plastics testing: reducing variablity in test results
DESCRIPTION
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.TRANSCRIPT
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ISO 527-2, ASTM D638
REDUCING VARIABILITY IN TEST RESULTS
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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.
![Page 3: Plastics Testing: Reducing Variablity in Test Results](https://reader034.vdocuments.net/reader034/viewer/2022042613/5479986fb4795995098b47b5/html5/thumbnails/3.jpg)
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…
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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
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0
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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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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
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CORRECTINGSPECIMEN ALIGNMENT
• Ensure specimen is vertical and centered in grips
• Available for most side-acting and wedge grips
Specimen Centering Aids
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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
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60
70
0 1 2 3
Tensi
le s
tress
[M
Pa]
Tensile strain (AutoX) [%]
Specimen 1 to 5
Specimen #
12345
0
10
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80
0 1 2 3
Tensi
le s
tress
[M
Pa]
Tensile strain (Clip- on) [%]
Specimen 1 to 5
Specimen #
12345
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
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THERMAL EFFECTS
Heat from fingers may:• Causes necking • Lower strain • Lower yield stress
0
1000
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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
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ELIMINATING HANDLING EFFECTS
GLOVES
SPECIMEN FINGERS
FULL AUTOMATION
TIME
COST
EFFECTIVENESS
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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
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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
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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
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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
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THANK YOU FOR YOUR TIME!
Please contact your local Instron® Sales Representative
with any questions.