Developments in Stress Relaxation and Lifetime estimation test methods

and instruments

Göran Spetz

Stress RelaxationStandardised test methods

Relaxation in compression, ISO 3384

a) measurement at test temperature

b) measurement at room temperature

c) measurement in liquids (rings)

Stress RelaxationStandardised test methods

Relaxation in tension, ISO 6914

a) continuous elongation

b) intermittent elongationthis is not relaxation, but measurement

of the change in modulus

Stress RelaxationWhat happens in the material

• Physical relaxation

• Thermal degradation

• Oxidative degradation

• Continued crosslinking

HistoryThe first relaxation rig

Relaxation rig and amplifier

Relaxation system

Relaxation rigsCompression, Tension, in liquids

Discontinuous relaxation

Discontinuous relaxation

Compression Tester

Control and measurement of:

• Temperature 0,1 °C• Force 0,1 N

• Displacement 0,001 mm

Discontinuous relaxationresults

Method A 168 h at 23 °C; % relaxationMaterial Mean Sr r SR R (R)A 10,9 0.80 2.22 1.21 3.40 31

Method A 168 h at 100 °C; % relaxationMaterial Mean Sr r SR R (R)A 50.5 0.85 2.37 2.15 6.03 11,9

Method B 168 h at 100 °C; % relaxationMaterial Mean Sr r SR R (R)A 67.5 2.07 5.80 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)(R) = reproducibility in %

Table 1 Precision

ITP ISO 3384 Stress relaxation,1998

Stress Relaxation b:9 2007-10-04Elastocon AB

RepeatabilityOne rig different test periods

RepeatabilityTwo rigs the same test period

Important factors

• Keeping a constant deformation

• Keeping a constant temperature

Deformation, mm

Force, N

1,50 1,52 1,54 1,56 1,58 1,60

0

10

20

30

40

45 ° IRH70° IRH

Compression curve

Stress Relaxation b:13 2007-10-04Elastocon AB

Manual compensation

Calculated compensation

Relaxation test in room temperatureInfluence of temperature variations

Relaxation test at elevated temperatureInfluence of room temperature variations

Stress Relaxation systemwith draught hood and room temperature box

Time, h

Relaxation, %

1 5 10 50 100 500 1000 5000 10000

0

20

40

60

80

100

M-ringG-ring

Sealing rings 70 °CRelaxation Test

Stress Relaxation b:20 2007-10-04Elastocon AB

Time, h

Relaxation, %

1 5 10 50 100 500 1000 5000 10000

0

20

40

60

80

100

M-ringG-ring

Sealing rings 70 °CRelaxation Test

Stress Relaxation b:21 2007-10-04Elastocon AB

Time, h

Relaxation index

1 5 10 50 100 500 1000 5000 10000

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

Relaxation - sealing rings

Stress Relaxation b:22 2007-10-04Elastocon AB

70 °C85 °C

100 °C

t1t2t3

Temperature,°C

Ln time, h

0 10 20 30 40 50 60 70 80 90 100 110 120

1e2

5e2

1e3

5e3

1e4

5e4

1e5

5e5

1e6

Arrhenius plot

Stress Relaxation b:23 2007-10-04Elastocon AB

t1

t2

t3

Lifetime

Max temperature of use

1/T

Temperature,1/T

Ln time, h

10

50

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-3

10

50

100

500

1 k

5 k

10 k

50 k

100 k

500 k

1 M

50 °C 100 °C 150 °C 200 °C

50 % reduction in force

FFKM 545Arrhenius plot

Lifetime estimation - Rubber Materials:33 2005-11-13Elastocon AB

1/T

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-3

100

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 force

Arrhenius plot for FKM

Lifetime estimation - Rubber Materials:31 2005-11-13Elastocon AB

1/T

ISO 3384-1 changes

• Divided into two partsPart 1 at constant temperaturePart 2 at cycling temperatures

• Technical revisions

• Addition of a calibration schedule

ISO 3384-2 Testing with temperature cycling

• Method A, Once a week at low temperature

• Method B, Continuous cycling

Temperature cycle for method A

Temperature cycle for method B

Example of a cycling test

Oven for cycling tests

Automatic Relaxation and Creep Tester

Automatic instrument

Control and measurement of:

• Temperature 0,1 °C• Force 0,1 N

• Displacement 0,001 mm

• Servo motor driven movement ofthe test rig

Relaxation test

Creep test

Stress Relaxation Tests

Stress relaxation tests are very

effective for conducting ageing tests,

as substantial amounts of information

result with little effort, especially

when using the continuous

measurements system