation mechanisms, which are diffusion-rate dependent, will have an effect on the life time of materials at high temperatures.

Creep is a performance-based behaviour since it is not an intrinsic materialsresponse. Furthermore, creep is highly dependent on environment including

temperature and ambient conditions. Creep can be defined as time-dependentdeformation at absolute temperatures greater than one half the absolute melting.

This relative temperature ( T (abs ) /Tmp (abs )) is know as the homologous temperate.Creep is a relative phenomenon which may occur at temperatures not normallyconsidered "high." Several examples illustrate this point.

a) Ice melts at 0C=273 K and is known to creep at -50C=223 K. The homologous

temperature is 223/273 = 0.82 which is greater than 0.5 so this is consistent with thedefinition of creep.b) Lead/tin solder melts at ~200C=473 K and solder joints are known to creep atroom temperature of 20C=293 K. The homologous temperature is 293/473 = 0.62which is greater than 0.5 so this is consistent with the definition of creep.

c) Steel melts at ~1500C=1773 K and is known to creep in steam plant applications

of 600C=873 K. The homologous temperature is 873/1773 = 0.50 which is equal to0.5 so this is consistent with the definition of creep.

d) Silicon nitride melts/dissociates at ~1850C=2123 K and is known to creep inadvanced heat engine applications of 1300C=1573 K. The homologous temperature

is 1573/2123= 0.74 which is greater than 0.5 so this is consistent with the definition

of creep.

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Conce

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What kind of tests do we carry out in order to assess the creep behaviour of amaterial?

There are two kinds of tests that are of interest to engineers. These are the creeptest and the stress rupture test. Both types of tests require similar setups.

When a creep test is performed, the specimen is subjected to a given stress andtemperature that are kept constant throughout the test. Its elongation is monitoredcontinuously up to the point where it fractures. In order to be able to verify that stressand temperature do not vary during the test, they are also monitored continuously.

The elongation is then plotted as a function of elapsed time. The slope of the curve isthe so-called strain rate. A creep curve usually exhibits three distinct sections:section I: where the strain rate stabilizes and hence decreases (primary or transientcreep),section II: where the curve is linear, i.e. the strain rate (slope of the curve) is constant

and minimum (secondary or steady state creep),section III: where the strain rate starts to increase gradually until the samplefractures (tertiary creep).

The part of the curve which is of interest to designers is the straight one. Steadystate creep often takes up most of the time in a creep test. The knowledge of thecreep rate allows to estimate how much time is needed until a component reaches acertain deformation and, for example, becomes incompatible with the geometry ofthe system it is part of. The jet engine is a good example. The gap between the tip ofthe rotating blades and the engine casing has to be very small in order to maximizethe engine efficiency in that the amount of gas that flows through the engine withoutacting on the blades is kept as small as possible. However, if a blade elongates toomuch as a result of creep, it ends up scraping against the inner wall of the casingcausing serious damage. J et engine manufacturers and users don't want this tohappen! If, however, the creep rate of the material is known along with the lifetimealready spent by the blades, these can be replaced before the irreparable happens.

The stress rupture test is easier to perform because it does not require a continuousmonitoring of the specimen elongation. With this test one basically wants to know

how long it takes for a specimen to break at a given stress and temperature.

How are the results of creep tests represented?

Usually, tests are performed at different stresses and temperatures. From the creepcurve, one can determine the steady state creep rate, either graphically or through asuitable evaluation software. The different steady state creep rates are plotted as afunction of the applied stress with the temperature as a parameter. Usually, on a log-log diagram, one obtains a sheaf of straight lines, each line corresponding to a

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different temperature. This allows to estimate e.g. what stress must be applied inorder to achieve a certain steady state creep rate.

The representation of the results of stress rupture tests consists of a diagram wherethe time elapsed until rupture lies on the abscissa and the applied stress on the

ordinate. The data points for a single temperature also lie roughly on a straight line ina log-log representation. However, the lines tend to curve downward at longer times.Hence, a linear extrapolation towards longer times turns out to be optimistic and canthus be dangerous!

What are the mechanisms that govern creep?

There are different mechanisms that control creep, depending on the applied stressand on the temperature at which the test is performed. These involve dislocation

motion and diffusion of vacancies and interstitials.-Dislocation glide is determined by dislocations moving conservatively along theirglide planes. It occurs if the stress is high enough for the dislocations to overcomeobstacles in the lattice.-Dislocation creep is the movement of dislocations outside their glide plane and isassisted by diffusion of vacancies. It occurs only at relatively high temperatures, i.e.when the influence of diffusion becomes significant.-Diffusion creep occurs at relatively low stress. The deformation of the material isdue to the flow of atoms and vacancies that causes them to rearrange themselvesalong the direction of load. At lower temperatures the diffusion occurs mainly alongeasier paths, such as e.g. grain boundaries (Coble creep), whereas at hightemperatures, atoms and vacancies diffuse across the bulk of the material (Nabarro-Herring creep).-Grain boundary sliding is also an important mechanism. In order to maintaincontinuity within the creeping material, the grains must rearrange themselves andthey can do so only by sliding along each other. Moreover, grain boundary sliding isimportant as, in the third part of the creep experiment, it determines the onset offracture.

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Equipment for Engineering Education

Experiment Instruction

WP600 Creep Testing Machine

G.U.N.T. Gertebau GmbH

PO. Box 1125

D-22881 Barsbttel Germany

Phone (040) 670854-0

Fax (040) 670854-42

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Experiment Instruction

Publication no.:912.000 00A 600 12 01/94

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Table of contents

1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Technical description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3 Creep test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1 Creep in metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.2 Creep in plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.1 Set the end stop. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.2 Clamp the sample in position. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.3 Insert the sample holder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.4 Load the sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.5 Mount the cold box. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5. Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.1 Performing the experiment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.2 Lead samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.3 Polyethylene samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6.1 Technical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156.2 Formula symbols and units used. . . . . . . . . . . . . . . . . . . . . . . . . . . 16

6.3 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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1 Introduction

With the WP600 creep testing machine, it is

possible to demonstrate the typical phenomena of

creep responses, such as periods of different

creep rate or temperature-dependent creep beha-

viour, in a simple creep rupture test.

In order to generate acceptable creep rates at

room temperature which are suitable for demon-

stration purposes, lead and plastic samples are

used. These materials indicate clear creep at

room temperature and low stresses. The test lasts

between a few minutes and one hour.

Creep tests with other materials may take weeks

or even months. Moreover, very high test tempe-

ratures are required, particularly with metals.

The device has a simple structure and is easy to

operate. Simple flat samples are used which the

operator can easily produce from other materials

if required.

The constant load is applied in a visible manner

using sets of weights.

Tests outside of room temperature can easily be

performed with the aid of a transparent cold box

with cooler elements.

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Extension of the sample is measured by a dial

gauge (9). The dial gauge is directly in contact

with the upper, movable sample holder (7). This

eliminates measurement errors caused by slack in

transfer elements.

An adjustable end stop (3) for the transmission

lever protects the dial gauge when the sample

fractures. It also prevents the load weight from

striking against the base plate.

A transparent housing (10) serves as a climatic

chamber for the sample. The temperature of the

sample can be lowered using cooler elements

(11). The temperature can be monitored by me-

ans of a digital push-in thermometer (12).

Flat samples made of lead or polyethylene (PE)

are supplied as samples.

The measured cross-section is 2 x 5 = 10 mm2,

the measured length 25 mm and the distancebetween the fastening holes 60 mm. They are

clamped to the sample holders using clamping

jaws.

3

12

10 11

9

7

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In a creep test, the sample is subjected to a con-

stant load at a certain, constant temperature. The

extension of the sample over time is recorded.

By recording elongation over time, the so-called

creep curve is obtained. Three sections of the

creep curve can be distinguished:

- Phase 1: Primary creep

Reduction in the initially extremely high creep

rate. At this point, the influence of material

hardening predominates.

- Phase 2: Secondary creep

Virtually constant creep rate. At this point, thecrystal recovery and material hardening are in

equilibrium. This section need not necessarily

occur in all experiments.

- Phase 3: Tertiary creep

As a result of increasing reduction of area after

fracture and a rise in the effective stresses, the

creep rate increases again, leading to fractu-

re of the material. In the case of low-ductility

fractures, phase 3 may be very short.Components are generally loaded in such a way

that they only enter the secondary creep phase.

This determines the life of the component.

Secondary creep is determined by various factors,

the most important of which are stress and tempe-

rature. The most common relationship between

creep rate .

and stress and/or temperature T is

as follows:

. =AneERT

In this equation, A and n are material constants, E

is the activation energy to trigger sliding processes

at the grain boundaries, and R is the universal gas

constant ( 8.31 J/mol K ).

Fracture

Time t

Elon

gation

1 2 3

4

Influence of stress on creep

Fracture

Time t

Elon

gation

1 2 3Plastic

Elastic

Creep curve

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Logarithmizing produces the following:

ln .=lnA+n ln

E

RT

.

At a constant temperature, recording on a graph

produces straight lines with the slope n, whilst

constant stress produces straight lines with the

slope E.

These relationships do not always apply. For ex-

ample, the stress exponent n also depends on the

stress itself. In practice, therefore, other formulae,in some cases considerably more complicated,

have been developed to describe creep.

The time until fracture of the sample for various

loads can be recorded in a creep diagram. This

then produces the creep strength curve.

ln

Time ln t

Stressatfracture

Creep diagram

Creep strength curve

T= 25Cln

.

Stress ln

Strainrate

T = 20C

Influence of temperature on creep

Slope n

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3.2 Creep in plastics

Creep curves in plastics are similar to those of

metals. Various plastics such as polypropylene(PP) or polyethylene (PE) also indicate pronoun-

ced creep behaviour at room temperature. How-

ever, the reasons for creep are different to those

of metals. In creep, the macromolecules are

straightened and extended. Here, too, creep de-

pends on stress and temperature. Because of

the different molecule strctures, it is difficult to

specify generally valid relationships for creep in

plastics. Empirical studies have produced the fol-lowing formula which is valid for most technical

plastics:

=0+Bmtk

In this formula, is the elongation after time t. The

constants B, m and k depend on the material.

Elastic elongation 0 can be calculated using the

modulus of elasticity. In many polymers, this ela-

stic part is so small in relation to elongation as a

whole that it can be ignored

=Bmtk.

Logarithmizing produces the following:

log =log B+m log +klog t

With a constant stress, recording log over log t

produces straight lines with the slope k.For plastics, the constant k is between k= 0.025

and k= 0.33. The constant is a measurement of

the proportion of elastic to viscous deformation.

In plastics, elongation after fracture is very large,

which means that experiments often do not conti-

nue until fracture. When the load is alleviated, the

log

Time log t

Elongation

Elongation over time for plastics

Slope k

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creep deformation largely recedes - relaxation

takes place. In this respect, the time response is

of the same magnitude as under load.

Elastic relaxation

Time t

Elongation

Plastic relaxation

Creep curve with recovery after alleviation for plastics

Load Alleviation

> 0 = 0

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4 Operation

4.1 Set the end stop

- Remove the dial gauge (1)

- Suspend the weight suspender (2) in the trans-

mission lever (3)

- After loosening the lock nut (5), adjust the end

stop (4) in such a way that there is 10 mm play

between the suspender and the base plate

- Re-lock the end stop

4.2 Clamp the sample in position

IMPORTANT! The lead sample is very soft

To avoid bending the sample, mount the sample

holder on an even table surface.

Exercise particular caution when tightening the

locking screw

- The upper and lower sample holders are iden-

tical

- Insert the sample in the groove of the sample

holder

- Mount the clamping plate and carefully secure

using the hexagon socket-head screw

- Carefully align the sample and the sample hol-

der

DANGER when handling lead samples!

Lead is poisonous and harmful!

Take care to ensure that no lead is absorbed by

the body.

- Do not eat, drink or smoke whilst handling

lead samples! Do not allow the samples to

come into contact with food!

10 mm

1

54

3

2

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- After handling lead, wash your hands tho-

roughly!

- Do not put used lead samples in the regularwaste, but dispose of correctly !

4.3 Insert the sample holder

- Remove the weight suspender and dial gauge

- Insert the sample with the sample holder (1) in

the V-groove of the lower dolly (2). The locking

screws on the sample holders should be poin-ting forwards

- Raise the transmission lever (3) and suspend

the upper sample holder in the V-groove of the

transmission lever (4)

- Gently lower the transmission lever, without

jolting, until the sample is under initial load

- Insert the dial gauge (5) and adjust in such a

way that the display reads zero. This guaran-

tees maximum measurement capacity. Preci-

sely adjust the display by rotating the scale

4.4 Load the sample

Transmission ratioThe sample cross-section and lever transmission

are harmonised in such a way that a load of 1N

corresponds to a stress of 1 N/mm2 in the sample.

Initial load

By virtue of its own weight, the transmission lever

with an empty weight suspender generates an

initial load of 5 N corresponding to 5 N/mm2.

34

1

2

5

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Weights

Weights are graduated in 2 x 5 N, 3 x 2 N, 3 x 1 N

and 2 x 0.5 N. This means that it is possible to seta load of between 5 N and 25 N in increments of

0.5 N.

As far as possible, the load should be applied to

the sample without jolting. For this reason, the

weight suspenders should be suspended gently.

4.5 Mount the cold box

Before use, freeze the cooler elements in the ice

box of a refrigerator.

IMPORTANT! It is essential that the contents are

completely frozen, otherwise the cooling output

will not be constant due to absorption of the heat

of fusion.

- Insert the sample holder as described under

4.3. Do not insert the dial gauge- Insert the frozen coolant elements (1) at the

sides of the cold box (2) in such a way that there

is sufficient space in the centre for the sample

- Place the cold box on the fastening plate (3).

Make sure that the centering pin at the bottom

right snaps into place

- Secure the cold box using the fastening screw

(4) at the top left

- Insert the thermometer (5)

- Insert the dial gauge (6) and set to zero

Do not begin the experiment until the temperature

change is less than 0.5 per 5 min.

IMPORTANT! Immediately prior to the experiment,

re-adjust the dial gauge.

6

5

4

2 1 3

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5. Experiments

5.1 Performing the experiment

As high creep rates can occur, particularly in pha-

ses 1 and 3, it is advisable to perform the experi-

ment with two people.

- One person monitors the stop-watch and gives

the command to read the dial gauge.

- The second person reads the dial gauge and

records the reading.

In the initial phase and around the time of fracture,

the readings should be taken every 15 seconds.

At lower creep rates, an interval of 1 or 2 minutes

is sufficient.

To ensure that the experiment runs smoothly, a

form should be prepared with the prescribed times

of the readings for recording the deflections.

- Insert the sample in the creep tester as descri-

bed in section 4

- Fit the weight suspender with weights accor-ding to the required load

- Have a paper and pencil ready to record the

deflections

- Have a stop-watch ready to measure the time

- Set the dial gauge to zero

- Gently suspend the weight suspender, without

jolting, and start the stop-watch

- Read and record the deflections in accordance

with the time schedule

NOTE: With low loads, a creep test may last a very

long time. In order to explore the behaviour of the

sample, an experiment should first be performed

with a medium load. The load can then be adju-

sted in small increments ( 0.5 - 1.0 N ) depending

on the creep behaviour.

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5.3 Polyethylene samples

Appropriate loads in this case are between 16 and

20 N, corresponding to a stress of 16 - 20 N/mm2.

As the material permits very large elongation,

fracture is unlikely. However, the effect of reco-

very after alleviation of the load (relaxation) can be

shown very clearly.

The diagram shows typical creep curves.

With a sample length of 25 mm, a 10 mm extension

corresponds to 40% elongation.

Load alleviation

= 19.5 N/mm2

= 19 N/mm2

= 17 N/mm2

10

8

6

4

2

00 5 10 15 20

Time in min

Creep curves for polyethylene

Sample: PET = 20.6 C

Elongationinmm

Elastic relaxation

Plastic relaxation

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6 Appendix

6.1 Technical data

Samples

Material: Lead, polyethylene (PE)

Cross-section: 2 x 5 mm2

Measured length: 25 mm

Settable tensile stress: 5 ... 25 N/mm2

Set of weights: 2 x 5 N

3 x 2 N3 x 1 N

2 x 0.5 N

Dial gauge

Measurement range: 0 ... 10 mm

Resolution: 1/100 mm

Dimensions L x W x H:

700 x 350 x 510 mm3

Weight: 23 kg

Sample form

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6.2 Formula symbols and units used

A: Constant (mm2/Ns)

B: Constant (mm2/Ns)

E: Activation energy (J/mol)

k: Constant

m: Constant

n: Constant

R: Gas constant (8.314 J/mol K)

t: Time (s)

T: Temperature, abs. (K)

: ElongationL

L0

.: Strain rate

Creep rate (1/s)

Tensile stress (N/mm2)

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6.3 Index

A

Adjustable end stop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3B

Base plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2C

Clamping sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Climatic chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Cold box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Cooler elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Creep curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6in plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4rupture behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4strength curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Crystal recovery temperature . . . . . . . . . . . . . . . . . . . . . . 4D

Dial gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,3E

End stop setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9F

Formula symbols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16I

Initial load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10L

Loading sample. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10P

Performing the experiment . . . . . . . . . . . . . . . . . . . . . . . 12Primary creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

R

Relaxation, elastic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

, plastic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8S

Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3holder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Secondary creep. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Support pillar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

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T

Technical data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Tertiary creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Thermometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Transmission lever . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Transmission ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

U

Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16W

Weights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

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