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Educational
PRODUCTS
~CQUIPMENT S1\!l06 CREEP MEAS_UREMENT APPARATUS
CONTENTS
SECTION
2.0
2.1
2.2
4.1
4.2
4.3
5.1
5.2
PAGE
LIST OF SYMBOLS
i
INTRODUCnON
1-1
THEORY
Creep in Metals
Creep in Plastics
2-1
2-1
2-3
DESCRIPTION OF APPARATUS
3-1
EXPERIMENTAL PROCEDURE
Lead Specimensat Room Temperature
Lead Specimensat Other Temperatures
4-1
4-3
4-5
4-6
Polypropylene Specimens
TYPICAL RESULTS
Lead Specimens
Polypropylene Specimens
5-1
5-1
5-6
TECQUIPMENT
SMI06 CREEP MEASUREMENT APPARATUS
UST OF SYMBOLS
~
Symbol
A, B, C
Constants
B
Activation energyfor creep
k
Time exponent(polymers)
m
Stressexponent(polymers)
n
Stressexponent(metals)
R
Universal gasconstant
t
Time
s
T
Absolute Temperature
K
a
Constant
&
Strain
£0
Initial (elastic) strain
E
Creeprate (Strainrate)
0'
Stress
kJ/mal
J/mol K
5-1
N/mm
1
2
TECQUIPMENT
1.0
SM1O6 CREEP MEASUREMENT
APPARATUS
INTRODUcrION
When a material like steelis plastically deformed at ambient temperatures,its
strength is increaseddue to work hardening. This work hardening effectively
prevents any further deformation from taking place if the stress remains
approximately constant. Annealing the deformed steel at an elevated
temperature removesthe work and restoresthe steel to its original condition.
However, if the steel is plastically deformed at an elevatedtemperature, then
both work hardening and annealing take place simultaneously. A
consequenceof this is that steel under a constant stress at an elevated
temperaturewill continuously deform with time, that is it is said to "creep".
Creep in steel is important only at elevated temperatures.In general creep
becomes significant at temperatures above about 0.4 Tm where Tm is the
absolute melting temperature. However, materials having low melting
temperatureswill exhibit creep at ambient temperatures.Good examplesare
lead and various types of plastic. For example, lead has a melting
temperature of 326°C (599K),and at 20°C (293K or about 0.5 Tm),it exhibits
similar creep characteristicsto thoseof iron at 650°C.
The SMIO6MkII Creep MeasurementApparatus is a simple unit designed for
demonstrating and investigating the creep characteristics of lead and
polypropylene specimens at room temperature. A temperature module is
provided to enableinvestigation of the effectsof temperatureon creeprate.
Page 1-1
TECQUIPMENT SMI06 CREEPMEASUREMENT APPARATUS
2.0
nIEORY
ConstantLoad or Stress
Failure
,#
Tertiary Creep
§
~
~
~
~
0
Secondary or
Quasi-viscous
Creep
Primary or
Transient
Creep
Plastic
2
.~
b
C/1
3
1
Elastic
Time
Figure 2.1 Typical Extension-Time Curve
2.1 Creep in Metals
A creep test is carried out by applying a constant load to a specimen and
observing the increasein strain (or extension)with time. A typical extensiontime curve is shown in Figure 2.1. Three regions can be readily identified on
the curve:
1 to 2 Primary Creep
- creep proceeds at a diminishing
rate due to work
hardening of the metal.
2 to 3 Secondary Creep - creepproceedsat a constantrate becausea balance
is achieved between the work hardening and annealing (thermal
softening) processes.
3 to 4 Tertiary Creep
- the
creep rate increases due to necking of the
specimenand the associatedincreasein local stress.Failure occurs at
point 4.
In terms of dislocation theory, dislocations are being generatedcontinuously
in the primary stage of creep. With increasing time, more and more
dislocations are present and they produce increasing interference with each
Page2-1
TECQUIPMENT SMI06 CREEPMEASUREMENT APPARATUS
others movement, thus causing the creep rate to decrease.In the secondary
stage,a situation ariseswhere the number of dislocationsbeing generatedis
exactly equal to the number of dislocationsbeing annealedout. This dynamic
equilibrium causesthe metal to creepat a constantrate. Eventually, however,
the creep rate increases due to localising necking of the specimen (or
component), voi.d and microcrack formation at the grain boundaries, and
various metallurgical effectssuch as coarseningof precipitates.
When in service, an engineering component should never enter the tertiary
stage of creep. It is therefore the secondary creep stage which is of prime
importance as a design criterion. Components which are subject to creep
spend most of their lives in the secondarystage,so it follows that the metals
or alloys chosenfor such componentsshould have as small a secondarycreep
rate aspossible.In generalit is the secondarycreeprate which determinesthe
life of a given component.
Secondary creep rate for a particular metal or alloy depends on several
variables, the most important of which are stressand temperature. The most
commonly used expressionfor relating secondarycreeprate & to suess0' and
absolutetemperatureT has the form:
;; = A an e-E!RT
where:
A and n are constants
E is the activation energy for creepin the metal
and
R is the universal gasconstant(8.31J/mol K)
The equation shows that the creeprate is increasedby raising either the suess
or the temperature.Taking natural logarithms gives:
1n S = lnA + nlna...
E
Thus for testsat constanttemperatureand varying stress,the stressexponent
n can be found by plotting In & against In 0'. Alternatively, if the stressis
kept constant and the temperature varied, E can be determined by plotting
In & against lIT. For the special caseof lead, the stress exponent n has a
value of about 10 for the relatively high levels of stress used in the SM106
MkII apparatus,and the activation energy E is approximately 120kJ/mol.
Page2-2
TECQUIPMENT
SM106 CREEP MEASUREMENT APPARATUS
Most metals have a stress exponent of about 5 and this value is also
applicable in the case of lead, but only when the stress is below about 5
N/mm2. At higher stress levels the exponent n increasesto about 10, and
eventually the simple power law of Equation 2.2 ceasesto apply. Instead an
exponential expressionmore adequatelyfits the experimental data:
& = B eaDe-E!RT
(2.3)
where B and a are constants.
A plot of In ;; against 0' will therefore yield a straight line of slope a. If the
stressis in units on N/mm2 (or lvIN/m2) the value of a is approximately 0.8
to 0.9 and alsovaries somewhatwith stresslevel.
The fact that exponentsn and a.vary with stressdemonstratesthe inadequacy
of simple laws for correlation of data over a wide range of stress levels. In
practice,more complicated equationsare used to correlateexperimental data.
For our purposes however, it is sufficient to use either of the Equations 2.1
and 2.3 since the resulting plots are very nearly linear for the stress levels
normally obtained with the SM106 Mk n apparatus. In this manual, the
power law of Equation 2.1 is used in the analysisof results.
Creep in Plastics
Plastics also creep at ambient temperaturesbut, compared to lead, they are
able to sustain much greater extensionsbefore failure. The creep curves are
similar in shape to those for metals, but the mechanism of deformation is
quite different because of the difference in structure of the material. A
polymer consists of long chain-like molecules in a tangled and coiled
arrangement;creep occursby chains untangling and slipping relative to one
another. The creep rate is still dependent on stress and temperature but
Equations2.1 and 2.3 no longer apply.
The complex processestaking place during creepmake it difficult to quote an
equation that describesthe creepbehaviour of all polymers. Many empirical
equationshave been proposed and one which applies to someof the common
engineeringplastics has the form:
8 = 80+ B am tk
Page 2-3
(2.4)
TECQUIPMENT
SMI06 CREEP MEASUREMENT APPARATUS
where &is the tensile creepstrain after a time t, cris the applied creepstress,&0
is the instantaneousor initial strain produced on loading, and 5, m, k are
constantsfor a given polymer. The elastic component of the initial strain can
be calculated by dividing the creep stress by the tensile modulus of the
polymer, which for polypropylene is 1250N/mm2. In many polymers this
initial strain is very small and canbe ignored, so that in thesecases:
f.=
Bcrmtk
(2.5)
A plot of log e againstlog t will thereforebe linear, and the slopewill give the
value of the exponentk. Values of k quoted in the literature range from 0.025
to 0.33.For polypropylene, k is in the range 0.1 to 0.2 and tends to increase
with stresslevel.
In caseswhere the stressexponent m is close to unity we have the situation
where 0'/8 is a constantas k ~ 0, in other words the material is behaving in
an elastic manner. Alternatively with high values of k, say k ~ I, then (0'/8)
is a constant and the material is behaving as a viscous fluid. The value of k
obtained from creepdata is therefore a measureof the relative contribution of
elasticand viscousdeformation to the creepprocess.
Finally, it should be noted that with polymer materials the primary creep
stage,where the creepis decreasing,is largely recoveredwhen the creepload
is removed. This behaviour is unlike that observedin most metallic systems,
and the effect canbe easily demonstratedusing the SM106Mkll apparatusby
removing the load after the polymer has been creeping for 7 to 15 minutes,
and continuing to take strain readings. It will be found that the elastic strain
is removed instantaneously,but that further recovery of strain takes place
over a period of several minutes. This time dependent effect is due to
recovery of the visco-elastic component of the creep strain. For the stress
levels used in the SM106 Mkll apparatus (typically 19 N/mm2),
approximately 40% of the creepstrain is recoveredafter 5 minutes.
Page2-4
~
TECQUIPMENT SMI06 CREEPMEASUREMENT APPARATUS
I
3.0
DESCRIPTION OF APPARATUS
I
I
..
.I
I
.
c~
I
I'
.
Figure 3.1Componentsof the SM106Mk n Apparatus
.I
Page3-1
TECQUIPMENT
SMI06 CREEP MEASUREMENT APPARATUS
Figure 3.2 Details of the Lever Arm
The SMI06 MkII Creep Measurement Apparatus, illustrated in Figure 3.1,
uses a simple lever to apply a steady load to the specimen. The specimen is
attached at one end to the lever mechanism by a steel pin and fixed at the
other end to the bearing block by another steel pin. To prevent deformation of
the specimen fixing holes, of the polymer specimens, during test, two 'U'
brackets are provided.
Loads are applied to the lever arm by placing weights on the weight hanger,
which is pinned to the lever arm. The weight hanger has two pinning
positions: the uppermost is used to pin the hanger in the rest position whilst
the lower hole is used to pin the hanger in the loaded position.
The lever arm has a mechanicaladvantageof 8. The massof the arm is 0.367
kg, the weight hanger mass is 0.16 kg, and the pins used for pinning the
weight hanger and specimenare 0.04kg each.The load on the specimencan
be found by taking moments about the pivot bearing as illustrated in Figure
3.2. If a mass m is added to the weight hanger then the tensile pull on the
specimen(F) is:
F = (2.84+ 8m) g Newtons
(3.1:
where
g
= accelerationdue to gravity
Note: The mass m does not include the mass of the hanger: this is
included in the constant 2.84.
The specimen extension is measuredby a dial test indicator (OTI). A tube
fixed to the bearing block is the housing for the DTI and a nylon pinch screw
is used to restrain the Dll under steadyload conditions. The top of the Dll is
attachedto the lever mechanismby meansof a grooved plate which is bolted
Page3-2
TECQUIPMENT SMI06 CREEPMEASUREMENT APPARATUS
to the lever arm. The arrangementis such that the groove in this plate is twice
the distancefrom the pivot than that of the centre of the specimen.Therefore
the extensiongiven by the DTI is twice the extensionof the specimen.It will
undoubtedly have been noted that the geometry of the system, whilst
permitting vertical movement of the DTI mechanism,constrainsthe extension
of the specimento follow a path which is the chord of a circle, whose centreis
the pivot and whose radius is the distancebetween the pivot and the centreof
the specimenpin on the beam. Hence the true extension of the specimen is
not exactly half of the extension indicated by the DTI; however, as the
maximum angle through which the beam pivots is only 20% the maximum
error in extensionshould be lessthan 1.5%.
It should be noted that when zeroing the DTI, the nylon pinch screw should
only be tightened finger tight, i.e. just sufficiently to prevent the DTI sliding
upwards when under steadyload. Over tightening could causedamageto the
DTI when the specimenbreaks.
Tests can be carried out above and below room temperature by using the
perspex encapsulation,which housesa thermometer,and the cold pack. Low
temperatures can be obtained by placing the cold pack in the freezer
compartment until the pack is frozen (usually 2 hours). Higher temperatures
are achieved by immersing the pack in hot water for 15 - 20 minutes. Note
that the water temperature MUSTNOT EXCEED
70°C to avoid damaging the
pack. To conduct a test, aboveor below room temperature,the pack is placed
to the rear of the bearing block closeto the specimen.Once the Dn has been
zeroed the perspex encapsulationcan be fitted. Before starting the test allow
10 to 15 minutes for the temperature inside the encapsulation to stabilise.
Once stabilised the temperatureshould remain relatively constantfor at least
30 minutes when hot and considerablylonger when cold.
The use of perspex encapsulationand cold pack is primarily designed for use
in demonstrations;however, when used with care, quantitative tests can be
carried out. When conducting a quantitative test, it is important to note that
the thermometer does not indicate the true specimentemperature. The only
sure way of obtaining the specimentemperature is to attach a temperature
sensor to the specimen. A thermocouple attached to the bottom of the
specimen with tape (to ensure a good thermal contact, a small quantity of
heat sink compound should alsobe used) canbe used successfully.
Page3-3
TECQUIPMENT SMI06 CREEPMEASUREMENT APPARATUS
4.0
EXPERIMENTAL PROCEDURE
40
30
20
3
0
5
10
15
20
Temperature(OC)
25
30
Figure 4.1 Typical Test Times for Lead Specimensat various Loads (m)
and Temperatures
Before starting an experiment a suitable load should be determined which
will produce a completecreepcurve in the time available. A suitable load can
be determined from Figure 4.1 which shows time to failure in terms of load
and temperature. Normally, the load should be chosento give a time of at
least 15 minutes, but this can be reduced for simple demonstrations. For
serious experimentsit is recommendedthat the specimensshould be labelled
and their cross-sectionsmeasured using a micrometer. This is to enable
subsequentcalculation of stress.
Page4-1
TECQUIPMENT SMI06 CREEPMEASUREMENT APPARATUS
n
I
8:
nl
~
E
-
.
I
I
Figure 4.2 Details of Specimen Loading Arrangement
Page4-2
TECQUIPMENT SMI06 CREEPMEASUREMENT APPARATUS
4.1
Lead Specimensat Room Temperature
1
Gently raise the lever arm and pin in the rest position.
2.
Remove the thumb nut retaining the grooved plate on the lever arm
and slacken the nylon pinch screw retaining the Dial Test Indicator
(Dn) in the tube.
3.
Using both hands, gently lift the Dn and the grooved plate dear of the
apparatus.Separatethe plate form the Dn and stow in a safeplace.
4.
Remove the specimenretaining pins from the lever arm and bearing
block.
NOTE:When fitting the specimenbetween the lever arm and bearing block,
caremust be taken not to bend the specimen.
5~
Measureand record the thicknessand width of the gaugelength of the
specimen.
6.
Fit the top of the specimeninto the lever arm and replace the specimen
retaining pin.
7.
Fit the bottom of the specimeninto the bearing block and replace the
specimenretaining pin (it may be necessaryto remove the rest pin to
allow somemovement of the lever arm; if this is done, then replace the
rest pin when the specimenhasbeenfitted).
8.
Refit the Dn and grooved plate but do not tighten up the nylon pinch
screw.
9.
Removethe rest pin and gently lower the lever arm to take up any free
movement. Zero the Dn and turn the nylon pinch screw until it is
finger tight.
NOTE:It cannot be over-emphasisedthat the nylon pinch screw should only
be tight enough to hold the Dn in position under steady load
conditions. Ensurethat the Dn travel limits are not exceededwhen the
specimenbreaks (i.e. when hanger contactsbaseof apparatus) to avoid
damaging the DTI.
Refit the rest pin.
Recordthe ambient temperatureand reset the stop watch to zero ready
to start the test.
Pal
TECQUIPMENT
SM106 CREEP MEASUREMENT APP ARA TUS
12.
Load the weight hanger with the required load, remove.the rest pin
and gently lower the lever arm to take up any slack.
13.
Raisethe hanger to the load position and refit the pin. Gently release
the load and start the stop watch.
14.
Record extension readings from the Dn every 15 seconds for the
primary stage of creep. When the extension rate slows down then
record readings every minute. As the test approachesthe tertiary stage
record readings every 15 secondsuntil fracture occurs or ~e weight
hanger bottoms.
Tests can be conducted at different loads in order to determine the stress
exponent n. Curves of strain (or extension)against time should be plotted to
determine the secondarycreeprate (the length of the test sectioncan be taken
as 2Ommfor calculating strains). The load on the specimen can be obtained
from Equation 3.1 and the stresscalculatedfrom the cross-sectionalarea.
Page4-4
~
TECQUIPMENT
4.2
SMIO6 CREEP MEASUREMENT APPARATUS
Lead Specimensat Other Temperatures
na
.
Figure 4.3 Arrangement for Test using the Temperature Module
Page4-5
TECQUIPMENT SMI06 CREEP MEASUREMENT APPARATUS
The sameprocedure can be used for tests at other temperatures,except that
the temperaturein the module must be allowed to stabilisebefore starting the
test If a direct comparison is to be made with a result at ambient
temperature, the test should be carried out at the same stresslevel. This can
be achieved by selecting a specimen of the same cross-sectionalarea and
using the same load. If more results are available, the load (and hence the
length of the test) can be chosen, using Figure 4.1/ by assuming that the
temperaturemodule will produce a temperaturechangeof about 3°C.
Procedure
1.
Heat or cool the pack asdescribedin Section3.
2.
Preparethe apparatusas describedin Section4.1 parts 1-10inclusive.
3.
Fit the pack and perspexencapsulationwith thermometer as described
in Section3.
4.
Allow 15 to 20 minutes for temperatureto stabilise.
5.
Carry out the procedure detailed in Section4.1 parts. 11-14inclusive.
Polypropylene Specimens
Tests on polypropylene specimenscan be conducted in the same way; but
when fitting the specimento the apparatus,the 'U. bracketsmentioned in the
first paragraph of Section 3 should be fitted over the specimen ends. The
polypropylene specimenscannotbe taken to breaking point, but by relieving
the load when the weight hanger is at the end of its travel, the effects of
elasticrecovery and creeprecovery canbe shown.
Pal