göran spetz elastocon ab - vkrt · heraueus5042 heraeus 5060e salvis tsw 60. ageing ovens ......

Lifetime estimation of Rubber materials

by stress relaxation tests

Göran SpetzElastocon AB

www.elastocon.com

Lifetime estimation of Rubber by stress relaxation tests:1

A study of ageing ovens

Development of ISO standards for relaxation

Development of a continuous stress relaxation system

Development of an ISO standard for lifetime estimation

This paper includes:


Old test tube ageing oven


Cabinet Ovens


Heraueus5042 Heraeus 5060E

Salvis TSW 60

Ageing ovens


Cell ageing oven, EB 01

Cabinet ageing oven, EB 04

12 laboratories, ITP 1988 mean s R (R)

Change in tensile strength, % -18 5,3 15 83Change in elongation at break, % -40 5,8 16 40Change in micro hardness, IRHD -13 3,8 10 77

s= std deviationR= reproducibility in actual units of measurements(R)= reproducibility in %

Ageing test reproducibility


temperature uniformity in time

temperature uniformity in space

air speeds

air exchange rates

ageing results in different ovens

Investigation of ovens


Temperature tolerances in ISO 471 are± 1 °C up to and including 100 °C± 2 °C 100 °C and up± 2 h time tolerance at test times 1 week or longer

1 °C wrong temperature corresponds to 10 % in testing time at an Arrheniusfactor of 2, or 15 % at a factor of 2,5.

This means that two laboratories can be 60 % from each other at a test at 125 °C and still be within the specification.

Temperature influence


temperatureair speed

Close investigations of the factorsHeat ageing


Heraeaus UT 5042

Heraueus UT 5060 E

Salvis TSW 60

Elastocon EB 01

Elastocon EB 04

Measured ovens


Oven 5042 5060E TSW 60 EB 01 EB 04°C 13,8 0,1 0,2 0,1 0,1

Temperature variations in timeHeat ageing


Location 5042 5060E TSW 60 EB 01 EB 04Inner 0,9 0,5 1,3 NA 0,4Centre 0,7 1,7 1,3 NA 0,3Outer 0,7 1,1 2,7 NA 0,2Total 1,2 1,7 3,1 0,5 0,4

Values in °C

NA= not applicableThe table shows the difference berween five points in each location and the total difference (all points all locations)

Temperature variations in spaceHeat ageing


Temp 5042 6060E TSW 60 EB 01 EB 04Set 99 97,5 97,2 100,1 100,0Shown 96-104 97,5 96,0 100,1 99,0Actual 96-103 100,0 100,0 100,0 100,0

Values in °CThe ovens were set to 100,0 °C in the center of each oven.

Set, shown and actual temperaturesHeat ageing


TSW 60Heat ageing


EB 04Heat ageing


Speed 5042 5060E TSW 60 EB 01 EB 04Min speed 0,5 0,0 0,4 <0,001 <0,001Max speed 2,6 4,5 3,0 <0,001 <0,001

Speed in m/s

Air speedsHeat ageing


Oven 5042 5060E TSW 60 EB 01 EB 04Open exhaust ~160 ~40 ~300 20 16Closed exhaust 0 0 ~20 0 0

Values in air changes per hour

Air exchange ratesHeat ageing


Time, hours

Hardness increase, m-IRHD

0 200 400 600 800 10000

5

10

15

20

25

TSW 60EB 01

NBR/PVC at 100 °CAgeing in different ovens


Temp, °C

Brotttöjning, %

90 95 100 105 1100

100

200

300

400

EPDMNR

Ageing, different temperatures


Time, weeks

Weight loss, %

0 1 2 3 4 5 6-10

-8

-6

-4

-2

0

3 m/s0,30,001

NBRAgeing, different air speeds


Time, weeks

Weight loss, %

0 1 2 3 4 5 6-2,0

-1,5

-1,0

-0,5

0,0

3 m/s0,30,001

EPDMAgeing, different air speeds


10 laboratories, ISO ITP 1997 mean r (r) R (R)

Change in tensile strength, % -7 8,5 121 11,7 167Change in elongation at break, % -24 9,2 38 12,2 50Change in micro hardness, IRHD -8,3 4,4 53 6,3 76

r= repeatability in actual units of measurementsr= repeatability in %R= reproducibility in actual units of measurements(R)= reproducibility in %

Ageing test precision


Ageing ovens


Cell oven

Triple cell oven Cabinet ageing oven

Relaxation in compression, ISO 3384 - measurement at test temperature, Method A- measurement at room temperature, Method B - measurement in liquid (rings)

Relaxation in tension, ISO 6914- continuous elongation- intermittent elongation

Stress Relaxation


Physical relaxation

Thermal degradation

Oxidative degradation

Continued cross linking

What happens in the materialStress Relaxation


The first relaxation rigHistory


Relaxation Rig


Relaxation System


compression, tension, in liquidRelaxation Rigs


Discontinuous Relaxation


Method A 168 h at 23 C°; % relaxationMaterial Mean Sr r SR R (R)A 10,9 0.795 2.22 1.21 3.40 31

Method A 168 h at 100 C °; % relaxationMaterial Mean Sr r SR R (R)A 50.5 0.845 2.37 2.15 6.03 12

Method B 168 h at 100 C°; % relaxationMaterial Mean Sr r SR R (R)A 67.5 2.07 5.8 8.66 24.3 36

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)

Table 1 PrecisionITP ISO 3384 Stress relaxation,1998


One rig - different test periodsRepeatability


Two rigs - the same test periodRepeatability


Keeping a constant deformation

Keeping a constant temperature

Important factors


Deformation, mm

Force, N

1,50 1,52 1,54 1,56 1,58 1,600

10

20

30

40

45 ° IRH70° IRH

Compression curve


Manual compensation


H= Orginal height of the test piece [mm]D= Compression during test [mm]d= Compression caused by the load cell [mm]F= Force [N]Fm= Maxium Force at deformation D [N]Fc= Compensation force [N]KLC = Load cell stiffness [mm/N]Fr= Real force after compensation [N]

compensating for the spring effectRelaxation Formula


D

d

F

H

Automatic compensation


Influence of room temperature variationsRelaxation test in room temperature


Stress Relaxation System with Draft Hood and Room Temperature Box


Time, h

Relaxation, %

1 5 10 50 100 500 1000 5000 100000

20

40

60

80

100

M-ringG-ring

Sealing rings 70 °CRelaxation Test


At a chosen test temperature, the variations in the numerical value of a chosen property, for example a mechanical or viscoelastic property, are determined as a function of time.

The testing is continued until the relevant threshold value of that property has been exceeded.

Further tests are carried out at at least two other temperatures.

Methods - principleISO 11346

Lifetime estimation


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

The life-time is estimated by extrapolation of the line to the temperature of use. The reaction rate at thattemperature will give the time to reach the threshold value.

The maximum temperature of use is estimated by extrapolation of the line to a specified reaction rate ortime to reach the threshold value. A time of 20 000 h is commonly used when establishing a general maximum temperature of use.

Methods - Results Arrhenius procedureISO 11346

Lifetime estimation


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

Temperature,1/T

Ln time, h

10

50100

5001 k

5 k10 k

50 k100 k

500 k1 M

-3,20e-3 -3,00e-3 -2,80e-3 -2,60e-3 -2,40e-3 -2,20e-310

50100

5001 k

5 k10 k

50 k100 k

500 k1 M

50 °C 100 °C 150 °C 200 °C

50 % reduction in force40 % reduction in force

FFKM 545Arrhenius plot


1/T

This point extrapolated

Temperature,1/T

Ln time, h

100

500

1 k

5 k

10 k

50 k

100 k

500 k

1 M

-3,20e-3 -3,00e-3 -2,80e-3 -2,60e-3 -2,40e-3 -2,20e-3100

500

1 k

5 k

10 k

50 k

100 k

500 k

1 M

50 °C 100 °C 150 °C 200 °C

Relaxation 50 % reduction in forceArrhenius plot for FKM


1/T

Stress relaxation tests are veryeffective for conducting ageing tests, as substantial amounts of information

result with little effort, especiallay when using the continuous

measurements system

Stress Relaxation


göran spetz elastocon ab - vkrt · heraueus5042 heraeus 5060e salvis tsw 60. ageing ovens ......

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