Stress Relaxation - Elastocon AB

Stress RelaxationTest methods, instruments and lifetime estimation

Technical report 98/1, 4:th edition Nov 2010

Göran SpetzElastocon ABSWEDEN

IntroductionStress relaxation tests are be-

coming more and more popular fordetermination of rubber properties.From the beginning stress relaxationtests were used mainly in scientificprojects at universities, but a grow-ing use has been shown in recentyears and this may be caused by theintroduction of stress relaxation testsin product standards, such as sealingrings for pipes (1). The automotiveindustry has also started to specifystress relaxation tests for critical seal-ing products in the cars. This paperwill describe the test methods, the in-struments, test results and how to usethe test data for lifetime estimations.

DefinitionsWhen a constant strain is ap-

plied to a rubber sample, the forcenecessary to maintain that strain is notconstant but decreases with time, thisbehaviour is called ”stress relaxa-tion”. Conversely, when a a rubbersample is subjected to a constantstress, an increase in the deformationtakes place with time, this behaviouris called ”creep”.

The processes causing stress re-laxation may be physical or chemicalin nature, and under all normal con-ditions both types of process will oc-

cur simultaneously. However, at nor-mal or low temperatures and/or shorttimes, stress relaxation is dominatedby physical processes, whilst at hightemperatures and/or long timeschemical processes are dominant.

Standardised test methodsMore than 25 years ago ISO

TC 45 started to standardise testmethods for stress relaxation tests. Asa result the standard ISO 3384 waspublished (2). Later the standard ISO6056 concerning testing of ring testpieces in liquids was published. An-other standard, ISO 6914 (3),describes the testing of relaxation intension.

Later ISO 3384 and ISO 6056were combined into one standard ISO3384, dealing with tests in both airand liquids. ISO 3384 has now beenrevised several times with the objec-tive of improving the precision whendoing relaxation tests.

The present standard ISO 3384,includes two methods A and B whichboth can be used in air or liquids. Thetest pieces are either a cylindrical discof diameter 13 mm, height 6,3 mmor a ring with square cross section 2x 2 mm and ID 15 mm .

In method A, the compressionis applied and all counter force meas-urements are made at the test tem-perature.

In method B, the compressionis applied and all counter force meas-urements are made at standard labo-ratory temperature (23 °C). The testpieces are stored at the test tempera-ture.

ISO 6914 describes the testingof stress relaxation in tension. Thiscan be done by two methods, eitheron continuously stretched samples orintermittent stretched samples.The test piece is a 1 mm thick stripelongated 50%. .

What happens in the testedrubber material

In a rubber material, tested dur-ing time, the following processes canbe observed:

Physical relaxation, due to re-location of the molecular chains andthe fillers, when subjected to defor-mation. Most of the physical relaxa-tion happens during the first momentsafter a deformation.

”Stress relaxation tests are veryeffective for conductingageing tests, as substantialamounts of information resultwith little effort, especiallywhen using the continuousmeasurement system”

Thermal degradation, due toincreased movements of the molecu-lar chains at increased temperature,causing chain scission.

Oxidative degradation, causeschain scission due to oxidation.

All the above processes causea reduction in the counter force whendoing relaxation tests.

Cross linking - the cross link-ing can continue in a rubber material,depending on the cure system and thestate of cure. There are always someremains of curing agent and in thecase of sulphur cures, polysulfidecross links can brake and form newcross links in the tested material.

When doing continuous stressrelaxation tests new cross linksformed are considered not to causeany new stress. However the crosslinking will show if testing intermit-tent stress relaxation, which is in factmeasuring the change in stiffness ofthe material. The cross linking willalso clearly show when doing com-pression set tests, as the new crosslinks formed in the deformed statewill restrain the rubber from recovering.

Instruments for testing ofstress relaxation

There are mainly two types ofinstruments for the testing of stressrelaxation available on the market.Continuous measurement

One type of relaxation instru-ment is measuring continuously andconsists of a small rig with a load cell.When testing at elevated temperaturethe rig is placed in a cell oven andwhen testing at room temperature therig is placed in a room temperaturebox, as temperature stability is ofgreat importance for this test.

The rigs are connected to a dataacquisition box connected to a com-puter, storing the force and tempera-ture data, see figure 1, 2 and 3.

Figure 2 Relaxation rigs in a cell oven

Figure 1 Stress relaxation system for continuous measurements

Figure 3 Relaxation rigs, arranged for testing in compression,tension and in liquids

Discontinuous measurement.The other type of relaxation in-

strument is using test jigs, like com-pression set. The counter force ismeasured in a tensile tester or a spe-cial relaxometer, figure 4, beforeplacing the jigs at the testtemperature. The jigs are taken outof the test temperature at intervals andthe counter force is measured. Tomeasure the counter force in thesetype of jigs it is necessary to apply aslight increase in compression.

PrecisionIn the beginnining of the 90’s

an Interlaboratory Test Program, ITP,was done within ISO TC 45 for stressrelaxation tests according to ISO3384. The intention was to be ableto include a precision clause in thestandard. The result from this ITP wasvery disappointing and showed greatvariation between participating labo-ratories. As a result of this, some workwas done in Sweden at the Univer-sity of Borås (4), where different fac-tors influencing the test result wereinvestigated. The investigationshowed that the two most importantfactors influencing the test resultwere to keep the temperature and thecompression constant during the test.

The importance of keeping aconstant temperature within ±0,25 °Cduring the test, can be explained bythe different thermal expansion ofrubber and steel. The rubber sampleis placed in a steel rig and as rubberexpands about 20 times more thansteel when the temperature increases,it is of vital importance to keep thetemperature constant during the test.

ISO has during 1988 repeatedthe ITP with the new information in-cluded in the revised standard. Theresults are now much better (but notgood), see table 1. A general conclu-sion made when analysing the test re-sults, was that commercial instru-ments showed much betterreproduceability than home build in-struments.

Figure 4 Stress relaxation system fordiscontinuous measurements using jigs

ITP ISO 3384 Stress relaxation,1998Table 1, Precision

Method A 168 h at 23 C°; % relaxationMaterial Mean Sr r SR RA 10,9 0.795 2.22 1.21 3.40

Method A 168 h at 100 C°; % relaxationMaterial Mean Sr r SR RA 50.5 0.845 2.37 2.15 6.03

Method B 168 h at 100 C°; % relaxationMaterial Mean Sr r SR RA 67.5 2.07 5.8 8.66 24.3

Sr = repeatability standard deviation, measured unitsr = repeatability, in measured units (i.e..%relaxation)SR= reproducibility standard deviation, measured unitsR = reproducibility in measured units (i.e..%relaxation)

Precision with the continuoustype of instrument

Figure 5 and 6 shows therepeatability of the relaxation rigs.Figure 5 is a graph from two tests ofthe same compound run at differenttest periods. Figure 6 is a graph fromtwo samples of the same compondrun at the same test period in differentrigs.

Figure 7 and 8 shows how tocorrelate for the spring effect in theload cell and rig, when doing very ac-curate tests. The spring effect iscaused by the expansion of the loadcell and rig when the force decreaseswith time. Figure 7 shows a manualcorrection done twice during the test,by manual adjustment of the com-pression. Figure 8 shows a correctiondone in the software by a mathemati-cal calculation. The correction maynot be necessary when doing com-parative tests.

Figure 5

Figure 8Figure 7

Figure 6

One rig - different test periodsRepeatability

Relaxation:11 1999-02-26Elastocon AB

Two rigs - the same test periodRepeatability


Manual compensation


Automatic compensation


Figure 10 Arrhenius plot

Figure 9 Stress relaxation test on sealing rings

Figure 11 Stress relaxation test on hydraulic hoses

Estimation of lifetime from relaxationtests

Stress relaxation tests are ideal for makinglifetime estimations using an Arrhenius plot.

How to do an estimation of lifetime of rub-ber materials using an Arrhenius plot isdescribed in the ISO standard ISO 11346 (5).

When doing an Arrhenius plot, tests aremade of a critical property at different times andat least at three test temperatures. The tests arenormally run until the properties are reduced to50 % of the original value, see figure 9. The timeto reach this level is determined for eachtemperature. The test temperatures are chosen sothe test time for the highest temperature is at leastone week and the time for the lowest temperatureis at least 9 months.

The times to reach the ”end of life” time foreach temperature are plotted in an Arrhenius plot,which is a diagram with ln time on the Y-axis and1/T on the X-axis, where T is the temperature inKelvin, see figure 10. A straight line is drawnthrough the points and extrapolated to the tem-perature of use, to get an estimation of the life-time of the tested material.

Testing of productsFor determination of the lifetime for a prod-

uct with thin walls and exposed to air, the stressrelaxation test in tension with a thin strip, extended50 % is the best test. This test is very sensitive forbroken molecular chains, showing as a decreasein force.

For thicker products and seals of differentkinds, stress relaxation in compression is moresuitable.

The standards normally prescribe the relaxa-tion tests to be done on cylindrical test pieces.Relaxation tests are however also easy to do onproducts or parts of products, such as O-rings,sealing rings for pipes, weather strips, hoses etc.

Figure 11 shows a test run on the inner tubeof hydraulic hoses made of three different com-pounds. The test pieces were 40 mm long parts ofthe inner tube compressed 75 % and the test wasdone in Oil no. 1.

Stress relaxation tests are very suitable fordoing ageing tests as they give a lot of informa-tion with very little operator time, especially whenusing the continuous measurement system, com-pared to make e.g. tensile tests at different times.

Time, h

Relaxation index

1 5 10 50 100 500 1000 5000 100000,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

Relaxation - sealing rings


70 °C85 °C

100 °C

t1t2t3

Time, h

Relaxation index

0,1 0,5 1 5 10 50 1000,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

Tested in Oil 1Relaxation - hoses


Temperature,°C

Ln time, h

0 10 20 30 40 50 60 70 80 90 100 110 1201e2

5e2

1e3

5e3

1e4

5e4

1e5

5e5

1e6

Arrhenius plot


t1

t2

t3

Lifetime

Max temperature of use

1/T

Test results from different materialsFigure 12 to 15 shows results from testing in tension and figure 16 to 19 results from testing in compression.

Time, h

Relaxation index

0.5 1 5 10 50 100 5000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

NBRRelaxation in tension


70 °C

100 °C

125 °C

Time, h

Relaxation index

0.5 1 5 10 50 100 5000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

HNBRRelaxation in tension


100 °C

125 °C

150 °C

Time, h

Relaxation index

0.5 1 5 10 50 100 5000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

EACMRelaxation in tension


100 °C

125 °C

150 °C

Time, h

Relaxation index

0.5 1 5 10 50 100 5000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

FPMRelaxation in tension


150 °C

175 °C

200 °C

Time, h

Relaxation index

0.5 1 5 10 50 100 500 1000 50000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

SBRRelaxation in compression


40 °C

60 °C

80 °C

Time, h

Relaxation index

0.5 1 5 10 50 100 500 1000 50000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

EPDMRelaxation in compression


40 °C

60 °C

80 °C

Figure 12 Figure 13

Figure 14 Figure 15

Figure 16 Figure 17

Figure 18Relaxation test of a perfluoro rubber at three temperatures

Figure 19Arrhenius plot of the test in figure 18

Relaxation in compression - Greene Tweed FFKM 545

hours

Time, h

0.5 1 10 100 1000 5000

Force

F/F0

0.00

0.20

0.40

0.60

0.80

1.00

Temp.

°C

150

160

180

200

220

240

260

280

300

320

340

350

Temp 160 °C

Temp 180 °C

Temp 200 °C

Force 160 °C

Force 180 °C

Force 200 °C

R-St

ress

_Rel

axa

tion_

05_1

012

Elastocon AB • Göteborgsvägen 99 • SE - 504 60 BORÅS • SWEDENPhone: +4633 - 22 56 30 • Fax:+4633 -13 88 71E-mail:[email protected] • www.elastocon.se

References:

1. EN 681-1: Elastomeric seals - Materials requirements for pipe joint seals used in waterand drainage applications - Part 1 : Vulcanized rubber

2. ISO 3384: Rubber, vulcanized or thermoplastic - Determination of stress relaxation incompression at ambient and at elevated temperatures (revision of ISO 3384:1991)

3. ISO 6914: Rubber vulcanized or thermoplastic - Determination of ageing characteristics bymeasurement of stress relaxation in tension.

4. Stress Relaxation in compression of Rubber, Peter Henricsson, Daniel Marcus, BoråsUniversity College, Institution for technology.

5. ISO 11346: Rubber, vulcanized and thermoplastic - Estimation of lifetime and maximumtemperature of use.

Key Words: Stress relaxation, ageing, rubber, lifetime

Documents

Stress Relaxation - Elastocon AB