ISO 527-2, ASTM D638

REDUCING VARIABILITY IN TEST RESULTS

The following slides are provided as a summary of the most common

explanations for test data variability. Users should have a full

understanding of any international testing standards specifications and requirements

prior to testing.

FACTORS THAT INFLUENCE RESULTS

Specimen Alignment

Thermal Effects Grip Type Extensometer

Type

Method Setup TechniqueSpecimen Measuring

Anvils

Procedural Errors

Grip Faces Extensometer Attachment

Weight of Extensometer Speed of Test

= most common

Accuracy of Equipment Data Rate Algorithm

Rigid v. Flexible Load

String

So you’re performing the tests to the standard and you’re still seeing variability? There are many factors that can contribute to

variability in results…

BALANCING LOAD BEFORE TEST

Zeroing out or auto balancing load after specimen insertion will result in incorrect load/stress data, which

impacts all stress-related calculations (like

modulus)

Load

Strain

“Real” Curve

Curve from test

20lbf

0

1000

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0 1 2 3 4

Load [

N]

Tensile strain (Strain 2) [%]

Specimen 1 to 10

Specimen #

12345678910

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1000

2000

3000

0 1 2 3 4

Load [

N]

Tensile strain (Strain 2) [%]

Specimen 1 to 10

Specimen #

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SPECIMEN ALIGNMENT

Proper Alignment leads to• Increased strain

• Better repeatability

• Higher yield stress

• Increased tensile strength

Affects proven on: PP (30% glass), ABS, various TPOs

Misaligned Specimen

Aligned Specimen

CORRECTINGSPECIMEN ALIGNMENT

• Ensure specimen is vertical and centered in grips

• Available for most side-acting and wedge grips

Specimen Centering Aids

When extensometers are not attached properly, it can negatively effect the results. Some extensometers (Automatic) are much more consistent and easily-aligned.

Some extensometers are more susceptible to misuse and misalignment (Clip-Ons).

EXTENSOMETER PLACEMENT/ATTACHMENT

0

10

20

30

40

50

60

70

0 1 2 3

Tensi

le s

tress

[M

Pa]

Tensile strain (AutoX) [%]

Specimen 1 to 5

Specimen #

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0 1 2 3

Tensi

le s

tress

[M

Pa]

Tensile strain (Clip- on) [%]

Specimen 1 to 5

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Automatic Extensometers • Modulus

• Better repeatability

• Higher mean

• Strain

• Better repeatability

Affects proven on: PP (neat), PEEK, ABS, Data based off PP (30% glass)

Aligned Extensometer

Misaligned Extensometer

THERMAL EFFECTS

Heat from fingers may:• Causes necking • Lower strain • Lower yield stress

0

1000

2000

3000

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Load [

N]

Tensile strain (Strain 1) [%]

Average curve specimen handling without Gloves

Specimen #

12345

0

1000

2000

3000

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Load [

N]

Tensile strain (Strain 1) [%]

Specimen 1 to 5

Specimen #

12345

Affects proven on: PP (neat and 30% glass) & PPSU

ELIMINATING HANDLING EFFECTS

GLOVES

SPECIMEN FINGERS

FULL AUTOMATION

TIME

COST

EFFECTIVENESS

DECREASING VARIABILITY IN MODULUS

VARIABILITY

Inconsistent gripping force

Inconsistent specimen alignment

Insufficient data capture

rate

Inconsistent extensometer alignment

Pneumatic grips; Apply a preload

Alignment aids; different grips

Increase data rate; modify calculation (overlap/region)

More care attaching

extensometer; Automatic

extensometer

SOLUTIONCAUSE

DECREASING VARIABILITY IN STRAIN AT BREAK

VARIABILITY

Inconsistent gripping force

Inconsistent specimen alignment

Insufficient data capture

rate

Inconsistent extensometer alignment

Pneumatic grips; Apply a preload

Alignment aids; different grips

Increase data rate; modify calculation (overlap/region)

More care attaching extensometer;

Automatic extensometer

CAUSE SOLUTION

Heat from fingers

Avoid excessive contact with

specimen: wear gloves, wait

ASTM D638 vs. ISO 527 – 2

Key differences related to producing different results include:• Different specimen sizes• Different test speeds• Multiple speeds vs. single speed• Tensile strength calculation differs• Nominal strain calculation differs• Micrometer anvil shapes vary – difference directly

linked to how much plastic “sink” exists

Expect different results even when testing the same material

CONFIDENCE IN RESULTS

Procedure reflects the latest standard

Equipment meets the latest requirements

Requirements are all interpreted correctly

NOW WHAT?

Based on this information, it is recommended that you check your lab for the following

THANK YOU FOR YOUR TIME!

Please contact your local Instron® Sales Representative

with any questions.